Shrinkage when casting in resin?

[Dog Star]

I am making a coach body as a master for casting in resin, probably to be cast by CMA and using the milky bar material. At some time in the past I did ask about the shrinkage and I recollect a figure of 1% when discussing a coach side of 280mm ( a 7mm model, prototype 40'0" over body). There is a chance that I have got the shrinkage rate wrong... and I would not be pleased to make a body 3mm longer than scale if that rate is not correct.

Anyone any experience which can help here?

regards, Graham

 

[Neil]

I'd get in touch with CMA again and see what they say; I don't bother with shrinkage rates for the home casting I do as everything pretty much comes out the size of the master but a professional operation may/should be a bit less hit and miss than my own bunglings.

 

[jamiepage]

Graham,
CMA did my G3 coach panels for me. When I visited them, they said that shrinkage could be up to 1%, but normally considered to be minimal.
I made the master panels to actual required size with no shrinkage allowance, and the castings I have used so far are all pretty much spot on. There is some very small variation in size- some are actually slightly bigger than original (!), which I can only put down to some bulging of a warm mould (?). But either way, they are close.
True, each of my components were not terribly large, certainly not a one- piece side, but FWIW accumulated error over a coach side of iro 440mm was about 1mm.
Mike Williams will hopefully see this and respond, because he has much more experience than me in this area.
All the best
Jamie
PS Have just measured 10 door castings. Should be 27.0mm. wide. Smallest was 26.96mm., largest 27.05mm.

 

[Dog Star]

Graham,
CMA did my G3 coach panels for me. When I visited them, they said that shrinkage could be up to 1%, but normally considered to be minimal.

Have just measured 10 door castings. Should be 27.0mm. wide. Smallest was 26.96mm., largest 27.05mm.

Interesting that you were given <=1%... and that the samples are 0.2%.

thank you, regards,Graham

 

[Simon]

I don't notice any shrinkage with any of my stuff. Such shrinkage as there is would only matter if you were working to close tolerances in which case you are probably using the wrong medium anyway.

Simon

 

[Steph Dale]

Graham,
CMA did my G3 coach panels for me. When I visited them, they said that shrinkage could be up to 1%, but normally considered to be minimal.
I made the master panels to actual required size with no shrinkage allowance, and the castings I have used so far are all pretty much spot on. There is some very small variation in size- some are actually slightly bigger than original (!), which I can only put down to some bulging of a warm mould (?). But either way, they are close.
True, each of my components were not terribly large, certainly not a one- piece side, but FWIW accumulated error over a coach side of iro 440mm was about 1mm.
Mike Williams will hopefully see this and respond, because he has much more experience than me in this area.
All the best
Jamie
PS Have just measured 10 door castings. Should be 27.0mm. wide. Smallest was 26.96mm., largest 27.05mm.

Jamie,

The increased size of the mouldings is chemical, not thermal. IIRC it's related to ambient humidity as the resin can take on water as it cures...

Interesting that you were given <=1%... and that the samples are 0.2%.
thank you, regards,Graham

One is a subjective measure (the 1%) and obviously worst case. The objective reality as Jamie shows is likely to be much less. The last time I spoke to CMA they suggested not worrying about shrinkage for milky-bar (polyester) resin.

Steph

 

[jamiepage]

Steph,
Thank you. Would never have thought of that.

Graham,
They could well have been responding to a leading question on my part eg "is 1% shrinkage correct?" or somesuch- probably trying to sound like I knew what I was talking about.
However it went, I came away from the conversation with the decision to build masters to final size - zero shrinkage allowance. And Steph's insight would confirm perhaps it is as likely to be plus a smidge as minus.
CMA were very approachable and would confirm for you one way or another by 'phone, I'm sure.

Jamie

 

[Dog Star]

Thank you Steph, Jamie, for your observations.

I shall not make any allowance for shrinkage per se. The prototype has a body length of 40' 3/4" with a (7mm) scale equivalent of 280.44mm - the method of constructing the body has several vertical joins between panels so I shall aim for an overall length of 281.0mm before use of solvent for making the joins.

regards, Graham

 

[tomstaf]

Graham,
CMA did my G3 coach panels for me. When I visited them, they said that shrinkage could be up to 1%, but normally considered to be minimal.
I made the master panels to actual required size with no shrinkage allowance, and the castings I have used so far are all pretty much spot on. There is some very small variation in size- some are actually slightly bigger than original (!), which I can only put down to some bulging of a warm mould (?). But either way, they are close.
True, each of my components were not terribly large, certainly not a one- piece side, but FWIW accumulated error over a coach side of iro 440mm was about 1mm.
Mike Williams will hopefully see this and respond, because he has much more experience than me in this area.
All the best
Jamie
PS Have just measured 10 door castings. Should be 27.0mm. wide. Smallest was 26.96mm., largest 27.05mm.

Hi Jamie,

I'm just preparing a master and am looking in to options of whether to get it professionally cast by someone like CMA or whether to try it myself. I'll drop CMA some images and dimensions soon but before I do I wonder whether you could tell me if they have minimum order quantities, and do they price by individual models or have an incremental price scale based on size?

Cheers

Tom

 

[jamiepage]

Hello Tom,
From my limited experience, it was either/or. Small numbers were quoted as a fixed cost for mould making plus a cost per unit. As the required number grew, at some point the moulding cost got hidden within a unit cost. Net result was a reducing unit cost overall, as one would hope.
The cost must I assume be based on mould complexity and amount of resin required. Fortunately, my bits were not large, and simple slab sided shapes. No undercuts etc.
I asked for a price for 10, 20, 30 etc, then chose the one that I was most comfortable with. Bear in mind also that you may perhaps be able to order more from them at a later date as well. Best check with them though.
All the best
Jamie

 

[tomstaf]

Hi Jamie, thanks for that. I was worrying I might have to order in multiples of 50 or something so it's reassuring to know they'll potentially supply small orders.

Cheers

Tom

 

[Mike W]

I think its all been said above Tom, but I've found CMA castings generally have no shrinkage at all. It does however depend on the material they use. The milky bar stuff is OK, but they once had supply problems and used a harder grey resin which did shrink by 1%. I have a pre-folded W iron unit which slots between solebars and you can imagine the problem that caused, having to scrape about 0.5mm from the insides of the solebars on every casting. Grrrrrr!

If doing it at home, I suspect the mould can shrink slightly over a few days, but then stabilises. You need to take care because you can stretch the open mould on the bench, maybe without realising it, and change the size.

If its a fairly simpe, single sided pattern, I suspect you could cast it at home without any problem. Or, people like me or Phil Traxsom could cast it cheaper than CMA (I'm not THAT experienced, but getting there!). It its complex or needs a double sided mould, then CMA win every time.

Mike

 

[Mike W]

Should have added note about quantities. I elect to pay CMA up front for the mould, and then per casting. Some people ask for an incusive price for the whole 50, which just means that CMA amortise the mould cost for you. They say a mould generally has a life of about 50 and is then worn out.

Mike