Centring Multiple Blow Pins at Same Station

When more than one pin is used on the same station, they must be set precisely at the same height or all the tops may not trim. The best way to set the height begins again by inverting the strikers and closing the moulds.

With the spanner nut snug, slowly and carefully lower the pins until the one of them contacts the piece of bar stock. Tighten this pin in place, loosen the spanner nuts and lower each remaining pin until it contacts the bar stock. Then tighten the spanners, check the centring and adjust if necessary. Finally, turn the strikers back over.

When More Than One Pin is Used, They must be Set Precisely at the Same Height
Use a similar method and bar stock to check a pin’s straightness. Place matched thicknesses of bar stock, in turn, under either side and then under the fronts and backs of the cutters on each pin.
Carefully apply pressure until the cutter contacts the piece. If the pin is straight, you will not easily remove one of the two pieces. If the pin is bent, you can readily pull out a piece. A bent pin may cause cutting problems and needs to be replaced.

Let’s review these two procedures we’ve just covered. Set all the pins at the same height and carefully lower them until the first one contacts the piece. Tighten it in place, lower the others until they too contact the pieces – tighten them as well. Check and adjust centring and turn the strikers over.

To check straightness, place the pieces under the cutter at points opposite each other. Try to remove one piece, if one can slip out easily, the pin may be bent. Bent pins can cause cutting problems.

Mounting Blocks
The mounting blocks secure the blow pins to the moving plate. The method differs from machine to machine. This machine uses a ‘T’ slot arrangement. By loosening the mounting bolts, the blocks can be slid into different centre distances to match the spacing of the mould cavities.

Mounting Blocks
Other machines have several tapped hole patterns which will accommodate the mounting blocks at different centre distances. On these particular machines, the moving plate usually acts as a blow air distribution manifold too. The air passes from one central connection through the plat and down into the blow pins.

Machines with Tapped Hole Patterns Which Accommodate the Mounting Blocks
When repositioning the mounting blocks on plates like these, it is necessary to ‘unplug’ the air ports at the correct centre distances and to block the ones that aren’t. After setting the blocks in the right positions, the mounting bolts are snugged down, and the centring is checked.

Repositioning the blocks is mostly done to change centre distances. It may be necessary though, if the pin can not be centred side to side suing the fine adjustment on the mounting block. If this is the case, loosen the mounting blocks and move the blocks until it can be centred within the range of the fine adjustment.

Repositioning the Blocks is done to Change Centre Distances
As stated previously, depending on the type of machine, the mounting blocks may be secured with a ‘T’ nut system – or they may bolt directly to the moving plate. By loosening the mounting bolts these may be repositioned over the correct hole pattern.
Before securing these, air ports must be opened, and the other plugged. The blocks are usually moved to match a different centre distance. They may also be moved if a centring adjustment is out of the range of the adjusting screws. At the base of the station is a fixed bottom plate. The plate itself or the attached stripper plates are designed to strip the top flash and the containers flow the blow pins.
When centre distances change it is required by some designs that the entire bottom plate be changed as well. These strippers are repositioned by loosening the bolts holding them onto the plate.

Lower the blow pins, centre the strippers, and tighten the bolts as you hold each plate in its new position. Depending on the design of the strippers and the tolerance between them and the blow pins, they may have to be loosened during the blow pins centring procedure. Be sure to retighten them afterwards.

Hydraulic Cylinders
The moving plate travels up and down as pressurized oil or air is directed to one or the other side of the calibration cylinder’s piston. Hydraulic cylinders have become the standard on calibration stations because oil power can be controlled better than air power.
Control of the cylinder’s movement is critical to the efficient production of quality containers. It is important to control the speed at which the cylinder travels. It is most important to control the distance that allows the blow pins to travel downwards. As the machine control directs pressurized air or oil to one side of the cylinder, what leaves the other can be throttled by adjusting a flow control valve.
Downward speeds should be set so that pins are inserted quickly but not so fast that they damage the mould. The upward motion should also be quick so that cycle time is saved. Some machines are equipped with proportional hydraulic controls that allow speed to be varied during different segments of the cylinder’s movement.
Rather than adjusting manual flow controls, dials are adjusted for the desired speed during each part of the station’s movement. In most cases the controls should be set so the beginning and the end of the stroke is slow and the middle part of the stroke is fast. This combination produces quality containers, maximizes the machine’s cycle time, and minimizes wear on the machine.
Whether manual valves or proportional controls are used, the cylinder’s speed should always be set so that minimizing wear on the machine is the first consideration.
The most important adjustment on the calibration cylinder is the ‘stop nut’. The ‘stop nut’ limits the downward travel of the blow pins. It must always be adjusted so that the cutting sleeves just barely contact the striker plates.

To set the stop nut, carefully jog the cylinder until the cutters first contact the mould. Loosen the lockdown and spin the nut clockwise until it bottoms out on the cylinder housing. Next, retract the cylinder rod slightly; lower the stop nut another half turn. Lastly, retighten the lockdown.
Once in production, readjust the nut until all the nuts are being cut.

The cycling may have to be stopped to make this adjustment safely. Always use extreme caution when adjusting the stop nut. Protect your personal safety and be careful never to raise the nut so much that cutters can travel beyond the strikers and in to damage the mould.

The stop nut adjustment should always be checked after cutters are changed. They will always have to be readjusted whenever moulds are changed as part of a new setup. If all the flash on the same side can’t readily be cut with a stop nut adjustment, check that all pins are adjusted to the same height using the pieces of bar stock.

Calibration Station Components

The calibration station is designed with adjustable mechanisms and interchangeable parts that allow desired neck dimensions to be moulded. Let’s look at the components of the station and how they work.

The blow pin assemblies do the neck finishing. They are held in place by the mounting blocks, which are secured to the moving plate in the station’s die setting. The system of plates and guide rods moves the blow pins as the calibration cylinder’s piston rod extends and retracts.

The frame or stanchion mounts the extension to the blowmoulder. The stanchion has adjustments to help set the position of the calibration equipment relative to the position of the moulds.
Here is a blow pin assembly. The stem is a precisely machined metal part. One end is made to accept the cutting sleeve and the blow pin tip. The cutting sleeve is a hardened steel ring with sharp edges. During blow pin insertion, it cuts through the plastic and finishes the very top of the container’s neck. Its inside diameter should be very close to the stem’s outside diameter so it can slide on but fit snugly. Its outside diameter approximately corresponds with the diameter of the neck finish.

