Know the plastic properties for plastic machining parts

As the precision of Plastic Injection moulding continues to increase, it is important to have the correct choice of materials and to be familiar with the properties of the selected plastics in order to obtain beautiful, flawless products, in addition to constantly updating and upgrading equipment. In this issue, Machining Of Plastic Parts SCZY and everyone to introduce the main plastic properties related to Plastic Injection Molding process.

1. Liquidity

After being heated, the molten state of the plastic, under pressure, the ability to fill the entire cavity is called liquidity, generally with professional liquidity test mould to test.

The fluidity of the same type of different grades of plastic are not the same, different structure of the plastic parts of the plastic fluidity requirements, plastic fluidity and molding temperature and pressure and other conditions are closely related.

Usually, if the fluidity is too large, on the one hand, the melt will not fill tightly, resulting in loose products, affecting product quality; on the other hand, the fluidity is too large, the Injection Molding parts around the easy to appear phi front (burr) sticky mold and nozzle flow delay, resulting in blockage of the water mouth. The fluidity of the plastic is too small, for the complex structure of the long process of plastic parts, injection molding flow difficulties, easy to appear lack of glue, shrinkage and other phenomena, resulting in a large number of scrap; or must be injected with high pressure conditions, easy to cause excessive internal stress in the plastic parts. We need to choose the right plastic flow according to the structure, size and thickness of the product.

2. Water absorption and volatile matter content

Thermoplastics contain more or less water and volatiles, and the right amount of water has a plasticizing effect. If the water and volatiles in the plastic exceed a certain proportion, many problems will occur during injection moulding (e.g. degradation, fogging, strength reduction, etc.), and in serious cases, bubbles (silver lines), surface roughness, and for transparent products, light transmission is destroyed (cloudy) and other undesirable phenomena, and it is difficult to ensure the accuracy of precision plastic parts. However, absolutely dry plastic will cause a reduction in fluidity, increased brittleness, mould filling difficulties, is also not available, this point should be paid special attention to, some people think that the more fully dried plastic the better, this is a wrong concept.

There are three main causes of moisture and volatiles in plastics;

A, the average molecular weight of plastic resin is low;

B. The plastic resin is not sufficiently dried at the time of production;

C, absorbent plastic due to improper storage and make it absorb the moisture in the surrounding air, different plastics have different drying temperature and drying time regulations.

3. Shrinkage rate

The main reasons for shrinkage are the following three aspects:

A. Plastics have a much larger thermal shrinkage than metals (an order of magnitude larger, about ten times).

B, plastic products in the hardening is not rigid, in the mold after the plastic parts have a certain elastic return.

C, just after demoulding, the pressure starts to decrease, but the Plastic Part is still attached to the mould wall, and the product undergoes plastic deformation.

Factors affecting shrinkage are the nature of the plastic, the moulding conditions and the design of the product. The shrinkage rate of amorphous plastic is less than 1%, while the shrinkage rate of crystalline plastic is more than 1%.