◆Characteristics of typical transparent plastics
Transparent plastics and their injection molding properties
With continuous advancements in materials research and development, an increasing number of optical materials are suitable for manufacturing transparent products. Taking optical lenses, widely used in optical products with high quality requirements and complex injection molding technology, as an example, as shown in the figure, the injection molding material for ordinary plastic lenses is often chosen for its low cost, high production efficiency, strong stability, and high transmittance. Compared to the plastic raw materials for ordinary plastic lenses, the requirements for injection molding materials for imaging optics plastic lenses are even more stringent. Not only must the plastic raw material possess characteristics such as high transparency, good mechanical properties, strong stability, and good flowability, but the material selection must also be based on specific performance characteristics such as refractive index.
Commonly used injection molding materials for imaging optical plastic lenses include polymethyl methacrylate (PMMA), polycarbonate (PC), and cyclic olefin polymers (COP). Among these, high-transparency PMMA is widely used in the injection molding of plastic lenses. PMMA is used not only in the manufacture of medium-to-low precision large-diameter plastic lenses, such as condenser lenses, Fresnel lenses, projection lenses, and automotive headlight lenses, but also in high-precision plastic lenses, such as laser printer lenses, camera lenses, mobile phone lenses, and CD/DVD reader lenses. PMMA possesses high strength and hardness, as well as optical properties such as high Abbe number and low birefringence. However, PMMA still presents several challenges as a plastic raw material for optical lenses. For example, under humid conditions, PMMA's water absorption causes changes in the refractive index of the plastic lens, resulting in volume expansion and alterations to the overall structural dimensions. Furthermore, PMMA's glass transition temperature is 100℃, and the heat resistance of plastic lenses is relatively poor, which significantly limits the application range of PMMA.
Polycarbonate (PC) is a commonly used optical material, widely used in automotive headlight lenses, beam-diffusing lenses, and other optical lenses. PC possesses high transparency, heat resistance, high strength, and a high refractive index, and is considered one of the plastic materials with the best impact strength. However, PC lenses still have significant drawbacks. For example, the photoelastic coefficient of PC lenses is several times higher than that of PMMA, resulting in a pronounced birefringence effect that severely impacts the optical imaging quality of the plastic lens. Furthermore, PC material has a large shrinkage rate, making plastic lenses highly susceptible to deformation, with a greater degree of deformation than PMMA.
Cyclic olefin polymers (COPs) are ring-finger polymers synthesized using cyclic olefins as monomers. COPs possess high transparency, low hygroscopicity, low birefringence, and good processability. Furthermore, COPs exhibit excellent environmental stability, remaining structurally unchanged even when absorbing moisture, and are frequently used as a substitute for PMMA in plastics. However, COPs are extremely expensive, costing approximately ten times more than PMMA. Using COPs as an injection molding material significantly increases the manufacturing cost of plastic lenses.
In general, lenses made of PC exhibit significant birefringence, making it difficult to guarantee image quality. COP and PMMA offer unparalleled advantages in terms of mechanical properties and structural stability. However, COP is extremely expensive, increasing manufacturing costs and violating economic principles. PMMA, on the other hand, is relatively inexpensive and has a large market supply, making it suitable for manufacturing low-cost, high-efficiency imaging optical plastic lenses. Furthermore, PMMA's melt flow properties can be enhanced by altering injection molding process parameters, allowing for precise replication of the injection mold's surface morphology.
Injection molding technology for transparent plastics
Because transparent plastics require high light transmittance, the surface quality of the plastic products must be strictly controlled. Defects such as blemishes, pores, whitening, haze, black spots, discoloration, and poor gloss are unacceptable. Therefore, the entire injection molding process requires meticulous attention and stringent, even specialized, requirements for raw materials, equipment, molds, and even product design.
Secondly, due to the high melting point and poor flowability of transparent plastics, fine adjustments to process parameters such as machine temperature, injection pressure, and injection speed are often necessary to ensure surface quality. This ensures the mold is filled completely without generating internal stress that could lead to product deformation and cracking. The following section analyzes the key considerations regarding raw material preparation, equipment and mold requirements, injection molding processes, and raw material handling.
Key technical points before injection molding
Raw Material Preparation and Drying
Since even the slightest impurity in plastic can affect the product's transparency, it is crucial to maintain a tight seal during storage, transportation, and feeding to ensure the raw materials are completely clean. In particular, the presence of moisture in the raw materials can cause deterioration upon heating, so thorough drying is essential. During injection molding, a drying hopper must be used for feeding. It is also important to note that the air introduced during the drying process should ideally be filtered and dehumidified to prevent contamination of the raw materials.
Cleaning of barrel, screw and accessories
To prevent raw material contamination and the accumulation of old material or impurities in the screw and its accessories, special attention should be paid to preventing the retention of old material with poor thermal stability. Before and after use, the injection molding machine should be thoroughly cleaned with a screw cleaner to ensure all injection components are free of impurities. If a screw cleaner is unavailable, PE or PS plastics can be used to clean the screw. During temporary shutdowns, to prevent degradation caused by prolonged exposure of the raw material to high temperatures, the dryer and barrel temperatures should be lowered. If producing PC, PMMA, or other plastics, the barrel temperature should be reduced to below 160°C. Furthermore, when using PC as the raw material, the hopper drying temperature should be reduced to below 100°C.
Issues to consider in mold design (including product design)
To prevent poor plastic molding, surface defects, and deterioration caused by poor reflow or uneven cooling, the following points should generally be considered during mold design.
01
The wall thickness should be as uniform as possible, and the draft angle should be sufficiently large.
02
The transition section should be gradually and smoothly rounded to prevent sharp corners and edges, especially for PC products, which must be free of notches.
03
The gate and runner should be as wide and short as possible, and the gate position should be set according to the shrinkage and solidification process. A cold slug well should be added if necessary.
04
The mold surface should be smooth with low roughness (preferably below 0.8µm).
05
There must be sufficient vents and channels to promptly remove air and gases from the molten material.
06
Except for PET, the wall thickness should not be too thin, generally not less than 1mm.
Issues to Consider in Injection Molding Processes (Including Injection Molding Machine Requirements)
To reduce internal stress and surface quality defects, the following aspects should be considered in the injection molding process:
Use an injection molding machine with a dedicated screw and a separate temperature-controlled nozzle.
01
Injection Temperature: A higher injection temperature is preferable, provided the plastic resin does not decompose.
02
Injection Pressure: Generally higher to overcome the high viscosity of the melt, but excessive pressure can generate internal stress, causing demolding difficulties and deformation.
03
Injection Speed: Generally lower, while ensuring mold filling, ideally using a multi-stage injection method of slow-fast-slow.
04
Holding Time and Molding Cycle: Shorter, while ensuring product filling and preventing sink marks and bubbles, to minimize the residence time of the molten material in the barrel.
05
Screw Speed and Back Pressure: As low as possible, while ensuring plasticizing quality, to reduce the possibility of delamination.
06
Mold temperature: The quality of product cooling has a great impact, so the mold temperature must be precisely controlled. If possible, the mold temperature should be higher.
07
Other issues
To prevent surface quality deterioration, mold release agents should be used sparingly during injection molding; the amount of recycled material should not exceed 20%. Except for PET, all products should undergo post-treatment to relieve internal stress. PMMA should be dried in hot air circulation at 70–80°C for 4 hours; PC should be heated in clean air, glycerin, liquid paraffin, etc., at 110–135°C for a time depending on the product, sometimes requiring more than 10 hours. PET must undergo a biaxial stretching process to achieve good mechanical properties.