Brittleness
Why molded parts break more easily than expected.
BrittlenessProcess guide
Annealing Molded Parts
A molded part can pass inspection and still crack at a boss a week later, or shrink out of tolerance after its first hot day. In both cases, stress frozen in during molding is looking for a way out. Annealing lets it out under control.
Where stress comes from
Molding freezes the plastic before it settles
Every molded part carries some stress. The question is whether it matters for the part.
Resin is pushed into the mold under high pressure, stretched as it flows, and cooled fast against the steel. The polymer chains are frozen in whatever stretched, packed arrangement they were in at that moment. That leftover strain is molded in stress. It is highest near gates, at sharp corners, and where thick sections meet thin ones.
Stress shows up in two ways. The part may crack when it meets a chemical, a solvent, or a screw, because stressed areas give way first. Or it may slowly change shape as the chains relax on their own, which is especially noticeable when the part gets warm in service.
By resin type
What annealing does for different plastics
The same oven cycle does different jobs depending on the resin.
| Resin type | Examples | What annealing achieves |
|---|---|---|
| Amorphous | Polycarbonate, acrylic, polysulfone, PEI | Relaxes molded in stress so parts resist cracking from solvents, cleaners, and fasteners |
| Semi crystalline | Nylon, acetal, PBT, PEEK | Finishes crystallizing that fast cooling cut short, so size and stiffness stop drifting later |
| High performance semi crystalline | PEEK and PPS in particular | Brings parts to full crystallinity so they hold up at the temperatures they were chosen for |
The cycle
How an annealing run is done
Temperatures and times come from the resin supplier's guidance for the specific grade.
- 1
Fixture if needed
Parts that could sag or warp are supported in fixtures that hold their shape while warm.
- 2
Heat slowly
The oven brings parts up to a temperature below the point where they soften, without shocking thick sections.
- 3
Hold
Parts soak long enough for heat to reach the thickest wall and for the chains to relax or crystallize.
- 4
Cool slowly
Controlled cooling, often inside the oven, so new stress is not created on the way down.
Limits
What annealing cannot fix
Annealing relieves stress. It does not repair a part that was designed or molded badly.
- A part that is already warped may warp further as stresses release, unless it is fixtured
- Sharp internal corners still concentrate load after annealing; the fix is rounding the corner in the steel
- Degraded or wet molded resin stays brittle
- Semi crystalline parts shrink a little as they crystallize, so critical sizes should be checked after annealing, not before
Signs
When a program should consider annealing
Try process and design changes first. Annealing is added when those are not enough.
Clear parts crack or craze after contact with alcohol, cleaners, or adhesives
Parts go on to solvent bonding or vapor polishing
Nylon or acetal parts change size after weeks in storage or in a warm environment
PEEK or PPS parts will run near their temperature limit
Metal inserts or press fits load plastic that already carries stress
Keep reading
Related guides
The defects stress causes and the processes that expose it.
Dimensional variation
Parts that drift out of size.
Dimensional variationSolvent bonding
A process that finds stress quickly.
Solvent bondingVapor polishing
Another solvent step that can craze parts.
Vapor polishingPEEK
Crystallinity decides its heat performance.
PEEKNylon
Moisture and crystallinity both move its size.
NylonProcesses hub
Every molding process and secondary operation.
Processes hubFAQ
Questions about annealing molded parts
Can molded in stress be seen?
In clear amorphous parts, yes. Between two polarizing filters, stressed areas show as colored bands. It is a quick check for gates and corners on clear parts before and after annealing.
Can the molding process reduce stress instead?
Often. A warmer mold, slower fill, lower pack pressure, and better gate placement all reduce stress. Annealing is added when those changes are not enough or would cost too much cycle time.
Is annealing the same as moisture conditioning nylon?
No. Conditioning adds water so nylon reaches its in service toughness. Annealing uses heat to relax stress or complete crystallization. Some nylon parts get both.
Does annealing change color?
It can slightly yellow some resins, especially at the high end of their range. Color critical parts should be checked after an annealing trial.
Next step
Tell us where parts crack or drift
Upload CAD with the resin and where cracking or size drift shows up. The review looks at gate position, corner radii, and wall changes that cut stress in the mold. NetProto manufacturing is ISO 9001 certified.