Pitfall 1: Mirror Corrosion & Rusting on Molds
Root Cause Analysis (Based on the Cause-and-Effect Flowchart for Mirror Corrosion Failure of Injection Molds)
Stage 1: Root Defect – Insufficient Mold Venting
Insufficient depth or width of vent slots on mold cavities, excessively long glue-sealing sections, and trapped gas at dead corners of ribs or bosses.
Stage 2: High-Temperature Combustion & Detonation at the Instant of Injection
During high-speed plastic filling, enclosed gas undergoes drastic adiabatic compression, heating up rapidly to 1000~2000°C and triggering gas autoignition.
Extreme high temperatures break down the flame retardant system, cracking flame retardant materials and releasing massive acidic free radicals. These acidic free radicals absorb moisture from ambient air and precipitate a strongly acidic liquid film on the mold mirror surface.
Stage 4: Electrochemical Galvanic Corrosion (Mold Rusting & Loss of Gloss)
The acidic liquid film forms micro galvanic cells with the iron-carbon mold steel, triggering continuous electrochemical corrosion. This results in pockmarks, white haze, rust stains and damaged polished layers on the mirror finish.
Improvement Countermeasures
Optimize mold design to completely eliminate gas trapping.
Promptly remove residual acidic crystalline deposits on molds with mild alkaline mold cleaner to prevent recurring corrosion.
Improvement Outcome
Zero rust formation on mirror-finished molds.
Pitfall 2: Material Residence Time
Anyone who processes flame-retardant PC resins knows that mid-run machine shutdowns pose major risks.
Brominated flame retardants or antimony-based composite additives rapidly degrade chemically at 280°C. This degradation weakens the mechanical properties of finished parts and releases highly toxic, severely corrosive hydrogen halide gas.
Operation Specifications
Failure to comply will damage molded products, corrode the barrel, and create health hazards for operators.
Pitfall 3: High Shear Viscosity
Take halogen-free flame-retardant glass-filled nylon as an example.
Halogen-free flame retardants require high loading levels, which drastically raise the shear viscosity of the compound. Excessive screw shear force breaks down flame retardants into acidic gas, causing yellow discoloration and silver streaks on molded parts.
Solutions
Reduce screw rotation speed. Minimize backpressure while maintaining uniform melt consistency (uniform melt temperature, density and viscosity inside the melting zone).
Lower injection speed to avoid shear overheating of flame retardants as material flows through the gate.
Pitfall 4: Improper Reuse of Sprue & Runner Scrap
Flame-retardant resins carry high material costs. To cut expenses, some manufacturers add excessive proportions of recycled sprue/runner scrap and reuse scrap indefinitely. This practice leads to soaring defect rates and degraded tensile strength & flame retardancy of finished components.
Root Cause Analysis
Flame retardants are small inorganic molecules. Repeated exposure to high injection pressure and temperatures of 200~300°C causes their precipitation and migration, which degrades flame retardant performance.
Multiple recycling cycles disrupt the material’s crystallization rate and molecular chain entanglement. If recycled scrap is processed under virgin material molding parameters, microscopic crystalline delamination occurs inside parts, amplifying stress concentration and reducing mechanical strength.
Improvement Countermeasures
Conduct verification tests on maximum acceptable scrap loading ratios and recycling cycles before mass use of recycled flame-retardant scrap.
Compile standardized work instructions for scrap handling and corresponding standard molding parameters only after confirming compliance of product strength and flame retardancy.
Centralize scrap collection and implement scientific injection molding recycling protocols.
