What is plastic injection moulding?

The Principal of Plastic Injection Moulding

The so-called plastic injection moulding process refers to: the plastic material in the barrel of an injection molding machine is plasticized into a melt by the sheer heat that is generated by external heating and screw rotation, and then the melt is fed into the cavity of a certain shape through the application of certain pressure, where, after cooling and solidification, a product is formed. So, this is plastic injection molding.

The 3 fundamental factors of plastic injection moulding: plastic injection machine, mold, and plastic material

The 5 Elements of Plastic Injection Moulding:

1. Temperature:

A. Oil temperature: For a hydraulic machine, the heat energy is generated by the frictional movement of the hydraulic oil in the machine, which is controlled by the cooling water. Before starting the machine, it is necessary to ensure that the oil temperature is around 45 °C. If the oil temperature is too high or too low, transmission of pressure will be affected.

B. Injection temperature: i.e., the temperature of the barrel. This temperature should be set according to the shape and function of the material and product. Set it according to rules specified in a document if there is any.

C. Mould temperature: This temperature is also an very important parameter, which has a great influence on the performance of a product. Therefore, the function and the structure of a product must be taken into consideration when setting the temperature, while other considerations include the material and the cycle.

2. Speed:

A. The setting of mould opening and closing speeds. The mould opening and closing speed setting generally follows the principle of slow-fast-slow, mainly by taking the machine, the mold and the cycle into consideration.

B. Ejector setting: It can be set according to the structure of the product. It is better to apply a slow speed to eject some of the complex structures, while some others may require faster speeds to reduce the molding cycle.

C. Injection speed: set according to the dimensions and structure of the product. If the structure is complex with a thinner wall, the speed can be set to be faster, and if the structure is simple with a thicker wall, the speed can be slower. Besides, the speed needs to be set from slow to fast according to material performance.

3. Pressure:

A. Injection pressure: According to product dimensions and wall thickness, set injection pressure from low to high. And, some other factors also need to be considered during debugging.

B. Holding pressure: The holding pressure is mainly intended to ensure the stable shape and size of a product, which is thus set as per the structure and shape of a product.

C. Clamping force: It refers to the compressive force required for the mould to close. Some machines allow the adjustment of clamping force, while others don’t.

4. Time

A. Injection time: The time must be set longer than the actual time needed. It can also play the role of injection protection. The value of the injection time needs to be about 0.2 seconds longer than the actual value. Coordination with pressure, speed and temperature needs to be taken into consideration during setting.

B. Cooling time: The time is usually set according to the size and thickness of a product, but the melting time should not be greater than the cooling time, so as to allow the product to be fully shaped.

C. Holding time: To ensure the dimensions of a product after injection molding is completed, the holding time allows the gate to be fully cooled before the melt flows back under holding pressure, which can be set according to the size of the gate.

5. Location

A. The mould opening / closing position can be set according to the speed of mold opening and closing. The key is to properly set the starting point of low pressure protection, i.e., the starting point of low pressure is most likely the point that is able to protect the mould without affecting the moulding cycle, and the ending point should be the position where the core and the cavity meet during slow mold clamping.

B. Ejector position: This position should be able to allow full ejection of a product. It should be set from low to high in an incremental manner. Note that, during mold installation, the position needs to be returned to “0”, or the mold would be damaged.

C. Melt position: Calculate material quantity according to product and screw sizes, and then set the corresponding position.