Because plastic is light in weight, has good toughness and is easy to form. Due to its advantages such as low cost, plastic is increasingly used instead of glass in modern industries and daily products. It is especially used in optical instruments and packaging industries, is developing particularly rapidly. However, due to requirement of good transparency, high wear-resistant parts, and good impact-resistant toughness, a lot of work must be done on plastic ingredients, the entire injection molding process, equipment, molds, etc. to ensure surface quality of these plastics used to replace glass (hereinafter referred to as transparent plastics) is good and meet requirements for use.

 

Currently, transparent plastics commonly used on market include polymethylmethacrylate (commonly known as acrylic or plexiglass, codenamed pmma), polycarbonate (codenamed pc), and polyethylene terephthalate (codenamed pet), transparent nylon, as (acrylonitrile-styrene copolymer), polysulfone (codenamed psf), etc. Among which the three plastics we are most exposed to are PMMA, PC and PET. Due to limited width, we will take these three plastics as examples to discuss characteristics and injection molding process of transparent plastics.

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

Note: (1) Due to wide variety of varieties, this is just 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, we can see that pc is an ideal choice, but due to its expensive raw materials and difficult injection molding process, pmma is still main choice (for products with general requirements), while ppt needs 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, surface quality of plastic products must be strict, and there must not be any defects such as markings, pores, whitening, halo, black spots, discoloration, poor gloss, etc. Therefore, during the entire injection molding process, great attention must be paid to raw materials, equipment, molds, even product design, and strict or even special requirements must be put forward. Secondly, since most transparent plastics have high melting points and poor fluidity, in order to ensure surface quality of product, it is often necessary to make fine adjustments to process parameters such as machine temperature, injection pressure, and injection speed, so that mold can be filled with plastic during injection without internal stress causing product deformation and cracking.
Following is preparation of raw materials. Talk about matters that should be paid attention to in terms of equipment and mold requirements, injection molding process and raw material handling of products.

During feeding process, attention must be paid to sealing to ensure that raw materials are clean. In particular, raw materials contain moisture, which will cause deterioration of raw materials after heating, so they must be dried. During injection molding, a dry hopper must be used for feeding materials. It should also be noted that during drying process, input air should preferably be filtered and dehumidified to ensure that it does not contaminate raw materials. Drying process is shown in Table 2.
Table 2. Drying process for transparent plastic:

In order to prevent raw material contamination and presence of old materials or impurities in recesses of screw and accessories, especially presence of resins 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 cleaning agent, 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 causing decomposition, temperature of dryer and barrel should be lowered. For example, 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 from causing poor plastic molding, surface defects and deterioration, generally following points should be noted when designing mold.
a) Wall thickness should be as uniform as possible, and draft angle should be large enough;
b) Transitions should be gradual. Smooth transitions to avoid sharp corners. Sharp edges are generated, especially PC products must not have gaps;
c) Gate. Flow channel should be as wide and short as possible, and gate position should be set according to shrinkage and condensation process. If necessary, a cold well should be added;
d) Mold surface should be smooth and clean with low roughness (preferably less than 0.8);
e) Vent. 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 lmm.

 

