Views: 0 Author: Site Editor Publish Time: 2025-05-20 Origin: Site
Have you ever wondered why some plastic parts last longer than others? The secret often lies in the material used to mold them. Choosing the right plastic mold material is key to part strength, safety, and performance. It also affects cost and production success. This guide is made for engineers, designers, and product creators. It includes real technical insights from industry practice. In this post, you’ll learn how to pick the best plastic mold material for your needs.
The plastic you pick for a mold isn’t just about looks. It decides how strong, safe, and reliable your part is.Let’s say a part carries weight or moves fast—like a gear or a car clip. If the material can’t handle stress, the part fails. Fast.And sometimes, it’s more serious. In life-saving tools or medical devices, a weak material could lead to disaster.Some plastics break when pushed too far. Others just bend. If your part must last under load or heat, you need the right plastic.
Look at this:
| Material | Load Strength | Impact Resistance | Safe for Critical Use |
|---|---|---|---|
| ABS | Medium | Good | Sometimes |
| Nylon 6/6 | High | High | Yes |
| Polycarbonate | High | Very High | Often |
| PEEK | Very High | High | Yes |
It’s not just strength. Some plastics crack when exposed to chemicals or heat. That’s why engineers test parts in real-world settings.Picking poorly can lead to cracking, snapping, or sudden failure—all things you don’t want.
The wrong plastic doesn’t just break. It costs more to fix.If your mold warps or wears out fast, you’ll spend time and money rebuilding it. That means production stops. Orders are late.Some plastics shrink unevenly. Others stick to molds. That means slower cycle times or more rejected parts.Here’s what happens when you choose wisely:
Less warping
Faster molding cycles
Fewer defects
Lower maintenance
Now compare:
| Material | Shrink Risk | Mold Cycle Time | Repair Frequency |
|---|---|---|---|
| PP | High | Fast | Medium |
| ABS | Medium | Fast | Low |
| POM (Acetal) | Low | Medium | Low |
| PC | Medium | Slow | High |
It’s all connected. Choose the right material early, and you avoid headaches later.
Before picking a plastic, you need to know how strong it is—and how it reacts under pressure.
Three key terms help here:
Tensile strength: how much force a material takes before it stretches or breaks
Elongation: how far it stretches before snapping
Modulus: how stiff it is when pulled or bent
But here’s a catch—just because a plastic can stretch to a certain point doesn’t mean it should.Operating a part near its yield point is risky. It may not break right away, but over time? It fails.So, we don’t design around the breaking point. We design below it. That’s how we keep parts safe and working longer.
Some plastics handle short bursts of stress. Others hold up for years.That’s the difference between tensile strength and creep resistance.Creep happens when a plastic slowly stretches under constant load.Even if the stress is low, over time? It adds up.Look at this real data for glass-filled nylon 6/6:
| Temperature (°C) | Tensile Strength (psi) |
|---|---|
| -4 | 36,500 |
| 23 | 30,000 |
| 77 | 17,500 |
| 121 | 12,500 |
See the drop? As heat rises, strength drops fast.That’s why it's key to check how a material behaves over time—not just at room temp.
Plastics age. They get weaker as heat and oxygen do their thing.We measure this using something called the Relative Thermal Index (RTI).It shows how long a plastic stays strong at certain temperatures.A higher RTI? Longer life. Lower RTI? Faster breakdown.There’s also a rule of thumb:Every 10°C increase in heat doubles the rate of aging.So if a part lasts 8 years at 140°C, it might only last 4 years at 150°C.Big deal in automotive, aerospace, or electronics.
Modulus tells you how stiff a material is.But modulus isn’t fixed—it changes when heat shows up.
Take a look:
| Polymer Type | Room Temp Modulus | Modulus at 120°C |
|---|---|---|
| Amorphous (PC) | High | Drops sharply |
| Semi-crystalline (Nylon 6) | High | Still usable |
Amorphous plastics (like PC) lose stiffness fast when heated.
