(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
Bubbles are cause from excessive moisture in the material since the water droplets turn to sacks of steam when heated in the injection unit, causing voided areas between molecules. This will cause bubbles to appear in the molded part.
Cure: Check moisture levels of material and dry if needed.
• Cause: Air entrapment is a cause of Bubbles. Air entrapment is caused when the molten material is not fully forced into the mold cavity. Thus, these trapped gases and air will form sacks, which will appear in the molded part.
Cures: Check vents and clean if needed. Reduce the clamp pressure to minimum amount needed. Increase the injection pressure.
2) Flashing:

The imperfection in the above figures is called Flashing. In the above diagrams, it is obvious that excessive and unwanted plastic appeared on the molded part. Eliminating this imperfection can cause huge amounts of savings, thus it is necessary to follow all the precautions before starting the injection molding. A plastic part is said to have flash when plastic extends beyond its normal shape. Flashing occurs at any point where two mold surfaces meet.
Possible causes and cures of the Flashing imperfection:

• Cause: Excessive melt or mold temperature.
Cure: Reduce melt or mold temperature.
• Cause: Low clamping force pressure.
Cure: Increase clamping pressure.
• Cause: Injection pressure is excessively high. If the injection pressure is too high, the clamp unit will not be capable of keeping