Introduction
PC (polycarbonate), as an engineering plastic with excellent comprehensive performance, is widely used in high-end fields such as 3C electronics, automotive parts, medical consumables, and optical products due to its advantages such as high strength, high light transmittance, impact resistance, and high temperature resistance.
However, the injection molding characteristics of PC material—high-temperature degradation that easily produces corrosive gases, high melt viscosity that requires high cavity smoothness, and glass fiber reinforced models that are highly abrasive—pose stringent requirements for injection mold materials.
Choosing the right mold material can not only ensure the surface quality and dimensional accuracy of PC products, but also significantly extend the mold life and reduce long-term production costs.
This article will analyze the optimal material selection scheme for injection molds in detail, taking into account different application scenarios of PC products.
Why does PC material have high requirements for mold materials?
Before selecting mold materials, it is essential to understand the processing characteristics of PC material, as this is the core basis for selection. The three main characteristics of PC directly determine the core requirements of the mold material.
1, High-temperature degradation can easily produce corrosive gases
PC has a melting temperature as high as 220-300℃. If the raw material is not dried sufficiently, the barrel temperature is too high, or the screw residence time is too long during the injection molding process, molecular degradation will occur, producing corrosive gases such as carbon dioxide and phenols.
These gases will continuously corrode the surface of the mold cavity. If ordinary mold steel is not specially treated, it will rust and pitting in a short period of time, resulting in defects on the surface of PC products.
Therefore, the mold material must have excellent corrosion resistance.
2, The melt viscosity is high, which places stringent requirements on the surface finish of the mold cavity
PC melt has poor fluidity, requiring high injection pressure and mold temperature (80-120℃) to fully fill the cavity. Insufficient surface smoothness of the cavity will increase melt flow resistance, leading to problems such as weld lines, shrinkage marks, and surface scratches on the product.
Especially for transparent PC products (such as optical lenses and lampshades), the cavity needs to achieve a mirror effect to ensure the product’s light transmission and aesthetics.
Therefore, the mold material must have good polishing properties to achieve a high-gloss surface.
3, Enhanced PC requires high wear resistance
Glass fiber reinforced PC (glass fiber content 10%-30%) commonly used in the industrial field contains hard glass fiber fillers that can cause rapid wear and dimensional deviations in the cavity surface during injection molding due to high-speed scouring and friction.
Therefore, for reinforced PC products, the mold material must have sufficient hardness and wear resistance.
How to choose the material for injection molds of PC products?
Based on factors such as production volume, precision requirements, transparency, and reinforcement of PC products, the selection of mold materials can be divided into three categories: “preferred solution,” “cost-effective solution,” and “special scenario solution.” The characteristics, applicable scenarios, and precautions for each type of material are as follows.
1, Preferred option: Stainless steel
Stainless steel, with its strong corrosion resistance, excellent polishing performance, high hardness, and good thermal fatigue resistance, is the preferred mold material for most mid-to-high-end PC products, especially suitable for applications requiring large batches, high precision, and stringent requirements. Commonly used grades include S136 and S136H.
2, Cost-effective solution: Ordinary mold steel + surface treatment
For PC products with small batches, low precision, and no special requirements, if the budget is limited, a solution of using ordinary mold steel (such as P20, 718H) with surface treatment can be chosen. Surface treatment can make up for the lack of corrosion resistance and wear resistance of ordinary mold steel, thereby reducing the cost of mold procurement.
3, Special scenario solution: hot work die steel
For some PC products that require ultra-high temperature injection molding (such as high-temperature resistant PC engineering parts with injection temperature ≥300℃), or PC molds that need to withstand high temperature-room temperature cycles for a long time, the thermal fatigue resistance of ordinary stainless steel may be insufficient. In this case, hot work die steel (such as H13) should be selected.
Principles and Recommendations for PC Mold Material Selection
1, Selection Principles
Matching production batch: For batches ≤ 300,000 cycles, P20/718H+ surface treatment is recommended; for batches between 300,000 and 1,000,000 cycles, S136H is recommended; for batches ≥ 1,000,000 cycles, S136 or H13 is preferred.
For products with suitable precision and appearance: S136 stainless steel must be selected for transparent and high-gloss parts; for non-transparent parts, 420 stainless steel or ordinary mold steel with surface treatment can be selected according to appearance requirements; for high-precision parts (±0.01mm), stainless steel is preferred.
Considering the characteristics of raw materials: glass fiber reinforced PC must be S136 or H13; ordinary PC can be flexibly selected according to batch size and budget; medical grade PC products must be made of stainless steel with strong corrosion resistance such as S136.
2, Recommendations
Clearly define requirements in the early stages: Before purchasing molds, clarify the production batch size, precision requirements, appearance requirements, and raw material type (whether reinforcement is required) of the PC products, and form a clear list of mold material requirements.
Collaborate with the mold manufacturer to confirm: Provide the requirement list to the mold manufacturer, requesting the mold manufacturer to provide a mold material selection plan based on the requirements, and provide material certificates and heat treatment reports. If necessary, a third-party testing agency can be commissioned to verify the authenticity and performance of the materials.
Avoid falling into traps by trying to save money: Do not choose inferior mold steel or omit surface treatment in order to reduce costs. It may seem like you are saving money in the short term, but in fact it may lead to short mold life, more product defects, and high rework costs, which will ultimately increase the overall cost.
Final Words From Diamond Mold
For most mid-to-high-end PC products, stainless steel (especially S136) is the best choice that balances quality and lifespan; for small-batch, low-precision products, ordinary mold steel with surface treatment is a cost-effective option; for high-temperature or special working conditions, hot work mold steel should be considered.
