Due to its advantages of being lightweight, tough, easy to form, and cost-effective, plastic is increasingly replacing glass in modern industrial and everyday products, especially in optical instruments and packaging industries, where it has seen rapid development. However, as plastic products need to meet high demands for transparency, wear resistance, and impact toughness, significant work must be done on the plastic’s composition, the entire injection molding process, equipment, and molds to ensure that these plastics (hereafter referred to as transparent plastics), used to replace glass, have good surface quality and meet usage requirements.
Currently, commonly used transparent plastics include polymethyl methacrylate (PMMA, also known as acrylic or organic glass), polycarbonate (PC), polyethylene terephthalate (PET), transparent nylon, acrylonitrile-styrene (AS), polysulfone (PSF), and others. Among these, PMMA, PC, and PET are the most frequently encountered. We will discuss the properties of transparent plastics and the injection molding process using these three materials as examples.
- Properties of Transparent Plastics
Transparent plastics must first have high transparency. Additionally, they need to have a certain level of strength, wear resistance, impact resistance, heat resistance, chemical resistance, and low water absorption. Only by meeting these requirements can the plastic maintain its transparency and functionality over time. PC is an ideal choice, but due to the high cost of raw materials and the more complex injection molding process, PMMA is still primarily used for products with general requirements. PET, on the other hand, needs to undergo stretching to achieve good mechanical properties, so it is mostly used in packaging and containers.
- Common Issues in the Injection Molding of Transparent Plastics
Since transparent plastics require high light transmittance, their surface quality must meet stringent requirements. The products must be free from defects such as streaks, air pockets, whitening, fogging, black spots, discoloration, or poor gloss. Therefore, special attention and strict requirements must be applied to the raw materials, equipment, molds, and even product design throughout the entire injection molding process.
Furthermore, because transparent plastics often have high melting points and poor flowability, it is necessary to make fine adjustments to process parameters such as barrel temperature, injection pressure, and injection speed to ensure that the mold is filled without causing internal stress, which could lead to deformation or cracking of the product.
Key Considerations Regarding Equipment, Molds, and Process Parameters
Drying of Materials
The preparation and drying of materials is critical, as any impurity in the plastic can affect its transparency. During storage, transportation, and feeding, it is essential to ensure that materials are sealed and kept clean, especially if there is moisture in the raw materials. Heating moisture-laden materials will lead to material degradation, so they must be thoroughly dried. During injection molding, a drying hopper should be used for feeding. Also, it is recommended that the air used for drying be filtered and dehumidified to avoid contaminating the material.
Cleaning of the Barrel, Screw, and Attachments
To prevent material contamination and to ensure that no old materials or impurities remain in the screw and attachments, particularly in areas with poor heat stability, it is necessary to clean these parts with a screw cleaning agent before use or after stopping the machine. If a cleaning agent is not available, other resins like PE or PS can be used to clean the screw.
During temporary shutdowns, to prevent material degradation due to prolonged exposure to high temperatures, the temperature of the drying machine and barrel should be lowered (for materials like PC and PMMA, the barrel temperature should be reduced to below 160°C, and for PC, the hopper temperature should be lowered to below 100°C).
Considerations in Mold Design (Including Product Design)
To prevent issues such as poor flow or uneven cooling, which can lead to molding defects and surface flaws, the following should be considered in mold design:
- Wall thickness should be as uniform as possible, and the draft angle should be sufficiently large.
- Transitions should be smooth to avoid sharp corners or edges, especially for PC products, which should not have notches.
- The gate and runner should be wide, short, and as free from resistance as possible. Gate locations should be set based on the shrinkage and solidification process, and cold wells should be added if necessary.
- Mold surfaces should be smooth, with low roughness (ideally under 0.8).
- Sufficient venting must be provided to expel air and gases from the melt.
- Except for PET, the wall thickness should not be too thin, generally not less than 1 mm.