Semi-crystalline plastics (like Nylon) hold on longer.
If you're building a part that has to carry weight while hot?
Go semi-crystalline. It holds up better.
Pull a plastic slow... it stretches more. Pull fast... it resists more.That’s strain rate sensitivity.This affects how we test and compare materials.If two suppliers use different strain rates? Their data won’t match.So when checking datasheets, always look at the test conditions.If they’re not the same, the numbers don’t tell the full story.
This one’s sneaky. A part looks great. Works fine. Then suddenly—crack.ESC happens when stress and chemicals team up.Even mild cleaners or oils can trigger it.
Check this out:
| Time (hours) | Working Stress @ 23°C | PC (no agent) | PC (with crack agent) |
|---|---|---|---|
| 0.01 | 9,200 psi | 9,200 | 7,900 |
| 10 | 8,080 psi | 8,080 | 6,050 |
| 10,000 | 7,100 psi | 7,100 | 4,200 |
Just adding a stress crack agent drops the strength by 40% over time.If your part touches chemicals? Choose a plastic that can resist ESC.
ABS is everywhere—phone cases, remote controls, LEGO bricks. It’s tough, easy to mold, and holds its shape well.But it’s also notch-sensitive. If there’s a sharp corner or cut, it can crack under stress.Over time, and under constant load or heat, its max stress drops. Add chemicals? It gets worse.Still, for low-cost consumer products, ABS is a solid option.
Key Points:
Good for cosmetic parts
Mild chemical resistance
Avoid sharp notches or high long-term stress
PC is tough and clear. It’s used in safety glasses, medical gear, and even bulletproof windows.It handles impact like a champ. And it stays strong in heat. But there’s a catch.It hates UV light. It doesn’t like chemicals either. And under constant flexing? It tires out.Stress cracking is a real concern—especially in harsh environments.
Ideal For:
High-impact parts
Optical lenses
Indoor electrical housings
Avoid If:
The part will be outdoors long-term
It’s exposed to oils or cleaners
Nylon’s a workhorse. It’s strong, slippery, and wears well.Used in gears, clips, bushings—pretty much anything that moves.But heat changes its game. Here's what happens at higher temps:
| Temp (°C) | Tensile Strength (psi) |
|---|---|
| 23 | 30,000 |
| 77 | 17,500 |
| 121 | 12,500 |
It also absorbs water. That means swelling, warping, or shifting tolerances.So, you’ve got to dry it before molding—and design for shrinkage.
Use It For:
High-load parts
Moving mechanical components
Under-the-hood applications
PBT and PET are strong, stable, and great for electrical use.They also resist oils and fuels, which makes them perfect for cars and appliances.But they don’t like acids or bases. Thin parts can warp fast.Glass-filled grades are stronger—but also more likely to twist if cooled too fast.
Best Suited For:
Connectors
Appliance parts
Automotive housings
Be Careful Of:
Thin-wall designs
Harsh chemicals
High shrinkage zones
These are the top-tier plastics. Expensive, yes—but when failure isn’t an option, they’re worth it.PEEK can survive in boiling water or near flames. PEI (Ultem) handles high heat and keeps its shape.PPSU? Great for medical tools that need to be sterilized over and over.They resist aging better than most. Check out how RTI plays a role:
| Material | RTI (°C) | HDT (°C) | Notes |
|---|---|---|---|
| PEEK | 160 | 160–170 | Excellent long-term stability |
| PEI | 150 | 170–180 | Slightly lower than PEEK |
| PPSU | 140 | 200+ | Great for sterilization cycles |
Use These Plastics For:
Aerospace and medical parts
Steam or chemical exposure
High-precision, high-value components
Just know—they’re not cheap. But they’re tough, stable, and built for tough jobs.
When you're picking the best mold material, the datasheet is your best friend—but only if you know what to look for.
