Plastic has advantages of light weight, good toughness and easy molding. Due to advantages of low cost and other advantages, more and more plastics are used instead of glass in modern industry and daily products, especially in optical instruments and packaging industries, development is particularly rapid. However, due to requirement of good transparency, high wear resistance and good impact toughness, components of plastic, process and equipment of entire injection molding process are required. A lot of work must be done to ensure that these plastics used to replace glass (hereinafter referred to as transparent plastics) have good surface quality, so as to meet requirements of use.

 

At present, transparent plastics generally used in the market are polymethyl methacrylate (commonly known as acrylic or plexiglass, codenamed PMMA), polycarbonate (codename PC), polyethylene terephthalate (codename PET) , transparent nylon, AS (acrylonitrile - styrene copolymer), polysulfone (code name PSF), etc. Among them, the three plastics we have most contact with are PMMA, PC and PET. Due to limited space, following three plastics are used as examples to discuss characteristics and injection molding process of transparent plastics.

First of all, transparent plastics must have high transparency, and secondly, they must have certain strength and wear resistance, impact resistance, good heat resistance, excellent chemical resistance, and lower water absorption. Only in this way can it meet requirements of transparency and remain unchanged for a long time in use. PC is an ideal choice, but it is mainly due to its expensive raw materials and difficult injection molding process, so PMMA is still main choice (for products with general requirements), and PPT can obtain good mechanical properties because it needs to be stretched. Therefore, it is mostly used in packaging and containers.

Due to high light transmittance of transparent plastics, surface quality of plastic products must be strictly required, and there should be no defects such as streaks, pores, whitening, halo, black spots, discoloration, poor gloss, etc. Therefore, in the entire injection molding process, we must pay great attention to, put forward strict and even special requirements for design of raw materials, equipment, molds, and even products.
Secondly, because most of transparent plastics have high melting point and poor fluidity, in order to ensure surface quality of product, it is often necessary to make fine adjustments in process parameters such as barrel temperature, injection pressure, injection speed, etc., so that mold can be filled with plastic during injection, internal stress will not cause product deformation and cracking.
In terms of equipment and mold requirements, injection molding process and raw material handling of products, we will talk about matters that should be paid attention to:

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

In order to prevent contamination of raw materials and existence of old materials or impurities in depressions of screw and accessories, especially resin with poor thermal stability, screw cleaning agent should be used to clean all parts before use and after shutdown, so that no impurities can be adhered. When there is no screw cleaning agent, resins such as PE and PS can be used to clean screw.
When temporarily shutting down, in order to prevent raw materials from staying at high temperature for a long time and causing degradation, temperature of dryer and barrel should be lowered, such as PC, PMMA and other barrel temperatures should be lowered to below 160 ℃. (Hopper temperature should drop below 100℃ for PC)
Problems that should be paid attention to in mold design (including product design)
In order to prevent poor reflow, or uneven cooling, resulting in poor plastic molding, surface defects and deterioration;
Generally, when designing mold, following points should be paid attention to:
Wall thickness should be as uniform as possible, and demolding slope should be large enough;
Transition part should be gradually and smoothly transitioned to prevent sharp corners and sharp edges, especially PC products must not have gaps;
Gate. Runner should be as wide and stubby as possible, and gate position should be set according to shrinkage and condensation process, and a cold well should be added if necessary;
Surface of mold should be smooth and low in roughness (preferably below 0.8);
Vent. Tank must be sufficient to discharge air and gas in melt in a timely manner;
Except for PET, wall thickness should not be too thin, generally not less than 1mm.
Issues that should be paid attention to in injection molding process (including requirements of injection molding machine)
In order to reduce internal stress and surface quality defects, following aspects should be paid attention to in injection molding process:
Special screw and injection molding machine with separate temperature control nozzle should be used;
Injection temperature should be higher on premise that plastic resin does not decompose;
Injection pressure: generally high to overcome defect of high melt viscosity, but too high pressure will cause internal stress to cause difficulty in demolding and deformation;
Injection speed: In the case of satisfying mold filling, it should generally be low, and it is best to use slow-fast-slow multi-stage injection;
Pressure holding time and molding cycle: In the case of satisfying product filling, no dents and bubbles; it should be as short as possible to minimize residence time of melt in barrel;
Screw speed and back pressure: under premise of satisfying plasticization quality, it should be as low as possible to prevent possibility of degradation;
Mold temperature: Cooling quality 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.

