Brittle and Cracked Extruded Plastic Profiles: Causes, Diagnosis and Fixes
A profile that snaps during cutting, splits at a punch hole, or cracks after a winter shipment is not always a material problem. In extrusion, brittleness has four separate origin points - the compound, fusion in the barrel and die, cooling and haul-off stress, and the profile geometry itself. Each one leaves a different signature on the broken part, so identifying which of the four you are looking at is what determines whether the fix sits with the formulation, the extrusion line, or the drawing.
This page covers extruded profiles and tubing. Injection-moulded parts fail brittle for a partly different set of reasons - gate shear, ejection stress, weld lines - and those are treated in a separate section at the end.
How to tell which kind of brittleness you have
| What you observe | Most likely origin | Where the fix sits | |
|---|---|---|---|
| 1 | Clean, glassy fracture face with almost no deformation; snaps at the saw or punch | Under-fused compound, or filler loading too high for the impact modifier level | Compound / extrusion process |
| 2 | Part is fine at room temperature, cracks in cold storage or winter installation | Impact modifier level or grade unsuited to the service temperature | Compound selection - specify a service temperature range |
| 3 | Yellowing, dark specks or streaks together with the brittleness | Thermal degradation from over-shear or insufficient stabiliser | Extrusion process |
| 4 | Cracks start at a punch hole, screw boss or sharp internal corner | Stress concentration in the profile geometry | Drawing / DFM |
| 5 | Outdoor part becomes brittle after months in service | UV degradation, or plasticiser migration in flexible PVC | Material specification - UV package or a different polymer |
| 6 | Cracks appear along the extrusion direction only | Residual haul-off and cooling stress, frozen-in orientation | Extrusion line setup |
Two of these - geometry and service environment - are decided before the tool is cut. The other four are supplier-side and can be corrected without changing your drawing.
Use extrusion process parameters and the tests that catch under-fusion to discuss process-side checks with your supplier. If the failure appears only in cold service, start with choosing an extrusion material for the service temperature. When cracks begin at corners, holes or wall transitions, review the profile design rules for corners and wall transitions before changing the compound.
Five questions to put in your RFQ
- Which polymer grade and which impact-modifier family is in the compound, and at what nominal loading range?
- What filler loading is used, and has it been validated against the impact requirement - not just the dimensional one?
- What is the lowest service temperature the profile has been validated for, and by what test?
- Is regrind used, at what percentage, and from which source?
- If the profile will be punched, drilled or bent after delivery, has a sample been tested in that condition rather than as-extruded?
Answers to 1–4 tell you whether the supplier controls its own compound or buys it blind. Question 5 catches the most common late failure: a profile that passes incoming inspection and then splits during your assembly.
For comparison: why injection-moulded parts turn brittle
If your part is moulded rather than extruded, the origin points shift. Gate and runner shear, ejection force, weld-line strength and metal-insert expansion have no equivalent in a continuous extrusion process, while haul-off stress and die fusion have no equivalent in moulding. The sections below cover the moulding side.
◆Burning (carbonization) and solutions
Brittleness and solutions
After injection molding, the impact resistance of the plastic part may decrease significantly compared with the raw material. This phenomenon is called brittleness. Excessive internal stress is one direct cause, but material degradation, weak weld lines and part geometry may also contribute.
The causes of brittle injection-moulded parts and corresponding solutions are as follows:
Material
① Set appropriate drying conditions before injection molding. If plastics are dried continuously for several days or at excessively high temperatures, although volatile substances can be removed, it can also easily lead to material degradation, especially for heat-sensitive plastics;
② Reduce the use of recycled materials and increase the proportion of virgin materials;
③ Select suitable materials, choosing high-strength plastics.
Mold Design
Increase the size of the main runner, branch runners, and gates, and avoid sharp corners in the runners. Too small a size of the main runner, branch runners, or gates, as well as sharp corners, can easily lead to excessive shear heat, which can cause polymer decomposition.
Injection molding machine
Choosing a suitable screw ensures more uniform temperature distribution during plasticization. Uneven material temperature can lead to excessive heat buildup in localized areas, causing material degradation.
