How come the screw boss crack after mold-in screw insert?

screw boss broken

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.

MFI (Mold Flow Index) value measurement equipment

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.

Molded-in screw boss diameter shall be 1.5 times teh insert diameter


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

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Trouble Shooting Guide for Plastic Injection

8 responses to “How come the screw boss crack after mold-in screw insert?”

  1. HC says:

    I was looking for some reference online to trouble shoot a problem which i am facing when i chance upon your site.

    I currently have a plastic housing that uses PC with glass fill 9% and has 4 mold in screw insert. the problem i have screw boss cracking and breaking. I will say put it as 2 “issue” as the crack propagate from the center towards the circumference and they are vertical. very similar to Hoop stress crack. secondly I have screw boss “shearing” off at the base of the insert , at half the boss height. After reading some reference and guide, i will say the first is due to hoop stress or poor insert quality which causes the boss to burst. the second failure , iam not sure as it may not have or usually do not have any crack. it just pops out after some time in a complete product. meaning there are axial stress constantly. hope you can offer some insight to what maybe causing this? i am guessing its material degradation, or sharp points on the insert.

    • WorkingBear says:

      1. You shall know this kind of screw insert boss broken was caused by stress.
      2. If your inset is mold-in process then you must check the item 4, to review the plastic screw boss diameter and insert parts diameter. Too much diameter of plastic screw boss will bring crack. I finally reduce the diameter of plastic screw boss then fix the problem. The stress will gradually make the plastic broken after couple weeks in a complete product.
      3. If you suspect plastic resin quality then you can check the MI (Mold Index) Value or call as MFI. Usually the MI value of complete product shall be less than 1.3 times of original value. As I knew that plastic supplier will add 20% of the re-grind resin. If this is true then the complete product MI value shall be less than 1.5 times of original value.

  2. HC says:

    Hi WorkingBear, Do you think the shape and sharp edges of the insert plays a part too? my current insert does not have chamfer or rounded edges. the break seems to start from the edge of the insert.

    • WorkingBear says:

      Well, Only if your screw insert parts apply ultrasonic to be pressed into the plastic boss. Otherwise I didn’t see the sharp edges of the insert will contribute the breaking. If you use ultrasonic press in then the chamfer must be designed on press in direction to make insert parts easily be pressed in without too much obstruction.
      May I know what is your process to put this insert into plastic boss? there are many solutions (ultrasonic, heat-staking, mold-in) can reach it and every solution has different requirements.

  3. HC says:

    Hi WorkingBear, we are currently doing molded in insert. the wall of the boss is 0.7mm for an insert with 2.4mm diameter. Initially we were thinking that by increasing wall thickness we might solve the issue, but now i am unsure as it seems it will get worse for circumferential stress

    • WorkingBear says:

      Here is my case. My original design show the boss diameter with 7.16mm for an insert with 2.38mm diameter.
      If your boss will be crack after screw then I do think you may need to increase boss diameter. If your boss crack without any screw then you may refer to my case below.
      Here is my case, the original boss diameter is 7.16/2.38=3.0 times the insert diameter and parts stored in warehouse over one week then found the crack without final assembly. The crack was terrible and it broken from inner boss to outside wall of plastic. I highlighted this issue to plastic supplier and asked them to do pre-heat for the insert parts before put it into the tooling. Thanks god the crack be fixed. That means there really something wrong in plastic supplier side but I can’t and hard to prove it.
      I finally change the design and reduced the boss diameter from 7.16mm to 5.25mm (considering the draft angle) and added radius on all core side and remove the pre-heating requirement for the insert parts. I was thinking the proposal of 1.5 times from design guideline is really small and would like to do the tool modification step by step gradually and not reduce it to 1.5 times at one time. Now 2 years pass and I never heard boss crack issue for this case.
      So. I will not say you can’t increase your boss diameter since your wall thickness is 0.7mm only. I do recommend you shall ask your plastic supplier why crack happen and make a QA plan to do screw torque test for the plastic parts. By the way, my plastic resin is PC and shall have more strength ability.

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