Troubleshooting · Defect guide
Brittle plastic parts
First find out when they turn brittle
Brittleness has many possible causes: material degrading in the machine, wet resin, the wrong grade, sharp corners, sunlight and chemicals. This guide starts with the question that rules out the most causes, when the part becomes brittle, then works through checks from cheapest to most expensive and is honest about where additives help.
Marketing Department, ADVANCEDCHEM SOLUTIONS CO., LTD. · 22 October 2026 · about 7 min read
When is it brittle? The question that halves the list
- Brittle straight off the machine: usually a production issue, such as degraded melt from excess heat or long residence, wet hygroscopic resin, a high regrind ratio, or a weld line sitting at a load point.
- Fine when moulded, cracks after storage or assembly: usually moulded-in stress plus a chemical the part touches, or clamping stress from screws and press fits.
- Fine indoors, brittle after outdoor use: UV degradation. The exposed face is often faded or chalky. See UV degradation.
- Cracks only in the cold: usually the grade, for example a PP homopolymer that turns brittle at low temperature.
Look at the fracture surface too. A smooth, glassy break with no stretching means a brittle fracture. White, stretched material around the break means the material is still ductile but the load exceeded what the part was designed for. Note where the crack started: a corner, a screw hole, a weld line or a dark speck in the material. The origin usually points most directly to the cause.
Mechanism: why a tough plastic breaks
Plastics get their toughness from long, entangled polymer chains that stretch and absorb energy before they break. A part turns brittle when one of three things happens: the chains get shorter (from heat, moisture or UV), the material is split into regions that do not bond (incompatible polymers mixed together, a poorly fused weld line), or the load concentrates at one point until it exceeds the material's strength (a sharp corner or notch).
The practical takeaway: if every part breaks in the same place, look at the geometry and weld lines first. If breaks are random or affect a whole lot, look at the material and the process first.
Four groups of causes
1. Part design
Inside corners with no radius, notches, thin walls around screw holes, and a layout that puts the weld line at a load point. Press fits and snap fits that keep the part under stress for its whole life belong here too.
2. Mould
A gate position that puts the weld line where the part is loaded, blocked vents that trap air at the weld, cooling that leaves the part with moulded-in stress, and hot-runner dead spots where melt hangs up and degrades. See Weld lines.
3. Process
Melt temperature too high, long residence because the shot is small for the barrel, restarting after a stop without purging the old melt, holding pressure high enough to lock in stress, and incomplete drying of hygroscopic resins.
4. Material
A grade that does not suit the service conditions, such as a PP homopolymer in a part that must take impact in the cold; a high or repeatedly recycled regrind ratio; recycled pellets containing other polymers; and wet PA, PC, PET or PBT. See Polymer basics for grades and MFI.
Step-by-step checks, cheapest first
- Record when the part turns brittle: off the machine, after storage, after assembly, after sun exposure or only in the cold, as in the first section.
- Find the crack origin. If it starts at the same corner, screw hole or weld line on every part, go to step 6. If it starts at a dark speck or a foreign particle, see Black specks.
- Mould virgin resin only, with no regrind or recycled pellets, and run the same drop or bend test. If parts become tough again, the problem is in the blend.
- Check heat and residence time. Compare barrel temperatures with the resin TDS, check whether each shot uses only a small part of the barrel, and see whether brittleness gets worse after a machine stop.
- For hygroscopic resins, check drying. Confirm the dryer reaches its setting, drying time is complete, and dried pellets are not left in an open hopper.
- Test under real service conditions. If parts break only in the cold, drop-test at that temperature. If they break after chemical contact, apply that chemical to a stressed part.
- Review the grade and the design. Adding corner radii, moving the weld line or changing the grade are the most expensive steps; do them after the other causes are ruled out.
