Elimination Method of Glare and Shadow on High-transparency Injection Molded Parts
High-transparency injection molded parts such as PC, PMMA, TPU and transparent ABS often have surface glare, local shadow, haze and uneven light transmission defects, which directly affect appearance transparency, optical imaging effect and product yield. Such problems are not caused by a single raw material or process, but concentrated on raw material cleanliness, injection molding temperature, exhaust state, mold surface condition, cooling uniformity and runner gate design. Only by adjusting item by item from the full dimensions of raw materials, molds, processes and on-site operation can glare and shadow be completely eliminated to achieve high-transparency and defect-free mass production.
I. Raw Material Control and Dehumidification Drying Treatment
High-transparency plastic materials are highly hygroscopic, prone to internal silver lines and surface haze after absorbing moisture. The drying temperature of PMMA is controlled at 75–80 °C with constant temperature drying for 3–4 hours; the drying temperature of PC is 100–110 °C with drying time no less than 4 hours. The moisture content must be controlled within 0.02% to prevent water precipitation from forming shadow.
Mixing of runner materials, colored recycled materials and different brand materials is strictly prohibited for high-transparency parts. The proportion of recycled materials is set to zero directly to avoid light refraction disorder caused by melt impurities and uneven molecules, resulting in visible glare and cloud-like shadow.
The whole process of feeding and material storage is sealed and dust-proof. Regularly clean carbon deposits and residual miscellaneous materials in hopper and barrel to prevent tiny dust and burnt particles from mixing into the melt, forming punctate astigmatism and local fog spots after molding.
II. Mold Surface and Cavity Optical Treatment
The cavity of high-transparency part molds must adopt ultra-mirror polishing with surface roughness controlled within Ra0.025 μm. Ordinary polishing lines will form diffuse reflection and directly cause surface glare; polishing lines must follow the demolding direction to avoid cross lines and local grinding marks.
Regularly clean residual carbon deposits, oil stains and release agent residues in cavities, runners and gates. Transparent parts are most sensitive to hidden surface stains, and slight carbon deposits will form flaky shadow. Frequent spraying of oil-based release agent is prohibited; special dry release agent for high transparency is selected when necessary.
Equipped with independent mold temperature machine for constant temperature control. The mold temperature of PMMA is stably maintained at 50–65 °C, and PC mold temperature is controlled at 80–95 °C. Uneven mold temperature will cause inconsistent cooling speed of surface layer, resulting in refraction difference and alternating light and dark shadow.
III. Optimization of Injection Molding Process Parameters to Eliminate Glare and Shadow
Too low temperature leads to incomplete plasticization, poor melt fluidity and weld line and shadow; excessively high temperature causes material decomposition and yellowing, generating gas glare. The melt temperature of PMMA is controlled at 210–230 °C, and PC at 270–290 °C. Segment temperature rise is adopted to avoid local overheating cracking.
High-speed jet glue feeding is strictly prohibited for high-transparency parts. Excessive speed generates melt turbulent flow and entraps air, forming internal micro-bubbles and surface glare. Adopt medium and low-speed layered filling, slow speed at gate section and constant speed in the middle section to reduce air entrapment and shear lines.
Excessively high holding pressure will leave internal stress, causing light refraction deformation and stress shadow. Appropriately increase screw back pressure to 0.6–0.9 MPa to improve melt plasticization uniformity and reduce astigmatism and haze caused by uneven molecular arrangement.
Properly extend cooling time to fully shape transparent parts and release internal stress; reserve sufficient exhaust time for each mold to prevent trapped air in the cavity from being unable to discharge and staying on the surface to form foggy shadow.
IV. Improvement of Runner, Gate and Exhaust Structure
Avoid direct facing the appearance surface with side gates. Priority is given to point gates and submarine gates for smooth glue feeding to reduce shear lines and glare caused by glue feeding impact; excessively small gates will increase shear heat and cause local material decomposition shadow. Match reasonable gate cross-sectional area according to product thickness.
Insufficient exhaust is the main cause of shadow on high-transparency parts. The depth of parting line exhaust grooves is controlled at 0.015–0.025 mm with evenly arranged multi-section exhaust to discharge cavity air and material cracking gas in a timely manner, preventing gas from being trapped on the surface to form whitish shadow.
Adopt rounded transition for runner corners to avoid stagnant material and overheating decomposition at right-angle corners, preventing hidden burnt material and melt stagnation from causing light transmission fog spots.
V. On-site Operation and Machine Maintenance Control
Thoroughly clean residual colored materials and miscellaneous materials in the barrel with special cleaning materials before producing high-transparency parts to prevent hanging material stagnation and gradual precipitation, resulting in persistent shadow and glare.
Keep the workshop dust-free and tidy to prevent dust falling into the cavity during mold opening and closing; use special dust-free wiping paper to wipe the mold cavity, and forbid ordinary gauze and dirty rags to avoid tiny scratches and fiber residues causing astigmatism.
Transparent parts are extremely sensitive to parameter stability. Fix temperature, speed and mold temperature parameters during mass production, and do not arbitrarily adjust the machine temporarily to avoid intermittent repeated shadow and glare caused by parameter fluctuation.
VI. Summary
The core causes of glare and shadow on high-transparency parts are concentrated in six aspects: residual water vapor, impure raw materials, insufficient mold mirror finish, poor exhaust, excessive filling speed and excessive internal stress. To eliminate defects, firstly complete raw material drying and cleanliness control, then upgrade mold ultra-mirror polishing and balanced mold temperature, cooperate with reducing filling speed, reasonable temperature control, optimizing exhaust and gate structure, and standardize on-site material change, cleaning and daily maintenance. Synchronous implementation of the whole set of measures can completely solve surface glare, haze, shadow and uneven light transmission problems of transparent parts, and stably realize high-quality mass production of high-transparency optical parts.
