Actually this is long ago issue and WorkingBear write down this article just for a record.
One day we received a complaint from field and complained our product had plastic house broken issue. Actually this crack can be saw from outside of the plastic case. So the crack on plastic is really terrible.
As checked the defect plastic parts and found the crack all locate at screw boss location and broke from inside to outside of plastic house. There is a mold-in screw INSERT parts was installed inside the plastic boss. We thought the potential cause shall come from this screw boss design and plastic inject condition.
Here WorkingBear will try to list out the screw boss crack potential causes for your reference and also take a memo for myself:
1. Using re-grind resin
We usually use the MFI (Mold Flow Index) value to judge plastic raw material with re-grind resin mixed or not. As experience show the molded plastic material will increase 25~30% of the MFI value as compare with the virgin resin. Once there is re-grind resin (already shot one time at least) mix into the virgin resin for the injection then the MFI value will be over 30% of original value.
2. Screw INSERT parts didn’t performance pre-heat process
As experience show the screw INSERT parts shall do pre-heat process before put it into the molding tool to reduce the inner stress and creep risk. If INSERT parts do not implement the pre-heat process then it will depend on plastic molding environmental temperature.
The molding cavity (PC resin) usually be preheated to 120°C and putting the 25°C (assume room temperature) of INSERT parts into cavity. Even cavity will heat the ISNERT parts rapidly to 120°C by contact, the cooper INSERT may not expand to its stable size in short time. The melted hot resin will go into the cavity around 310°C to heat the INSERT parts again. After few seconds the plastic to swap to cooling stage. Unfortunately, The INSERT parts are still expanding its size and plastic parts are going to cool and reduce size. The inner stress will happen on the INSERTS parts surface conjunct with plastic material.
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
Trouble Shooting Guide for Plastic Injection