Five major issues that need attention when forming transparent injection molded parts, please check!

Time：2020-12-10 11:17:51 / Popularity： / Source：

Due to high light transmittance of transparent plastics, strict requirements on the surface quality of plastic products must be required, there should be no defects such as markings, pores, whitening, haze, black spots, discoloration, and poor gloss. Therefore, in the entire injection molding process, we must pay great attention to and put forward strict or even special requirements for design of raw materials, equipment, molds, and even products. Secondly, because transparent plastics mostly have high melting points and poor fluidity, in order to ensure surface quality of products, it is often necessary to make fine adjustments to high temperature of machine, injection pressure, injection speed and other process parameters, so that mold can be filled with the plastic during injection, and no internal stress will cause product to deform and crack.

Let's talk about matters that should be paid attention to in terms of raw material preparation, equipment and mold requirements, injection molding process and product raw material processing.

Raw material preparation and drying

Since any impurity in the plastic may affect transparency of product, it must be sealed during storage, transportation, and feeding to ensure that raw materials are clean.
Especially raw material contains moisture, which will cause raw material to deteriorate after heating, so it must be dried, and when injection molding, a drying hopper must be used for feeding. It should also be noted that during drying process, input air should preferably be filtered and dehumidified to ensure that raw materials will not be polluted.

Cleaning of barrel, screw and accessories

In order to prevent contamination of raw materials and presence of old materials or impurities in recesses of screw and accessories, especially presence of resin with poor thermal stability, screw cleaning agents should be used to clean all parts before use and after shutdown to prevent impurities from sticking. When there is no screw cleaning agent, screw can be cleaned with pe, ps and other resins.
When temporarily shutting down, in order to prevent raw materials from staying at high temperature for a long time, causing solution to drop, temperature of dryer and barrel should be reduced, such as pc, pmma and other barrels should be reduced to below 160 ℃. (Temperature of hopper should drop below 100°C for pc)

Mold design (including product design)

In order to prevent occurrence of poor flow back or uneven cooling, resulting in poor plastic molding, surface defects and deterioration, generally following points should be paid attention to during mold design:
Wall thickness should be as uniform as possible, and draft angle should be large enough;
Transition part should be gradual. Smooth transition to prevent sharp corners. Sharp edges, especially pc products must not have gaps;
Gate and runner should be as wide and short as possible, gate position should be set according to shrinkage and condensation process, and a cold well should be added if necessary;
Mold surface should be smooth, with low roughness (preferably lower than 0.8);
Vent holes and grooves must be sufficient to discharge air and gas in the melt in a timely manner;
Except pet, wall thickness should not be too thin, generally not less than 1 mm.

Injection molding process and injection molding machine

In order to reduce internal stress and surface quality defects, following aspects should be paid attention to in injection molding process:
An injection molding machine with a special screw and a separate temperature control nozzle should be used;
Injection temperature should be higher if plastic resin does not decompose;
Injection pressure: generally higher to overcome defect of high melt viscosity, but too high pressure will cause internal stress and cause difficult demoulding and deformation;
Injection speed: In the case of satisfying mold filling, generally it should be low, and it is best to use slow-fast-slow multi-stage injection;
Holding pressure time and molding cycle: under condition that product does not produce dents or bubbles when filling mold; it should be as short as possible to minimize residence time of melt in barrel;
Screw speed and back pressure: on the premise of meeting plasticization quality, it should be as low as possible to prevent possibility of degrading;
Mold temperature: Cooling of product has a great impact on quality, so mold temperature must be able to accurately control process. If possible, mold temperature should be higher.

Other issues

In order to prevent deterioration of surface quality, generally use as little release agent as possible during injection molding; when using recycled material, it should not exceed 20%.
Except pet, all products should be post-processed to eliminate internal stress. PMMA should be dried in 70-80t hot air circulation for 4 hours; PC should be heated in clean air, glycerin, liquid paraffin, etc., at 110-135°C. Time depends on product, and it takes up to 10 hours. Pet must undergo a biaxial stretching process to obtain good mechanical properties.

Gud Mould Industry Co., Ltd