Plastics can take many different forms and have a variety of properties, but all end products start off as the raw material: resin. The process can be complicated and take many forms, but there are a few basic steps which all plastic extrusion processes follow.
The raw materials are fed into a hopper which goes to the extruder; this is located at the top of the machine. This means that gravity will feed the pellets into the barrel of the extruder. At this moment in the process, any additives or colourants are added into the mix when appropriate for the outcome.
Additives are added to alter the properties that the final plastic will have; different additives are used for different outcomes. The raw pellets are passed through a rotating screw which regulates the number of raw materials carried to the machine as well as mixing any additives in.
The material enters the barrel out of the hopper and is then forced out of the other end of the barrel by the rotating screw. The screw is inside of the barrel, it rotates and pushes the material forward out of the front of the barrel into the die.
The barrel itself is heated, using heater bands, which melt the plastic granules while they pass along the screw.
All plastics require the correct amount of heat to melt the granules but not burn them. There is an optimum temperature for all plastic types to achieve this and also to provide fast running speeds. The barrel is continually heated and cooled in order to achieve the optimum temperature.
Once the materials have been heated, they will have a consistency similar to thick bubble gum. There is a breaker plate at the end of the barrel, this filters out any contaminants through a screen. The breaker plate also changes the motion of the plastics from rotational to longitudinal.
The die is the hollowed shape for the final shape that the plastic will take. After the plastic has gone through the breaker plate, it is then forced into the die to set it into the correct shape. This is where the most amount of attention needs to be paid in the process.
When designing the die heavy consideration has to be placed on how the plastics will flow through; small faults can result in warping of the final plastic product.
There needs to be consistency throughout the flow of the die to keep design integrity for the end product.
Once the plastic has passed through the die, it needs to cool in this shape, so it sets in shape. Plastic cools much slower than other materials created from extrusion. This means that plastic must undergo a much more effective cooling process; it is fed straight into water baths from the die.
Inside the water baths is the calibration equipment that supports the section whilst it is being cooled. This forms the final shape and dimensions of the section.
The last step is the haul-off, which pulls the extrusion out of the calibration equipment towards the final process. For example, this could be adhesive strips, punching holes, cutting into lengths up to 5 metres, bending, drilling, printing and coiling.
This is just a brief summary of the plastic extrusion process and an insight into how the final product that you may see on a daily basis is produced. Each process is slightly different depending on the type of plastic that is needed and their properties.