In today's product design, transparent plastic parts are increasingly used, from automotive headlights to medical devices, from electronic device panels to daily packaging containers. However, production of transparent injection molded parts presents a complex technical challenge, involving multiple steps, including material selection, product design, mold manufacturing, and process control. This article will systematically introduce key technical points for transparent injection molded parts made of PC, PS, PMMA, and other materials.

Transparent plastics come in a wide variety of types, each with distinct properties. Understanding these properties is fundamental to successful production. PMMA (polymethyl methacrylate, commonly known as organic glass) has high light transmittance and is superior to PC, but it has poor fluidity and is difficult to mold.
PC (polycarbonate) is known for its excellent toughness and impact resistance, but it is more expensive and notch-sensitive.
PS (polystyrene) offers lower costs and good moldability, but relatively weaker mechanical strength.
When selecting a material, consider intended use of product: PMMA is preferred for applications requiring high optical performance (such as lenses); PC is more suitable for applications requiring high impact resistance (such as face shields); and PS is an economical choice for cost-sensitive, everyday products that require transparency.

Design of transparent plastic parts directly impacts quality and production efficiency of final product.
Wall Thickness Uniformity: Wall thickness uniformity is primary principle of transparent part design. Uneven wall thickness leads to uneven cooling shrinkage, generating significant internal stress, which not only affects transparency but can also cause product deformation and cracking. Generally, wall thickness of transparent parts should be no less than 1mm. Small products are typically controlled at around 1.2mm-1.8mm, and medium-sized products at around 2.2mm-2.8mm.
Draft Angle and Transition Processing: Draft angle is crucial for transparent parts. Due to high molding pressure and strong clamping force of PC materials, a draft angle of no less than 2 degrees is recommended. PMMA, on the other hand, is more brittle, so a draft angle of no less than 3 degrees is recommended, especially for products with high or deep cavities. All transitions should be smooth, avoiding sharp corners and edges. In particular, PC products must be free of notches.

Mold is the key to transparent injection molding. Proper mold design can effectively prevent various defects.
Gating System Design: Gate design has a significant impact on quality of transparent parts. Gate should be located in the thickest part of part wall to ensure smooth and complete mold filling. Plastic filling process should be minimized to reduce pressure loss and facilitate mold venting.
For PMMA, runner should be curved into an S-shape, with radiused corners to ensure smooth flow. A long cold well should be designed at the end of runner. For PC, on the other hand, runner should be short and thick. This is difference between two transparent material mold designs. For runner and inlet design, runner should be trapezoidal and inlet fan-shaped. Runner should be polished to a very high finish.

 

Choice of gate type is also important. Transparent plastic products often use a protective gate to avoid defects such as serpentine marks. A point gate is a common gate type used in transparent injection molds. It allows plastic melt to be injected into cavity at a higher velocity, facilitating melt flow and filling, and reducing weld marks.
Common side gate design parameters:
a. Gate width W is 1.5-5.0 mm, generally W = 2H. This can be increased for large and transparent plastic parts.
b. Depth H is 0.4-1.5 mm. Specifically, for common ABS and HIPS, H = (0.4-0.6)δ, where δ is basic wall thickness of part. For PC and PMMA, which have poorer flow properties, H = (0.6-0.8)δ.

 

Venting System Design: Adequate venting is essential. A general rule of thumb is to ensure venting wherever there are corners and transitions. Venting slots should be located at final filling point of part to promptly expel air and gases from melt, preventing defects caused by trapped gases.

 

Cooling System Design: Uniform cooling is crucial for molding of transparent parts. Uneven cooling can lead to residual stress within part, affecting transparency and dimensional stability. Mold's cooling system should be uniformly designed, with cooling channels close to cavity surface and evenly distributed. For complex transparent parts, multiple cooling circuits may be required, achieving uniform cooling by adjusting cooling water flow rate and temperature.

Mold's surface treatment has a decisive impact on quality of transparent parts.
Steel Selection: For molds with production volumes under 100,000 units, NAK80 from Japan can be used. For larger production volumes, S136 or French SMV3W high-mirror heat-treated steel, heat-treated to HRC 48-52, is essential. High-transparency molds should use imported S136, quenched to HRC 45-52.
Surface Polishing and Precision: Surface polishing is a core process for molds made with transparent parts. Both fixed and movable mold cavities must be polished to a mirror finish, followed by a nitriding treatment. Nitriding layer depth is 0.03-0.05mm, and surface hardness is HRC 55-60. Polishing process requires manual, graded polishing using optical abrasives: #5 abrasive → #2.5 abrasive → #1 abrasive, until surface is mirror-finished. Molds require extremely high surface accuracy. Surface roughness of cavity and core should reach Ra0.01-0.05μm to ensure a smooth, flawless finished product. Parting surfaces and insert mating surfaces must also be machined with high precision to avoid defects such as flash and burrs.