In order to prevent deterioration of upper surface quality, generally use as little release agent as possible during injection molding; when using recycled materials, it should not exceed 20%.
Except for PET, all products should be post-treated to eliminate internal stress. PMMA should be dried in hot air at 70-80℃ for 4 hours; PC should be heated at 110-135 ℃ in clean air, glycerin, liquid paraffin, etc. Time depends on product, and it takes more than 10 hours at most. PET must go through process of biaxial stretching in order to obtain good mechanical properties.

Process characteristics of transparent plastics
In addition to above common problems, transparent plastics also have some process characteristics, which are described as follows:

PMMA has high viscosity and slightly poor fluidity, so it must be injected with high material temperature and high injection pressure. Influence of injection temperature is greater than that of injection pressure, but increase of injection pressure is conducive to improving shrinkage rate of product.
Injection temperature range is wide, melting temperature is 160 ℃, and decomposition temperature is 270 ℃, so material temperature adjustment range is wide, and processability is good. Therefore, to improve fluidity, injection temperature can be started. Poor impact resistance, poor wear resistance, easy to scratch, easy to brittle, so mold temperature should be increased to improve condensation process to overcome these defects.

PC has high viscosity, high melt temperature and poor fluidity, so it must be injected at a higher temperature (between 270-320℃). Relatively speaking, material temperature adjustment range is narrow, and processability is not as good as PMMA. Injection pressure has little effect on fluidity, but due to high viscosity, injection pressure is still large. Correspondingly, in order to prevent internal stress, pressure holding time should be as short as possible.
Shrinkage rate is large and the size is stable, but internal stress of product is large and it is easy to crack. Therefore, it is advisable to increase temperature instead of pressure to improve fluidity, and to increase mold temperature, improve mold structure and post-processing to reduce possibility of cracking. When injection speed is low, gate is prone to corrugation and other defects. Temperature of radiation nozzle should be controlled separately, mold temperature should be high, resistance of runner and gate should be small.

PET molding temperature is high, and material temperature adjustment range is narrow (260-300 ℃), but after melting, fluidity is good, so processability is poor, and an anti-spreading device is often added to nozzle. Mechanical strength and performance are not high after injection, performance must be improved by stretching and modification.
Mold temperature is accurately controlled to prevent warping. Deformation is an important factor, so it is recommended to use a hot runner mold. Mold temperature should be high, otherwise it will cause poor surface gloss and difficulty in demolding.

 

Its defects are probably following:

Influenced by anisotropy of internal stress during mold filling and condensation process, stress generated in vertical direction causes resin to orientate in flow direction, and refractive index is different from that of non-flow orientation, resulting in shiny silk lines. When it expands, it may cause cracks in product. In addition to paying attention to injection molding process and mold, it is best to anneal product. For example, PC material can be heated to above 160℃ for 3-5 minutes, and then cooled naturally.

Water vapor and other gases mainly in resin cannot be discharged, (during mold condensation process) or due to insufficient filling of mold, condensation surface condenses too quickly to form a vacuum bubble.

Mainly due to large roughness of mold, on the other hand, premature condensation makes resin unable to copy state of mold surface, all of which make surface uneven and make product tarnish.

It refers to dense corrugations formed from sprue as center. Reason is that due to excessive viscosity of melt, front end material has condensed in cavity, and then material breaks through condensation surface, resulting in vibration lines on the surface.

It is mainly caused by dust falling into raw materials in the air or water content of raw materials being too large.

It is mainly caused by decomposition or deterioration of resin in barrel due to local overheating of plastic in barrel.