I. Background: The Critical Impact of Black Specks and Bubbles on VR Glasses
VR glasses are precision optical devices, and the injection molding quality of their lenses and structural components directly impacts the user experience.
Black Specks: Tiny impurities or carbides can reduce light transmittance, distort images, and even cause dizziness.
Bubbles: Bubbles larger than 0.05mm in diameter can disrupt optical uniformity, cause light scattering, and reduce immersion.
Industry data shows that the scrap rate of VR lenses due to black specks and bubbles is as high as 8%-15%, posing a significant cost control challenge for injection molders and manufacturers.
II. Causes and Solutions of the Black Speck Problem
1. Cause Analysis
Raw Material Contamination: Plastic pellets contain metal debris, dust, or insufficiently dried moisture (moisture content must be <0.02%).
Mold Residue: The transfer of an oxide layer on the mold wall or residue from previous production (such as carbonized plastic) to the new part.
Process Out of Control: Excessive screw temperatures (e.g., >280°C) can cause material decomposition and carbide formation.
2. Solution Strategy
Raw Material Screening: Use optical-grade resins (e.g., Zeonex, PMMA) with magnetic screening equipment to remove metallic impurities, and use a vacuum drying system to control the moisture content to below 0.01%.
Mold Optimization:
Use mirror-polished molds (surface roughness Ra ≤ 0.01μm) to reduce the risk of adhesion.
Use ultrasonic cleaning and nitrogen purging after each batch to prevent residue accumulation.
Process Parameter Control:
The screw temperature is controlled in three stages (e.g., for COP material: 200°C in the feed stage → 240°C in the compression stage → 230°C in the metering stage) to prevent local overheating.
Use a "slow-fast-slow" injection speed control to reduce frictional heating.
III. Root Causes and Treatment Techniques for Bubble Problems
1. Vacuum Bubbles (Shrinkage-Type Bubbles)
Cause: Uneven lens wall thickness (e.g., center thickness > 3mm), inadequate center filling due to external cooling and internal shrinkage.
Solution:
Gating System Design: Gate height should be 50%-60% of the wall thickness, and a fan-shaped gate should be used to disperse the melt flow.
Package Optimization: Increase the packing pressure to 80% of the injection pressure and extend the packing time to 1.2 times the gate solidification time to compensate for shrinkage.
2. Volatile Bubbles (Gas Release-Type Bubbles)
Cause: Inadequate drying of the raw materials (e.g., PMMA requires 6-8 hours of pre-drying) or high-temperature decomposition of the resin to generate gas.
Solution:
Nitrogen Shielded Injection Molding: Nitrogen is introduced into the barrel and mold to isolate oxygen and prevent oxidation and decomposition of the material.
In-Mold Vacuum Technology: Evacuate the mold to -0.08 MPa before closing to expel gas from the cavity.
IV. Full-Process Quality Control System
To ensure the yield rate of VR glasses injection molding, a "three-stage inspection" system is required:
Raw Material Incoming Inspection: Resin purity is analyzed using an infrared spectrometer, and impurity content must be less than 50 ppm.
Online Process Monitoring: In-mold pressure sensors monitor the filling status in real time and automatically adjust injection parameters.
Finished Product Optical Inspection:
Black Spot Detection: A 10-megapixel CCD camera is used, with an accuracy of 5μm.
Bubble Detection: A laser interferometer measures the lens surface shape, and bubbles exceeding the specified diameter are automatically sorted and removed.
ZIBO HUIWEN Plastics offers a full range of plastic molding capabilities to efficiently mass-produce parts specializing in injection molding and have served customers in variety of industries such as medical, automotive, packaging, construction, home appliances , and agriculture. The company has two factories which occupy a total area of more than 18,000 square meters, and a total of 102 sets of injection molding machines .
COmpetitive products:
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