Controlling mold temperature is a big deal in injection molding. You can have a great machine and a well-made mold, but if the temperature isn’t right, your parts won’t come out the way you want.
Why Mold Temperature Matters
When hot plastic goes into the mold, it starts cooling right away. At the same time, it heats up the mold surface. If the mold stays too hot, the plastic fills the mold easily, but it takes longer to cool. That means slower production.
If the mold is too cold, the plastic cools too fast. It might not flow into every corner, so the part might be incomplete or weak. On the plus side, the cycle time is shorter.
Getting the temperature just right helps you:
- Make better parts
- Avoid warping or defects
- Shorten cooling time
- Speed up production
- Lower costs
What This Blog Covers
In this post, you’ll learn:
- Why mold temperature affects your parts
- How to pick the right temperature
- Ways to control the temperature
- Common issues and how to fix them
By the end, you’ll see how better temperature control leads to better results—and helps save time and money.
Mold Temperature: Why It Matters
Mold temperature means the heat of the mold surface—the part that touches the plastic. This temperature plays a big role in how well your part turns out.
The right mold temperature depends on a few things:
- What type of plastic you’re using
- The size and shape of your part
- How strong or detailed the part needs to be
- Other settings like melt temperature, injection speed, and pressure
What Happens When the Mold Is Too Hot or Too Cold?
If the mold is too hot, the plastic flows better. It fills the mold more completely and makes the part look smoother. But it takes longer to cool. This can slow down production and sometimes damage the part when removing it.
If the mold is too cold, the plastic cools too fast. That makes it harder to fill the mold, which leads to problems like:
- Internal stress
- Dull surfaces
- Silver streaks
- Weak weld lines
Why Temperature Control Is So Important
Different plastics need different molding temperatures. Also, parts with special surfaces or shapes need different heat settings.
If you want your parts to be the right size, shape, and quality every time, you need to keep the mold temperature steady. A stable mold temperature helps you:
- Avoid defects
- Keep dimensions consistent
- Improve part appearance
- Speed up production
So when designing a mold, don’t forget to think about how you’ll control the temperature. It’s just as important as the mold design itself.
Why You Need to Control Mold Temperature
Mold temperature affects a lot more than just how the plastic flows. It changes the way your part shrinks, how strong it is, and how it looks on the outside.
If the mold gets too hot or too cold—or if the temperature keeps changing—you may end up with parts that bend, crack, or just don’t meet your standards.
That’s why it’s important to:
- Keep the mold temperature within a set range
- Make sure it stays steady over time
- Keep all cavities in a multi-cavity mold at the same temperature
Let’s look at three key reasons why mold temperature control really matters:
1. Better Appearance
Today’s customers care about how things look. One way to improve the surface of your parts is by adjusting mold temperature.
This is especially true with parts reinforced with glass fiber—a material added to plastics to make them stronger. If the mold is too cold, those fibers can float to the surface and make the part look rough or cloudy.
That’s a problem in parts like gears, where glass fiber content can be as high as 50%.
2. Size Accuracy
For precision parts, like small gears, looks aren’t enough—they also need to hold their shape.
Things that affect size stability include:
- A stable molding process
- Consistent mold temperature
- Even material mix
- Balanced water flow in the mold cooling system
- Accurate temperature control from the mold heater
If any of these go off track, your part might warp or shrink too much.
3. Strength and Clarity
The mold temperature also affects how strong or clear your part is.
For crystalline plastics like PA, PP, PE, and POM, using a high mold temperature lets the plastic form a better internal structure. That means:
- Higher crystallinity
- More shrinkage
- Better strength and stiffness
But there’s a tradeoff—higher temperatures mean longer cooling time, which can slow down production and raise costs.
For clear plastics, like PC or PS, you often want the opposite. A lower mold temperature helps reduce crystallinity, which keeps the part more transparent.
Also, for materials that tend to trap stress inside the part, such as PC or ABS, raising the mold temperature slightly can help. It makes it easier for that stress to escape and reduces the chance of cracking later.
Controlling mold temperature is not just about speed—it’s about getting the right look, size, and strength in your plastic parts. Keeping it stable is key to making high-quality parts every time.
How Mold Temperature Affects the Quality of Injection Molded Parts
Mold temperature plays a big role in how your final plastic part turns out. It affects how the part looks, how strong it is, and whether it warps or shrinks too much after molding.
