(i) Injection Molding Demonstration

Battenfeld HM 40/130 is the machine used in the laboratory session to mold Polypropylene. Thermoplastic materials such as nylon, polystyrene, polythene and polypropylene can be used in a process called injection molding.

How injection molding works?

1. The hopper is filled with granules of plastic powder. The hopper stores the material until it is needed.
2. The heater starts heating up the tube until it reaches a high temperature, then a screw thread starts turning.
3. The granules are pushed along the heater section by a motor, which pushes the granules along the heater section. The heater melts the granules until they reach the liquid state. This liquid is then forced under pressure into a mold where it cools down the liquid and solidifies it, which then turns into a shape.
4. The mold then opens and the ejector pins remove the molded part.

In one molding cycle, the maximum amount of plastic injection mold that can be injected is called the “shot size”. For best quality, parts must use about 60% to 70% of a machines rated shot size. Larger shot sizes will not allow sufficient melt cushion for packing. Smaller shot sizes produce greater irregularities and loss of precision.

The relatively high melting point of polypropylene allows continued use at 104°C. The resin begins to soften at about 121°C.

The polymer “Polypropylene” (C3H6)n properties:

Property Value
Molecular weight of repeat unit 42.08 g/mol.
Young’s modulus 1.5-2 Gpa
Impact strength 3.0 - 30.0 KJ/m²
Melting temperature 130-171°C
Density 0.855 g/cm3, amorphous
0.946 g/cm3, crystalline

Polypropylene, which is a thermoplastic polymer, was molded in the laboratory session. Polypropylene has its own special characteristics, as shown below:
• High tensile strength and electrical resistance.
• Very flexible.
• Can be