Considerations in Injection Molding Process (Including Injection Machine Requirements)
To reduce internal stress and improve surface quality, the following issues should be considered in the injection molding process:
- Use specialized screws and injection machines with individual temperature control for the nozzle.
- Injection temperature should be set as high as possible, provided that the resin does not decompose.
- Injection pressure should generally be higher to overcome the high viscosity of the melt, but excessive pressure can cause internal stress and lead to difficulty in mold release and deformation.
- Injection speed should generally be low to ensure mold filling, with a slow-fast-slow multi-stage injection recommended.
- Holding pressure time and molding cycle should be as short as possible, provided that the product is fully molded and no sink marks or bubbles are formed.
- Screw rotation speed and backpressure should be as low as possible while ensuring adequate plasticizing quality, to avoid material degradation.
- Mold temperature has a significant impact on product quality. Therefore, mold temperature must be precisely controlled, and if possible, higher mold temperatures are preferred.
Other Considerations
To prevent degradation of surface quality, it is recommended to use minimal mold release agents. When using recycled materials, the amount should not exceed 20%.
For products other than PET, post-processing should be done to eliminate internal stress. PMMA should be heat-cycled in hot air at 70-80°C for 4 hours. PC should be heated at 110-135°C in clean air, glycerin, or liquid paraffin, with the time adjusted based on the product (up to 10+ hours if necessary). PET must undergo a biaxial stretching process to achieve good mechanical properties.
- Injection Molding Process for Transparent Plastics
Process Characteristics of Transparent Plastics
In addition to the common issues mentioned, transparent plastics also have specific process characteristics:
- PMMA Process Characteristics
PMMA has high viscosity and poor flowability, so it requires high material temperature and high injection pressure. The injection temperature has a greater effect than injection pressure, but increasing the pressure helps improve product shrinkage. PMMA has a wide injection temperature range (160°C for melting temperature and 270°C for decomposition temperature), which provides better processability. To improve flowability, injection temperature adjustments should be made. PMMA has poor impact resistance, wear resistance, and is prone to scratching and brittle fractures, so mold temperature should be increased to improve the cooling process and overcome these issues.
- PC Process Characteristics
PC has high viscosity and melt temperature, with poor flowability, so it requires a higher injection temperature (270-320°C). The temperature range for adjusting the material is narrower, and the processability is not as good as PMMA. Injection pressure has less impact on flowability, but due to its high viscosity, higher injection pressure is still required. To prevent internal stress, holding pressure time should be kept as short as possible. PC has high shrinkage, good dimensional stability, but significant internal stress, which makes it prone to cracking. Therefore, improving flowability through increased temperature, improving mold temperature, mold structure, and post-processing are essential to reduce the likelihood of cracking. When injection speed is low, defects such as ripples can form at the gate. Nozzle temperature should be independently controlled, and mold temperature should be high to reduce flow resistance.
- PET Process Characteristics
PET has a high molding temperature and a narrow material temperature adjustment range (260-300°C). However, once melted, it has good flowability, making it difficult to process, and a flow control device may need to be added to the nozzle. The mechanical strength and performance of PET after injection molding are not ideal and must be improved through stretching and modification processes. Accurate mold temperature control is critical to preventing warping and deformation, so hot runner molds are recommended. Mold temperature must be high to avoid poor surface gloss and difficulty in mold release.
- Defects in Transparent Plastic Parts and Solutions
Common injection molding defects include:
- Silver streaks: Caused by anisotropic internal stress during filling and cooling, resulting in flow orientation differences and refractive index discrepancies. These can lead to cracking. To prevent this, the product should undergo annealing treatment.
- Bubbles: Result from moisture or gases trapped inside the resin or insufficient filling.
- Poor surface gloss: Due to mold roughness or premature solidification during cooling, causing the resin to fail to replicate the mold surface.
- Black spots: Due to overheating or decomposition of the resin in the barrel.
