Polypropylene is one of the most versatile polymers known. It serves two fundamental functions, one as a plastic and the other as a fiber. As a plastic, it is used to make food containers capable of being microwaved and washed in a dishwasher.
This is feasible because its melting point is 171ºC. As a fiber it is used to make indoor and outdoor carpeting, the kind you always find around swimming pools and miniature golf courses. It works well for outdoor carpets because it is easy to make colored polypropylene and because polypropylene, unlike nylon, does not absorb water.
Polypropylene Processing
Polypropylene has an excellent balance of properties and processability, so it is perfectly suited to all types of injection molding machines. Polymers with a wide range of molding conditions, good flow and good welding characteristics in the molten state are molded practically and reproducibly under tight tolerances by carefully controlling temperatures, clamping pressures and injection speeds.
Polypropylene can be processed on common piston machines, piston machines with preplasticizer and reciprocating screw machines. Molding conditions can be widely varied and resins with good flow and welding characteristics are available. It does not require preliminary drying, since it does not absorb moisture.
As the load approaches the limit value, the temperature of the cylinder must increase because the injection speed of the material will be higher.
For a reciprocating screw machine it is recommended to start under the following conditions:
Cylinder temperature, 216 to 243ºC.
Mold temperature, 7 to 38ºC
Injection pressure, 60% of available pressure.
The molding cycle is an important factor in determining processing conditions, but it is not possible to define it with start-up conditions. The cycle also depends on part design, part dimensions and other factors.
Regardless of the machine used, the goal should be to reach a temperature of 260°C in the molten mixture.
Subsequent increases in cylinder temperature are usually not done to have a higher melt temperature, but mainly to compensate for the higher material flow required for the higher production rate.
Applications:
Polypropylene finds its widest applications in household products, appliances, packaging, laboratory utensils, and various types of bottles. High-impact polypropylene copolymers have replaced rubbers in transportation and building heat accumulators. Polypropylene homopolymers are widely used as protective and reinforcing materials in the transportation of goods. They are used in the manufacture of sacks and are used directly as bags for soft goods, due to their gloss and good tenacity.