Introduction of transparent plastic injection molding process

1、The characteristics of commonly used transparent raw materials

Transparent plastics, because of the high light transmission rate, must require strict surface quality of plastic products, there can not be any mottling, porosity, white, haze, black spots, discoloration, poor luster and other defects, and thus the entire injection molding process of raw materials, equipment, molds, and even product design, should be very careful and put forward strict and even special requirements.

Secondly, because transparent plastics are mostly high melting point and poor liquidity, in order to ensure the surface quality of the products, it is often necessary to make fine adjustments in the higher temperature, injection pressure, injection speed and other process parameters, so that the injection plastic can be filled with mold, and will not produce internal stress and cause product deformation and cracking.

Since transparent plastics are mostly high melting point and poor flowability, in order to ensure the surface quality of the products, it is often necessary to make fine adjustments in the higher temperature, injection pressure, injection speed and other process parameters, so that the injection plastic can fill the mold, but will not produce internal stress and cause product deformation and cracking.

Dry goods: transparent plastic injection molding process introduction

2、Problems to be noted in the process

In order to reduce the internal stress and surface quality defects, the following aspects of the injection molding process should be noted.

- 1) Injection temperature: it is advisable to use higher injection humidity provided that the plastic resin does not decompose.

- 2) injection pressure: generally higher to overcome the defects of high viscosity of the melt, but the pressure is too high will produce internal stress caused by difficult release and deformation.

- (3) injection speed: in the case of meeting the mold filling, it is generally desirable to be low, preferably using slow-fast-slow multi-stage injection.

- (4) pressure-holding time and molding cycle: in the case of meeting the product filling and not producing depressions and bubbles; it is advisable to keep it as short as possible in order to minimize the residence time of the melt in the barrel.

- (5) screw speed and back pressure: in the premise of meeting the quality of plasticization, should be as low as possible, to prevent the possibility of decompression.

- (6) mold temperature: good or bad cooling of the product, the quality of great impact, so the mold temperature must be able to accurately control the process, if possible, the mold temperature should be higher good.

- (7) In order to prevent the deterioration of the surface quality, the general injection molding as little as possible with mold release agent; when using reuse material should not be more than 20%.

Dry goods: transparent plastic injection molding process introduction

3, commonly used transparent raw materials injection molding process injection molding

In addition to the above common problems, transparent plastics also have some process characteristics, is divided into the following.

1, PMMA viscosity, liquidity is slightly poor, so it must be high material temperature, high injection pressure injection, where the impact of injection temperature is greater than the injection pressure, but the injection pressure to improve the shrinkage rate of the product.

The injection temperature range is wide, the melting temperature is 160℃, and the decomposition temperature is 270℃, so the material temperature adjustment range is wide and the processability is good. Therefore, to improve the fluidity, we can start from the injection temperature. Poor impact, wear resistance, easy to scratch, easy to brittle crack, so the mold temperature should be increased, improve the condensation process, to overcome these defects.

2, PC viscosity, high melt temperature, poor liquidity, back to this must be a higher temperature injection (between 270-320T), relatively narrow range of material temperature adjustment, the process is not as good as PMMA. injection pressure on the liquidity of less, but because of the viscosity, still need to be larger injection pressure, the corresponding in order to prevent internal stress, holding time to be as short as possible.

The shrinkage rate is large and the size is stable, but the product has high internal stress and is easy to crack, so it is advisable to improve the flowability by increasing the temperature rather than the pressure, and to reduce the possibility of cracking by increasing the mold temperature, improving the mold structure and post-treatment. When the injection speed is low, the gate is prone to ripples and other defects. The temperature of the radiation nozzle should be controlled separately, the mold temperature should be high, and the flow channel and gate resistance should be small.

3, PET molding temperature is high, and the material temperature adjustment range is narrow (260-300 ℃), but after melting, the fluidity is good, so the process is poor, and often in the injection nozzle to add anti-delay flow device. Mechanical strength and performance after injection is not high, must be through the stretching process and modification to improve performance. Accurate control of mold temperature is to prevent warpage. Deformation is an important factor, so hot runner molds are recommended. Mold temperature is high, otherwise it will cause poor surface gloss and difficult to release the mold.

4、Defects and solutions of transparent plastic parts

- 1, silver stripe: by the mold filling and condensation process, the internal stress anisotropic influence, the vertical direction of the stress generated, so that the resin occurs flow on the orientation, and and non-flow orientation to produce different refractive index and raw glitter filigree, when it expands, may make the product appear cracks. In addition to the injection molding process and mold attention, the best products for annealing treatment. Such as PC material can be heated to 160 ℃ for 3-5 minutes, and then natural cooling can be.

- 2、Bubbles: Due to the water and other gases in the resin can not be discharged, (in the process of mold condensation) or due to insufficient filling, condensing surface and too fast condensation and the formation of "vacuum bubbles.

- 3、Poor surface gloss: mainly because of the roughness of the mold, on the other hand, the condensation is too early, so that the resin can not copy the state of the mold surface, all these make the surface of the tiny bump, and the product lost gloss.

- 4、Shock ripple: It refers to the dense ripple formed from the center of the straight gate, which is caused by the excessive viscosity of the melt, the front-end material has condensed in the cavity, and later the material breaks through this condensing surface, and makes the surface appear shock ripple.

- 5, white, haze: mainly due to dust in the air into the raw materials or raw materials too much water content and caused by.

- 6, white smoke, black spots: mainly due to plastic in the barrel, due to local overheating and the barrel resin decomposition or degradation and the formation of.