Process conditions
① Reduce the temperature of the barrel and nozzle if the material shows signs of thermal degradation;
② Reduce back pressure, injection pressure, screw speed, and injection speed to reduce excessive shear heat and prevent polymer decomposition;
③ If the brittleness is caused by insufficient weld-line strength, the weld-line strength can be improved by increasing the melt temperature and injection pressure within the material supplier's processing limits;
④ Reduce mold opening speed, ejection speed, and ejection pressure.
⑤ Part design: the sections through which the melt flows should not have excessively thin walls. Avoid sharp corners, inserts, notches, major thickness transitions and other structures that are prone to stress concentration.
Cracking and crazing in injection-moulded parts
Cracking means visible fractures in the plastic part. Crazing is a fine stress-whitening or microvoid pattern that can precede a crack; the two terms are related but are not interchangeable. Excessive holding time, improper gate shape or position, excessive injection or holding pressure, forced ejection, internal stress and molecular orientation can all produce these defects. The specific analysis is as follows.
The specific analysis is as follows:
The residual stress is too high
External forces cause residual stress concentration
Generally, these types of defects occur around the ejector pins. When such defects appear, the ejection device should be carefully inspected and adjusted. The ejector pins should be placed at the locations of greatest demolding resistance, such as bosses or reinforcing ribs. If the number of ejector pins cannot be increased due to limitations in the ejection area, multiple smaller ejector pins can be used. If the draft angle of the mold cavity is insufficient, scratches and wrinkles may also appear on the surface of the plastic part.
The coefficients of thermal expansion of the molding material and the metal insert differ
Metal inserts should be preheated, especially when surface cracks in the plastic parts occur immediately after startup, as this is mostly caused by the inserts being too cold. Furthermore, when selecting insert materials, materials with coefficients of linear expansion close to those of the plastic should be used whenever possible.
When selecting molding materials, high molecular weight plastics should be used whenever possible. If low molecular weight molding materials must be used, the plastic thickness around the insert should be designed to be thicker.
Improper or impure raw materials
Select the molding material according to the part's mechanical and service requirements. During production, take special care to avoid mixing incompatible plastics such as polyethylene and polypropylene, because contamination can cause weak interfaces and cracking. Release agents are also foreign substances in the molding system; improper use can contribute to defects, so their dosage should be minimized.
The plastic part has an unreasonable structural design
Sharp corners and notches in the shape and structure of plastic parts are prone to stress concentration, leading to cracks and breakage on the surface. Therefore, external and internal corners should be rounded with the largest practical radius permitted by the part's function and manufacturing constraints.
Cracks in the mold are reflected on the surface of the plastic part
During injection molding, the mold is repeatedly subjected to injection pressure, causing fatigue cracks to develop on sharp edges within the cavity, particularly near cooling holes. When cracks on the mold cavity surface are reflected onto the plastic part surface, the cracks on the part surface appear continuously in the same location with the same shape. When such cracks appear, the corresponding cavity surface should be checked for identical cracks. If the cracks are due to reflection, the mold should be repaired by machining.
PS and PC products may be prone to cracking under certain processing and part-design conditions. Where the material supplier recommends it, annealing may reduce cracks caused by excessive internal stress.
Causes and solutions for cracks
| Cause Analysis | Solution | |
|---|---|---|
| 1 | Injection pressure is too large or the injection speed is too fast | Reduce the injection pressure or reduce the injection speed |
| 2 | Holding pressure is too high or holding time is too long | Reduce the holding pressure or shorten the holding time |
| 3 | Melt temperature or mold temperature is too low or uneven | Increase the melt temperature or mold temperature within the material supplier's processing limits and correct uneven heating |
| 4 | Gate is too small, or its shape and position are improper | Enlarge the gate or change the gate shape and position |
| 5 | Release slope is insufficient, mold surface is not smooth or there is ejection resistance | Increase the release slope, repair the mold surface and eliminate ejection resistance |
| 6 | Too much or too little material is held in the barrel | Adjust the shot size and barrel residence conditions to the machine and material recommendations |
| 7 | Ejection speed is too fast | Reduce the ejection speed |
| 8 | Metal insert temperature or dimensional fit is unsuitable | Preheat the metal inserts where appropriate and check the insert fit |
| 9 | The proportion of reclaimed material in the feedstock is too large or uncontrolled | Reduce or eliminate reclaimed material and control its source |