 

Injection molding process for transparent plastics requires precise control, all parameters must be coordinated and optimized.
Temperature Control: Barrel temperature should be raised as high as possible within resin's thermal stability range to reduce melt viscosity and minimize shear stress during mold filling. For example, when molding PC, increasing barrel temperature from 280℃ to 300℃ improves melt uniformity and can increase product's light transmittance by 2%-3%.
Mold temperature significantly affects quality of transparent parts. Thick-walled parts (wall thickness >5mm) require gradient temperature control: raising cavity surface temperature above resin's glass transition temperature delays surface curing and reduces internal stress. For PC materials, movable and fixed molds should be heated with oil at a temperature of 70-120℃; for PMMA materials, fixed mold should be heated with oil at a temperature of 50-80℃, and movable mold should be heated with room-temperature water.
Pressure and Speed Control: Injection pressure is generally higher to overcome disadvantage of high melt viscosity. However, excessive pressure can generate internal stress, leading to product deformation and cracking. Injection speed should be adjusted appropriately based on product structure and size, as well as mold's gate pattern and number, to ensure uniform melt filling and avoid defects such as underfill and bubbles.
Raw Material Handling: Transparent plastics are extremely sensitive to impurities and must be thoroughly dried. PC drying conditions are 100-200℃ for 3-4 hours; PMMA drying conditions are 90℃ for 2-4 hours. During drying process, input air should be filtered and dehumidified to prevent contamination of raw materials.
Cleanliness of barrel, screw, and accessories is also crucial. To prevent raw material contamination, clean components with a screw cleaner before and after use. During temporary downtime, to prevent degradation of raw materials caused by prolonged exposure to high temperatures, dryer and barrel temperatures should be lowered.

A variety of defects can occur during transparent injection molding, requiring targeted solutions.
Weld Lines: Weld lines are common defects in transparent parts. Causes include low melt temperature, low injection pressure, and low mold temperature. Solutions include increasing molding temperature, injection pressure, and mold temperature, improving runner configuration, and reselecting gate location.
Bubbles and Vacuum Voids: Bubbles and vacuum voids are caused by moisture and other gases trapped within resin, or by rapid condensation on condensation surface due to insufficient mold filling. Solutions include pre-drying raw materials, appropriately reducing molding temperature, increasing injection pressure, extending hold time, and increasing mold temperature.
Moire and Haze: Moire and haze are often caused by low melt temperature, low mold temperature, and low injection pressure. Molding temperature, mold temperature, and injection pressure should be appropriately increased to thoroughly remove impurities from raw materials or replace with new materials.
Poor Surface Gloss: Poor surface gloss may be caused by factors such as low melt temperature, low injection pressure, low mold temperature, and poor mold cavity surface finish. Appropriate measures include increasing molding temperature, injection pressure, and mold temperature, as well as grinding and polishing mold surface to improve surface finish.
Black Spots and Streaks: Black spots and streaks are primarily caused by impurities in raw material, melt decomposition caused by excessively high or low barrel temperatures, and poor mold venting. Raw material impurities should be thoroughly removed, barrel temperature should be adjusted appropriately, and mold venting should be improved.

With technological advancements, transparent injection molding technology is also evolving. Micro-nanostructured surface molding technology uses laser engraving of micron-scale prisms and lens arrays on mold surface to impart optical features to surface of part while maintaining high light transmittance.
Supercritical fluid-assisted injection molding technology injects carbon dioxide or nitrogen into melt to create a uniform microporous structure, reducing part weight while regulating transparency through "light diffuse reflection" effect of micropores. Online inspection technology, through installation of a laser transmittance tester, monitors product's transmittance in real time, automatically adjusting process parameters and achieving closed-loop control, significantly improving quality stability.

Production of transparent injection molded parts is a systematic project, requiring meticulous control throughout the entire process, from material selection, product design, mold manufacturing, process control, to post-processing. With continuous emergence of new materials, processes, and technologies, quality of transparent injection molded products will continue to improve, and their application areas will further expand. By mastering key technical points mentioned above and flexibly applying them in accordance with actual production conditions, you can produce high-quality, highly transparent plastic products.