Deciding plastic type / mould type

[seppederoeck]

Hi all,

I am fairly new to plastics engineering and we are developing a small bike frame in plastics.
As from the prototyping we did, it is quite hard to test the strength of the bike in each different material we are doubting which plastic to make.
I know these are probably questions we should have answered at the start of the project, but we did. We decided for PP, but are not sure if it will be strong enough and we'd like our tooling investment to be able to get us the right final product.

Our preferred option is still PP for obvious cost reasons. Other options might be ABS or PA but will drive up the cost.
As we need to start make tooling, are there recommended stages to try out all materials ?
Can we f.ex. start with a PP tooling and try to mold a PP version, ABS version, PA version ? And when we have tested the initial out-of-tooling samples, modify or finish the molds to best suite the final chosen material ?

Or should we start off the other way around with an ABS tooling and modify it afterwards to PP/PA if needed ?

Any recommendations to this ?

tx,
Seppe

 

[patprimmer]

ABS is in my opinion a very bad choice.

For cheap use PP

For tough use Nylon 6

For extra strength with more like PP mould shrinkage use nylon 6.6

For extra tough use Supertough nylon.

For extra tensile and rigidity use GF nylon 6.

The GF nylon is the only one likely to give problems with a mould designed for PP

If the mould is designed for PP, when you mould nylon, it is likely the parts will be slightly larger and the gates might be to small.

If you use GF nylon, the parts will be a lot to big and may warp a lot due to large difference between direction of flow and across flow shrinkage from GF orientation.

To get a real idea of final properties and size for nylon parts they need to be moisture conditioned after moulding. Normal moisture uptake from the air by nylon makes a significant change to dimensions and an extreme difference to physical properties, especially impact, elongation and modulus.

Regards
Pat
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[seppederoeck]

Hi Pat,

Thanks, but why would you not recommend ABS ? Personally, I didn't prefer ABS as to a consumer it sounds/looks more brittle (it actually is, although rigidity is stronger than PP). For this item I prefer the "softer" kind of plastic.

But I am considering ABS as it can be more shiny, it looks more high quality to a consumer, etc.
Also, as we have quite some structural ribs on the inside, I am afraid sink marks will be more visible in a PP/PA part than in an ABS part (not sure if that is correct though).

Here's an item we made in ABS (most of the frame, fork, etc.) : [URL unfurl="true"]http://www.youtube.com/watch?v=F-r-dO0zwGI&feature=g-upl[/url]

So for our new item we are more likely to use PP, but I am still wondering why ABS would not be a good choics.

tx,
Seppe

 

[patprimmer]

ABS is not so good for chemical resistance, fatigue and abrasion. It is also not so good for UV resistance. Both Nylon and PP are proven tough in a variety of rough environments like power tool housings and under bonnet (hood) applications.

Regards
Pat
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