Because plastic is light in weight, has good toughness and is easy to form. Due to its low cost and other advantages, plastics are 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, since it requires good transparency, high wear resistance, and good impact toughness, a lot of work must be done on plastic ingredients, the entire injection molding process, equipment, molds, etc. to ensure that plastics used to replace glass (hereinafter referred to as transparent plastics) have good surface quality and meet requirements for use.
Currently, transparent plastics commonly used on the market include polymethylmethacrylate (commonly known as acrylic or plexiglass, code-named PMMA) and polycarbonate (code-named PC). polyethylene terephthalate (code name PET), transparent nylon, AS (acrylonitrile-styrene copolymer), polysulfone (code name PSF), etc. Among them, the three plastics we are most exposed to are PMMA, PC and PET. Due to limited space, these three plastics will be used as examples to discuss characteristics and injection molding process of transparent plastics.

 

Transparent plastics must first have high transparency. Secondly, it must have certain strength and wear resistance, be able to resist impact, have good heat resistance, have excellent chemical resistance, and have low water absorption rate. Only in this way can it meet requirements of transparency and remain unchanged for a long time during use. 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. Therefore, it is often used in packaging and containers.

Due to high light transmittance of transparent plastics, surface quality of plastic products must be strict, and there must be no markings, pores, or whitening. Defects such as halo, black spots, discoloration, poor gloss, etc., thus affecting raw materials and equipment during the entire injection molding process. Design of molds and even products requires great attention and strict or even special requirements.
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 barrel temperature, injection pressure, and injection speed, so that mold can be filled with plastic when plastic is injected, without internal stress causing product deformation and cracking.
Let’s talk about things that should be paid attention to in terms of equipment and mold requirements, injection molding process and raw material handling of products:
Dry
Preparation and drying of raw materials. Since any impurity 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 materials contain moisture, which will cause deterioration of raw materials after heating, so they must be dry, and during injection molding, a dry 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 raw materials.
Cleaning of barrel, screw and accessories
In order to prevent raw material contamination and presence of old materials or impurities in recesses of screw and accessories, especially 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 degradation, 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)
Issues that should be paid attention to in mold design (including product design)
In order to prevent poor backflow or uneven cooling from causing poor plastic molding, resulting in surface defects and deterioration;
Generally, when designing molds, you should pay attention to following points:
Wall thickness should be as uniform as possible, and draft angle should be large enough;
Transition part should be gradual and smooth to avoid sharp corners. Sharp edges are generated, especially PC products must not have gaps;
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;
Mold surface should be smooth and clean with low roughness (preferably less than 0.8);
Vent. Tank must be sufficient to discharge air and gas in melt in a timely manner;
Except for PET, wall thickness should not be too thin, generally not less than 1mm.
Issues that should be noted in injection molding process (including injection molding machine requirements)
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 special screw and a separate temperature-controlled injection nozzle should be used;
As long as plastic resin does not decompose, a higher injection temperature should be used;
Injection pressure: Generally high to overcome defect of high melt viscosity, but if pressure is too high, internal stress will be generated, causing difficulty in demolding and deformation;
Injection speed: When mold filling is satisfied, it should generally be low, and it is best to use slow-fast-slow multi-stage injection;
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;
Screw speed and back pressure: On the premise of meeting plasticizing quality, they should be as low as possible to prevent possibility of degradation;
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.
Other issues
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 70-80℃ for 4 hours; PC should be heated to 110-135℃ in clean air, glycerin, liquid paraffin, etc. Time depends on product, and it takes up to more than 10 hours. PET must go through a biaxial stretching process 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 below:
1. Process characteristics of PMMA
PMMA has high viscosity and poor fluidity, so it requires high material temperature and high injection pressure for injection molding. 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 brittle cracking. Therefore, mold temperature should be increased and condensation process should be improved to overcome these defects.
2. PC process characteristics
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 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, pressure 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 gate. Temperature of radiating 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.

 

Its shortcomings probably include following:
Silver streak
During mold filling and condensation process, internal stress anisotropy affects stress generated in vertical direction, causing resin to flow upward, and refractive index is different from non-flow orientation, resulting in sparkling silk patterns. When it expands, it may cause cracks in product. In addition to paying attention to injection molding process and mold, it is best to anneal product. For example, PC material can be heated to above 160℃ for 3-5 minutes, and then allowed to cool naturally.
Bubble
Because 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 vacuum bubbles.
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 cause slight unevenness on the surface, which makes product lose its luster.
Seismic marks
It refers to the dense ripples formed from sprue as center. Reason is that due to excessive melt viscosity, front-end material has condensed in mold cavity, and later material breaks through this condensation surface, causing vibration marks on the surface.
White fog
It is mainly caused by dust in air falling into raw materials or raw materials containing too much moisture.
White smoke black spots
It is mainly caused by decomposition or deterioration of barrel resin due to local overheating of plastic in barrel.