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Polycarbonate (PC) is a high-transparency, high-impact, and high-strength engineering plastic widely used in electronics, automotive parts, optical products, security accessories, and structural components. PC material is sensitive to processing conditions such as moisture, temperature, shear, and cooling. Improper control can easily lead to problems such as silver streaks, bubbles, cracking, weld lines, warping, and decreased transparency. To ensure the appearance, dimensional accuracy, and mechanical properties of the finished product, injection molding must adhere to standardized process control and operational guidelines.
Raw material drying: the most critical pre-process in PC injection molding
PC material has a certain degree of hygroscopicity, absorbing moisture from the air during storage and handling. If it is injected directly without being sufficiently dried, the moisture will vaporize in the high-temperature barrel, leading to defects such as silver streaks, bubbles, blemishes, and increased haze in the product. It will also cause polymer degradation, resulting in decreased product strength, brittleness, and increased cracking. Therefore, drying is an indispensable and non-simplistic first step in PC injection molding.
The recommended drying temperature for PC raw materials is 100℃～120℃, with a typical drying time of 4～6 hours. If the raw materials have been stored for a long time or the ambient humidity is high, the drying time should be appropriately extended to ensure that the moisture content is reduced to below 0.02%. Dehumidifiers should be used preferentially for drying, ensuring stable airflow with a low dew point. Avoid using ordinary hot air hoppers for prolonged high-temperature drying to prevent oxidation and yellowing of the raw materials. The dried hopper should be kept sealed and insulated to prevent the raw materials from absorbing moisture again during transport. During production, if the machine is stopped for more than 2 hours, the drying status should be rechecked to avoid batch defects due to moisture absorption.
Key points for setting the temperature of the barrel and nozzle
PC is a non-crystalline engineering plastic with a relatively high melt temperature and moderate flow properties, making it quite sensitive to temperature. Too low a temperature can lead to poor plasticization, poor flowability, difficulty filling molds, and high internal stress; too high a temperature can cause degradation, discoloration, strong odor, and brittleness in the finished product.
The barrel temperature is generally set to be slightly higher in the middle and slightly lower at the front and back, with recommended ranges:
Back section: 260℃～270℃
Middle section: 270℃～285℃
Front section: 275℃～290℃
Nozzle: 270℃～285℃
The actual temperature should be adjusted appropriately based on the product wall thickness, gate size, and mold structure. For thin-walled, long-flow products, the temperature can be increased by 5℃ to 10℃; for thick-walled products, it should be decreased appropriately to avoid overheating and degradation. The nozzle temperature should be stable to prevent cold material from entering the mold cavity and causing defects, while also avoiding excessive temperature leading to drooling. Prolonged idling at high temperatures is strictly prohibited during production; reduce the residence time of the melt in the barrel to prevent thermal decomposition of the material.
Mold temperature and cooling system control
Mold temperature directly affects the internal stress, transparency, dimensional stability, and surface gloss of PC products. If the mold temperature is too low, the melt cools too quickly, resulting in severe molecular orientation, increased internal stress, and a tendency for cracking, whitening, and noticeable weld lines. If the mold temperature is too high, cooling time is prolonged, production efficiency decreases, and problems such as sticking, shrinkage marks, and deformation may occur.
The recommended temperature for PC injection molds is 80℃～100℃. For products requiring high transparency, high gloss, and high appearance, this can be increased to 100℃～120℃. The mold must be equipped with a stable temperature control system to ensure uniform temperature in the cavity and core, avoiding deformation caused by localized temperature differences. The cooling channel design should be reasonable, close to the cavity surface, and evenly distributed to ensure cooling efficiency. The cooling time should be set according to the product thickness, aiming for a product that does not deform or whiten upon demolding. Too short a cooling time can lead to deformation, while too long a cooling time will reduce efficiency.
Key points for controlling injection speed, pressure, and back pressure
PC melt has a high viscosity and moderate fluidity. Improper injection parameter settings can easily lead to problems such as insufficient mold filling, high internal stress, scorching, and air bubbles.
The recommended injection pressure is medium to high, adjusted within the range of 80–140 MPa depending on the product structure, ensuring smooth mold filling without excessive pressure. Excessive pressure increases internal stress, leading to cracking and warping after demolding. Medium to high injection speeds are recommended. For thin-walled and transparent parts, the speed can be appropriately increased to avoid flow marks and gate halo; for thick-walled parts, the speed should be reduced to prevent trapped air, scorching, and air pockets.
Multi-stage injection is a common method for stabilizing PC molding: low speed in the gate area, high speed during filling, and low speed again before the cavity is fully filled, reducing jetting and air bubbles. Back pressure should not be too high, generally 5–15 MPa, aiming for uniform plasticization and no air bubbles. Excessive back pressure increases shear heat, leading to material degradation. Moderate screw speed is crucial to avoid high shear causing localized overheating, which affects transparency and toughness.