The Calibration Station
The blow pin assembly is designed so that the cutter can be replaced easily. It eventually dulls and leaves a ragged edge on the top of the neck or stops cutting the plastic altogether. In either case, the cutter needs to be changed on a regular basis.

The Cutter Needs to be Changed on a Regular Basis
The blow pin tip screws onto the stem and holds the cutter in place. A hex slot is usually provided in the end of the tip. Moderate torque is applied with a hex wrench. These parts are designed so that there is no gap between them once they are tightened.
The tip is designed with the container’s eye dimension in mind. Its size and shape largely determine the finished dimensions inside the bottle neck. A larger diameter on the tip will increase the eye dimension, while a smaller diameter on the tip will decrease it. The tips are always made slightly larger than the desired eye dimension because the plastic shrinks slightly as cooling continues after calibration.

Certain caps require the necks have special shapes forming their eye dimensions. Liner less caps do not have the cardboard inserts to act as a seal; but have a ridge of plastic that forms a seal against the special radius moulded inside the neck. This radius is formed by a specially shaped tip.

Caps with Necks that have Special Shapes Forming
Besides shaping the inside of the neck, the tip also plays a part in forming the wall and threads on the outside. As the tip inserts, it forces hot plastic outward and into the threads. The ‘E’ dimension is the outside dimension of the container neck. It’s measured from the root of the thread on one side to the root of the thread on the opposite side.

“E” Dimension Refers to Outside Dimension of the Containers Neck
The ‘T’ dimension is the overall thread diameter. The dimension to which the metal is cut into the mould’s thread insert largely determines the containers ‘T’ and ‘E’. Both dimensions however can be varied by changing the blow pin tip’s diameter. Increasing the diameter may create larger dimensions, while decreasing it may make them smaller. Remember that any change in the blow pin tip’s size will have a definite effect on the eye dimension.

‘T’ Dimension is the Overall Thread Diameter
The pressure at which the blow air is set may also have an effect on the outside neck dimensions, especially on thin-walled necks. In these cases an adjustment to the blow air pressure can increase or decrease outside dimensions. Be careful never to adjust the pressure so high that it overcomes the clamp pressure. This can show up as a heavy seam at the next parting line.

Centring Blow Pin
Good cooling is critical in forming good necks. If cooling is poor, necks can wrap as they continue to cool on their own. A separate supply hose and a separate drain hose should be connected to each blow pin. The other end of the stem is machined to fit into the mounting blocks. The mounting design can vary with different blow pin designs.
Here the pin is held in place by tightening the two spanner nuts. The mounting blocks have two or more adjusting screws for positioning the pin precisely in the centre of the cavity.

Centering of the Blow Pin is Critical
Positioning must be exact, front to back and side to side, or the pin will enter the parison ‘off centre’. This may cause the plastic to be stuffed into the neck. In the case of extreme misalignment, the wall and threads may be left incomplete.

The blow pin is centred visually using a combination of adjustments to the mounting block adjustment screws. The best method for doing this begins by inverting the striker plates in the top of the mould. Then close the moulds and lower the pins very slowly and carefully until they are right above the mould.

Blow Pin is Centred Visually
Visually check and assure that they can insert without interference. If one or more pins can’t, then adjust the screws appropriately. Then lower the tips until they are half way in the strikers.
Adjust the blow pin until the space between each tip and its striker seems even all the way around. Finally, snug down all the adjustment screws and complete the procedure by putting the strikers back in the right way. Lastly, tighten down all the bolts.

Let’s go over the procedures for centring the blow pin. First, invert the strikers and slowly and carefully lower the pin. Adjust the screws so that they can insert without interference and lower them half way into the strikers. Adjust the screws so that the space is even, snug down all the screws and turn the strikers.

The mounting blocks also provide the adjustment for setting the initial height of the pins.

Decoration

Consumer packaging containers usually have some kind of decoration. Paper labels are sometimes applied by special auxiliary equipment that works in conjunction with the blowmoulder, but most often, labelling takes place on the filling line.

Containers can also be decorated by machines that apply ink directly to the container wall. Decorating is done by a variety of machines that are either operated manually, with a silk screening machine, or semi-automatically, by a high speed silk screening machine or heat transfer labelling equipment.

Whatever the type, they all have two things in common. First, they can not decorate defective containers. Second, a functionally defective article may jam or damage equipment.
Printing puts pressure on the walls of the container as the ink is applied through the screen. Many containers are printed continuously around the outside. The thickness of the walls must be fairly uniform and the surface must be smooth and free of bumps and valleys. And since ink will not adhere to dirt or grease, all articles must be very clean.

A container is held in the jigs, which are designed to exactly match their specifications. If the container is defective and fits poorly in the jig, it could shift and ruin the screen. Heat transfer labelling machines operate at fairly high speeds, and have sophisticated parts and timing.

The ink label has been printed on wax paper. As the containers pass through the transfer station, a heater melts the wax and presses the ink onto the container. Like the silk screeners, it has jigs and fixtures that precisely orient the containers. These are also designed around the container’s specifications.

The ink label has been printed on wax paper. As the containers pass through the transfer station, a heater melts the wax and presses the ink onto the container. Like the silk screeners, it has jigs and fixtures that precisely orient the containers. These are also designed around the container’s specifications.

The quality and efficiency of the decoration process is very dependent on the quality of the blowmould containers. Defective or off-spec articles can not be decorated properly and can severely damage auxiliary equipment.

Filling Line
The last of the product manufacturing steps usually takes place on the filling line. It is a series of high speed sophisticated machines that are all interconnected by conveyers.
Bottles are sent into a unit called an ‘unscrambler’. It automatically orients them so they can be stood up on to conveyer and sent down stream.
The bottles are metered into a carousel unit for filling. As they make the trip around the circle, the fill tubes are inserted and the product is pumped in. The fill tubes are retracted, and the bottles are placed back on the conveyer.
Next, they enter the station which applies and tightens the caps. From here, they move to the labeler. Several paper labels can be applied simultaneously, each containing the manufacturing lot or the date of production.

Inspectors visually check the product to assure that each is properly filled, labelled on both sides, and capped before hand packers or machines place the products in shipping containers. These are sealed and sent away from the line to be palletized and shipped to stores.

