Common Defects and Improvement Solutions of PC Injection Molding in Chinese Plastic Molds

In the manufacturing system of Chinese plastic molds, PC material is widely adopted for transparent components, electronic housings, structural accessories and home‑appliance parts for its high transparency, outstanding impact resistance and reliable heat‑resistance performance. Nevertheless, PC is a high‑viscosity and highly hygroscopic engineering plastic, which shows high sensitivity to raw‑material drying condition, mold venting, mold temperature and injection process parameters. During actual mass‑production in Chinese plastic mold factories, PC molded parts tend to generate various typical molding defects such as silver streaks, internal bubbles, residual‑stress cracking, prominent weld lines and sink marks. These frequent quality issues will damage surface appearance, dimensional stability and mechanical properties of finished products. It is necessary to analyze root causes of different defects and carry out standardized improvement measures, so as to raise finished‑product rate and production stability for PC injection‑molding projects in domestic plastic mold industry.

1. Silver streaks and bubble defects improvement

Silver streaks and enclosed internal bubbles rank as the most frequent defects for PC products produced by Chinese plastic molds. Excessive residual moisture inside PC pellets acts as the primary inducement. PC features strong moisture‑absorbing capacity. Even if pellets look dry visually, invisible water molecules still remain inside material particles. Without adequate drying before production, moisture vaporizes rapidly inside high‑temperature barrel, creating water vapor and turbulent airflow during melt filling, and further producing surface silver streaks and sealed bubbles inside thick‑wall sections.

Apart from raw‑material moisture, over‑high barrel temperature triggers thermal degradation of PC molecular chains and generates pyrolysis gas. Improper injection speed brings air entrapment, while blocked mold vent slots cause trapped‑air pressure, which will aggravate silver streaks and bubble problems. Most Chinese plastic mold manufacturers implement standardized drying parameters from 110℃ to 120℃ with drying time over four hours. The feeding hopper shall keep sealed and heat‑insulated to avoid secondary moisture absorption. Segmented injection speed shall be adjusted to reduce air entrainment caused by shear effect. Mold vent slots need regular cleaning, and vent size at melt‑flow terminals shall be properly enlarged for smooth gas evacuation.

2. Internal stress and stress cracking solutions

PC injection‑molded parts easily retain considerable internal stress during forming process, and cracks may occur under assembly force, chemical‑solvent contact or low‑temperature working environment. In mass‑production practice of Chinese plastic molds, unreasonable process setting and defective product‑mold structure are major triggers. Excessive injection and packing pressure force melt filling and result in severe molecular orientation. Too low mold temperature makes melt solidify rapidly without sufficient molecular relaxation. Abrupt wall‑thickness transition and improperly arranged gates will also lead to local stress concentration.

For improvement, domestic mold factories moderately cut down injection and holding pressure, raise mold temperature to 80℃‑110℃ and extend cooling period to realize molecular stress release. Product structure shall be optimized by adding large radius fillets at sharp corners to avoid dramatic wall‑thickness difference. Gate locations need adjustment to prevent stress accumulation on force‑bearing surfaces. For high‑requirement PC components, post‑mold annealing treatment can be applied to further eliminate residual stress and lower cracking risks in subsequent application.

3. Weld line elimination and optimization

Due to high melt viscosity of PC, obvious weld lines are easily formed where separated melt streams converge, impairing both visual appearance and local mechanical strength. In Chinese plastic mold production, multi‑gate layout, hole features and dense rib structures often force melt to divide and re‑combine. Low mold temperature weakens melt fusion activity, and trapped air at bonding interfaces makes weld lines more visible.

Effective countermeasures contain optimizing gate layout to reduce unnecessary melt shunting. Appropriately increase barrel and mold temperature to promote melt fusion. Open sufficient vent slots corresponding to weld‑line positions to exhaust trapped gas. Set proper injection speed to shorten filling cycle and retain melt temperature. Cold‑slug wells arranged behind weld‑line zones can intercept low‑temperature cold material and greatly weaken weld‑line marks on finished parts.

4. Sink mark and short shot countermeasures

Although PC owns relatively low shrinkage rate, sink depressions still appear on surfaces behind thick‑wall zones, screw bosses and reinforcing ribs. Insufficient packing pressure, short packing duration and undersized gate cannot transmit feeding pressure before gate freezing. Over‑high mold temperature slows cooling speed of thick sections and worsens shrinkage depressions. For improvement, reduce local glue accumulation on product design, enlarge gate and runner cross‑section, extend packing time and strengthen cooling circuits for thick‑wall corresponding mold positions. When short shot occurs caused by high melt viscosity, raise barrel and mold temperature properly instead of simply boosting injection pressure, and optimize runner and venting structure to realize complete cavity filling.

5. Summary

For PC production supported by Chinese plastic molds, raw‑material drying management shall be placed in the first position for defect prevention. Most quality failures originate from inadequate drying, poor venting and unreasonable temperature setting. Blindly raising injection pressure and speed will bring new defects such as stress cracking and flash. Combining product DFM optimization, precise mold manufacturing and standardized process debugging can effectively control typical PC molding defects and guarantee stable mass‑production output.