Six points of injection molding of PP plastic bottle

The processing process of plastic raw materials is mainly the melting, flowing, and cooling of the rubber granules to become the finished product. process.

1. Melting

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

In addition, the good effect and the characteristics of PP's high thermal cracking are that it is best to allow the raw materials to smoothly flow to the die during production, so as to avoid insufficient filling or reflux phenomenon. The increase in average flow efficiency is equal to the increase in MFR, which is one of the methods available for processing, but it also causes the abnormal distribution of MFR, which may lead to increased instability, which may lead to an increase in the defective rate.

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

2. Screw

Most of the PP processing relies on the screw to drive the fluidity, so the design of the screw has a great influence. The size of the caliber affects the output, the compression ratio affects the pressure value, and also affects the output and the effect of the finished product. This also includes a variety of materials. (color masterbatch, 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 small screw compression ratio drives the flow to be small, and the rotational speed must be increased, resulting in 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 the raw material is not just the heater, but also the frictional heat and the suffocation time must be taken into account.

So this is a practical matter, and experience contributes to production problem solving 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 each section of different types of screws is divided to achieve various mixing effects.

3. Mould or die

Plastic reshaping relies on the mold or die. The injection molding product is three-dimensional, and the mold is more complex, and the shrinkage rate must be considered. Others are flat, strip, and needle-shaped continuous product dies. It is classified as a special shape and needs to pay attention to the problem of immediate cooling and shaping.

Most of the plastic machines are designed like a syringe barrel. The extrusion force driven by the screw will cause huge pressure at the small outlet, improving production efficiency. When the die head is designed to be flat, how to distribute the raw materials evenly on the entire surface, the design of the hanger die head is very important, and the attention to the extrusion opportunity increases the stable supply of raw materials by the gill pump.

4. Cooling

In addition to pouring raw materials into the sprue gate, the injection mold also has a cooling water channel to cool the raw materials. 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 sprayed on the blow bag, and the cooling method such as hollow air blowing.

5. Extension

Reprocessing and extension of the finished product will enhance the effect. For example, the different driving speeds of the front and rear rollers of the packing belt will cause the extension effect. Molecular weight distribution also affects the elongation effect during high-speed production. All extruded products, including fibers, have unequal elongation. Vacuum and air pressure forming can also be regarded as another form of elongation.

6. Shrink

Any raw material has the problem of shrinkage. The shrinkage is caused by thermal expansion and cold contraction and the internal stress generated during the formation of crystallization.

Generally speaking, it is easier to overcome thermal expansion and cold contraction. The cooling time can be extended in processing, and the pressure can be maintained continuously. Crystallized raw materials often have a larger shrinkage difference than amorphous materials, which is about 1/1000 in terms of PP. 16. But ABS is only about 4/1000. The difference is very big. This part needs to be overcome on the mold, or often by adding additives to reduce the shrinkage rate. LDPE is often added to the extruded flat plate to improve the problem of necking.