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Injection molding process for producing transparent plastic products

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Transparent plastic box isolated on white background.

Processing Focus

Due to the high light transmittance of transparent plastics during injection molding processing, plastic products must have strict surface quality requirements and must not have any defects such as markings, pores, whitening, halos, black spots, discoloration, poor gloss, etc. Therefore, during the entire injection molding process, great attention must be paid to raw materials, equipment, molds, and even product design, and strict or even special requirements must be put forward.

Production Preparation

Secondly, since most transparent plastics have high melting points and poor fluidity, to improve the surface quality of the product, it is often necessary to make fine adjustments to process parameters such as machine temperature, injection pressure, and injection speed, so that the mold can be filled with plastic while not filling the mold. Internal stress will be generated, causing product deformation and cracking. Therefore, strict operations must be carried out from raw material preparation to equipment and mold requirements, injection molding process and product raw material handling.

Material Properties

Preparation and drying of materials. Any impurities contained in the plastic may affect the transparency of the product, thus affecting storage and transportation. During the feeding process, attention must be paid to sealing to keep the raw materials clean. In particular, the raw materials contain moisture, which will cause deterioration of the raw materials after heating, so they must be dried, and during injection molding, a dry hopper must be used for feeding.

Manufacturing Processes

It should also be noted that during the drying process, the input air should be filtered and dehumidified to ensure that the raw materials will not be polluted. The drying process is such as the drying process of transparent plastic:

PMMA process parameters

PC process parameters

PET process parameters

Cleaning of the barrel, screw and accessories. In order to prevent raw material contamination and the presence of old materials or impurities in the recesses of the screw and accessories, especially the presence of resin with poor thermal stability, screw cleaning agent should be used before use and after shutdown. Clean all parts to prevent impurities from sticking to them. When there is no screw cleaning agent, PE, PS and other resins can be used to clean the screw.

When the machine is temporarily shut down, in order to prevent the raw materials from staying at high temperatures for a long time and causing degradation, the temperature of the dryer and barrel should be lowered. For example, the barrel temperature of PC, PMMA, etc. should be lowered to below 160°C. (The hopper temperature should drop below 100℃ for PC)

Mold Factors

Issues that should be paid attention to in mold design (including product design) In order to prevent poor backflow or uneven cooling from causing poor plastic molding, resulting in surface defects and deterioration, generally the following points should be paid attention to when designing molds.

a: The wall thickness should be as uniform as possible, and the draft must be large enough;

b: Transitions should be gradual. Smooth transition to prevent sharp corners. Sharp edges are generated, especially PC products must not have gaps;

c: gate. The flow channel should be as wide and short as possible, and the gate position should be set according to the shrinkage and condensation process. If necessary, a cold well should be added;

d: The mold surface should be smooth and clean with low roughness (less than 0.8 is recommended);

e: Exhaust hole. The tank must be sufficient to discharge air and gas in the melt in a timely manner;

f: Except for PET, the wall thickness should not be too thin, generally not less than lmm.

BFY Mold integrates product conception with product forming and mainly undertakes plastic mold product development and design, mold design and manufacturing, product injection molding, product surface treatment, and other services. With more than 20 years of production experience in mold design and mold manufacturing, the injection molding processing industry can create competitive molds for you. Contact us today for free drawing optimization.

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