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.