Introduction: As one of the widely used general plastics, PP can be seen everywhere in daily life. It has higher purity than ordinary pc. Although it does not have the high coloration of ABS, PP has higher purity and color rendering. In the industry, PP material is often used in packaging materials such as plastic bottles, bottle caps, cream bottles, etc. This article shares the six key points of the PP injection molding process. The content is for your reference:
The processing of plastic raw materials is mainly the melting, flow, and cooling of the rubber particles into finished products. It is a process of heating and then cooling. It is also a process in which the plastic changes from particles to different shapes. The following will explain the processing from the perspective of each stage process.
The heater of the device 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 flow smoothly 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 is equal to an increase in MFR, which is one of the available methods for processing, 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.
Most of the PP processing is driven by the screw to drive 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 raw material flow mainly depends on the heater, but the frictional heat of the raw material will also produce 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, which will cause more frictional heat energy 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 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.
Mold or die head
Plastic reshaping relies on the mold or die head. The injection molded product is three-dimensional, and the mold is more complex, and shrinkage must be considered. The others are flat, strip, and needle-like continuous product die heads. If it is a special shape, It is classified as heterogeneous 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 to 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, the design of the hanger die head is very important, and the exquisite pressing opportunity increases the stable raw material supply of the fish gill pump.
In addition to the sprue gate pouring raw materials, the injection mold also has a cooling water channel cooling raw material design. The 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.
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 finished product is strengthened and not easy to tear, but it is easily torn 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.
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. It can be done by extending the cooling time in processing, and continuing to maintain the pressure. The crystalline raw material often has a larger shrinkage difference than the amorphous material, 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 additives to reduce shrinkage are often added to overcome the problem of LDPE to improve the necking of the extruded plate.