As with the decoration machinery, the equipment on the filling line is designed with the container specifications in mind. Efficiency is dependant on the total absence on functionally defective bottles. If the bottom flash is left on the containers, they can fall over and the others to jam up.
If a neck finish is incomplete, or the opening is too small, the fill tubes can get ruined as they insert. Fill tubes are fragile, expensive assemblies that can only be replaced while the filling line is down. A leak in a container requires that the line be stopped, so the mess can be cleaned before operations resume.

Cappers operate at a certain torque which can not overcome a lot of resistance. If the neck dimensions are over specification, the cap can not be applied. Conversely, if necks are under spec, the caps will be too loose. This may allow the product to leak.

The quality and efficiency of the filling line operations are dependant on the quality of the containers. After leaving the filling plant, the product goes under the most important inspection. The ultimate test for container and product quality comes at the hands of the consumer and may be the deciding factor on whether or not they will repurchase the product.

Quality Standards

Blowmoulding is not an absolute science, nor is the process exact. As with most other manufacturing processes where machines and people are mixed, problems can arise. Today’s blowmoulding machinery is extremely reliable, yet processing drifts can still be expected. Even with the best maintenance, machines wear.
Furthermore, the performance of these machines relies on the skill levels of the machine mechanics, the setup personnel, the process engineers, and other workers and managers involved in production.
Any deficiencies with the equipment or with the operating personnel can show up as low product quality. Because these instances are unavoidable, blowmoulders use procedures to analyze and report product quality during manufacturing. These tasks are usually referred to as ‘quality assurance procedures’. Other tasks are used to prevent defective containers from leaving the production process. These are called ‘quality control procedures’.
The objective of both is the same; to assure that products meet the quality standards that have been set. Quality standards are those things about a blowmoulded item which are desired or required by the end users.
On plastic articles, these things, or attributes, fall into two general categories; visual and dimensional attributes.
Visual attributes are those that deal with appearance. Since consumers often look at the packaging and make an unconscious decision about the quality and value of the product inside – it is imperative to achieve the specified attributes. These include; colour, clarity, cleanliness, and other visual things.
Dimensional attributes are those which deal with the size, shape, and construction or the blown article. Blowmoulders need to meet the certain dimensional requirements so their parts will work with others to form an end product.
The structural requirements assure product is held without deforming. And since containers are used to hold a specific volume of product – the thickness must also be ‘in-spec’.


Specifications
Specifications, or specs, can be thought of as a language for clearly communicating ideas about quality standards. For example, the quality idea may be that this bottle should hold a little bit more than fifteen ounces of water. A specification could clarify that idea, it could state that the volume be fifteen and one-half ounces – a little more must be interpreted. Since fifteen and one-half ounces can be measured, it means the same to everyone who understands liquid volume.

Blowmoulded Part Specifications
Specifications like this one quantify something important about a blowmoulded product. A numeric value is assigned to attributes which can be measured or weighed, Specs for size, shape, weight, and volume are usually assigned numeric specs based on inches, or thousandths of an inch, gram weights, liquid measures.
Visual attributes can be quantified. However, expensive and sophisticated testing equipment is needed. Some companies can not justify the large expense, and will simply compare a visual attribute of the blowmoulded article to a target sample.
The quality idea is ‘this bottle should be blue’. The specification is that ‘the bottle colours match an approved colour sample’. While less finite than a measurement, this eliminates nearly all interpretation about the required colour and shade of blue.
Let’s review what we’ve covered so far about specifications. They form a language for clearly communicating quality ideas. Most dimensional attributes can be quantified; their specifications can be set in measurements or weights. Most visual attributes can be compared to approve samples.
Specifications usually include a tolerance. A tolerance is an accepted deviation from the desired quality standard. In our example using volume we set the specification at fifteen and one-half ounces of water, but we know that within the blowmoulding process it will be nearly impossible to make every container’s volume exactly fifteen and one-half ounces.

Include Tolerance in Specifications
Thus a tolerance is added to the spec. Our example becomes fifteen and one-half ounces, plus or minus one-quarter ounce. Some quick math tells us that the spec requires the container’s volume to range from a minimum of fifteen and one-quarter ounces to a maximum of fifteen and three-quarters ounces of liquid.

Ranges are established for the visual attributes which can not easily be measured. The specification says the blue bottle is to match the blue sample. Creating an approved colour range establishes a tolerance – the colour may range in shade from the light limit to the dark limit.

Ranges Established for the Visual Attributes
Because blowmoulding is not an exact manufacturing process, tolerances are added to specifications. They are provided so visual and dimensional attributes can fluctuate over an exceptional range, yet still meet quality standards.

Critical Container Attributes
Some critical container attributes that are usually assigned specifications include the dimensions for an article’s size, it’s height, it’s width from side to side, and it’s depth from front to back. These are normally referred to as the ‘body specifications’.
On containers intended to work with caps or special closures, several specifications for the neck are called out. For example, the outside diameter of the threads, the height of the neck, and it’s inside diameter are referred to as the ‘neck finish specifications’.
Another spec is set for the thickness of the plastic that makes up the container’s wall. Specifications for these dimensional attributes are called out in inches or thousandths of inches, or in metric equivalents.
Other dimensional attributes are evaluated through measurements of weight, or liquid measure. The volume of liquid which the container must hold is sometimes called out through two specifications – the ‘fill line’ and the ‘overflow line’. These are sometimes measured in liquid ounces but more often in cubic centimetres, or ‘CC’s’, which are more accurate units of measure. The amount of plastic required in the article is it’s net weight. Weight is usually expressed in grams.
The most common visual attributes are those we’ve already covered. The colour spec is set so that the shade falls within an established range. Bottle clarity must allow certain print sizes to be read through the container wall. Speckles and contaminants in the plastic are usually limited in size, number, and location.
Those we’ve discussed are the critical specifications. There are many other dimensional and visual attributes that depend on the intended use of the product and on the end user’s requirements.

To quickly review the critical dimensions – specifications are those that are measurable and are set for dimensional attributes like size and shape and also for neck finished, volume and container weight, and for wall thickness. The specs set for visual attributes must be evaluated by quality personnel and include; colour, clarity and cleanliness.