Let’s break down the key ways mold temperature impacts part quality:
1. Surface Appearance
If the mold is warm enough, the melted plastic flows better. This helps the part come out with a smoother, shinier surface. It also improves the look and strength of weld lines—the places where two flow fronts meet inside the mold.
For parts with surface textures (like etching), low mold temperature makes it harder for the plastic to reach deep into the grooves. That means the final part may look flat or glossy instead of showing the full texture. Raising the mold and material temperature helps transfer the full texture onto the part.
2. Internal Stress
Internal stress builds up during cooling when the plastic shrinks at different speeds. The outside cools first, hardens, and shrinks. Then the inside cools and shrinks later. This difference creates stress inside the part.
If that stress is too high, the part may crack—especially if exposed to certain chemicals.
In see-through plastics like PC (polycarbonate) and PMMA (acrylic), stress shows up as compression on the outside and tension on the inside. These stresses depend on how quickly the surface cools. A colder mold makes the plastic cool faster and builds more stress.
Even a small change in mold temperature can make a big difference in internal stress. That’s why each plastic and product has a recommended minimum mold temperature to help avoid these problems.
For parts with thin walls or long flow paths, use a higher mold temperature than usual to reduce stress.
3. Warping and Bending
When cooling isn’t even, parts can warp or bend after they come out of the mold.
To avoid this, pay attention to temperature differences between:
- The two halves of the mold
- The mold core and mold wall
- Mold walls and inserts
If one side cools slower, it shrinks less. That makes the part curve or bend in that direction.
For parts that are symmetrical in shape, try to keep mold temperatures equal on both sides. This helps the part cool evenly and stay straight.
4. Molding Shrinkage
Shrinkage means the plastic gets smaller as it cools and hardens.
When mold temperature is low, the plastic freezes faster. This forms a thick frozen layer near the surface, and there’s less time for the plastic to shrink—so the overall shrinkage is smaller.
If the mold is hotter, the melt stays soft longer, giving more time for the material to relax and form crystals. That makes the part shrink more.
5. Heat Distortion Problems
For crystalline plastics, molding at too low a temperature locks the molecules into place too soon. Later, if the part is exposed to heat or goes through another process, the molecules may shift around and keep crystallizing. This can cause the part to shrink more or even bend—well before it reaches its rated Heat Distortion Temperature (HDT).
To avoid this, use the mold temperature recommended by the resin maker. It should be close to the material’s crystallization temperature. That way, the part fully crystallizes during molding and won’t change shape later.
Key Takeaways
Controlling mold temperature helps you:
- Improve surface finish
- Reduce internal stress
- Prevent warping
- Control shrinkage
- Avoid heat-related deformation
When you get the temperature right—and keep it stable—you get better quality parts with fewer problems down the line.
Ways to Control Mold Temperature
Getting the mold temperature right is key to making high-quality plastic parts. Here are some practical ways to manage and control that temperature:
1. Separate Cooling for Cavity and Core
The cavity (front half of the mold) and the core (back half) heat up differently. So, they need separate cooling channels. This lets you adjust the temperature of each side to match its heat load. It helps prevent uneven heating or cooling.
2. Use Different Temperatures for Different Sides
Sometimes, using different temperatures for the front and back of the mold—or between the cavity and core—can help stop parts from warping or bending after molding.
3. Use Turbulent Flow in Cooling
To cool the mold more evenly, make sure the water or oil flows in a turbulent way—not smooth and slow. There are two main ways to set up your cooling circuits:
- Parallel setup: Easier flow, but not always even. Some areas may get cooler water than others.
- Series setup: Water moves through one area after another. It can cause temperature differences between the first and last sections.
If you can keep the water temperature difference below 5°C—and ideally under 2°C—you’ll get much better temperature balance across the mold.
4. Arrange Cooling Around the Cavity
The best way to spread heat evenly is to build your cooling layout in concentric circles around the main flow channel. Use smart cavity placement and balanced cooling paths. This helps reduce differences in shrinkage between parts and keeps your production consistent.
5. Use Magnetic Thermometers
Stick magnetic thermometers to the front and back of the mold. Use them to check and log the actual mold temperature often.
6. Go Fully Automatic if You Can
Fully automatic machines use robots to remove parts. That keeps the mold open for the same amount of time each cycle, helping maintain a steady temperature. Manual (semi-automatic) work creates small timing differences that can throw off your mold’s heat balance.
7. Control the Water Temperature
One of the easiest and most accurate ways to control mold temperature is by adjusting the temperature of the cooling water.
