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What are the injection molding processes of PP (polypropylene)?

Views:9     Author:China Xicheng     Publish Time: 2021-07-30      Origin:China Xicheng

The molding process of PP plastic raw materials is mainly plasticization, mold filling, cooling and shaping to become a finished product. It is a process of heating and then cooling. It is also a process of changing the plastic from pellets to different shapes. The different stages will be described below. To illustrate the processing process from the perspective of:


1. Screw

Most of the PP processing is driven by the screw to drive the fluidity, so the design of the screw has a great influence. The size of the diameter affects the output, and the compression ratio affects the pressure value and the output and the effect of the finished product. This also includes a variety of materials. (Color masterbatches, additives and modifiers) mixing effect. The flow of raw materials mainly depends on the heater, but the frictional heat of the raw materials will also generate frictional heat to accelerate the fluidity, so the screw compression ratio is small and the flow is small, and the rotation speed must be increased. The frictional heat will be more than that of 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 only a heater, but also the frictional heat and suffocation time. So this is a practical problem, and experience helps solve production problems and efficiency. If the screw needs to have a particularly good mixing effect, sometimes two-stage different screws or twin-shaft screws are designed and different types of screws are divided into sections to achieve various mixing effects.


2. Melting

The heater allows the raw material particles to gradually melt into a fluid flow, which is mainly suitable for temperature adjustment of different raw materials. Increasing the temperature will tend to speed up the flow of raw materials, which can increase efficiency but may not guarantee yield. It must be suitable Balance.

In addition, the good effect and the characteristics of high thermal cracking of PP are that it is best to make the raw materials smoothly flow to the die during production to avoid insufficient filling or backflow. Backflow means that the flow of raw materials is faster than the output rate. An increase in average flow efficiency equals an increase in MFR, which is one of the available processing methods, but it also causes abnormal MFR distribution, which may lead to increased instability, which may increase the defect rate. However, the PP product is not a product with high dimensional precision because of the application, so the impact is not big.


3. Mould or die head

Plastic reshaping relies on the mold or die head. The injection molded product is three-dimensional, and the mold is more complicated, and shrinkage must be considered. The others are flat, strip, and needle-shaped continuous product die heads. If it is a special shape, It is classified as a special shape, and needs to pay attention to the problem of cooling and shaping immediately.

Most of the plastic machines are designed like injection syringes, and the extrusion force driven by the screw will cause huge pressure on the small outlets and improve production efficiency. When the die head is designed as a flat surface, how to make the raw materials evenly distributed on the entire surface is very important.


4. Cool

In addition to the sprue gate pouring raw material, the injection mold also has a cooling water channel to cool the raw material design. Extrusion molding relies on the cooling water channel in the roller to achieve the cooling effect, except for the air knife, the cooling water is directly poured on the blowing bag, and the cooling method such as hollow blowing.

5. Extend

The reprocessing and extension of the finished product will enhance the effect. For example, the strapping belt is driven by the front and rear rollers at different speeds to cause the extension effect. The tensile strength of the oriented extension part of the finished product is strengthened and it is not easy to tear, but it is very easy to tear in the horizontal direction. The molecular weight distribution also affects the extension effect during high-speed production. All extruded products including fibers have different extensions. Vacuum and pressure forming can also be regarded as another form of extension.


6. Shrink

Any raw material has the problem of shrinkage. The shrinkage is caused by thermal expansion and cold shrinkage and internal stress generated during crystal formation. Generally speaking, thermal expansion and contraction are easier to overcome, and the cooling time can be prolonged in processing, and the pressure can be maintained continuously. Crystalline raw materials tend to have larger shrinkage differences than non-crystalline materials, which is about one thousandth for PP. Sixteen, but ABS is only about four thousandths. This part of the difference is very large. This part must be overcome on the mold, or the shrinkage reduction additives are often added to overcome the extrusion plate. LDPE is often added to improve the necking problem.


Typical application range

Automotive industry (mainly using PP containing metal additives: mudguards, PP ventilation ducts, PP fans, etc.), appliances (dishwasher door gaskets, dryer ventilation ducts, washing machine frames and covers, refrigerator door gaskets, etc.), Japan Use consumer goods (lawn and garden equipment such as lawnmowers and sprinklers, etc.).

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