Specifications are Those that are Measurable

Preventing Loss

The accumulation of excess regrind must be minimized to prevent resin loss. While flash is likely to account for thirty percent or more of what is extruded, there may be unavoidable periods where one hundred percent of what is extruded is converted back to regrind.
For example, during start-ups and shutdowns or while in production, an in-process defect may require one or even all of the containers to be rejected until the correction is made.
During these periods, the proportion of regrind being added to the closed loop systems should be raised slightly to keep the regrind at a manageable level. Regrind levels must be held in check. Any regrind backlog is, in fact, valueless waste until it is converted into product. An accumulation increases resin expense and lowers profits.
Producing overweight containers is the most overlooked cause of resin loss. Each blowmoulded part normally requires that it be made with a specified amount of plastic. This amount is usually expressed in weight.

Consistent Monitoring of Product Weight
Variations in the extrusion process often make the weights drift. Weights should be monitored on a consistent basis, and any overweight condition should be adjusted so that resin isn’t needlessly wasted. If left unchecked, this too can increase resin expense and lower profitability.

Contamination

Part of every employee’s job is to prevent resin losses through these three major causes: contamination, excess regrind and overweight containers. Resin in any form must be free of contaminants like dirt, metal, or any foreign matter and can not be processed through a blowmoulding machine unless it is perfectly clean.
This includes virgin pellets, flash, or out-of-spec containers. The most frequent cause of contamination occurs because something other than clean plastic is inserted into the granulator.
When this happens, it too gets ground up and blended with the regrind and ultimately gets sent through the extruder. Unfortunately, this problem is usually detected after it has caused others.

Contaminants
Contaminants can get stuck in the extruder and if deemed necessary, production must be stopped while the machine is dismantled and cleaned. If dirty material does manage to get extruded, the parts made with it will have to be discarded.
Machine downtime hurts production efficiency and discarded articles affect resin expense. Contamination can be avoided by careful inspection of anything hand fed into the granulator. Whether it’s flash, out-of-spec containers, or purgings – a check for cleanliness is time well spent.
Automatic flash conveying systems include magnets located to catch ferrous metal parts or tools which may find their way into the system.
Floor sweepings should never be ground up, as even a small amount of dirt could be harmful to the process. Another measure to prevent contamination is to maintain tight fitting guarding under press sections on part conveyers and around inspection stations.

Never Ground Up Floor Sweepings
Contamination also occurs when dissimilar resins are mixed. Combining one resin with another ruins both and makes the batch unusable. Such losses can be prevented by thoroughly cleaning the material system when changing over from one resin to another or from one colour to another. Extreme care should be taken to ensure that the correct resin and colour goes into the right system.

Resin Flow within the Plant

As you learned earlier in the series, a continuous extruder converts the resin into a fluid-like mass, and ultimately a parison, by melting and mixing the pellets. The parison is then closed in moulds and blown out to a given shape.
When the moulding is complete, two things remain; the finished hollow plastic article and some extra plastic which was the part of the parison pinched outside of the mould. This extra or excess plastic is referred to as ‘flash’.

Flash
As a normal part of the continuous extrusion process, around thirty percent of the plastic put into the extruder ends up as flash. A higher percentage can be expected when moulding handled or irregular shaped containers – sometimes as much as fifty or sixty percent.

Since resin is so expensive, it is advantageous and feasible to reclaim virtually all of the flash produced. This is done by granulating, or grinding, it down to pellet size and feeding it back into the process. Off-spec parts and purgings made by the extruder during startups, shutdowns, and adjustments can also be reclaimed.

Recycling by Granulating and Grinding
Let’s examine the flow of resin throughout a blowmoulding facility in greater detail. The closed-loop resin handling system begins with a vacuum unit to transfer the virgin pellets out of storage. This usually consists of a major network of pipes and vacuum pumps so that different types of resins can be transported to different locations.
The first stop for the virgin pellets is the ‘blending station’. This area also contains the granulated reclaimed material called ‘regrind’. And since most resin in its virgin state is colourless, colouring agents are also added to meet the containers cosmetic specifications.
The blender is equipped with controls and systems to automatically combine a set percentage of the virgin, regrind and colorant. The finished blend is distributed to the vacuum loader finds its way to the extruder hopper – where it is ready to begin its trip through the blowmoulding process.
At the end of the trip, the resin either leaves the closed loop as a finished product or is fed back into the loop to make another trip.
Flash is normally vacuumed directly to a granulator. Defective parts and extruder purgings are usually hand fed into the grinder. This material is chopped up to a size similar to the pellets and is then blown to the blending station.
The equipment does the bulk of the mixing and conveying, but it is the good resin management habits of the production personnel that ultimately maximize the use of the material.

General Resin Information

Resins used to blowmould containers may differ in name and formulation, but they all have a few things in common. Material is expensive; the largest single cost incurred when producing a blowmoulded container is usually resin.

Costs in a Typical Container
In fact, the expense of the resin often exceeds all other operational costs combined. One truckload of resin can cost twenty to thirty thousand dollars depending on the material type.
Productivity is adversely affected when foreign substances are introduced to the resin handling system. Imperfections caused by contamination result in lost time and increased work because of the need for closer inspection.
A successful operation can not tolerate financial losses – and poor resin handling or excessive waste of materials can turn potential profits into major losses. To understand more about resin, let’s examine how it is used in an extrusion blowmoulding plant.
Virgin pellets arrive in one of three ways; by truckload, in cardboard boxes called ‘gaylords’, or by bulk tanker trucks or railroad hopper cars. When purchased and delivered by the ‘truckload’, forty thousand pounds of resin arrives at a facility and the material is transferred to an onsite silo.

Materials Arrives by Truckload and Transferred to Silo
If purchased by the ‘gaylord’, the resin arrives in one thousand pound cardboard boxes lined with plastic bags that are sealed by the resin manufacturer. The material is then integrated into the handling system using vacuum loaders.

Cardboard Boxes Lined with Plastic Bags
Pellets can also be shipped in bulk tanker trucks or in railroad cars. When these arrive at the blowmoulding plant, the pellets are vacuumed into a storage silo that is part of the closed-loop resin handling system.

Pellets Transferred in Bulk Tanker Trucks or Railroad Cars

Common Blowmoulded Materials

There are thousands of distinctively different kinds of plastics produced in the world – each is a unique complex formulation of gases, chemicals, minerals, plant and animal by-products, as well as other ingredients. A given formulation possesses specific properties that make it perfect for certain applications, but often unusable for others.

