PP injection molding six key points

The plastic raw material processing process is mainly molten rubber particles, flow, shaping and cooling into finished products, is a process of heating and then cooling, but also the process of changing the plastic from particles to various different shapes, the following will be on each of the different stages of the perspective to explain the processing process.

1. Melting

The device heater (Heater) allows the raw material particles to gradually melt into a fluid flow, mainly with different raw materials suitable for temperature adjustment, adjusting the temperature will drive the flow of raw materials faster, can increase efficiency but not necessarily guarantee the yield, must achieve the right balance.

Another good effect and the characteristics of PP cracked by high heat, it is best to let the raw material flow smoothly to the die when producing, in order to avoid the phenomenon of insufficient filling or reflux, reflux represents the flow of raw material faster than the output rate, and finally, it will cause the average flow efficiency to increase equal to the MFR, which is one of the methods available for processing, but it also causes the MFR distribution to be non-normal, which may lead to increased instability, resulting in the possibility of increased defective rate.

However, PP finished products are not products with high dimensional precision because of the application, so the impact is not big.
 

2. Screw

The majority of PP processing is driven by the screw mobility, so the design of the screw has a great impact, the size of the diameter affects the output, the size of the compression ratio affects the pressure value also affects the output, and the effect of the finished product, which also includes a variety of materials (masterbatch, additives, and modifiers) mixing effect.

Raw material flow mainly by the heater, but the raw material turning friction will also generate frictional heat to accelerate mobility, so the screw compression ratio is small to drive the flow of small, the speed must increase the frictional heat caused by the larger than the compression ratio of the screw must be more.

Therefore, it is often said that there is no master of plastic processing, and the person who understands the performance of the machine by heart is the master. The material is heated not only the heater but also the frictional heat and choking time must be included.

So this is a practical issue, experience helps to solve production problems and efficiency. If a screw needs to be mixed particularly well, sometimes a two-stage different screw or biaxial screw is designed and divided into sections of different forms of screws to achieve various mixing effects.
 

3. Molds or die heads

Plastic reshaping relies on the mold or die, injection molding products are three-dimensional, the mold is also more complex to consider shrinkage issues, other than flat, strip, needle-shaped continuous product die, if the special shape is classified as shaped, need to pay attention to the immediate cooling and shaping issues.

Most of the plastic machine designs are like injection syringes, the extrusion force driven by the screw will cause huge pressure in a small outlet to improve production efficiency. When the die head is designed for a flat surface, how to let the raw materials evenly distributed throughout the surface, the design of the hanger dies head is very important, the sophisticated press out will increase the fish gill type help to stabilize the supply of raw materials.
 

4. Cooling

Injection mold in addition to the sprue gate filling raw materials, there is also cooling water channel cooling materials design. The press molding relies on the cooling water channel inside the roller to achieve the cooling effect, except there is also an air knife, cooling water directly on the blowing bag, and hollow blowing, and other cooling methods.
 

5. Extension

Finished products and then processing extension will enhance the effect, for example, packing tape by the front and rear roller drive rate is different that causes the extension effect, the finished product with the extension of the part of the tensile strength to strengthen not easy to tear, but the horizontal is very easy to tear. Molecular weight distribution will also affect the extension effect of high-speed production, all pressed-out products, including fibers, have varying extension, vacuum and pressure hollow molding can also be regarded as another form of extension.
 

6. Shrinkage

Any raw material has the problem of shrinkage, shrinkage from thermal expansion and contraction, and crystallization caused by the formation of internal stress.

Generally speaking, thermal expansion and contraction are easier to overcome, can be processed to extend the cooling time, continuous pressure can be good, crystalline raw materials than non-crystalline often have a greater difference in shrinkage, in terms of PP about 16 parts per thousand, but ABS only about 4 parts per thousand, the difference is a very large part of the mold to overcome, or often add additives to reduce the shrinkage rate to overcome, pressed out of the flat plate is often added LDPE to improve the problem of neck shrinkage.