linear shrinkage for nylon 66 1

[toddb]

I've been designing and making many molds for small products made for nylon 66. My tolerances has been excellent on these small pieces(around .25 in3 in volume) But I have been noticing that as my molds have been getting longer that my shrinkage ration has been changing. Now I'm doing a large piece 15" x 3.5" at the widest(for us it's large), it's looking alot like a hand held metal detector scanner at airports and about the same size with 1/8" wall. My concern is shrinkage the long way versus across the shorter because I have screw holes to match up.

Does anyone has advice on what I should be doing in this instance so I less chance of rework. I have some leeway on the screw holes but closer the better as usual.

thank-you

 

[patprimmer]

Many factors influence the final dimensions of nylon 6.6

Some are design oriented and will be consistent for that mould, but others are process controlled and therefore variable.

If the material is unfilled, the along and across the flow will be fairly similar, with slightly higher shrinkage across the flow, but this will depend on the degree of pack and on injection speed.

My advice is to make the mould with the holes undersized, so that the location can be corrected when the hole size is increased after the actual shrinkage is measured.

Remember accelerated water conditioning in water or steam at over 60 deg C can give a different final result than conditioning at less than 60 deg C

Regards

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