Complex Formulation of Plastics
To understand this better, let’s examine some of the plastics used in extrusion blowmoulding. Polyethylene, or P.E., is probably the most common type of blowmoulded plastics.
A harsh substance like bleach can be packaged in polyethylene because of its chemical resistance properties. Other substances like shampoo, motor oil, and laundry detergent take advantage of the lightweight and fairly strong characteristics of PE. Blowmoulded containers made from natural PE are normally off-white or opaque, similar to the virgin pellets, and have a dull surface appearance.

Another material that is commonly used in the industry is polypropylene. Polypropylene is similar to PE, but has increased chemical and heat resistance. PP is also off-white and opaque in its virgin form. Natural polypropylene products look similar to finished PE containers cut are usually much shinier.

PE and PP Material Product
When it is desirable to blowmould a product that has a clear, glossy, glass-like appearance, the material polyvinylchloride, or PVC, is often selected as the resin of choice. PVC moulds into strong and impact resistant structures that are transparent and very shiny. Because of its transparency, the product inside the PVC container can be clearly seen – unlike PE or PP containers. In its virgin state, polyvinylchloride pellets appear bluish in colour and are somewhat transparent.

PVC Product
Although the three resins mentioned are the major materials used in continuous extrusion blowmoulding, there are other resins used. New materials are constantly being developed and introduced to the blowmoulding industry to produce superior products.

The Clamping Unit

The clamp section is the second unit of the blowmoulding machine and consists of: a clamp which holds, opens, and closes the blowmoulds, a carriage system which transports the clamp and moulds, a cutting device to sever the parison, a blow station – to blow the container out to its final shape, and controls to sequence the machine. As stated before, the clamping section is the part of the blowmoulding machine which captures the parison and blows it into a hollow article.

The Clamping Unit
In order to do this, the clamping section performs seven functions:
• opens and closes the moulds
• shuttles moulds from the extruder to the blow station
• cuts the parisons
• inflates the parisons
• cools them to the shape of the mould
• trims excess plastic from the moulds article
• ejects the flash and parts from the machine
Let’s take a closer look at these seven functions. The moulds are mounted to large plates called platens. Steel rods, called tie bars, pass through the platens and keep them in line. Linkage is connected to the back of the rear platen and to a plate between the back ends of the tie rods. A hydraulic cylinder or electric motor is connected to the linkage. As the cylinder or motor operates in one direction or the other it causes the linkage to open or close the platens and thus the moulds.
The clamp assembly is mounted onto the carriage. The carriage is a mechanical unit that also uses an electric motor or a hydraulic cylinder and a linkage system for an up and over or over and down motion.
The carriage shuttles the moulds between the parison pickup position under the extruder and the parison blowing position under the blow station. Positioning must be exact – the mould must stop precisely so that the parison is captured within the mould cavity. It must also stop exactly where the blow pin can be inserted properly.
Sensor switches sense the position of the carriage as it shuttles up and down. They tell the machine control system to slow down and stop the motor or hydraulic cylinder which is moving the carriage. Mechanical breaking devices also help locate the carriage’s stopping points.
As the clamp section cycles, the mould is moved into this position in time to pickup a parison which is the proper length. The machine control system is told by a sensor switch that the mould is in the proper position to be closed around the parison and the mould close system is activated.
Another sensor switch tells the machine control when the mould has closed. A switch fires a parison cutting device, what was captured in the mould is severed from the parison that continues to extrude.
Almost at the same moment that the cutting device is activated, the control tells the carriage system to move to its down position. It must move quickly to prevent the severed parisons from becoming reconnected. When the machine control is signalled that the carriage is in position, the blowing sequence is activated.
Let’s review some of the functions of the clamping section of the blowmoulder. The moulds are opened and closed and moved up and down by mechanical systems. These systems are operated by hydraulic cylinders or electric motors, the motion of which is directed by the machine control system. The control system receives instructions from switches, as the positioning of the carriage and clamp assemblies are detected. They also tell the control system to operate the parison cutting device.
Now we’ll look at how the parison is inflated, cooled, stripped, and ejected from the machine. These functions begin when the machine control is told that the carriage is in the down position.
First, the blowing station is activated. The blow station is a mechanical unit operated by a hydraulic or compressed air cylinder. The downward motion inserts blow pins into the mould.
The same instruction which activates the blow station cylinder also begins a timing sequence in the machine control. This timing sequence controls duration for which compressed air will pass through a valve, down through the blow pins and into the parison.
The compressed air inflates the parison until it contacts the mould surface. Air flow is maintained until the parison has set up in the shape of the mould. The mould is cooled by chilled water circulating below its surface. When the blowing timer stops it activates another timer that allows the air in the newly blown article to decompress prior to opening the mould.
The blowing time and the decompression time can be adjusted to meet the cooling requirements of each blowmoulded product. These timers are found either as part of the blowmoulding machine’s microprocessor control system or as an individual. Settable instruments are installed as part of an analog control system.
When blowing and decompression are done, the controls signal the clamp section to strip excess flash from the part and to eject the flash finished object. A device called a ‘tail puller’ is activated. It is mounted on the bottom of the mould and at the same time the blow station lifts out of the top of the mould. These two operations separate the portion of the parison that was pinched outside the mould from the blown article.
Moments later, the moulds open and signal the blow station to retract. A finished hollow product falls onto a conveying system which will remove it from the inside of the press section. Switches signal that the moulds and blow pins are in place to begin another cycle.
Let’s review the functions of the clamp section. Switches instruct the machine control system to insert the blow pins. Timers turn the compressed air on and off to pressurize the parison. Under pressure, the hot plastic cools against the mould surface and takes on its final shape. The flashing devices operate to strip excess plastic from the finished article. With the blow pins retracted and the moulds opened – the machine’s control system begins another cycle.
The overall cycle time of the press section is determined by yet another timer. It is appropriately called the ‘cycle timer’ and must be set so that enough time is allowed for the clamps to complete all of their operations before attempting to begin another cycle.
In addition to the controls that maintain the press section and automatic cycle, machines are also equipped with controls so that they can be operated manually. These switches allow technicians and mechanics to set up the moulds and to make adjustments. Most have safety circuits which do not allow manual operations unless the machine gates are closed.
We are watching a twin sided shuttle machine in automatic cycle. It has two complete clamping sections that move independently to capture the parison and sever it with a cutting device that is common to both sides. The moulds go to their respective blowing stations where the parisons are blown and ejected.