Here’s a quick breakdown of what matters most:
Tensile strength — how much pulling force a plastic takes before it breaks.
Modulus (tensile or flexural) — how stiff it is under stress.
Elongation — how far it can stretch before snapping.
Vicat softening point — the temp where it starts to soften under pressure.
HDT (Heat Deflection Temp) — the temp where it begins to bend under load.
Creep data — how it deforms over time under constant stress.
Time-to-failure curves — how long it lasts under load at different temperatures.
Look for these values, especially if your part will deal with heat, weight, or time.
Let’s look at a real case: polycarbonate (PC).It’s strong, but tricky under long-term stress or heat—especially when chemicals are nearby.Check out this chart:
Table: PC Working Stress Over Time
| Time (hours) | Stress @ 23°C (psi) | Stress @ 60°C (psi) |
|---|---|---|
| 0.01 | 9,200 | 7,200 |
| 1 | 8,450 | 6,850 |
| 10,000 | 7,100 | 5,100 |
Now add a stress-cracking agent like oil or cleaner:
| Time (hours) | PC + Agent @ 23°C | PC + Agent @ 60°C |
|---|---|---|
| 0.01 | 7,900 | 5,050 |
| 1 | 6,800 | 3,150 |
| 10,000 | 4,200 | 1,200 |
Big drop, right?If your part deals with heat, chemicals, or long-term stress—you can't skip this kind of data.Now check how stiffness changes when heat goes up:
Modulus vs. Temperature (Simplified View)
| Material Type | Room Temp Modulus | High Temp Modulus |
|---|---|---|
| PC (Amorphous) | High | Drops fast |
| Nylon 6 (Semi-crystalline) | High | Holds better |
This tells you: don’t just look at one number.Check how it performs over time and temperature.
Need a plastic to handle brief heat spikes?Use HDT and Vicat softening point to guide you.These numbers show how the plastic bends or softens under short heat exposure.
| Material | HDT (°C) | Vicat (°C) | Use Case Example |
|---|---|---|---|
| ABS | ~95 | ~105 | Consumer electronics |
| PC | ~135 | ~150 | Safety gear, lenses |
| PEEK | ~160 | ~170 | Aerospace, medical |
Short-term = minutes or hours. Perfect for fast molding or brief high-temp use.
If your part needs to last for years—think ovens, hot water, engines—you’ll need to check the RTI.RTI stands for Relative Thermal Index, and it’s tested to predict how long a material stays strong under heat.Here’s a rule of thumb engineers use:
Each 10°C rise above RTI cuts lifespan in half.
So if RTI is 140°C:
At 150°C → 4 years
At 160°C → 2 years
At 170°C → 1 year
That’s why RTI matters more for long-life parts.
| Material | RTI (°C) | Good For |
|---|---|---|
| PEEK | 160 | Long-life, high-heat parts |
| PPSU | 140 | Sterilized tools, hot water |
| PEI | 150 | Electrical & medical housings |
Always use RTI over HDT when planning for long-term exposure.
If you're an engineer, guessing isn’t an option. You need data—and lots of it.Start with a full 3D model. Plug it into simulation software.Check how your part behaves under real loads, heat, and pressure.
Use tools like:
FEA (Finite Element Analysis) for stress and strain
Moldflow for melt behavior and cooling
Creep and fatigue models for long-term use
Why? Because two materials can look similar on paper—but one might crack under load.
Simulation shows that before you waste time or money.
Also check:
RTI for long-term thermal use
Modulus vs. temperature curves
Chemical compatibility for the working environment
Quick Checklist for Engineers:
| Step | Why It Matters |
|---|---|
| 3D CAD model | Accurate stress paths |
| Load case definition | Real-world pressure and forces |
| Heat impact simulation | Avoid part warping or creep over time |
| Material database selection | Match exact specs, not just generic resin |
| Prototype testing | Validate before final tooling |
If failure isn’t an option—go deep, simulate it, then decide.
