What Is the Typical Tolerance for Injection Molding?
Your CAD model says 25.00 mm. Your first molded parts come back at 24.93 mm. Is that a problem, or is it normal? The answer depends on the tolerance you…
A part passes inspection and ships out on time. Weeks later, it cracks after a solvent wipe or a tightened bolt. Nothing was wrong with the design. The stress was locked in as the part cooled.
Environmental stress cracking is the leading cause of field failures in plastic parts. It shows up most often in amorphous plastics, such as polycarbonate. The good news is that much of this stress can be taken out before a part ever ships.
This guide explains the process of annealing plastic, step by step. You’ll learn what it does to a part and when your parts need it. We mold and machine plastic parts, so we’ll keep it practical.
First, we’ll show why stress forms during molding and machining. Then we’ll walk through the five steps, plus temperatures and methods for common plastics. After that, we’ll cover the benefits. We’ll finish by helping you decide if your part needs annealing at all.
Annealing plastic is a controlled heat treatment that relaxes stress locked into a part during molding or machining. The process has five steps:
Done right, annealing lowers the risk of cracking. It also keeps parts stable in size and helps them hold up to chemicals.
Annealing takes out stress that builds up inside a part while it’s being made. To see why that matters, it helps to know where the stress comes from.
When hot plastic fills a mold, it starts cooling right away. The outer skin sets first, while the core is still soft. As the core cools and shrinks, it pulls against a shell that’s already firm. That pull stays locked inside the part.
Machining adds stress too. Cutting tools create heat and push on the plastic as they remove material. Both can leave strain behind in the finished part.
You often can’t see this stress when a part is new. It shows up later as:
Annealing heats the part and holds it below the point where it would soften or melt. That gives the plastic’s molecules time to relax into a calmer state. When the part cools slowly, much of the locked-in stress is gone.
Some of these problems can also start in the mold itself, which we cover in our guide to common plastic molding defects. Now that you know where the stress comes from, here’s how annealing takes it out.
Every annealing cycle follows the same five steps, whether the part was molded or machined. Each step controls how heat moves into and out of the part.
The steps stay the same, but the numbers change with every plastic. Next, we’ll look at those numbers by resin type.
There’s no single annealing temperature for all plastics. Each resin has its own softening or melting point, so each one needs its own soak temperature. Wall thickness then sets how long the part stays there.
For most materials, a common guide for plastic stock shapes is about 30 minutes per 1/4 inch of thickness. Some resins use their own formula. Polycarbonate needs at least 30 minutes at temperature, plus 5 minutes per millimeter of wall. So a 3 mm polycarbonate wall would soak for about 45 minutes.
| Resin | Reference Temp | Soak Guide | Source |
| Polycarbonate (PC) | 121–135°C (250–275°F) | 30 min minimum + 5 min per mm of wall | Plastics Technology |
| ABS | Heat deflection temp at 264 psi, about 77–116°C (170–240°F) | 1–2 hours | Despatch Industries |
Thick sections require additional soak time. For polycarbonate walls thicker than 6 mm, the soak time is typically doubled. These are reference ranges, not settings for your part. Always confirm temperature and time with the resin maker’s data sheet for your exact grade.
High-performance plastics can take far longer. Polyamide-imide (PAI) is heated in stages over several days. The final stage alone can run 3 to 10 days.
Your resin choice shapes the whole cycle. If you’re still picking a material, get help choosing the right resin before tooling is built.
There’s more than one way to deliver the heat. The right method depends on how many parts you need and how fast you need them.
| Method | Speed | Best For | Drawback |
| Batch oven | Slower | Short runs | Each load waits for a full cycle |
| Conveyor hot air | Faster, continuous | Production runs | Built for steady, higher volume |
| Infrared | Fastest | Quick surface heating | Shadowed areas can heat unevenly |
| Oil bath | Varies | Very even heating | Pollution controls and costly cleanup |
Batch ovens are the most common method. Parts sit on racks, and the whole load runs through one cycle. It’s slower, but it suits short runs well.