Controlling the Parison Rate

The last function of the extruder is to control the rate at which the parison flows. Extrusion rate is determined by the speed at which the screw rotates inside the barrel. The faster the screw turns, the faster the parison flows from the tooling.

Barrel Screw
The screw is turned by a motor which is capable of variable speeds. The speed is established by manually setting the motor speed control. The tachometer indicates the speed that screw is turning. There is a limit to the speed at which the screw can be turned, and it is not necessarily a limitation imposed by the motor or the drive system.
Screw rotation must be restricted to a speed that will allow the plastic to make a smooth, gradual transformation from pellets to parison. For this reason, different size extruders are designed to process different volumes of plastic.

Extruder Volume
Extruder volume is expressed in pounds of plastic output per hour. Extruder size is normally expressed in terms of the size of the screw it uses. This extruder is an 80/24:1 extruder. That means that the screw is 80 millimetres or about 3 inches in diameter. The 24 to 1 part means that the length of the screw is 24 times its diameter, or about six feet long.

Expression of Extruder Size
On this extruder, the screw is turned by a sixty horsepower motor and a belt driven gear reducer. It has an output of up to 300 pounds per hour. Thus, the last function of the extruder is to control the rate of plastic output. A manual setting establishes the drive motor’s speed. Through a gear reducer, the motor causes the screw to rotate inside the barrel. The speed of this rotation determines the rate at which the parison extrudes from the tooling.
Let’s go over what’s been covered so far. An extruder transforms plastic pellets using external and internal heat sources. It maintains temperature profiles by automatically adding heat or by taking it away. It uses the extruder head, head tooling, and parison programming to form parisons. And finally, it provides for control of the rate at which the parisons can be extruded – up to a maximum design output.

Overview of the Machine

Let’s start with an overview of the machine. It is actually two separate units assembled together on one machine frame. The job of the first unit, the extruder, is to convert plastic pellets into one or more continuous hot plastic parisons.

The clamp section then captures the hot parison and permanently converts it into the shape of the blowmould. Each of these two units are considered independent because each machine contains its’ own exclusive control system.

The extruder controls set the rate at which the parison extrudes. The clamp section controls establish the rate at which the unit goes through one complete operation cycle. The proper operation of the blowmoulding machine, as a whole, requires the rate of extrusion to match the rate at which the press section cycles.

The Extruder
An extruder is made up of a barrel, an extruder screw that precisely fits inside the barrel, a motor and drive system which turns the screw, a hopper to feed the pellets to the screw, and the extrusion head and head tooling which convert the melted pellets into parisons. The extruder must perform four major functions to convert pellets into hot plastic parisons and ultimately into hollow parts.

Melting of Pellets
It must first melt the plastic pellets into viscous fluid-like mass. The extruder must control temperatures during the extrusion process in order to form the melted plastic mass into centerless hose-like parisons. The fourth function is to control the rate at which these parisons are extruded.

The Barrel Section
Let’s take a closer look at each of these four functions. Plastic pellets are melted by heat which comes from two sources. One of these forms is from the electric heaters wrapped around the exterior of the barrel. Heat is conducted through the metal barrel wall to the interior surfaces; this heat only begins to soften the pellets.

Electric Heater Bands
The main heat source comes from friction. Internal heat is developed from the friction of the pellets pressing together as they are pushed toward the front of the barrel by the extruder screw. The threads of the screw, called flights, are responsible for causing this internal friction.

Internal Friction Heating
The screw is designed so that the area between the flights is deepest where the plastic feeds in from the resin hopper. This section is named ‘the feed zone’. Where the feed zone ends, the thickness of the screw shank, or root diameter, begins to increase until the flights are very shallow. This tapered section is called ‘the transition zone’. It is here that the screw compresses the pellets together until there is no space between them.

General Extruder Screw
As the pellets collide and compress they generate tremendous pressures and frictional heat, this causes them to melt. The remainder of the screw is called ‘the metering zone’; it is here that the final melting and mixing take place.

Feed Zone, Transition and Metering Zone
As the pellets travel the length of the screw they are transformed into a smooth, viscous, fluid-like mass of hot plastic. It is important to remember that the external heaters only add heat to help maintain a consistent temperature throughout the melting process. It is the design of the screw that causes the transformation from pellets into a fluid-like mass.

Transformation from Pellets into a Fluid-like Mass

Temperature Control
The second function of the extruder is to control temperatures. It is equipped with systems that can maintain desired temperatures. The pellets require a certain amount of heat to transform them properly. However, too much heat can be harmful and too little can result in the transformation being incomplete.
Certain heat profiles are favourable to processing the plastic through the extruder. The temperature control system has two capabilities; it can add heat as it is needed with the heater bands, and it can also remove any excess frictional heat through a cooling system.

To understand the concept of temperature control in the extruder, think of the climate control system in a house or an apartment. A thermostat is set for a desired temperature, if the area is colder than the set point the thermostat triggers the heating unit, and if the area is warmer than the set point, the thermostat signals the air conditioning to start.
On the extruder, sensors called thermocouples feed temperature information to instruments which operate like a thermostat. If a thermocouple signals that an area is too cold the instrument sends electricity to the heater band over that area.

Thermocouple
Beneath the heater are jackets or coils through which a fluid can pass. They are connected to a unit which pumps cool oil. If an area becomes too hot, the heat-controlled instrument sends electricity to open a valve. This allows cooled oil to flow in, absorb the heat, and then carry it back to the pumping unit. The valve will close once the thermocouple signals that temperature has been lowered.


Cooling Jackets or Coils
Depending on the length of the extruder, it may have three or four temperature controlled zones. Each of the zones can be maintained at a different temperature.

Temperature Controlled Zones
On blowmoulding machines equipped with microprocessor-based controllers, the brains for extruder temperature control are included as part of the unit. The extruder’s own numbers and set points are entering using a keypad and temperature information can be viewed on a display screen.

Microprocessor-Based Controller
Machines with analogue control systems are equipped with individual heat control units. Most state-of-the-art systems can maintain extruder temperatures within a few degrees of the set point.

Analogue Controlled System
As you have seen, extruder temperature profiles can be maintained automatically by the heat control system. Heat can be added by the heater bands and can be removed by the cooling system. Both of these functions are managed by controllers which get temperature feedback from thermocouples.

