The choice of equipment for plastic injection (Part 2)
Principles for selecting injection molding machines (Part 2)
When selecting equipment for injection molding, it is also worth considering the chemical and physical characteristics of the polymer. Each material or polymer composition behaves differently at the same melting temperature and under different working processes. To obtain guaranteed quality products, you need to know these indicators.
Polymers with a smooth transition from solid to liquid (cast) state belong to the first group. The change in viscosity is quite slow and ensures temperature stability during processing. These substances include:
- low density polyethylene (LDPE, LDPE, LDPE);
- polystyrene copolymers;
- polysulfin and a number of other polymers.
Polymers of the second group are characterized by a sharp transition from the solid to the liquid phase. These include polypropylene, high density polyethylene, a number of polyamides.
The choice of injection molding machine depends on what type of material will be processed. The differences in the machines relate to the shape of the screw, the design of the feeding zone, the length of the plasticizing section. For the first group of polymers, machines with a feed zone length of 5÷6D are used, as well as dosing zones. In this case, the plasticization section is 3÷4D.
The second group of polymers is processed at installations with the corresponding indicators 6÷7D. The third group of polymers, substances with low temperature stability, require the use of machines with a plasticizing zone length of 5÷6D, and a dosing section of 5÷6D. These are PVC, polyformaldehyde and other plastics with similar characteristics.
Thus, knowing how the selected polymer will behave in the process of manufacturing products and taking into account the technical characteristics of the working equipment, the production of a quality product that meets all requirements is achieved.