Case study for the mold-in insert screw boss broken

Case study for the mold-in insert screw boss broken

Last time WorkingBear mentioned the potential root causes to bring the screw boss crack after mold-in screw nut insert. Actually we received the complaint from the field then asked EMS and plastic supplier to double check their inventory and found about 1% of the screw boss cracked after mold-in screw nut insert before box build assembly. That means no extra force and screw driver give to these screw bosses and the crack happen just from its inner stress. The defect rate increase to 30% after screw in the plastic case together.

So, what make this happened? Workingbear had pointed out 4 potential root causes  to bring the screw boss crack for the mold-in nut screw insert process.

  • Using too much of the re-grind resin
  • No pre-heat on the screw Insert parts
  • The improper molding parameter
  • Design risk 

Since we had proved the boss crack really happened from plastic parts itself. We held these poor quality plastic parts and question the plastic supplier why crack happen? The interesting is that plastic supplier challenged me back that they didn’t change any parameter for this plastic parts and they really don’t know what happen and how come the screw boss crack now?

Well, I have a think in my mind that I met a really poor plastic supplier this time. I have to do something myself and consider to close this supplier in the future.

This is a really interesting supplier and they not only product the plastic parts but also help customer to do the box build assembly and packing. They do the box build assembly by pure manual. Even screw the top and bottom case together by manual screw driver in production line. And yes, this product original build in this supplier too. We moved the product  out of this supplier two years ago and just keep the plastics parts there. (WorkingBear found almost EMS won’t accept the business of small quantity order. That’s why this kind of supplier can be survival.)

Obviously, this supplier has attitude problem and lack of engineering technology background. Their engineer only know how to build the parts and products and don’t why do this way. Their management also do not like to face the problem and want to get out of this trouble as soon as possible.

1. Supplier said they usually do the same way without any crack happened in the past but now happen in other EMS factory?

Case study for the mold-in insert screw boss broken

This supplier claimed that they applied 1.5kgf-cm torque usually by manual screw driver to lock these Philips type of M2 machine screws between top and bottom case together. This supplier also challenge the crack may be caused by large torque that another EMS applied 2.0~3.0Kgf-cm by semi-auto screwdriver to lock it. Workingbear must say that even apply larger screw torque on the screw then it will bring stripped screw and not crack on the plastic boss.

I must say this supplier is lucky and didn’t get any complaint in the past. As time passed this crack will become serious gradually. It does not means no crack was found for their original products but the crack does not break to outside only.

2. Why needs to do pre-heat the INSERT nut since the plastic injection process tooling will be pre-heated to around 110°C already?

Case study for the mold-in insert screw boss broken

Almost of the INSERT parts are made by cooper material and its Coefficient of thermal expansion (CTE) is 16.4 µm/m-°C while stay at 20°C~100°C environmental. The plastic resin is PC and its CTE is 66.6 µm/m-°C while stay -40°C~+40°C. There is about 4 times of CTE different.

Of course, the injection machine can do the pre-heating for the tooling and the INSERT nut parts. The question is how can we make sure the INSERT nut parts really be heating to its temperature. By the way, the only contact between plastic tooling and INSERT parts is the bolt only. We really can’t guarantee plastic tooling pre-heating process will heat the INSERT parts to its position in a short time.

While the high temperature melted resin flow into the cavity of tooling, it will make the  INSERT parts increase its temperature and expansion its size. Sooner and later the resin start to become solid and shrink its size by the cooling system of tooling but the INSERT parts may still stay at high temperature.  The expansion rate is different between the resin and INSERT parts. Inner stress will happen at the interface of resin and INSERT parts. Once the plastic strength is less than inner stress then the crack/broken will be occurred from the weak point.

We usually ask INSERT parts be pre-heated to 50°C~60°C and keep 10 minutes at least then put it into the tooling. Keeping 10 minutes is in order to make the INSERT parts reach its stable expansion size. Only pre-heat to 60°C is for safety to prevent operator from scalding.

Basically, more screw boss diameter higher risk to make screw boss crack for the mold-in process. Because of the resin dimension will shrink during cooling stage. The outer and inner diameter will be shrinkage at same time but the INSERT nut will be shrinkage a little only. The INSERT nut will against the resin shrinkage stress. The more screw boss diameter more shrinkage dimension. Once the plastic strength is less than inner stress then the crack/broken will be occurred from the weak point.

So, WorkingBear took below actions to fix this screw boss crack issue

1. Ask plastic supplier to do pre-heating for the INSERT nut at 60°C temperature and keep 10 minutes at least then put it into the injection molding tool.

Actually, the problem was fixed and no crack show after this action. I did use 8Kgf-cm semi-auto screw driver to lock the screw into the INSERT nut for test and didn’t find any crack. I tracked it after one day, one week, two weeks, one month.  

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 personal think supplier shall remove the re-grind resin adding too and just won’t tell me the truth.

2. Do tooling modification for the mold-in screw boss to reduce its diameter from 7.16mm to 5.25mm.

Screw_Insert_uut_modificationMy original case show the outer boss diameter is 7.16mm and INSERT nut has outer diameter of 2.38mm. The boss diameter is 7.16/2.38=3.0 times the insert diameter. 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. So I reduce the diameter from 7.16mm to 5.25mm (there still have 5.25/2.38=2.2 times and considering the draft angle) and added 0.3mm radius on all core side corner and remove the pre-heating requirement for these INSERT nut process.

Now 2 years had been passed at least and I never heard any screw boss crack issue for this case.

PS: One year later that I asked to move this molding tool out this supplier. I would like to end the nightmare with this supplier.

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