Initial capability for injection molding part

Jafri

Involved In Discussions
#1
Hello,

I have machining background but currently working on development of an injection molding part from a supplier.

I am currently confused about what kind of features should I ask them to do initial process capability?

The part is ABS. I gave them a list of features (pins, holes etc.) to do Cp on both size (e.g. dia) and location (e.g. position, perpendicularity etc.).
But they pushing back saying that they are unable to do Cp on so many dimensions. The dimensions total to about 25.

I am now planning to reduce this list so as to remove the location dimensions, and only ask them to do sizes (length, width, dia, etc.).

However, I am wondering what is the industry norm in initial Cp of injection molding parts?

Thanks.
 
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Ron Rompen

Trusted Information Resource
#2
Not sure about 'industry norm' as far as the number of features identified for initial process capability.

Process capability requirements should be driven by features that are critical to YOUR process - whether it be diameters, positions, or any other feature.

You can also take into account past history with OTHER similar parts (whether from this supplier or another) and any issues you have had that would have possibly been caught with an initial process capability analysis.

I don't understand how they can 'push back' at this stage, I am assuming that the features requiring capability were identified on the print at the time of quotation. If that did NOT happen, then they do have at least some grounds for complaint.
 

Jafri

Involved In Discussions
#4
I don't understand how they can 'push back' at this stage, I am assuming that the features requiring capability were identified on the print at the time of quotation. If that did NOT happen, then they do have at least some grounds for complaint.
Yes, that is the root of the problem. Thanks for identifying.
The design team took away all critical dimensions from the print for some reason. I wasn't involved at that time.
Now the supplier is making the point that we didn't agree to these dimensions in the first place. Don't there were no criticals mentioned on print, so I picked dimensions myself based on what was important for us from assembly point of view.
 

leftoverture

Involved In Discussions
#6
Sorry I missed this question when you asked it. But I disagree with the poster who says it should be based on what critical to you, because that is only somewhat the case. This is an initial PROCESS study. It measures the capability and stability of the process. Your injection molding supplier is the process expert here so do listen to them and leverage their expertise.

Molding is not like machining. One to three dimensions are all that is needed to assess the stability of the process. In general the desired dimensions will include overall length, width, and height. In other words, in the direction of plastic flow, across plastic flow, and across parting line.

Smaller dimensions such as diameters are not likely to reveal much about the stability of the process. And yes, 25 dimensions is way too many for injection molding. By insisting on that many dimensions you are only adding cost and lead time to your project and you're not getting added value.

Best practice for injection molding: 3 piece per cavity full ISIR and capability study on 2-3 process sensitive dimensions selected with agreement from the molder.
 
N

ncwalker

#7
Will agree with leftoverture on this:

1) Pick a diameter that has a tight tolerance. This will show you what kind of tolerance to expect. And diameters are easy to measure.
2) Pick a diameter that is very different is size from diameter 1. This is because you are dealing with a solidification process, which means a large contributor to variability is initial size due to shrinking associated with cooling. A larger dimension will have more variability than a smaller one.
3) Pick a cross parting line dimension that goes from the A side to the B side. Another molding factor is the consistency with which the mold closes shot to shot. You want a cross parting line dimension is your study to demonstrate this. Key point: the dimension doesn't even have to make sense in the assembly, you want one that is easy to measure. The goal is understanding the molding process.
4) IF the tool has slides, you want to do the same thing as 3 above to the slides. Hopefully, your initial datums are all made by one piece of mold steel. You want a dimension from this piece of steel to every other molding piece. At a minimum, you want to check to at least one moving piece in the A side and one in the B side. Your part may or may not have this complexity. And again, this chosen dimension should be picked as one convenient to measure.
 
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