Injection molding is a commonly used manufacturing process for producing plastic parts. A key component in this process is the injection mold, which gives shape to the molten plastic. Given their importance, it’s critical injection molds remain in good working order. This raises an important question – does injection mold need to be heat treated?
As a professional plastic injection molding manufacturer, I set out to uncover the answer. In this complete guide, I’ll walk you through everything you need to know about heat treating injection molds.
Does Injection Mold Need to Be Heat Treated?
Yes, heat treatment is absolutely vital for injection molds tasked with producing millions of consistent, high tolerance plastic parts over years of service. The upfront investment is well worth it when you consider the value provided in parts produced, reduced repairs, and improved uptime.
Why Heat Treat Injection Molds?
Before jumping into the details, it’s important to understand why heat treatment is done in the first place.
There are two primary reasons injection molds are heat treated:
1. Increase Hardness and Wear Resistance
The repeated injection of hot, viscous plastic puts a lot of stress on the mold. Over hundreds of thousands of cycles, even hardy tool steel will slowly wear away.
By heat treating the mold, hardness and wear resistance is improved significantly. This translates to:
- Longer overall mold lifetime
- More consistent part quality over lifespan
- Less unplanned downtime for repairs
For high volume applications, the value of increased mold longevity easily justifies the extra cost and time of heat treating.
2. Dimensional Stability
As the mold repeatedly heats up and cools down, internal stresses start to build up. If left unchecked, these stresses can lead to small amounts of deformation over time.
Proper heat treatment relieves these internal stresses, leading to excellent dimensional stability of the mold. This is crucial for holding tight tolerances part-to-part across production volumes in the millions.
So in summary – yes, heat treatment of injection molds is absolutely vital! The extra hardness and dimensional stability are well worth the investment for virtually all production applications.
Now let’s explore the most common heat treating processes.
Overview Of Injection Mold Heat Treating Processes
There are many ways to heat treat tool steel to transform its properties. However, a few key processes make up the bulk of injection mold heat treatments:
Normalizing
- Heating mold steel above critical temperature
- Holding long enough for microstructural changes
- Air cooling to room temperature
- Refines grain structure
- Relieves internal stresses
Annealing
- Heating mold steel to suitable temperature
- Holding long enough for microstructural changes
- Slow cooling, typically in the furnace
- Refines grain structure
- Relieves internal stresses
Hardening
- Heating mold steel above critical temperature
- Holding until fully austenitized
- Rapid quenching in oil or water
- Forms very hard martensitic structure
Tempering
- Heating quenched steel to below critical temperature
- Holding to allow decomposition of martensite
- Air cooling to room temperature
- Reduces brittleness from quenching
- Adjusts final hardness
Now let’s walk through when you would use each process and why.
Pre-Machining Stress Relief
Before any heavy milling or machining takes place, the mold steel is in an annealed state. This means it has low hardness to allow for easy cutting, but has high internal stresses leftover from material processing.
To relieve these stresses before machining, normalizing or annealing heat treatments are done first. Remember – normalizing involves faster cooling for some grain refinement, while annealing uses slow cooling for maximum softening.
Which one you choose depends mainly on the specifics of the mold steel composition and machining needs. As a general guideline:
- For mold steel with >0.45% C, use normalizing
- For mold steel with <0.45% C, use full annealing
Once stress relieving is complete, the steel can be safely machined without risk of distorting.
Final Property Optimization
After the mold is fully machined to its final dimensions, the last step is to optimize its properties for functionality – hardness, strength, wear resistance and temperature capability.
This is accomplished with a specialized sequence of hardening and tempering.
Hardening Treatment
The machined mold first goes through an aggressive quench from the austenitizing temperature, which could be up to 1900°F based on composition. As soon as mold steel hits this critical temperature, time is critical.
A quick quench in oil or water rapidly pulls heat from the steel, forming a very hard martensitic structure. However, this same structure is quite brittle until tempering is done.
Tempering Treatment
The final tempering step serves two important purposes:
- Adjust hardness to specifications
- Reduce brittleness for durability
This is accomplished by gently reheating the mold to a precise temperature below the critical point, allowing some decomposition of the martensite into tempered structures.
The exact temperature and time selected dramatically impacts the final mold properties. As such, tempering is customized carefully using empirical data to meet the needs of the application.
And after final tempering, the injection mold is complete! The tailored microstructure provides an exceptional balance of hardness, strength, and temperature resistance to churn out millions of high quality plastic parts.
Real-World Heat Treating Examples
Now that you understand why heat treating is vital for injection molds, let’s look at some real world examples.
Two of the most commonly used mold tool steels are:
- P20 Steel
- H13 Steel
So how are these alloy steels typically heat treated?
P20 Steel Heat Treatment
P20 steel is considered a “pre-hardened” mold steel, meaning it achieves increased hardness through alloying rather than a full hardening and tempering process.
A typical P20 treatment process looks like:
- Stress relieving anneal
- Rough machining
- Final machining
- Polishing
The annealing step relieves internal stresses from previous processing and makes the steel easier to machine. P20 is delivered pre-hardened to ~34 HRC, so no further hardening is done.
H13 Steel Heat Treatment
For more demanding applications, H13 hot work steel is a popular choice. Here is a typical multi-step heat treat process:
- Stress relieving normalize
- Rough machining
- Final machining
- Austenitize harden at 1900-2000°F
- Oil quench rapidly to room temperature
- Double temper at 1000-1100°F to achieve 50-52 HRC
- Finish grinding
The initial normalization provides stress relief and grain structure control. Final properties are tailored through an aggressive hardening and tempering cycle to maximize wear resistance and temperature capability up to ~1000°F.
This allows H13 molds to run high duty cycle applications with abrasive-filled plastics that would quickly degrade a P20 mold.
Conclusion
Mold heat treating transforms the microstructure of tool steels to achieve incredible hardness, strength and temperature resistance. This capability simply can’t be matched through alloying alone.
Now I’d like to hear from you:
- What tool steel do you typically use for injection molds?
- What heat treating process works best in your experience?
Let me know in the comments section below!
