We have a part that is molded from Low Density Polyethylene (LDPE) with added black color, and we have been using this part for almost 1-1/2 years.
We recently had a few pieces (2-3%) crack after assembly, while in storage (not installed, still in the package). The parts are molded flat and are slightly stressed once installed in an arced position, but not excessively. The cracking was always located in the weld/knit line and therefore caused by weakness in the bonding of the plastic at that line. We originally thought it may be caused by a silicone release agent that was occasionally sprayed on the injection mold, but testing with minimal agent and later built-in slip agent showed no change. We then moved the gate and therefore eliminated the weld/knit line. There is now only one line, located at the opposite end of the part. This line existed before along with the weak line, but it is now the only line and has never been weak.
Although we have fixed the problem, we are concerned that we have not defined why the weld line was weak and therefore cannot guarantee that it will not occur again. Research tells us that the temperature of the injection mold and plastic can cause this issue, but only in general terms and with no specifics. A lab analysis showed no defects, degradation, or contamination in the material, even at the crack location. Other than affecting the location of the weld line, the geometry of the part does not affect the cracking and does not have any excessive stress risers (grooves, holes, etc.). Per ASTM D1693, we have tested for internal stresses and could not get a crack or slit to propagate.
We did test paneling the part after receiving them, and the part bent/warped due to the release of internal stresses.
Now that the gate has been moved, the problem (cracking) has not appeared, but we would now like to test 100% of existing and old parts to see if we can weed out any parts that will crack…but we do not have a reliable and simple way to test every part. We have been assembling parts and letting them sit for 5 days. Old parts show 2-3% crack, new parts don’t crack. We are not sure 5 days is enough. We have also exposed a small number of parts to Igepal detergent per ASTM testing, but none cracked (old and new parts). I am continuing to work with the vendor and resin supplier, but are unable to define the definitive cause and/or find a bullet proof test method! All the theories presented are great, but I have found no easy way to positively identify the true cause or test the existing parts. We will be molding more parts and manipulating the conditions to reproduce the failure.
We spoke with a local injection molder who is very experienced and produces high end plastic parts for the medical industry. When they examined the parts, they noted the flow path, how it creates and positions the weld line, and was almost sure that the flow was trapping air in the part, at the weld line. There was a very slight discoloration along the weld line, but no voids or bubbles in the part. The air is compressed at the weld line and weakens the line. Several mold design features can be incorporated to resolve or prevent this (some of which we have incorporated into the mold modifications):
- move the gate to change the flow direction and path (done in the modification of our injection mold)
- add an ejector pin at the site of the trapped air, to vent the air (added pin at the remaining weld line after gate was moved, ensuring venting).
- vent the whole perimeter of the part mold.
- locate the gate at the thickest section of the part.
They were unaware of any test that could effectively detect the potential for cracking, other than the existing ASTM test, but noted that the changes we made would certainly fix the problem and our testing is showing it has (so far).