8. Use a Hot Runner System
A hot runner system keeps the plastic inside the runner hot. This keeps the mold at a more stable temperature and shortens the cycle time.
9. Heat the Mold with Gas
You can also use gas heating. This is another precise way to heat molds, especially for special molding needs.
10. Insulate the Mold
Use insulation plates or other materials to keep heat from escaping. This helps the mold stay at the set temperature and improves product consistency—especially for parts that need specific sizes or crystal structures.
Here’s what to keep in mind:
- Use separate cooling for cavity and core
- Keep water flow turbulent
- Watch for temperature differences
- Use thermometers to check real temperatures
- Automate if possible
- Try hot runners, gas heating, or insulation if needed
By managing mold temperature carefully, you’ll get more stable parts, better surface quality, and fewer problems during molding.
How to Choose and Control Mold Temperature
Choosing the right mold temperature is important if you want your plastic parts to come out looking good and working well. Different materials and mold designs need different temperatures. Here are the key things you should keep in mind:
Basic Principles
- Use the right temperature for the material.
Each type of plastic needs its own mold temperature. For example, harder-to-flow plastics like PC or PPO need a higher mold temperature. Easier-to-flow plastics like PP, PE, or PA can use a lower one. - Match the mold temperature to the surface design.
Molds with detailed textures or spark finishes may need different temperature control setups than smooth ones. - The front mold should be slightly hotter.
Usually, the front mold is about 2–3°C warmer than the back mold. This helps with better surface finish. - Textured molds need even more heat.
For detailed surfaces like spark patterns, the front mold may need to be up to 40°C hotter if using hot water or oil. - If heat from the plastic isn’t enough, add mold heating.
When designing the mold, check if the heat from the plastic melt is enough. If not, you may need a mold heater.
Important Reminders
- Use water or oil to carry away heat.
Most of the plastic’s heat is removed by the circulating water or oil—not just by radiation or contact. - Keep the temperature even.
Avoid hot or cold spots in the mold. Uneven temperature can lead to defects in your parts. - Stay below the plastic’s heat distortion point.
The mold should not be hotter than the point where the plastic starts to soften or deform. - Thick parts need care.
Avoid using low mold temperatures for thick-walled parts. That can trap bubbles or cause stress inside the part.
Quick Reference Tips
- Higher viscosity plastics (like PC, PSU) → Use higher mold temps
- Lower viscosity plastics (like PE, PP) → Use lower mold temps
- PA + glass fiber → Use 80–120°C mold temps
- Thick parts → Avoid cold molds
When you pick the right mold temperature—and keep it steady—you’ll get better part quality, fewer defects, and more stable production.
Mold Temperature Needs for Different Plastics
Each plastic type needs a certain mold temperature to work well during injection molding. If the mold is too hot or too cold, your parts might not come out right.
Getting the mold temperature right helps the plastic flow better, fill the mold, and cool evenly. It also keeps the part from shrinking or warping too much after it’s molded.
Here’s a quick guide for common plastics and their recommended mold temperatures:
Mold Temperature Guide
- PP (Polypropylene):
40–80°C (50°C is ideal) - PPS (Polyphenylene Sulfide):
120–180°C - PE-HD (High-Density Polyethylene):
50–95°C - PC (Polycarbonate):
70–120°C - PBT (Polybutylene Terephthalate):
40–60°C - PA6 (Nylon 6):
80–90°C for thin or large parts
Over 80°C for glass-filled types - PA12 (Nylon 12):
30–40°C for basic plastic
80–90°C for thin or large parts
90–100°C for glass-filled versions - ABS (Acrylonitrile Butadiene Styrene):
25–70°C
When in doubt, check the material supplier’s datasheet. They’ll list the best temperature range for each plastic.
Conclusion
Controlling mold temperature is a big part of making good plastic parts. If the mold is too hot or too cold, your parts may not turn out the way you want.
When you understand how mold temperature works—and use the right tools to manage it—you can make parts that are stronger, more accurate, and look better.
Mold temperature is one of the first things to think about when designing a mold. It affects everything from how the plastic flows, to how the part cools, to how it holds up over time.
Good mold temperature control helps you:
- Make better quality parts
- Reduce waste
- Run production more smoothly
- Stay consistent from part to part
Once you know the basics, it gets easier to adjust for different materials, part shapes, and mold designs.
If you’re in injection molding, learning to manage mold temperature well is one of the best ways to improve your results.
