Because plastic has advantages of light weight, good toughness, easy molding and low cost, more more plastics are used to replace glass in 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, a lot of work must be done on plastic components, process, equipment, and molds of the entire injection molding process to ensure that surface quality of these plastics (hereinafter referred to as transparent plastics) used to replace glass is good, so as to meet requirements of use.

 

At present, transparent plastics generally used in the market include polymethyl methacrylate (commonly known as acrylic or plexiglass, codenamed pmma), polycarbonate (codenamed pc), polyethylene terephthalate (codenamed pet), transparent nylon, as(acrylonitrile-styrene copolymer), polysulfone (code-named psf), etc. Among them, the three plastics we have most contact with are pmma, pc and pet. Due to limited width, following three plastics are used as examples to discuss characteristics and injection molding process of transparent plastics.

Transparent plastics must first have high transparency, and secondly must have certain strength and wear resistance, impact resistance, good heat resistance, excellent chemical resistance, and low water absorption. Only in this way can it meet requirements of transparency and remain unchanged for a long time in use. Table l is listed below to compare performance of pmma, pc and pet.
Table 1 Comparison of transparent plastic properties

Note:
(1) Due to wide variety of varieties, this is only an average value, and actual data of different varieties are different.
(2) Pet data (mechanical aspect) is data after stretching.
From data in Table 1, it can be seen that 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 (generally required products), ppt is mostly used in packaging and containers because it needs to be stretched to obtain good mechanical properties.

Due to high light transmittance of transparent plastics, surface quality of plastic products must be strictly required, there should be no defects such as streaks, pores, white haze, black spots, discoloration, poor gloss, etc. Therefore, in the entire injection molding process, great attention should be paid to design of raw materials, equipment, molds, and even products, strict or even special requirements should be put forward. Secondly, since 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 to process parameters such as high temperature, injection pressure, injection speed, etc., so that mold can be filled when plastic is injected, product will not be deformed and cracked due to internal stress.
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 raw material processing of products.

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 the raw materials will not be polluted. Its drying process is shown in Table 2:
Table 2 Drying Process for Transparent Plastics

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 attached. 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 material from staying at high temperature for a long time and causing degradation, temperature of dryer and barrel should be lowered, such as barrel temperature of pc, pmma, etc. should be lowered to below 160 ℃. (Hopper temperature should drop below 100℃ for pc)

 

In order to prevent poor backflow or uneven cooling, resulting in poor plastic molding, surface defects and deterioration, following points should be paid attention to in mold design:
a) Wall thickness should be as uniform as possible, and demolding slope should be large enough;
b) Transition part should be gradually and smoothly transitioned to prevent sharp corners and sharp edges, especially PC products must not have gaps;
c) Gate. Runner should be as wide and stubby as possible, gate position should be set according to shrinkage and condensation process, and a cooling well should be added if necessary;
d) Mold surface should be smooth and low in roughness (preferably lower than 0.8);
e) Exhaust holes. Tank must be sufficient to discharge air and gas in melt in a timely manner;
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 paid attention to in injection molding process.
a) Special screw and injection molding machine with separate temperature control nozzle should be used;
b) Under premise that plastic resin does not decompose at injection temperature, a higher injection humidity should be used;
c) Injection pressure: generally high to overcome defect of high melt viscosity, but too high pressure will generate internal stress to cause difficulty in demolding and deformation;
d) Injection speed: in the case of meeting mold filling, it should generally be low, and it is best to use slow-fast-slow multi-stage injection;
e) Holding time and molding cycle: When meeting product filling, it should be as short as possible without producing dents and bubbles, so as to minimize residence time of melt in barrel;
f) Screw speed and back pressure: on the premise of satisfying plasticizing quality, they should be as low as possible to prevent possibility of degradation;
g) 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 quality of upper surface, 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 circulation 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 be biaxially stretched in order to obtain good mechanical properties.

(1) 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 beneficial to improve shrinkage rate of product. Injection temperature range is wide, melting temperature is 160℃, 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 manufacturability 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 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 defects such as ripples. Temperature of radiation nozzle should be controlled separately, mold temperature should be high, resistance of runner and gate should be small.

 

Molding temperature of pet is high, and material temperature adjustment range is narrow (260-300℃), but after melting, fluidity is good, so craftsmanship is poor, and an anti-spreading device is often added to nozzle. Mechanical strength and properties are not high after injection, properties must be improved by stretching and modification. Accurate control of mold temperature is an important factor to prevent warpage and deformation, so it is recommended to use hot runner mold. If mold temperature is too high, it will cause poor surface gloss and difficulty in demolding. In summary, we can list injection molding process parameters for your reference when injecting.
Table 3 Transparent plastic injection molding process parameters

Due to space constraints, only defects affecting product transparency are discussed here. Its defects are probably the following:
(1) Silver streak: Influenced by anisotropy of internal stress during filling and condensation process, stress generated in vertical direction causes resin to undergo a flow orientation, and refractive index is different from non-flow orientation to generate a light silk streak. After its expansion, it may cause cracks in product. In addition to paying attention to injection molding process and mold (see Table 4), it is best to anneal product. For example, pc material can be heated to above 160℃ for 3-5 minutes, and then cooled naturally.
(2) Bubble: Main reason is that water vapor and other gases 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". Its overcoming methods are shown in Table 4.
(3) Poor surface gloss: mainly due to large roughness of mold, and on the other hand, premature condensation, so that resin cannot copy state of surface of mold, all of which make surface uneven and make product lose its luster. Its overcoming methods are shown in Table 4.
(4) Vibration pattern: It refers to dense corrugation formed from sprue as center. Reason is that due to excessive viscosity of melt, front end material has been condensed in cavity, and then material breaks through condensation surface, resulting in the appearance of vibration pattern on the surface. Methods of overcoming it are shown in Table 4.
(5) Whitening. Fog halo: Mainly caused by dust falling into raw material in the air or too much moisture content of raw material. Its overcoming methods are shown in Table 4.
(6) WHITE SMOKE. Black spots: Mainly due to decomposition or deterioration of resin in barrel due to local overheating of plastic in the barrel. Its overcoming methods are shown in Table 4. In order to clearly illustrate measures taken to overcome these deficiencies, Table 4 is presented for your reference.
Table 4: Defects of transparent products and how to overcome them