In order to reduce internal stress and surface quality defects, following aspects should be paid attention to in injection molding process.
a) An injection molding machine with a special screw and a separate temperature-controlled injection nozzle should be used;
b) Injection temperature should use higher injection humidity on premise that plastic resin does not decompose;
c) Injection pressure: generally higher to overcome defect of high melt viscosity, but if pressure is too high, internal stress will be generated, causing difficulty in demoulding and deformation;
d) Injection speed: When mold filling is satisfied, it should generally be low. It is best to use slow, fast and slow multi-stage injection;
e) Holding time and molding cycle: Under condition that product is fully filled and no dents or bubbles are generated, it should be as short as possible to minimize residence time of molten material in barrel;
f) Screw speed and back pressure: On the premise of meeting 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 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, products should be post-processed to eliminate internal stress. PMMA should be dried in hot air circulation at 70-80t for 4 hours; PC should be heated in clean air, glycerin, liquid paraffin, etc. at 110-135℃. Time depends on product, and it may take up to more than 10 hours. And PET must go through a biaxial stretching process 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 below:
1. Process characteristics of pmma: pmma has high viscosity and slightly poor fluidity, so it must be injection molded with high material temperature and high injection pressure. Impact of injection temperature is greater than injection pressure, but increasing injection pressure will help improve shrinkage rate of product. Injection temperature range is wide, melting temperature is 160℃, and decomposition temperature reaches 270℃, so material temperature adjustment range is wide and processability is good. Therefore, to improve fluidity, we can start with injection temperature. It has poor impact resistance, poor wear resistance, easy scratching, and brittleness. Therefore, mold temperature should be increased and condensation process should be improved to overcome these defects.
2. Process characteristics of pc: PC has high viscosity, high melt temperature, and poor fluidity. Therefore, it must be injected at a higher temperature (between 270-320t). 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 still needs to be higher. Correspondingly, in order to prevent generation of internal stress, holding time should be as short as possible.
Shrinkage rate is large and dimensions are stable, but internal stress of product is large and it is easy to crack. Therefore, it is better to increase temperature rather than pressure to improve fluidity, and to reduce possibility of cracking by increasing mold temperature, improving mold structure and post-processing. When injection speed is low, defects such as ripples are prone to occur at the gate. Temperature of radiation nozzle must be controlled separately, mold temperature must be high, resistance of runner and gate must be small.
3. PET process characteristics
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-flow device is often added to nozzle. Mechanical strength and performance are not high after injection, properties must be improved through stretching process and modification. Accurate control of mold temperature is to prevent warpage. Deformation is an important factor, so it is recommended to use a hot runner mold. Mold temperature must be high, otherwise it will cause poor surface gloss and difficulty in demoulding.
To sum up, we can list molding process parameters in Table 3. Transparent plastic injection molding process parameters (Table 3) for your reference during injection.
Table 3 Transparent plastic injection molding process parameters:

Due to space constraints, only defects affecting product transparency will be discussed here. For other defects, please refer to company's product manual or other materials. Its shortcomings probably include following:
(1) Silver streaks: Due to influence of internal stress anisotropy during mold filling and condensation process, stress generated in vertical direction causes resin to flow upward, and refractive index is different from non-flow orientation, resulting in glittering filaments. After it expands, it may cause cracks in product. In addition to paying attention to injection molding process and mold (see Table 4), it is best for product to be annealed. For example, PC material can be heated to above 160℃ for 3-5 minutes, then allowed to cool naturally.
(2) Bubbles: Mainly due to water vapor and other gases in resin that cannot be discharged (during mold condensation process) or due to insufficient mold filling and condensation surface condenses too quickly to form "vacuum bubbles". Overcoming methods are shown in Table 4.
(3) Poor surface gloss: Mainly due to large roughness of mold. On the other hand, premature condensation prevents resin from copying state of mold surface. All of these will produce tiny unevenness on the surface, causing product to lose its luster. Overcoming methods are shown in Table 4.

 

(4) Seismic marks: refers to dense ripples formed from sprue as center. Reason is that melt viscosity is too large, front-end material has condensed in cavity, and later material breaks through this condensation surface, causing seismic marks to appear on the surface. Overcoming methods are shown in Table 4.
(5) Whitening and halo: mainly caused by dust in the air falling into raw materials or raw materials containing too much moisture. Overcoming methods are shown in Table 4.
(6) White smoke and black spots: Mainly caused by decomposition or deterioration of barrel resin due to local overheating of plastic in barrel. Overcoming methods are shown in Table 4.
In order to clearly illustrate the measures taken to overcome these deficiencies, Table 4 is listed for your reference.
Table 4: Defects of transparent products and methods to overcome them: