Polycarbonate Injection Molding Temperature

Core Processes in Polycarbonate Injection Molding:

Processing optical-grade polycarbonate (PC) requires a barrel melt temperature range of 280°C to 320°C, combined with a constant mold temperature range of 80°C to 120°C. Prior to plasticization, PC pellets must undergo continuous pre-drying at 120°C for 4 to 6 hours in a dehumidifying dryer to reduce residual moisture below the 0.02% threshold. Deviating from this thermal baseline by ±5°C directly triggers hydrolytic degradation and uncontrolled product shrinkage.

 

In-depth Analysis of Thermal Stability:

 

Melt Viscosity Control within the 280°C–320°C Range

Polycarbonate is an amorphous thermoplastic with a glass transition temperature of approximately 147°C. Unlike semi-crystalline polymers, PC exhibits high melt viscosity and is extremely insensitive to shear rates; consequently, melt flow relies primarily on thermal input rather than injection pressure.

At temperatures below 275°C, the melt front becomes rigid. The injection molding screw is subjected to excessive torque, resulting in high shear stress, short shots, and frozen-in internal stresses. Temperatures exceeding 330°C trigger thermal chain scission, leading to the formation of carbon dioxide bubbles and dark silver streaks. For thin-walled parts (<1.5 mm), engineers utilize the upper temperature limit (310°C–320°C) to prevent premature gate freeze-off; conversely, thick-walled structural components (>3.0 mm) require a lower thermal range (280°C–295°C) to control the formation of internal shrinkage voids.

 

 

Barrel Zone Optimization and Mold Cavity Temperature Control

 

Mold temperature is the critical active variable for managing internal stress. Setting the mold temperature below 70°C forces the surface layer of the plastic melt to solidify the instant it contacts the mold steel; this rapid cooling effect locks molecular chains into a high-energy, stretched, and oriented state. Maintaining the cavity temperature between 90°C and 120°C prolongs the molten state of the core layer, allowing holding pressure to be transmitted smoothly and ensuring uniform compaction of the part's geometry.

 

Processing Zone	Standard PC (MFR 10-15)	Optical Grade PC (MFR 5-8)	Glass-Filled PC (30% GF)	Primary Defect Risk
Rear (Feed Zone)	260°C – 280°C	270°C – 290°C	275°C – 295°C	Pellet bridging / premature melting
Center (Compression)	280°C – 300°C	290°C – 310°C	295°C – 315°C	Unmelted solid core capture
Front (Metering)	290°C – 315°C	300°C – 320°C	305°C – 325°C	Polymer chain degradation
Nozzle Tip	285°C – 305°C	295°C – 315°C	300°C – 320°C	Cold slugs / nozzle drool
Mold Cavity	80°C – 100°C	95°C – 120°C	90°C – 110°C	Surface delamination / sink marks

 

Residual Internal Stress Suppression and Birefringence Control

 

In transparent PC applications-such as automotive headlight lenses or housings for medical diagnostic equipment-uncontrolled thermal gradients manifest directly as optical birefringence. Under polarized light inspection, areas of stress concentration exhibit anisotropic refraction, revealing colorful interference fringes. These "frozen-in" optical distortions correspond directly to mechanical weak points in the material; consequently, the part becomes highly susceptible to environmental stress cracking when exposed to cleaning solvents or esters.

 

 

Engineering Solutions to Eliminate Birefringence:

 

Strictly control the temperature differential between the melt and the mold so it does not exceed 190°C.
Implement a multi-stage injection rate profile: use 60% injection speed at the gate and reduce it to 20% during the final stage of cavity filling.
Apply a holding pressure equivalent to 50%–70% of the maximum injection pressure before the gate fully solidifies.

To validate gate designs and predict shear-induced stress concentration points prior to mold cutting, our engineering team utilizes Moldflow CAD analysis to provide precise thermo-mechanical insights.

 

Shrinkage Control (0.5%–0.7%) and Geometric Mold Tolerance Allocation

 

Polycarbonate exhibits isotropic volumetric shrinkage ranging from 0.5% to 0.7% (0.005–0.007 mm/mm). Due to the absence of a crystalline structure, PC offers highly predictable shrinkage characteristics compared to materials like Nylon (PA66) or Polyoxymethylene (POM). However, dimensional repeatability across batches depends entirely on the thermal balance of the mold assembly.

Draft Angle Allocation: Molds must be designed with a minimum draft angle of 1.0° to 1.5°. Ribs and locating bosses require a draft of 2.0° to prevent "whitening" or puncture by ejector pins.

Pre-drying Specification: 0.02% Maximum Moisture Threshold

Polycarbonate is highly hygroscopic; atmospheric moisture chemically bonds to the resin molecules. If virgin pellets contain >0.02% moisture, the entrapped water instantly converts to superheated steam at 300°C, triggering ester-group hydrolysis.

Mechanical consequences of molding PC with high moisture content include:

Measurable degradation of the polymer's weight-average molecular weight.
Surface appearance of characteristic silver streaks, spots, or internal voids/bubbles.

Standard hot-air circulating ovens cannot achieve this level of dryness. Production facilities must utilize molecular sieve desiccant dryers capable of a -40°C (-40°F) dew point and an operating temperature of 120°C, with a minimum pellet residence time of 4 hours. Once dried, pellets exposed to ambient shop air will reabsorb excessive moisture within 30 minutes.

Achieving zero-defect polycarbonate manufacturing requires absolute synchronization between raw material thermodynamics, high-pressure injection molding equipment, and precision mold tooling. Our production facilities deeply integrate ISO 9001:2015 certified injection molding units with precision CNC machining capabilities, creating an in-house closed-loop process for hardened steel mold cutting and complex aluminum insert manufacturing.

 

 

 

 

Frequently Asked Questions

 

1.What are the standard CMM inspection specifications for polycarbonate injection-molded parts?

We strictly implement 100% First Article Inspection (FAI) in accordance with ISO 9001:2015 standards, utilizing high-precision Coordinate Measuring Machines (CMM). Before inspection, injection-molded parts undergo a mandatory 24-hour thermal stabilization period at 23°C (±2°C) to ensure geometric dimensions strictly comply with tolerance specifications.

 

2.What is the lead time for the initial T1 sample of a custom polycarbonate mold?

The standard lead time for T1 sample delivery is 15 to 22 working days following the final sign-off on the DFM report. This process encompasses CNC rough machining of P20 or NAK80 steel inserts, high-speed 5-axis finish milling, and EDM cavity corner clearing.

 

3.What mold lifespan guarantee is provided for high-volume PC production?

We guarantee a service life of at least 1,000,000 molding cycles. Mold cores are constructed from hardened H13 or S-7 tool steel (48–52 HRC) with diamond optical polishing, and internal conformal cooling channels undergo fully automated pressure decay testing.