addenroy
Mechanical
- Jan 12, 2011
- 11
Imagine a 100mm long part which is wider at one end and tapers down from 30mm to 15mm dia over a length of 30mm and then is effectively straight for the remainder of the length - see attached sketch. The wide end is hollow and has a constant wall thickness (3mm) following the outside taper for 30mm and then it stops and the wall thickness jumps to around 9mm (i.e. a short cone shaped hollow). Beyond this, the wall thickness is fairly constant for around 70mm (which is in fact the handle of the part).
The part is gated at the open cone end and flows towards the thicker handle end, so the melt does flow around the core forming the cone shaped hollow and forms a small weld line as the flow fronts meet at the tip of the cone.
The wall thickness differences and gating into a narrower wall cannot be changed unfortunately - we know this design goes against guidelines, but we have to go this way.
We are finding that a small amount of parts are cracking near the tip of the cone shaped hollow, where the wall thickness goes from 3mm to 9mm. The break is cup shaped and shiny, apart from outer surface, approx. 1mm of skin. The break is actually through the 3mm wall thickness, approx 1mm from the end of the inner cone shaped hollow - see attached sketch.
These breakages are not occurring after moulding, also other items are assembled onto these parts and then they are packaged, all without breakages. It is only when they arrive at our distributors that some are found to be either broken, or else a white line is visible on the surface (most of these are moulded in dark colours or black) and it just takes a light knock to break the part.
We acknowledge differential shrinkage is occurring at the junction of the thin to thick wall and probably creating built in stresses. The cracking however does not happen in the majority of the parts (we've cut a few and they look fine). The parts are packaged and boxed adequately.
Sorry for the ramble, hope the above isn't too exhausting. The parts are made from Novodur P2HAT ABS. Does anyone know what could be causing this type of crack and if there are any ways to resolve (e.g. are there any other ABS grades less likely to crack)?
Thanks in advance for any assistance.
Andrew.
The part is gated at the open cone end and flows towards the thicker handle end, so the melt does flow around the core forming the cone shaped hollow and forms a small weld line as the flow fronts meet at the tip of the cone.
The wall thickness differences and gating into a narrower wall cannot be changed unfortunately - we know this design goes against guidelines, but we have to go this way.
We are finding that a small amount of parts are cracking near the tip of the cone shaped hollow, where the wall thickness goes from 3mm to 9mm. The break is cup shaped and shiny, apart from outer surface, approx. 1mm of skin. The break is actually through the 3mm wall thickness, approx 1mm from the end of the inner cone shaped hollow - see attached sketch.
These breakages are not occurring after moulding, also other items are assembled onto these parts and then they are packaged, all without breakages. It is only when they arrive at our distributors that some are found to be either broken, or else a white line is visible on the surface (most of these are moulded in dark colours or black) and it just takes a light knock to break the part.
We acknowledge differential shrinkage is occurring at the junction of the thin to thick wall and probably creating built in stresses. The cracking however does not happen in the majority of the parts (we've cut a few and they look fine). The parts are packaged and boxed adequately.
Sorry for the ramble, hope the above isn't too exhausting. The parts are made from Novodur P2HAT ABS. Does anyone know what could be causing this type of crack and if there are any ways to resolve (e.g. are there any other ABS grades less likely to crack)?
Thanks in advance for any assistance.
Andrew.