Plastic extrusion is a very common practice in the polymer production industry. Very important products are produced via plastic extrusion machines. A typical plastic extrusion machine includes a screw mechanism inside a barrel that is filled with a polymer melt. This polymer melt is drawn into the die with the rotation of the screw inside the machine. In here, you can find the information about;
There are three main categories for conventional plastic extrusion processes available in the industry; Wire and cable coating, production of colid cross-sections from polymers, and hollow products such as tubes or pipes.
Cable and wire coating is one of the most important applications of plastic extrusion machines. The working principle of these machines is actually very simple. There are two feeding sources at the die section. One of them is plastic melt feeding which provides coating material, and bare wire or cable feed. Plastic melt comes from the screen pack section which directs the streamlines to obtain laminar flow at the dying opening. At die opening, the bare wire feed is coated with the plastic melt, with the laminar opening and exiting of coating and bare wire at the same time. The vacuum is also applied for adhesion between bare wire and coating.
In this application of plastic extrusion, the polymer melt is drawn into the die opening with the application of pressure. There are a screen pack and breaker plate just like at wire coating process. The size of the dire opening is constant, but obtained extrudate has a bigger cross-section because of the die swell. Die swell must be considered and calculated in the design required to die for a specific product. The extrudate will be soft after the die opening. To harden the extrudate, water sprays or air blow can be applied. Different cross-sections of dies must be designed to obtain different solid extrudate shapes from polymers. The most important product types are; door and window frames, automobile trimmings, etc.
For the production of tubes or hollow shapes, mandrel structures are used. With these mandrels, an internal hollow shape is obtained. To obtain tubes, spider legs structures are used in the die opening section. Inside the mandrel structure, the air is blown to ensure the hollow shape is obtained after the die opening. Screen pack and breaker plate mechanisms are also used like other plastic extrusion techniques.
The most encountered defects in polymer extrusion processes are; sharkskin, bambooing, and melt fracture.
Sharkskin occurs because of the occurrence of the velocity profile at the polymer melt inside the die hole. Velocity profile occurs because of the high friction between the die and flowing polymer melt. If the friction increases further, the bambooing problem occurs on extrudate. So, the solution is the minimize the friction between the die opening and extrudate.
Melt fracture is another problem results from the uneven flow profile inside between the die opening and screw. Uneven profile means turbulent flow, which leads to uneven geometries of the extrudate. To compensate for it, converging entrance geometry for the die and the use of breaker plate and screen pack geometries are recommended.
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