Hello, I would like to ask if any of you experienced cracking of PC part after few days. Part is made of polycarbonate and there is a small rectangular shaped part. whole part is about 50×50 mm. Critical rectangular part is 5x7mm. Wall thickness af whole part is cca 1mm. During assembly of electrical motor this critical part is bend down by 5° and hold the motor in proper place. After assembly the bend is 3-4°. The scrap rate is about 10%. Do you have any ideas why it does not crack immediately but later? The issue occurs the first at the moment but I am worried about future lots. Process settings are more or less the same. All proposals are welcome.

You must check the crack location, if it is in a weld line portion, you must improve your molding parameters or the tooling condition. You must check also the degradation of your material, the residence time of resin in the screw cylinder must be in a short timing. You can use the melt flow analysis to check the degradation of your end products. It must be less than 20% of your raw materials.

Hi,
Only knowing that your article is quite small, need more tooling detail for better advise:
1) How many cavities and cavity positioning in the tool?
2) What is the machine tonnage?
3) Need the parameter for the speed and screw positioning, such back and injection pressure, machine barrel or hot runner temperature setting
4) Actual tool temperature.
5) Is there sub inserting at bottom of article for air releasing?
6) Pre-heating of resin: temperature and period?
Thanks.

Jiri, One of the typical causes for your issue results from moisture in the PC. I suggest that you check the material delivery steps and verify if the PC moisture content at proper levels.

Gents, thanks for quick reactions. Basically we have this problem by our supplier and we are trying to find a root cause.The crack is not in a weldline area. If material is degraded by cylinder residence time you will probably see white lines by the gate area and if caused by regrind the whole lot or more parts than 10% should be infected. The crack has marks of brittle failure.
The mold has 1 cavity
Machine is about 150 tons
As I check the process temperatures they we according to recommended parameters (by resin supplier).
Venting is fine. There are no "burn marks". Of course It might be drying but in that case you should see silver lines which are specific for this kind of failure.

If there would be a small notch in the cracked area it might a problem. But if it is there for years why would cracking occurs after couple days and why not before but now in few lots. The parts are stored in room temperature.

over packing the part, causing internal stress within the polymer chains.

Hi Jiri, You may be at the limits of strength for the material. Just not enough to imediately break, but very close. Also, if any hydrocarbon based chemical comes in contact with the PC plastic, that will weaken the plastic strength. Try another material like acetal (POM) that has a very good tensile strength and yield properties - plus it resists chemical attack. Try 30 samples and see if that fixes the issue.

Best of luck,
Kirk

Hello Kirk,
I was also thinking about chemical reaction which where PC can partially lose its properties. I have been told that such chemicals should not be present during molding by supplier or at our shopfloor during assembly. But I can try to ask once more.