Forming of Parisons
The third function of the extruder is to change the hot plastic mass into hose-like parisons. This is accomplished by the extruder head and the head tooling. The head is an assembly of precision machined metal components. It contains channels through which the plastic is forced to flow by the continuous pumping action of the extruder screw. The channels contain special parts that force the plastic mass to become centerless.

Head Tooling
Toward the very bottom of the channels are parts that establish the final diameter and shape of the parison, these parts are known as the head tooling. The die bushing forms the outside diameter of the parison and the mandrel pin forms the inside diameter
As the plastic flows over the tooling and out of the head, it takes on the shape of the area between the die bushing and mandrel pin, thus these two parts determine the thickness and shape of the parison.

Die Bushing and Mandrel Pin
For the container in this example, the die bushing would be sized to make a parison small enough to fit inside the neck of the mould yet large enough to support itself until the blow pin is inserted. The mandrel pin would be sized so there is enough plastic in the parison wall to blow out to the extremities of the container.

Blow Pin Inside the Mould Neck
The distribution of the plastic through the parison, and thus through the container is controlled by a system called ‘parison programming’. By varying the gap between the die bushing and mandrel pin parisons are structured to have thinner and thicker sections through their length. In this example, the parison is profiled to have a thicker wall through this section, even though it will have to blow out further ̶ the wall thickness of the container will end up as thick as in this section where the parison does not have to blow out as far.

Parison Programming
Changing the tooling gap is done by a mechanical assembly which receives instructions from an electronic unit appropriately called a parison programmer. The mechanical assembly uses a hydraulic cylinder to vary the relationship between the die bushing and the mandrel pin. The electronic unit can be programmed with information about the desired parison profile.

Verification between the Die Bushing and Mandrel Pin
On a blowmoulding machine with a microprocessor control system a portion of the unit handles parison programming tasks. The instructions are entered on a keypad and can be reviewed on a small screen.

Screen Displaying the Parison Programming Tasks
Some machines are fitted with parison programming units which are separate from the machine control. In these cases the parison profiles are usually established by setting sliders to points on a grid. All programmers are interconnected to machine circuits which synchronize the parison profiling with the press section’s pattern of movement. This assures that the same profiling is programmed into every newly extruded parison.

Separate Parison Programming Controller
Let’s go over the third function that an extruder performs. The plastic mass delivered by the screw pushes through channels and over the special parts that make up the extruder head. Once made centerless, the plastic is formed into its final shape by the tooling and by parison programming.

Conclusion

After viewing this program, your awareness of the wide variety of products made using the continuous extrusion blowmoulding process has been enhanced. You should be familiar with the blowmoulding process which converts raw materials in their virgin forms to usable finished products.
You can now recognize the blowmoulding machinery and other required support equipment. And finally, you be aware of the vital role played by plant production workers.
You should next participate in program 2 of the Blowmoulding Series, entitled ‘The Blowmoulding Machine’.

Blow moulding Personnel

Blowmoulding not only requires hardware like an extruder, a clamp section, and support equipment, but also relies on the most important ingredient – trained personnel. Blowmoulding is something less than an absolute science. Though the machines and equipment run automatically, qualified technical and semi-skilled workers are necessary to setup and refine the machinery’s operation, monitor and correct process conditions, and make repairs. They also perform quality inspections, visually inspect and pack blowmoulded parts, and handle and care for expensive resins.

Trained Personnel
Managers are needed to coordinate the activities of all the groups and individuals involved in this process. One group is the setup mechanics and process technicians who ready blowmoulding equipment for production. Moulds, blow pins, and tooling must be precisely installed in the machine. Temperatures and machine timing must be set and adjusted until optimum quality and production rates are achieved. Once the machine is up and running, we are not on automatic pilot, so to speak.

Extrusion rates will fluctuate with the density of the resin, parisons will drift slightly out of position, mould pinch areas wear, deflashing equipment creeps out of adjustment, colours go light and dark, and tooling requires cleaning.
Usually, three groups of workers are on staff to deal with the numerous processing drifts. The machine mechanics anticipate problems and make adjustments like parison centring, changing worn deflashing sleeves, and extruder speed adjustments.
The packer inspectors sort out any visually defective parts like dirt or foreign material in the resin or parts that haven’t been completely deflashed, and an occasional hole. They pack the good ones in shipping containers and grind the bad ones in the granulator.
The quality insurance inspectors measure and test samples to assure that the parts produced are being made to certain specifications. Details like legal volumes and dimensions of an area that must work with another part, like a cap, are verified.

Quality Insurance Inspectors

There are other groups of personnel that have responsibilities in the manufacturing process. The material handlers assure a continuous supply of properly dried and blended to the machines. They adjust blender settings and monitor and handle excess regrind accumulations. They move in supplies of shipping containers and move out loads of finished goods.
Maintenance personnel perform the preventative maintenance routines on the equipment and the moulds and tooling. They also do heavy duty repair jobs like motor replacements and equivalent overhauls.

Maintenance Personnel
The vital link between the production team and management is the production supervisor. The supervisor must coordinate everyone’s activities and make decisions to assure that manufacturing proceeds profitably and safely. The supervisor monitors proper usage of raw material while maintaining quality standards and production output. He or she prepares various reports and communicates to upper management the needs of the blowmoulding department.

Support Systems

Blowmoulding machines need the help of other equipment and systems in order to keep running. These are generally called support systems or auxiliary plant equipment. They include heavy duty transformers and switchgear which are needed to handle the electric demands.
Each twin sided shuttle machine contains heaters, large electric motors, and the machine control system – all consume large amounts of electric energy. Also required is water cooling system to provide cold water to the machine. The moulds must be cooled as most of other things on the machine, like oil units. This ‘processed water’ as it is called, is generally supplied by water chilling units like these which cool all the blowmoulding machines in the plant. They are similar in principle to a large air conditioning unit except they cool water instead of air.
The blowing process also needs large volumes of compressed air which is made on a continuous basis by these types of air compressors. The blowmoulding process requires a continuous supply of plastic at the extruder hopper.

Automatic vacuum systems generally referred to as resin or material handling systems take care of this chore. Virgin pellets are used along with a percentage of a material called ‘regrind’. Regrind is the name given to the flash which is ground using a granulator. The granulator reduces the size of the flash back down closer to the size of the pellets.

Granulator
Virgin and regrind along with colour additives are mixed in units called material blenders, then back in to weigh the extruder hopper.

