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How to achieve better surface gloss in optical component injection molding?

2025/08/29 By le zhan

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In optical component injection molding, gloss is more than just a cosmetic concern. It is directly related to surface finish, light transmittance, stray reflections, and the perceived quality of automotive, medical, and consumer optical parts. Therefore, gloss must be considered as a holistic process in production, with material, melt state, mold surface, machine precision, and process control all optimized simultaneously. Key considerations include resin selection and drying, melt uniformity, mould surface finish, and temperature control, as well as injection and holding pressure strategies, machine accuracy, and a contaminant-free production environment. Each of these variables influences micromorphology and residual stresses, ultimately determining gloss and optical performance.

Material Selection, Conditioning, and Melt Preparation in Optical Component Injection Molding

In optical component injection molding, the right raw materials are fundamental. Selecting resins with inherent optical clarity and low haze requires optimization, including PMMA for the highest gloss, polycarbonate for superior toughness, and COP/cyclic olefin polymers for excellent optical properties. Avoid resins containing flame retardants, high recycled content, or incompatible additives, as these additives can negatively impact the surface finish.

Optical polymers are typically hygroscopic. Moisture in the melt can create bubbles and microvoids, which reduce gloss and increase haze. Therefore, we require an injection molding dryer with a proven inlet dew point (typically ≤ -40°C for polycarbonate). Melt uniformity must also be ensured. Uniform melt temperature and low melt pressure pulsation minimize flow marks and shear-induced surface defects. Specialized optical screws are required in production, along with optimized compression and mixing zones to reduce unmelted particles and color streaks. Fine filtration upstream of the hot runner manifold or gate nozzle can also remove foreign matter.

PMMA

Mold, Polishing, and Coating Preparation

Achieving high surface gloss in optical component injection molding requires meticulous mold design and process-friendly surface treatment. We recommend starting with high-grade tool steel. Optical-grade finishes typically require multiple polishing cycles using progressively finer abrasives to achieve single-digit nanometer Ra values ​​and a mirror-like appearance. For ultra-fine replication, electroformed inserts or nickel plating on polished mandrels provide superior replication fidelity and extended tool life. Hard coatings, such as DLC or ceramic-like PVD, enhance wear resistance and preserve the initial polished finish in high volumes.
Regarding texture and gloss, designers should avoid microtexturing unless they explicitly require it for anti-glare purposes. Even microetching can reduce gloss and increase light scattering. If you genuinely require patterned effects, use precision laser etching technology, along with process control and prototype validation.

Maintaining mold temperature uniformity is also crucial, as gloss is very sensitive to surface temperature during filling and packing. For mold release, choose a release agent compatible with the optical polymer and, whenever possible, use air or electric release to minimize adhesive contact time.

Advantages of All Electric Injection Molding Machines in Optical Component Injection Molding

Machine performance fundamentally determines your ability to control the gloss-critical stages of the molding cycle effectively. In optical component injection molding, all electric injection molding machines provide the repeatability and fine control required to mass-produce glossy surfaces.

They enable precise metering, ensuring repeatable shot-to-shot delivery. The all electric injection molding machine utilizes an electrically driven screw. This configuration achieves tight shot weight tolerances, minimal screw return variation, and highly stable melt conditions. Furthermore, Topstar’s ally electric injection molding machines feature high holding pressure control and multi-stage holding pressure, enabling more precise and programmable holding pressure profiles, as well as rapid transitions from injection to holding pressure.

High-precision mold plate control and rapid injection-to-compression transitions enable controlled compression, slowly heating the cavity to final thickness while the melt contacts and replicates the mold surface. Compared to hydraulic systems, electric drive produces smoother velocity and acceleration profiles, reducing vibrations that can cause surface defects or micro-marks. An optical-specific screw and precision nozzle combine to achieve superior surface quality.

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Optimizing the Overall Injection Molding Process

Optimizing the molding cycle is key to translating theory into production reality. For optical component injection molding, injection speed, velocity-pressure (V/P) switching, compression, and holding pressure strategies are crucial for achieving a mirror-like finish. In the injection molding of optical components, rapid filling is necessary to minimise the window for surface cooling, which typically enhances replication rates. Switching from velocity-controlled filling to pressure-controlled holding at precise points can effectively prevent weld lines and internal voids.

As previously mentioned, injection-compression technology molds the part in a slightly open cavity and then compresses it to the final thickness. This minimizes the flow length across the surface and reduces shear marks. High holding pressure is then applied to ensure the melt remains close to the cavity surface as it begins to solidify, requiring a controlled, decreasing pressure profile.

Part, Post-Molding Handling, and Quality Control

Even with optimal injection molding machine and mold settings, gloss can be compromised by downstream handling, contamination, or inadequate inspection. Gentle ejection and carefully controlled platen time during part handling can prevent scouring or drag marks. For delicate surfaces, consider using vacuum or soft-grip clamps. Keep ejection speeds as low as possible, but ensure that you avoid sticking.

In addition, optical surfaces are susceptible to particle contamination. Cleanroom or clean zone procedures should be implemented for mold polishing, molding, and post-molding processing. Use lint-free gloves, filtered air, and enclosed conveyors. Strict protective clothing and standard operating procedures (SOPs) should be implemented to control airborne particles, oil, and fingerprints. If necessary, post-molding coatings can be applied to improve scratch resistance, anti-reflective properties, or fine-tune gloss.

Ensuring Molding and Yield of Precision Optical Components

Achieving superior surface gloss in optical component injection molding requires the use of appropriate materials and drying processes, precise mold surface engineering, temperature control, all-electric injection molding machines, and rigorous process optimization and part handling. Topstar’s all electric injection molding machines deliver precise metering, high holding pressure control, injection compression, and optical screws, resulting in high surface gloss, dimensional stability, and repeatable production.

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