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.