The Clamp Section

The clamp section is the part of the blowmoulding machine in which the actual blowmoulding of the parisons is done. It is automatically controlled and repeats the same sequence over and over again with split second timing accuracy.
The clamps are fitted with precisely machined mould halves, which resemble the reverse of the part to be made. The moulds are mounted to the clamping plates, called ‘platens’.


The clamp section’s main job is to transport the mould between the parison pickup point under the extruder head and the parison blowing point under the blowing station. It also opens and closes the moulds at specific times during the machine cycle.

Remember that the parisons are continuously extruding. A parison of appropriate length must be quickly captured in the closing mould, severed by a cutting device, when whisked out of the way of the parison which continues to extrude in preparation for the next cycle.


The clamp section then positions the mould under the blowing station, and the blow pins are inserted into the parison which is now hanging in the mould. Compressed air is turned on and the hot plastic expands to fill the mould cavity there by taking its final shape.


Enough blowing time is applied to allow the parison to cool and cure against the walls of the cold mould. When the blowing is completed, the compressed air is relaxed, the mould is opened, and the blow pin is withdrawn. A newly blown hollow article falls away to a conveyer that will remove it from inside the machine. The clamp section is now ready to begin another cycle.
The blowmoulding machine shown here in full front view is a two sided shuttle machine. Each set of the platens contains a set of mould halves. They are alternately transported to capture the continuously extruded parisons. There is a blowing station on each side of the machine.
This one high output extruder is all that is required to process enough plastic into the three parisons being captured and blown in six mould cavities. This machine is typical of most continuous extrusion blow moulders; two-sided, multi-parison, high output extruder, and multiple mould cavities.

Two-Sided, Multi-Parison, High Output Extruder and Multiple Mould Cavities.
This one high output extruder is all that is required to process enough plastic into the three parisons being captured and blown in six mould cavities. This machine is typical of most continuous extrusion blow moulders; two-sided, multi-parison, high output extruder, and multiple mould cavities.

Inherent in continuous extrusion blowmoulding is some excess unblown plastic. This excess is called ‘flash’ and is the top and bottom of the parison which sealed outside of the mould.

Flash
Normally, the flash is automatically removed inside the machine by equipment attached to the mould or the blow pins, or by a secondary trimming unit. Sometimes it is removed after the part leaves the machine by a secondary deflashing machine or by a manual operation.

The Extruder

The blowmoulding process begins in the extruder portion of the machine. An extruder is made up of a long, hollow metal tube called a ‘barrel’. On the outside of the barrel are electric heaters. Into the barrel fits a threaded shaft, which resembles a large screw, in fact it is known as the ‘extruder screw’. It is turned inside the barrel by a motor and drive system.


Plastic pellets are fed into the back end of the extruder, the turning motion of the screw pushes the pellets forward, the electric heaters and the mixing action of the screw soften the pellets as they move toward the front of the barrel. When they reach the end, the pellets have melted and mixed into a hot fluid-like matter.

Attached to the end of the extruder barrel is a unit designed to form the hot plastic mass into parison. This unit is called the ‘extruder head’ and is a series of precisely machined metal components containing flow channels and special parts. These parts, referred to as the ‘head tooling’, form the plastic mass into centerless hot plastic posts, called parisons.


When it is desirable to blowmould multiple containers simultaneously, the extrusion head is where the plastic mass can be divided into two or more streams, thus two or more parisons. This is an extruder seen from one side – it is an intricate part of extrusion blowmoulding machine. This is the motor which turns the screw, located inside the barrel. The electric heaters that help soften the pellets are these shiny silver bands.


The plastic pellets fall from the hopper into an opening in the barrel onto the screw. The pellets are melted and mixed as they push forward toward the extrusion head. Shown here from the front of the extruder, this head has divided the plastic mass into three streams and has formed three continuously extruding parisons.


Thus, the first essential component for blowmoulding is an extruder which converts plastic pellets into one or more continuous hot parisons.

Continuous Extrusion Blow moulding

There are several different methods for blowmoulding and all of them are based on the over-simplified concept shown earlier. Since it would be impossible to review all of the different types of machinery and processes in this one presentation – we will closely examine just one method, ‘Continuous Extrusion Blowmoulding’.
Continuous extrusion blowmoulding is the process normally selected to make large quantities of precision parts that must work in close tolerance with other parts, such as caps. Also, continuous extrusion is suited for processing several different kinds of plastic.
Examples of containers that are made through the continuous extrusion method of blowmoulding are shampoo, mouthwash, and cooking oil bottles. To produce any blowmoulded item there are four essentials. The extruder, which is a machine that prepares plastic into parison form. A clamp section, a unit which holds and moves moulds and blows the parison. Support systems supply electricity, resin, compresses air, and chilled water to the machine. The manufacturing staff, the groups and individuals that run the blowmoulding process on a production basis.

Blowmoulding Concepts

To understand the concept of blowmoulding, imagine that this is a balloon. Further imagine a hollow plastic ball cut in half and closed around the balloon. By pinching the bottom and blowing air in the balloon, the balloon inflates and takes on the shape of the ball. Blowmoulding isn’t balloons and plastic balls - but in principle, is quite similar to what you have just seen.
Blowmoulding begins with a metal mould that is almost exactly the same size and shape as the article to be made. The mould consists of two halves, each designed to allow cold water to be circulated below its surface.


The mould halves close around hot plastic that has been formed into a centerless hose-like tube. This hose-like tube is called a ‘parison’. As the moulds close they pinch one end of this parison, sealing it shut. Into the other end of the parison a pin is inserted which compresses the plastic between itself and the mould. Thus creating a seal on this end of the parison as well.


Compressed air is then blown in via a channel through the centre of this pin, generally called a ‘blow pin’. The air pressure inflates the hot plastic parison outward until it contacts the mould surface, much like the balloon did in the demonstration.


The moulds, called ‘blowmoulds’, are cooled by the circulating water that is a temperature much lower than that of the hot plastic. Thus in contacting the cold mould surface, the hot plastic cools and sets up in the shape of the mould. The blow pin holds the object on centre as the mould halves part and break free from the plastic. The pin retracts and the hollow plastic article falls away.


Let’s review the basics of this principle. A cold two-part mould is closed around a hot plastic parison. Compressed air blows the parison out to the mould surface where the plastic is cooled into the form of the mould.