Due to advantages of plastics such as light weight, good toughness, easy molding, and low cost, they are increasingly replacing glass in modern industry and daily products, especially in optical instruments and packaging, where their development has been particularly rapid. However, because they require good transparency, high wear resistance, and good impact toughness, a lot of work must be done on composition of plastic, the entire injection molding process, equipment, molds, etc., to ensure that these plastics used to replace glass (hereinafter referred to as transparent plastics) have good surface quality and meet requirements for use.

 

Currently, commonly used transparent plastics on the market include 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 PSF), etc., among which PMMA is the most commonly encountered. Due to space limitations, this discussion will focus on PC and PET as examples to illustrate characteristics and injection molding process of transparent plastics.

Transparent plastics must first possess high transparency. Secondly, they must have sufficient strength and abrasion resistance, impact resistance, good heat resistance, excellent chemical resistance, and low water absorption. Only in this way can they maintain their transparency requirements over a long period of use. PC is a relatively ideal choice, but due to its expensive raw materials and complex injection molding process, PMMA is still primary choice (for products with general requirements). PPT, because it requires stretching to achieve good mechanical properties, is mostly used in packaging and containers.

Because transparent plastics require high light transmittance, surface quality of plastic products must be strictly controlled. There must be no defects such as blemishes, pores, whitening, haze, black spots, discoloration, or poor gloss. Therefore, the entire injection molding process requires careful attention and strict, even special, requirements regarding raw materials, equipment, molds, and even product design.
Secondly, because transparent plastics generally have high melting points and poor flowability, fine adjustments are often needed to process parameters such as barrel temperature, injection pressure, and injection speed to ensure product surface quality. This ensures plastic fills mold completely without generating internal stress that could cause product deformation and cracking.
Regarding equipment and mold requirements, injection molding processes, and raw material handling, following points should be noted:
Drying
Raw material preparation and drying: Even a small amount of impurities in plastic can affect product's transparency; therefore, proper drying is crucial during storage and transportation.
During feeding process, it is essential to maintain a tight seal to ensure raw material is clean. In particular, if raw material contains moisture, heating can cause deterioration, so it must be dried. A drying hopper must be used for feeding during injection molding. It is also important that air introduced during drying process is filtered and dehumidified to prevent contamination of raw material.
Cleaning of Barrel, Screw, and Accessories
To prevent raw material contamination and accumulation of old material or impurities in recesses of screw and accessories, especially resins with poor thermal stability, all parts should be thoroughly cleaned with a screw cleaner before use and after shutdown to ensure no impurities remain. If screw cleaner is unavailable, PE or PS resins can be used to clean screw.
When temporarily shutting down, to prevent prolonged exposure of raw materials to high temperatures and subsequent degradation, dryer and barrel temperatures should be lowered. For example, barrel temperature for PC and PMMA should be lowered to below 160℃. (Hopper temperature for PC should be lowered to below 100℃.)
Issues to Consider in Mold Design (including Product Design)
To prevent poor backflow or uneven cooling, which can lead to poor plastic molding, surface defects, and deterioration;
Generally, following points should be considered during mold design:
Wall thickness should be as uniform as possible, and draft angle should be sufficiently large;
Transitions should be gradual and smooth, avoiding sharp corners. Sharp edges, especially for PC products, must be avoided;
Gate. Runner should be as wide and short as possible, its location should be designed according to shrinkage and solidification process. A cold slug well should be added if necessary;
Mold surface should be smooth with low roughness (preferably below 0.8);
Ventilation. Vents must be sufficient to allow for timely removal of air and gases from molten material;
Except for PET, wall thickness should not be too thin, generally not less than 1 mm.
Issues to Consider in Injection Molding Process (Including Injection Molding Machine Requirements)
To reduce internal stress and surface quality defects, following aspects should be considered in injection molding process:
A dedicated screw and an injection molding machine with a separate temperature-controlled nozzle should be selected.
Injection temperature: A higher injection temperature is preferable, provided plastic resin does not decompose.
Injection pressure: Generally higher to overcome high viscosity of melt, but excessive pressure can generate internal stress, causing difficulty in demolding and deformation.
Injection speed: Generally lower, while ensuring mold filling, preferably using a slow-fast-slow multi-stage injection method.
Holding pressure time and molding cycle: As long as product fills mold and does not produce sink marks or bubbles, time should be as short as possible to minimize residence time of molten material in barrel.
Screw speed and back pressure: As long as plasticizing quality is met, speed should be as low as possible to prevent potential degradation.
Mold temperature: Quality of product cooling has a significant impact on quality, so mold temperature must be precisely controlled. If possible, a higher mold temperature is better.
Other Issues
To prevent surface quality deterioration, mold release agents should be used sparingly during injection molding; recycled materials should not exceed 20%.
For products other than PET, post-treatment is necessary to eliminate internal stress. PMMA should be dried in hot air circulation at 70-80℃ for 4 hours; PC should be heated in clean air, glycerin, liquid paraffin, etc., at 110-135℃ for varying times depending on product, sometimes requiring more than 10 hours. PET must undergo biaxial stretching to achieve good mechanical properties.

