Because plastic has advantages of light weight, good toughness, easy molding, low cost, etc., so in modern industry and daily products, more and more plastics are used instead of glass, especially in optical instruments and packaging industries, development is particularly rapid. However, because it requires good transparency, high abrasion resistance, and good impact toughness, a lot of work has to be done on composition of plastic, entire process of injection molding, equipment, molds, etc., to ensure that these plastics used to replace glass (hereinafter referred to as transparent plastics) have good surface quality to meet requirements for use.

 

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

Transparent plastics must first have high transparency, secondly they must have certain strength and abrasion resistance, impact resistance, good heat resistance, good chemical resistance, and low water absorption. Only in this way can it meet requirements of transparency and remain unchanged in use. PC is an ideal choice, but mainly because of its high raw material price and difficult injection molding process, PMMA is still main choice(for products with general requirements), and PPT has to be stretched to obtain good mechanical properties. So it is mostly used in packaging and containers.

Due to high light transmittance of transparent plastics, strict requirements on the surface quality of plastic products must be required, and there should be no defects such as markings, pores, whitening, haze, black spots, discoloration, and poor gloss. Therefore, in 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 point and poor fluidity, in order to ensure surface quality of product, it is often necessary to make fine adjustments to barrel temperature, injection pressure, injection speed and other process parameters, so that mold can be filled when plastic is injected, and internal stress will not be generated to cause product to deform and crack.
In terms of equipment and mold requirements, injection molding process and product raw material processing, talk about matters that should be paid attention to:

Preparation and drying of raw materials
Because any impurity in the plastic may affect transparency of product, it is stored and transported.
During feeding process, you must pay attention to sealing 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. 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.

In order to prevent contamination of raw materials, presence of old materials or impurities in the 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 cleaner, PE, PS and other resins 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 and cause degradation, temperature of dryer and barrel should be lowered, such as PC, PMMA and other barrels should be reduced to below 160 ℃. (Hopper temperature should be lowered to below 100℃ for PC)

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 are generated, especially PC products must not have gaps;
Gate. 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. Tank must be sufficient to discharge air and gas in the melt in time;
Except for PET, wall thickness should not be too thin, generally not less than 1mm.

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 dedicated 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.However, if pressure is too high, internal stress will result in 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: in the case of satisfying product filling without dents and bubbles; 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.

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

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

PMMA has high viscosity and poor fluidity, so high material temperature and high injection pressure must be used for injection molding. Influence of injection temperature is greater than injection pressure, but increase in injection pressure will help improve shrinkage of product.
Injection temperature range is wide, melting temperature is 160°C, and decomposition temperature is 270°C, so material temperature adjustment range is wide and processability is better. Therefore, to improve fluidity, start from injection temperature. Impact resistance is poor, wear resistance is not good, it is easy to scratch and brittle, so mold temperature should be increased, and condensation process should be improved to overcome these defects.

PC has high viscosity, high melt temperature and poor fluidity. Therefore, it must be injection molded at a higher temperature (between 270-320°C). Relatively speaking, temperature adjustment range of material is narrow and manufacturability is not as good as PMMA. Injection pressure has little effect on fluidity, but due to high viscosity, injection pressure is still higher. Correspondingly, in order to prevent generation of internal stress, pressure holding time should be as short as possible.
Shrinkage rate is large, size is stable, butproduct has high internal stress and is easy to crack. Therefore, it is better to increase temperature instead of pressure to improve fluidity, and increase mold temperature, improve mold structure and post-processing to reduce possibility of cracking. When injection speed is low, defects such as ripples are prone to occur at gate, temperature of radiating nozzle should be controlled separately, temperature of mold should be high, resistance of runner and gate should be small.

PET molding temperature is high, material temperature adjustment range is narrow (260-300℃), but after melting, it has good fluidity, so it is poor in manufacturability, and it is often necessary to add anti-spreading devices in nozzle. Mechanical strength and performance are not high after injection, performance must be improved through stretching process and modification.
Accurate control of mold temperature is to prevent warpage. An important factor of deformation, it is recommended to use hot runner molds. Mold temperature should be high, otherwise it will cause poor surface gloss and difficulty in demolding.

Defects are as follows:

Influenced by internal stress anisotropy during filling and condensation process, stress generated in vertical direction causes resin to flow upward orientation, and refractive index is different from non-flow orientation to produce flashing silk patterns. When it expands, it may crack product. In addition to paying attention to injection molding process and mold, it is best to make product annealed. For example, PC material can be heated to above 160°C for 3-5 minutes, and then cooled naturally.

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

Main reason is that roughness of mold is large. On the other hand, resin can not copy state of mold surface due to premature condensation, all of which make surface produce small irregularities and make product lose its luster.

It refers to dense ripples formed from straight gate as center. Reason is that due to excessive viscosity of melt, material at the front end has been condensed in cavity, and then material breaks through condensing surface, causing surface to appear shock lines.

It is mainly caused by dust falling into raw material in the air or raw material has too much water content.

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