How to produce qualified and stable transparent plastic parts?
Through over 10 years of production experience, Kingsjeng has identified key measures. The core of producing qualified transparent plastic parts (such as PMMA, PC, PS, etc.) lies in achieving the three core goals of "no appearance defects", "qualified light transmittance", and "no internal stress or low stress". Strict control is required in multiple links including raw materials, molds, processes, equipment, and environment. The key points are as follows:
I.Strict Selection and Pre-treatment of Raw Materials
- High-purity raw materials are fundamental
Transparent plastic parts have extremely high requirements for the purity of raw materials. It is necessary to select low-impurity, high-light-transmittance special materials (such as medical-grade PC, optical-grade PMMA) and avoid mixing dust, metal debris, colored particles, or excessive proportion of recycled materials in the raw materials (recycled materials are prone to contain degraded impurities, which will reduce light transmittance).
Note: Different materials have different characteristics (e.g., PC is impact-resistant but prone to hydrolysis; PMMA has high light transmittance but is easy to scratch). The raw materials should be matched according to the performance requirements of the product.
- Thorough drying to eliminate moisture interference
If transparent plastics (especially hygroscopic materials such as PC and PA) contain moisture, they will undergo "hydrolysis" or "gasification" due to high temperature during injection molding, resulting in silver streaks, bubbles, and foggy whitening on the products. It is necessary to dry strictly according to the requirements of raw materials (e.g., PC needs to be dried at 120-130℃ for 4-6 hours; PMMA at 80-90℃ for 3-4 hours) to ensure the moisture content is ≤0.02%.
II.Mold Design and Processing Precision
- Ultra-high surface finish
The surface of the mold cavity must reach mirror-level polishing (Ra≤0.02μm) to avoid any scratches, depressions, rust, or processing lines. Tiny defects on the mold surface will be directly copied to the product surface, causing light scattering and reducing light transmittance.
Tip: Use processes such as chrome plating and mirror grinding (e.g., polishing with diamond grinding wheels below W3.5) to improve surface quality.
- Optimized venting and runner design
- Poor venting will cause air to be trapped during melt filling, forming bubbles and burn marks (air is compressed and ignited at high temperatures). Vent grooves with a depth of 0.01-0.03mm should be set at the positions where the melt fills last (such as corners and places with sudden changes in wall thickness).
- The runner should adopt a circular cross-section (to reduce flow resistance), and its inner wall also needs mirror polishing. The gate position should avoid facing the cavity wall (to prevent eddy currents caused by melt impact from trapping air). Submarine gates or pin point gates can be used if necessary.
- Uniform cooling to reduce internal stress
The mold cooling system must fit the contour of the cavity to ensure rapid and uniform cooling of the melt (e.g., PC parts require sufficient cooling time to avoid internal stress concentration due to temperature differences). Uneven cooling will cause inconsistent local shrinkage of the product, resulting in warpage or internal stress cracks (which may crack later due to stress release).
III.Precise Control of Injection Molding Process Parameters
- Temperature: avoiding decomposition and poor plasticization
- Barrel temperature: It should match the melting point of the raw material (e.g., PC processing temperature is 260-320℃; PMMA is 180-230℃). Too low temperature will cause uneven plasticization (resulting in a foggy appearance of the product); too high temperature will decompose the raw material (producing black spots and yellowing, especially PC is prone to degradation at high temperatures).
- Nozzle temperature: It should be slightly higher than the temperature at the end of the barrel to prevent the melt from cooling and solidifying at the nozzle to form "cold slugs", which will be brought into the cavity and cause defects.
- Pressure and speed: balancing filling and venting
- Injection pressure: It needs to be sufficient to overcome the melt flow resistance (transparent parts usually have uniform wall thickness, so high pressure should be avoided to prevent flash), but excessive pressure may aggravate internal stress.
- Injection speed: Too fast speed is easy to trap air and form bubbles; too slow speed will cause the melt to cool in the runner and lead to insufficient filling. A "segmented speed" (slow at the initial stage, fast in the middle, and slow at the later stage) should be adopted to ensure stable filling.
- Packing and cooling: controlling shrinkage and stress
- Packing pressure: It needs to be sufficient to compensate for the cooling shrinkage of the melt to avoid surface sink marks on the product (especially at thick-walled parts), but excessive packing pressure will increase internal stress.
- Cooling time: It should be sufficient (e.g., the cooling time for PC parts is usually 2-3 times the injection time) to ensure that the product is completely solidified before demolding, reducing post-demolding deformation.
IV.Equipment Cleaning and Condition Maintenance
- Thorough machine cleaning to avoid cross-contamination
Before producing transparent parts, the barrel, screw, and nozzle must be thoroughly cleaned with the same material or special purge compound (such as PMMA purge compound) to remove residual colored materials, degraded materials, or impurities. Otherwise, the product will have black spots and streaks.
- Guarantee of equipment precision
The clearance between the screw and the barrel must be uniform (excessive wear will cause uneven plasticization), and the check ring must be well-sealed (to prevent melt backflow from generating cold slugs). Otherwise, the melt stability will be affected, leading to fluctuations in product quality.
V.Internal Stress Control and Post-treatment
Transparent plastic parts (especially PC and PMMA) are prone to internal stress due to improper processes, resulting in cracks during storage or use (e.g., when in contact with solvents or temperature changes). It can be controlled through the following methods:
- Process optimization: Reduce injection pressure and extend cooling time to reduce stress concentration.
- Annealing treatment: Bake the finished products at low temperature (e.g., PC is baked at 120-130℃ for 2-4 hours; PMMA at 70-80℃ for 1-2 hours) to release internal stress.
- Inspection: Use a polarimeter to check the stress distribution and avoid stress concentration areas (showing obvious light and dark stripes under polarized light).
VI.Environmental Cleanliness and Quality Inspection
- Clean production environment
The workshop must be dust-free (Class 10000 or higher cleanliness is recommended) to prevent dust and fibers from falling into the mold or product surface, forming impurity points. Operators must wear cleanroom clothing and gloves to prevent oil pollution.
- Full inspection of key indicators
- Appearance: No scratches, black spots, bubbles, silver streaks, or burn marks.
- Light transmittance: Test with a light transmittance meter (e.g., high-quality PMMA has a light transmittance of ≥92%; PC is ≥89%).
- Dimensional accuracy: Avoid assembly problems caused by warpage and shrinkage.
- Stress test: Polarization method or solvent immersion method (e.g., wipe PC with carbon tetrachloride, and stress concentration areas will crack).
In summary, Kingsjeng's years of production experience have proven that the production of transparent plastic parts is the result of the combination of "raw material purity + mold precision + process stability + environmental cleanliness", which is somewhat challenging. Any omission in any link may lead to reduced light transmittance or appearance defects, requiring refined control throughout the entire process.