Not every project needs a full simulation.Sometimes, you just need something that works most of the time.Here’s a simple material guide if you're not running stress tests:
Quick Pick Material Guide:
| Need | Try This Material |
|---|---|
| General purpose | ABS |
| Cheap and simple | PP |
| Strong and tough | PC |
| Looks clear and clean | PMMA |
ABS: Great all-rounder. Easy to mold, strong enough for most jobs.
PP (Polypropylene): Lightweight, cheap, chemical-resistant. Good for toys, containers, and lids.
PC (Polycarbonate): Handles hits. Use it when parts need to be tough—like tool housings or covers.
PMMA (Acrylic): Looks like glass. Use it when you want shine, clarity, or clean edges.
But even if you're skipping the math—test it first.
Try this:
Make a few parts using CNC machining or 3D printing
Hit them. Bend them. Drop them.
If they break? Try the next material
Prototyping before molding helps avoid costly mold changes later.Don’t guess once your mold is made. That’s the expensive part.
Not all plastics shrink the same. Some pull away from the mold more than others.Some warp when they cool unevenly. That messes up tolerances and fit.If your part needs tight dimensions, you must know how much the plastic shrinks.A small error in shrink rate? It can make holes too tight or clips too loose.You can't just swap materials in the same mold.Each resin has a different shrink percentage. One mold = one resin type.Typical Shrink Rates by Material:
| Material | Shrink Rate (%) |
|---|---|
| ABS | 0.4 – 0.7 |
| Nylon 6/6 | 1.0 – 1.5 |
| Polypropylene (PP) | 1.5 – 2.5 |
| Polycarbonate (PC) | 0.5 – 0.7 |
| Acetal (POM) | 1.8 – 2.1 |
Biggest issue? Warping.Thin walls, uneven cooling, or glass-filled plastics twist if not designed right.What helps:
Uniform wall thickness
Radii instead of sharp corners
Predictive warpage simulation in CAD
Design it wrong, and your part won’t fit—even if the mold is perfect.
Mold temperature isn’t just a setting. It changes everything.Too cold? The part sticks, sinks, or warps.Too hot? Cycle time stretches, and costs go up.Each plastic has its sweet spot. Go outside that range, and problems stack up.Let’s compare two common materials:
ABS vs. PC Molding Temperatures
| Property | ABS | PC |
|---|---|---|
| Mold Temp Range | 40–80°C | 80–120°C |
| Ideal Mold Temp | ~60°C | ~100°C |
| Cycle Time | Shorter | Longer |
| Surface Finish | Good | Excellent |
| Warping Risk | Medium | High (if cooled fast) |
ABS is more forgiving. PC needs heat to look good and avoid stress issues.Tips for better molding:
Match mold steel to resin shrink
Use heated molds for PC or PMMA
Keep mold temperature stable—avoid spikes
Use cooling channels to balance heat across the tool
Choosing the right mold temp can reduce defects and speed up production.
A: PEEK or PPSU. They resist high heat, chemicals, and mechanical stress—ideal for aerospace or medical parts.
A: Not recommended. Each plastic has its own shrink rate and flow behavior. One mold won’t fit all.
A: HDT shows when plastic bends under load. Vicat tells you when it starts to soften. Vicat is usually higher.
A: It depends on RTI and creep data. Yield strength isn’t enough—check how it performs over time and temperature.
A: Environmental stress cracking. Stress plus chemicals weakens parts over time, even if loads seem low.
A: Fast loading makes plastics seem stronger. Always compare data tested at the same strain rate.
A: Both matter. Modulus shows stiffness. Tensile strength shows break point. Use both to predict performance.
There’s no one best plastic for every mold. It depends on strength, heat, chemicals, and cost.Use data sheets, RTI, and modulus curves to guide your choice—especially for parts that must last. For complex or high-risk parts, talk to an expert. And always prototype before making the final mold.