Conveyor systems move parts through zones of forced hot air. Parts go in and come out in a steady stream. That makes them a good fit for production volumes.
Infrared is the fastest method, but shadowed areas of a part can heat more slowly. Oil baths surround the part with even heat. They come with a catch, though. They need pollution controls, and cleaning the parts afterward is costly.
Warm water baths are also used to moisture-condition nylon, which we cover below. For low-volume work, a batch oven is usually the simple choice. For production runs, a continuous method can cut the time spent on each part.
Annealing pays off most when a part has to hold up over time. It does have limits, though. It can change a part’s size, and it can make semi-crystalline plastics, like polypropylene, stiffer. Here’s what you can expect it to improve.
Stress cracking happens when tensile stress in a part meets a chemical. Neither one alone is usually enough to crack the part. That’s why a part may look fine for weeks, then crack after its first contact with a cleaner.
Crazing is often the early warning, with fine marks forming near the surface. Annealing lowers the stress those cracks start from. With less stress inside, it takes more to push the part past its limit.
Cleaners, solvents, and oils tend to attack where stress is highest. That’s why a part may crack right after a wipe-down. Extra stress from threads, fasteners, or machining can make it worse. Taking that stress out helps your part stand up to the chemicals it meets in daily use.
Locked-in stress tries to release on its own. As it does, a part can slowly shrink, warp, or drift out of tolerance. Annealing lets that movement happen in a controlled way, before the part ships.
That matters for parts that mate with other parts or form a seal. There, a small shift can mean a loose fit or a leak.
Plastic stock shapes are usually stress-relieved by the maker before you buy them. For very tight tolerances, parts are often rough-machined, annealed, then finish-machined. That way, stress from the first cuts doesn’t pull the finished part out of spec. Few machined parts need another anneal after the final cuts.
So does your part actually need this step? Not every part does. It depends on the resin, the shape, and what the part faces once it’s in use.
| Good candidates | Usually skip |
| Polycarbonate, acrylic, or ABS parts that will touch chemicals or cleaners | Most polypropylene (PP) parts |
| Tight-tolerance machined parts | Low-stress parts with no chemical contact |
| Parts with thick walls | |
| Parts with metal inserts | |
| Parts that see temperature swings |
Here’s a common case. A clear polycarbonate or acrylic part looks perfect, then crazes after contact with a cleaner or solvent. Molded-in stress alone can be enough to cause this, and annealing insert-molded polycarbonate parts greatly improved their resistance to stress cracking.
Annealing also has costs. Each cycle adds time before parts can ship. Oven space is limited, so large runs may need to go through in batches. All of that adds to the cost of each part.
Annealing is not the same as moisture conditioning. Nylon absorbs water from the air, and its size and stiffness change as it does. Conditioning brings nylon parts to a steady moisture level, using a humidity chamber or warm water bath. It’s a separate step, and some parts need one, the other, or both.
Heat and moisture both affect a part’s final size. Plan them alongside how much plastic shrinks after molding, so your part lands where your print says it should.
At Freeform Polymers, one local team in Logan handles molding, machining, annealing, and assembly. Your parts don’t have to ship to another shop for heat treatment. That cuts down on handling and keeps your schedule in one place.
We plan annealing during design review, not after problems show up. Our annealing and conditioning process starts with a check of your resin, wall thickness, geometry, and any inserts. Then our team picks the method, temperature, and dwell time. ABS, polycarbonate, nylon, and acetal each have their own target range.
Annealing often pairs with post-mold machining and trimming. When a molded part needs machined features, annealing can come before or between those steps. That helps the finished part hold its tolerances.
Businesses across Utah and even beyond rely on us as a local molding partner. If your parts crack, craze, or drift out of tolerance, talk to our team about your part. You can also call us at (435) 774-9090.