Cause → check → fix table
| Cause | Check | Fix |
|---|---|---|
| Thermal degradation | Compare temperatures with the TDS · worse after machine stops · part MFI higher than pellet MFI | Lower temperature, cut residence time, purge after stops |
| High or repeatedly recycled regrind | Mould virgin only for comparison · step through regrind ratios | Limit and fix the ratio; segregate multi-pass regrind |
| Wet hygroscopic resin | Check dryer and drying time · compare with freshly dried resin | Dry as the resin maker specifies; feed straight from the dryer |
| Grade brittle at low temperature | Drop-test at service temperature | Switch to an impact copolymer or an impact-rated grade |
| Sharp corners, notches | Cracks start at the same corner on every part | Add inside corner radii; rework the mould |
| Weld line at a load point | Short-shot series to locate where flow fronts meet | Move the gate, adjust temperature and speed, clean the vents |
| Moulded-in stress + chemicals | Cracks after assembly or contact with oil or cleaners | Reduce holding pressure, adjust mould temperature, change the contact chemical or the grade |
| UV degradation | Exposed face faded or chalky | Design UV protection for thickness and service life |
| Mixed polymers in recycled pellets | Layered or flaky fracture surface · ask for lot test results | Better sorting; use a compatibiliser where needed |
How much can additives help?
They help with some causes and not at all with others. These are the material options that actually exist, including ones ACS does not sell:
- A different grade or an impact modifier, such as a PP impact copolymer or an added thermoplastic elastomer. These suit parts that break from impact or cold. Stiffness drops somewhat; confirm the part still meets its specification.
- Antioxidants reduce degradation during melting, especially with regrind or recycled pellets that have seen several heat histories. They do not repair chains that are already broken.
- UV stabilisers for parts that turn brittle after outdoor use, such as a UV Masterbatch designed for the part's thickness and service life. See UV degradation.
- Compatibilisers help different polymers mixed in recycled pellets bond to each other. They work when the contaminating fraction is limited and controlled. See Recycled PCR resin.
What additives cannot fix: sharp corners and notches in the design, wet hygroscopic resin, and a grade that never suited the service conditions.
Technical caution
Let-down ratios and technical data on this page are typical reference values, not a guarantee of properties. Confirm by trial in your own process before production use.
FAQ
Brittle part questions we hear often
Why do PP parts crack in cold weather or cold storage?
PP homopolymer turns from ductile to brittle at a much higher temperature than PE, so its impact resistance falls sharply as it gets cold. A part that performs well at room temperature can crack when dropped in a cold room or shipped early in the morning. The most direct fix is a PP impact copolymer or a grade the resin maker rates for low-temperature impact, then a drop test at the real service temperature.
Parts got brittle when we added more regrind. What is the maximum percentage?
There is no single number. Every melt pass cuts some polymer chains and consumes some of the stabiliser in the resin. The effect depends on the resin, how many heat histories it has seen and how hot the barrel runs. Step through regrind ratios and drop-test or break parts against virgin-only parts, then keep the ratio constant every shift. If regrind is recycled many times, a rising MFI is a sign the material has already degraded.
Parts are fine when moulded but crack after a few weeks in storage. Why?
A common cause is moulded-in stress combined with a chemical the part touches, such as oil, a cleaning agent or an assembly lubricant. This is environmental stress cracking. Cracks usually start at corners, screw holes or press-fit points. The other cause is storage in sunlight; if the exposed face is chalky, look at UV degradation.
Our PA or PC parts are brittle but the surface looks normal. Is drying the issue?
Very likely. PA, PC, PET and PBT absorb moisture. If they are melted while wet, water cuts the polymer chains (hydrolysis), and parts can be brittle even without splay on the surface. Dry the resin at the temperature and time the resin maker specifies, check that the dryer actually performs, and do not leave dried pellets in an open hopper long enough to pick moisture back up.
Parts cracking and no clear cause?
Send photos of the fracture surface and where it broke, plus the resin grade, regrind ratio and machine settings, on LINE. Our technical team will help you separate material, process and design causes.