 

Process Characteristics of Transparent Plastics
In addition to common issues mentioned above, transparent plastics also have their own unique processing characteristics, which are described below:
1. Process Characteristics of PMMA
PMMA has high viscosity and slightly poor flowability, therefore, high material temperature and high injection pressure are required for injection molding. Injection temperature has a greater impact than injection pressure, but increasing injection pressure helps improve shrinkage rate of product.
Injection temperature range is wide, with a melt temperature of 160℃ and a decomposition temperature reaching 270℃. Therefore, material temperature adjustment range is wide, resulting in good processability. Improving fluidity can be achieved by adjusting injection temperature. However, it has poor impact resistance, poor wear resistance, is prone to scratching, and is easily cracked. Therefore, increasing mold temperature and improving cooling process are necessary to overcome these defects.
2. Processing Characteristics of PC
PC has high viscosity and a high melt temperature, resulting in poor fluidity. Therefore, it must be injection molded at a relatively high temperature (between 270-320℃). Relatively speaking, material temperature adjustment range is narrower, and its processability is not as good as PMMA. Injection pressure has a relatively small impact on fluidity, but due to its high viscosity, a relatively high injection pressure is still required. Correspondingly, to prevent internal stress, holding time should be as short as possible.
It has a high shrinkage rate and dimensional stability, but product has high internal stress and is prone to cracking. Therefore, it is advisable to improve fluidity by increasing temperature rather than pressure, 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 gate. Nozzle temperature must be controlled separately, mold temperature must be high, runner and gate resistance must be low.
3. Processing Characteristics of PET
PET has a high molding temperature and a narrow material temperature adjustment range (260-300℃). However, after melting, it has good fluidity, resulting in poor processability. Anti-spreading devices are often added to nozzle. Mechanical strength and performance are not high after injection; stretching and modification are necessary to improve performance.
Accurate mold temperature control is crucial to preventing warping and deformation. Therefore, hot runner molds are recommended. High mold temperature is essential; otherwise, poor surface gloss and difficulty in demolding will occur.

Defects generally include following:
Silver streaks: During filling and cooling process, anisotropy of internal stress causes vertical stress, leading to resin flow orientation. Non-flow orientation produces different refractive indices, resulting in shimmering streaks. When these streaks expand, they may cause cracks in product. Besides paying attention to injection molding process and mold, it's best to anneal product. For example, PC material can be heated to above 160℃ and held for 3-5 minutes, then allowed to cool naturally.
Bubbles: These are mainly caused by moisture and other gases within resin not escaping (during mold solidification) or by insufficient filling, leading to excessively rapid solidification on solidified surface, forming vacuum bubbles.
Poor Surface Gloss: This is mainly due to high mold roughness and premature solidification, preventing resin from replicating mold surface condition. All of these factors create micro-unevenness on the surface, resulting in a loss of gloss.
Vast Ripples: These refer to dense ripples forming around sprue. Cause is excessive melt viscosity; material at the front has already solidified in cavity, and subsequent material breaks through this solidified surface, resulting in ripples on the surface.
White Haze: This is mainly caused by dust falling into raw material from air or excessive moisture content in raw material.
White smoke and black spots: This is mainly caused bythe decomposition or deterioration of resin in barrel due to localized overheating of plastic inside barrel.