Heat Treatment for Plastic Injection Moulding
Imparting Soul to Steel
Heat treatment is a critical process in the manufacture of plastic injection moulds. It enhances the mechanical properties of tool steels, improving hardness, wear resistance, toughness, and dimensional stability. Proper heat treatment ensures that mould components can withstand the demanding conditions of injection cycles, including high pressure, temperature fluctuations, and abrasive polymer flow.
Why Heat Treatment ?
In mould making, the primary objectives of heat treatment are:
- Increase surface hardness for wear resistance and longer mould life
- Relieve internal stresses introduced during machining or EDM
- Improve toughness to prevent cracking or chipping under load
- Stabilize dimensions before final finishing and assembly
- Enhance corrosion resistance when used with stainless tool steels
Stages of Heat Treatment
1. Preheating
Before hardening, the mould steel is slowly preheated (typically 400–800°C, depending on the grade) to prevent thermal shock and distortion. This stage equalizes the temperature throughout the component.
2. Hardening
The steel is heated to its critical temperature (around 850–1050°C) to transform its microstructure into austenite. It is then rapidly cooled (quenched) in oil, air, or vacuum to form martensite, achieving high hardness.
3. Quenching
The quenching medium is selected based on steel type and component geometry:
Oil Quench: Moderate cooling rate for alloy steels
Air or Gas Quench: For air-hardening steels like H13
Vacuum Quench: Used to minimize oxidation and distortion
4. Tempering
Immediately after quenching, tempering relieves internal stresses and adjusts hardness to a workable level. Typical tempering temperatures range from 150–600°C. Most mould steels are double, or triple tempered to achieve uniform properties.
5. Stress Relieving
After rough machining and before final finishing, stress relieving at 550–650°C helps prevent movement during hardening and machining
Advanced Heat Treatment Methods
Vacuum Heat Treatment
Provides a clean, oxidation-free surface ideal for polished moulds and tight tolerances.
Contact UsCryogenic Treatment:
Exposing the steel to sub-zero temperatures (-80°C to -196°C) converts retained austenite into martensite, enhancing wear resistance and dimensional stability.
Contact UsNitriding / Gas Nitriding
Forms a hard, wear-resistant surface layer (up to 70 HRC) without altering the core hardness. Ideal for slides, pins, and inserts.
Contact UsInduction Hardening
Selectively hardens specific areas such as wear zones, parting lines, or gate areas.
Contact UsHeat Treatment and Mould Performance
- Uneven hardness and dimensional instability
- Properly heat-treated moulds show:
- Reduced wear and maintenance requirements
- Better part quality and surface finish
- Longer tool life cycles
- Improved consistency during high-volume production
- Enhanced resistance to stress cracking and deformation
- Incorrect heat treatment can lead to:
- Warping or distortion
- Premature cracking or soft spots