SLA/SLS/etc shrinkage 3

[ctopher]

I have used a few prototyping companies to print my parts.
Most of them have a shrinkage value that I should build into my models.
Unfortunately none of them work. It varies due to thickness, length, etc.

Curious, has anyone used a shrink factor for prototypes? Especially for dowel pins?
So far it seems to be trial and error for me.

Chris
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[MadMango]

I've never had good luck with this. There are so many variables, even somethings as simple as orientation of the part will affect shrinkage. If you are looking to press two parts together, it is better to make your pins larger and holes smaller, then you can sand teh pins and drill out the holes to fit.

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[Eltron]

Yeah, I rely on secondary machining operations to make sure critical fits are at least close.

Dan

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[ctopher]

Thanks.
I have been having the machinist open the holes to match.
It seems that the prototype companies would have a better handle on the shrinkage by now, they don't.
Sometimes secondary machining does not make my boss happy.

Chris
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[ajack1]

On Z-corp machines you can calibrate a test piece and scale and change the bleed comp to get the required result, well at least in theory.

In reality because the machine builds things in layers around 0.1mm thick you will never get a good slide fit like you would with a dowel pin and a reamed hole.

As with casting you cannot achieve the sort of limits you can with machining, there are just so many variables.

Whilst technology moves on I am not aware of any rapid prototyping printer that can offer “slide fit” type tolerances.

 

[peppinu]

Chris,
I am not a solidworks user but am in charge of feeding our SLA. I never ask for any shrinkage on the STL files.
The machine has mainly two variables that need checking from time to time to calibrate it.
It is the shrinkage of the material; this depends on the type of resin being used.
And the beam compensation; this depends on the beam with.
The process as with any other process has its tolerances. We produce automotive switches here and to say it all we have a prototype demartment that does the fitting of the parts. As Dan said you still would need to touch the parts a bit and sometimes some grease will do miracles.

 

[ctopher]

peppinu,

Thanks. But, depending on materials and size, I have seen shrinkage vary a lot.
I recently had a part made from GF Nylon thicker than usual. It shrank approx .03in on the sides.
A smaller part, but longer, didn't shrink on the sides but did considerably on the length.
Holes I can manage because they have to be drilled to size anyway.
I have tried other materials on the same parts, the shrinkage varies too much. Some parts were rotated on the x-y, the shrinkage wasn't as much on some parts.
It's not consistent. I would like to see the 3D printer suppliers test the material and come up with a table with some shrinkage. It would be helpful.

Chris
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[CorBlimeyLimey]

With sheet metal parts, it is often better to let the fabricator apply the bend allowance to suit the equipment they have. Similarly with RP parts it is better to let the RP house define the shrinkage required. With the few parts I've had RP'd, I've supplied the model per the finished product size required. That model defines the product I'm expecting them to produce.

 

[ctopher]

CBL,
Which RP house do you use?
The one's I have used tell me they don't know what their shrinkage is.

Chris
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[peppinu]

Another aspect is; what process you are using. I was referring to SLA by 3DSystems.
If I understood you correctly you are referring to 3D printers. 0,03 in is way too much.
Some processes have wider tolerance and the supplier should inform you what tolerances they can offer. We are offering our SLA for outside work but we have the machine mainly for us. As I told you we produce very small components. If I gave them parts with the tolerance you mentioned they would really complain. A service bureau would not go into much detail. With SLA however the choice of material is limited. I know there are even ceramic type resins but it will never be like injected plastic.

 

[CorBlimeyLimey]

The one I used (years ago) was QuickParts

 

[ctopher]

Thank you.

Chris
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[MadMango]

We use an Objet printer here for plastics and an EOS direct metal laser-sintering (DMLS) machine for metals. We haven't had to calibrate the Objet yet, but we have calibrated the DMLS machine. Just print out a know item and measure, then adjust the beams. We print a .5"x1"x2" block for this purpose.

If you're doing this all with an outside vendor, they should be able to account for the shrinkage and your selected material. There's a great shop in Valencia, CA that has really grown over the years, I think they are called Solid Concepts. You might try to give them a holler.

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[ctopher]

I have used Solid Concepts a lot. They will not allow for shrinkage. I have tried working with them on this, it's up to me.
Sometimes very frustrating that they can't tell me what there own process can do.

Chris
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[MadMango]

I guess things have changed with them in the last 5yrs.

"Art without engineering is dreaming; Engineering without art is calculating."

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