Due to advantages of light weight, good toughness, easy molding, and low cost, plastics are increasingly used in modern and daily-use products to replace glass, especially in optical instrument and packaging industries. However, due to requirement for transparency, wear parts should be high, and impact resistant parts should be good. Therefore, a lot of work must be done on composition of plastic, process of injection manufacturing process, equipment, mold to ensure that these plastics used for replacing glass (hereinafter referred to as transparent plastics) have good surface quality, and met requirements for use.
Currently, transparent plastics commonly used on the market are polymethyl methacrylate (commonly known as acrylic or plexiglass, pmma), polycarbonate(pc), Polyethylene terephthalate (pet), transparent nylon, As (acrylonitrile-styrene copolymer), polypyrene (psf), etc. Among them, we have the most contact with pmma, pc and pet. Due to limited number of frames, these three plastics are used as examples to discuss characteristics of transparent plastics and injection manufacturing process.

Transparent plastic must first have high transparency. Secondly, it must have certain strength and wear resistance. It can resist impact, heat-resistant parts, chemical resistance, and water absorption. Only in this way can it meet transparency requirements and remain unchanged for a long time. Table 1 below compares performance of pmma, pc and pet.
Table 1: Comparison of transparent plastics performance

Note:
(1) Due to wide variety, this is only an average value, actual data of different varieties is different.
(2) Pet data (mechanical aspect) is stretched data.
From data in Table 1, it can be seen that pc is an ideal choice, but it is mainly because of high price of raw materials and injection manufacturing process. Therefore, pmma is still main choice (for general requirements), and pet is used in packaging and containers because it has to be stretched to obtain good mechanical properties.

Due to high light transmittance of transparent plastics, surface quality of plastic products is required to be strict, and there should be no defects such as streaks, pores, whitening, halo, black spots, discoloration, poor gloss, etc. Therefore, in entire injection manufacturing process, raw materials, equipment, molds, and even design of products must pay great attention to and put forward strict or even special requirements. Secondly, since transparent plastics have high melting point and poor fluidity, in order to ensure surface quality of products, it is necessary to finely adjust process parameters such as machine temperature, injection pressure and speed so that plastic injection can not only fill mold without generating internal stress, causing deformation and cracking of product.
Here are some things to be aware of in terms of raw material preparation, equipment, mold requirements, injection manufacturing process, and raw material handling of product.

Since any impurities contained in the plastic may affect transparency of product, it is necessary to pay attention to sealing during storage, transportation and feeding to ensure that raw materials are clean. In particular, raw material contains moisture, which causes deterioration of raw material after heating, so it must be dried. When it is injected, 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 it does not contaminate material. Drying process is shown in Table 2.
Table 2, drying process of transparent plastic:

In order to prevent contamination of raw materials, presence of old materials or impurities in recesses of screws and accessories, especially resin with poor thermal stability, Therefore, screw cleaning agent should be used to clean parts before and after shutdown, so that they do not stick to impurities. When there is no screw cleaning agent, screw can be cleaned with a resin such as pe, ps. When temporarily stopping, in order to prevent raw material from staying at high temperature for a long time, causing solution to fall, temperature of dryer and barrel should be lowered. For example, temperature of barrel such as pc and pmma should be reduced to below 160℃. (Hopper temperature should be reduced to below 100℃ for pc)

In order to prevent occurrence of poor back flow, or uneven cooling, resulting in poor molding of plastic, surface defects and deterioration, generally in molding design, we should pay attention to following points.
a) Wall thickness should be as uniform as possible, and draft angle should be large enough;
b) Transition should be gradually smooth transition to prevent sharp corners. Sharp edges are produced, especially PC products must not have gaps;
c) Gate. Flow path should be as wide and thick as possible, and gate position should be set according to shrinkage condensation process. If necessary, a cold well should be added;
d) Surface of mold should be smooth and roughness is low (preferably less than 0.8);
e) Vent hole. Tank must be sufficient to expel air and gas in melt;
f) 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 noted in injection manufacturing process.
a) Special screw and injection molding machine with separate temperature control nozzle should be used;
b) Injection temperature should be high on premise that plastic resin does not decompose;
c) Injection pressure: generally high to overcome defect of high viscosity of melt, but if pressure is too high, internal stress will be generated to cause bad demoulding and deformation;
d) Injection speed: in the case of filling mold, it is generally low. It is better to use slow, fast and slow multi-stage injection;
e) Holding time and molding cycle: in the case of satisfying product filling, no dents or bubbles; it should be as short as possible to minimize residence time of melt in barrel;
f) Screw speed and back pressure: under premise of satisfying plasticizing quality, it should be as low as possible to prevent possibility of depreciation;
g) Mold temperature: Cooling of product has a great influence on product quality. Therefore, mold temperature must be able to accurately control process. If possible, mold temperature should be high.

