How much do you know about PP injection molding?

The processing of plastic raw materials mainly involves the melting, flowing, shaping and cooling of rubber particles into finished products. It is a process of heating and then cooling, and it is also a process in which plastic changes from particles to different shapes. The following will explain the processing process from different perspectives of each stage.

1. Melting

The heater is installed to gradually melt the raw material particles into a fluid flow. The temperature is mainly adjusted according to the different raw materials. Increasing the temperature will speed up the flow of raw materials, which can increase efficiency but may not guarantee the yield. A proper balance must be achieved. In addition, the good effect and the characteristics of PP's cracking when exposed to high heat are that it is best to allow the raw materials to flow smoothly to the die head during production to avoid insufficient filling or reflux. Reflux means that the flow of raw materials is faster than the output rate, which will eventually increase the average flow efficiency, which is equivalent to an increase in MFR. It is one of the methods that can be used in processing, but it also causes the MFR distribution to be abnormal, which may lead to increased instability and a possible increase in the defect rate. However, due to the application of PP finished products, they are not products with high dimensional precision, so the impact is not significant.

2. Screw

Most of the PP processing relies on the screw to drive the fluidity, so the design of the screw has a great impact. The diameter size affects the output, and the compression ratio affects the pressure value, which also affects the output and the effect of the finished product, including the mixing effect of various materials (masterbatch, additives and modifiers). The flow of raw materials mainly depends on the heater, but the friction of the raw materials will also generate friction heat to accelerate the fluidity. Therefore, the small compression ratio of the screw drives the small flow, and the speed must be increased to generate more friction heat than the screw with a large compression ratio. Therefore, it is often said that there is no master in plastic processing, and the person who carefully understands the performance of the machine is the master.

The heating of raw materials is not just the heater, but also the friction heat and residence time. So this is a practical issue, experience helps to solve production problems and improve efficiency. If the screw needs to have a particularly good mixing effect, sometimes a two-stage screw or a twin-shaft screw is designed and each stage has a different type of screw to achieve a variety of mixing effects.

3. Mould or die head

Plastic reshaping relies on molds or die heads. The finished product of injection molding is three-dimensional, and the mold is more complicated and the shrinkage rate must be considered. Others are flat, strip, needle-shaped continuous product die heads. If it is a special shape, it is classified as a special shape, and it is necessary to pay attention to the immediate cooling and shaping problem. Most of the designs of plastic machines are like injection syringes. The extrusion force driven by the screw will create huge pressure at a small outlet, improving production efficiency. 

When the die head is designed as a plane, how to evenly distribute the raw materials on the entire surface, the design of the hanger die head is very important. The exquisite extruder will increase the gill pump to stabilize the supply of raw materials.

4. Cooling

In addition to pouring materials through the runner gate, the Injection Mold also has a cooling water channel design to cool the materials. Extrusion molding relies on the cooling water channel inside the roller to achieve the cooling effect. In addition, there are also cooling methods such as air knife, cooling water directly sprayed on the blow bag, and hollow air blowing.