Well, this is a long-ago history that WorkingBear writes this article just for a record.
One day we received a complaint from the customer about one of our products had a plastic house broken issue. There is a crack show on the outside of the plastic house. So the symptom of crack on plastic is quite terrible.
As checked various defect plastic parts, then found all of the cracks locate at the position of screw-boss. This broken show from the inside to outside of the plastic house. There is a mold-in screw INSERT that was designed and installed in the plastic boss. We thought the potential cause should come from this screw boss design and plastic inject condition.
Here WorkingBear is trying to list out the potential cause of screw boss crack for your reference and also take a memo for me:
1. Using regrind resin
Mixing the regrind with vergin resin will degrade the plastic mechanical property in the injection process.
We usually use the MFI (Mold Flow Index) value to judge the plastic raw material with re-grind resin (already shot one time at least) mixed or not. The experience tells MFI value will increase by about 25-30% for the molded plastic material compares with the virgin resin. Once the virgin resin mixes with the re-grinding one for the injection molding, then its MFI value will be higher than about 30% of the original value.
2. Screw INSERT didn’t performance pre-heat process
Experience tells that the screw insert shall be preheat before put it into the injection mold. It will prevent inner stress and creeping from cold inserts damaging the plastic screw boss due to thermal expansion and contraction.
Imagine a process that a copper material of insert-part is put into the injection mold and heated from 25°C (environmental temperature) to about 300°C then cooled. When its body size is still expanding gradually continuously, not only the tooling temperature has begun to drop, but also the plastic resins have begun to solidify. Internal stress will occur between the swelling of the inside screw boss and the shrinking of the outside boss. The screw boss will start to crack from the conjunction between the insert part and resin when the internal stress is greater than the strength of the plastic resin.
3. Plastic molding parameter effect
If apply lower packing pressure and time then the inner stress around the boss will be reduced and prevent the potential crack on boss. But lower packing pressure will also bring the sink marks on the thicker area and degrade the strength ability.
4. Design risk
According the design guidelines (refers to DUPONT document of “Molded-In Inserts: Precautions and Guidelines) for the mold-in parts. The molded-in boss diameter shall be 1.5 times the INSERT diameter except for insert with a diameter greater than 12.9mm (0.5”). More thickness of boss wall, the more stress happen around the INSERT due to material expand hotter and contract when cooler. Same as pre-heating section reason.
We finally take short-term solution to do pre-heat INSERT parts before plastic injection and take long-term solution by design change to enlarge plastic screw boss inner diameter. Refers to this post for more detail information: Case study for the mold-in insert screw boss broken
This article translates from https://www.researchmfg.com/2013/01/mold-in-screw-crack-potential-cuase/
Trouble Shooting Guide for Plastic Injection