In order to prevent deterioration of surface quality, mold release agent should be used as little as possible during injection molding; when it is used, it should not be greater than 20%.
For products other than pet, product should be post-treated to eliminate internal stress. Pmma should be dried in 70-80t hot air for 4 hours. Pc should be heated at 110-135℃ in clean air, glycerin, liquid paraffin depending on product, maximum time is more than 10 hours. And pet must undergo a biaxial stretching process 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 a high viscosity and a slightly poor fluidity, so it must be injected with high temperature and injection pressure. Influence of injection temperature is greater than injection pressure, but injection pressure is increased to improve shrinkage of product. Injection temperature range is wide, melting temperature is 160℃, and decomposition temperature is 270℃, so temperature adjustment range is wide and processability is good. Therefore, improving fluidity can be started from injection temperature. Poor impact and wear resistance, easy to scratch and brittle, so mold temperature should be improved, condensation process should be improved to overcome these defects.

Viscosity of pc is large, temperature of melt is high, and fluidity is poor. Therefore, it must be injection molded at a relatively high temperature (between 270 and 320 t), and temperature adjustment range is relatively narrow, and process is not as good as pmma. Injection pressure has little effect on fluidity, but due to high viscosity, it still requires a large injection pressure. In order to prevent internal stress, pressure holding time should be as short as possible. Shrinkage is large and size is stable, but internal stress of product is large and easy to crack. Therefore, it is advisable to increase temperature rather than pressure to improve fluidity, and to improve mold temperature, improve mold structure and post-treatment to reduce possibility of cracking. When injection speed is low, gate is prone to ripples and other defects. Temperature of nozzle should be controlled separately, mold temperature should be high, runner and gate resistance 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 anti-ducting device is often added in nozzle. Mechanical strength and performance are not high after injection and must be improved by stretching and modification. Accurate control of mold temperature is an important factor to prevent warpage and deformation. It is recommended to use hot runner mold. Mold temperature is high, otherwise it will cause poor surface gloss and hard demoulding. In summary, we can list process parameters of plastic injection moulding process in Table 3 for your reference.
Table 3: Transparent plastic injection moulding process parameter table:

Due to length of relationship, only defects affecting transparency of product will be discussed here. For other defects, please refer to company's product manual or other materials. Defects are probably following:

During filling and condensing process, internal stress anisotropy is affected, and stress generated in vertical direction causes resin to be oriented on flow, and non-flow orientation produces a different refractive index and produces a flashing silk pattern. When it is expanded, it may cause cracks in product. In addition to attention paid to injection molding process and mold (see Table 4), it is preferable to anneal product. If pc material can be heated to above 160℃ for 3-5 minutes, then it can be cooled naturally.

Water vapor and other gases in resin are not discharged (during mold condensation process), or condensation surface is too fast to condense due to insufficient filling, forming a "vacuum bubble". Method of its overcoming is shown in Table 4.

Mainly due to large roughness of mold, on the other hand, condensation is too early, so that resin can not copy surface of mold, all of which makes surface of cattle slightly uneven, and product is tarnished. Method of its overcoming is shown in Table 4.

Refers to dense corrugation formed from sprue center. Reason is that melt viscosity is too large, front end material has been condensed in cavity, and then material is broken through condensation surface, causing surface to appear seismic pattern. Method of overcoming is shown in Table 4.

mainly caused by dust falling into raw material in air or moisture content of raw material is too large. Method of its overcoming is shown in Table 4.

It is mainly formed because plastic is in barrel and barrel resin is decomposed or deteriorated due to local overheating. Method of its overcoming is shown in Table 4. In order to clearly illustrate measures taken to overcome these deficiencies, Table 4 is listed for your reference.
Table 4: Defects and Overcoming Methods for Transparent Products: