Injection moulding with thin wall thickness

[dogbural]

Hi,

We have supplier to inject mould parts (cylindrical flange) with PPS40 GF and lately we received samples, measured out of tolerance.
Supplier told us that they had concern with thin wall thickness with only 0.5mm so they increased mould cavity by 0.25mm (by 0.5mm in diameter).
The mould is made of steel. You may see the attached cross section view.

The diameter was supposed to be 56.2 (+/- 0.05mm) but measured 56.5mm.
Since we do not have any room in mating part, we cannot allow additional thickness.

Q1. What is the minimum wall thickness recommended in injection moulding?

Q2. I do understand our tolerance is quite tight, but is it a common practice in injection moulding to follow DIN ISO 2768 as a general tolerance?

Q3. Any way to resolve this issue? try different material of mould, rather than steel?

Any advice would be much appreciated

 

[3DDave]

ISO 2768 is likely not appropriate for injection molded parts.

"It applies to the dimensions of workpieces that are produced by metal removal or are formed from sheet metal."

https://www.iso.org/standard/7748.html

I am unsure what DIN ISO 2768 is about as ISO 2768 seems to be the replacement for DIN 7168

 

[Pud]

Ask what the tool surface temperature was/is. Should be 135°C minimum. Try it at +150°C - this will increase crystallinty and shrinkage. Might get you down into tolerance.

Politicians like to panic, they need activity. It is their substitute for achievement.

 

[dogbural]

Thanks 3DDave, i will check that again with supplier.

Pud, supplier tried moulding at 150 deg, which gave the same result as the final trial at 140 deg.

Now we are talking about modifying the inner diameter of the existing mould, need to reduce the diameter to be close to 56.2mm.
What is the common practice to fill up the space? Just weld them around?
Or machine out inside and put an insert (like thin tube) and weld around to be more precise?

 

[Pud]

Tools are often laser welded to give a hole-free weld. In the UK the welders tend to be speciality shops. Cavity can then be recut/resparked. The welding will most likely leave a witness mark on the finished part, but I guess as it's in 40%GF PPS a high cosmetic finish isn't required anyway


Politicians like to panic, they need activity. It is their substitute for achievement.

 

[hendersdc]

3DDave, Note 2 allows for ISO 2768 to be used for other processes: "These tolerances may be suitable for use with materials other than metal." We have used this standard frequently for injection molded parts produced in Asia at the suppliers request.

dogbural, In the quote you got from the supplier did they add a statement like "tolerance per ISO 2768-mK"? I have seen that in quotes before, where the supplier deviates from the tolerances on the drawing and substitute their own preferred standard.

 

[LouisL]

Just share my experience.

Q1. What is the minimum wall thickness recommended in injection moulding?
ANS: It somehow depends on how long the plastic should flow and any feature affects its flow. For glass-filled material, I think 0.5mm is really too thin and agree with your supplier to increase it to 0.75mm. You can also start from adding 0.1mm each time until you get the good or still acceptable result.
Meanwhile, it is steel safe to remove material from the mould (=add material on plastic). No need to cut or weld the steel. Normally the tool shop only has to EDM the steel if you want to add material to plastic (in general case, not all cases).

Q2. I do understand our tolerance is quite tight, but is it a common practice in injection moulding to follow DIN ISO 2768 as a general tolerance?
ANS: General tolerance could not be applied to every dimension of plastic molded part per my experience. It is always limited by your supplier's ability, defect rate and your product design. There are couple of things you can consider to do. For example, you can ask supplier to mold some samples not filling up the mould so that you could review how the plastic really flows inside the mould and help you to analyze the real problem (or pass it to expertise with their molding parameter).
In some cases, you can also consider to use sizing. Put the plastic part into a steel to fix its size just after molding and do heat treatment to release its internal stress. But your supplier may ask for higher unit cost.

Q3. Any way to resolve this issue? try different material of mould, rather than steel?
ANS: I think the main factors in tool design are the gate type and gate position instead of the mould material. Of course you can ask for changing to different grade of steel but it only affects the life cycle of the mould and not used to fix your problem.

 

[Pud]

Can you try a PPS with less glass content? Get some unfilled and mix

Politicians like to panic, they need activity. It is their substitute for achievement.