Casting for precision parts.

[lastspartacus]

Hello folks. The good news is that I've finally got my kit together and my casting so far has been bubble free. The bad news is that the next items on my list to cast have got me wondering if I've been pursuing the right way of replicating these things.

I have designed with the help of my partner a segmented ruler for a naval tabletop game I've been working on. The problem is that the pieces need to be very accurate after casting, as they will fit together very precisely.

http://www.4shared.com/video/oYvxQzbG/rulerdemo.html

Relevant information shown up to about 55 seconds. The more complex pieces are seperated into two parts because I couldn't think of how to cast them otherwise. So my question is, how, if possible, should I go about this? Which casting method would be most feasible, if any?

Also, how accurate are the shrinkage specs listed on a resin bottle? Is it very accurate or just an approximation? Perhaps I could have the masters reprinted to where it comes out to exactly 1" ruler segments on the second generation cast if I do my math right.

[Blindhorizon]

From my experience with the tap plastic resin i use you shouldn't be able to notice the shrinkage. I think you could do a split mold or a 2 part mold. The thing about a 2 part mold that i really like especially if you want to create a bunch of this item youll need to worry about mold degradation, so you can create a mold plate. A plate is when you take the silicone halfs and pour resin into the silicone, this will make 2 exact replicas of your master molds. What that gives you is when your mold degrades and completely breaks down you don't need to set everything in play-doe or whatever you use again. You can pour both halfs together and they will always fit because you made them from your first mold.

[lastspartacus]

Thanks for the insight horizon. I figured it wouldn't be very visible, but that it might matter as far as precise fitting goes, over a few generations.

Maybe I did my math a bit wrong, put the decimal in the wrong spot. It looks like with my Mold Max 30 and Smooth-Cast 305, after two generations of replication I'm looking at 0.017 shrinkage, so a bit under 2%. That is pretty negligible if I've come to the right conclusion.

I guess now my only concern in that area is the accuracy of those numbers, but I should trust it, because SCIENCE, right? :p

[Blindhorizon]

Thats why you make mold plates your replicating the same generation as your mold deteriorates.

[mangozac]

Shrinkage really will be negligible and hard to notice.

The ruler segments I'd do in a split mould. The base could be an open top mould or split mould. The stem for the base I'd do as a hybrid:

Glue a bunch of stems to a sprue/reservoir in a line. Pour the silicone like a normal split mould. When cutting the split you only cut to the base of the stem itself - no further. This allows you to produce the stems with no visible mould lines and thus no cleanup. Sure, you can't get the mould release down in there very well, but with such a simple, smooth, tapered shape the mould degradation will be about as low as you can get. Here's a diagram:

Stem Diagram.png (11.57 KiB) Viewed 6271 times

One side note: the square peg on the bottom of the stem is a bit too small IMO. I'd increase the size so that the diagonal is the same as the diameter of the base of the stem. Why did you choose a square peg?

If doing single sided moulds for anything else mould plates will be beneficial. But they don't really make any difference to the shrinkage (you'll always make new moulds from either the original parts or first gen production masters).

[lastspartacus]

Thanks for the diagram, thats a really good idea.

When you say 'the bottom square peg' I assume you mean the proper bottom rather than the upside down diagram. Or do you mean the square peg at the bottom of the mold will be difficult to cast.

What glue do you use for this did you say, by the way? Elmers?

Edit: For those who watched the ruler vid, do you agree in the way I split the pieces with the 'hollow' receiving end that takes the circular head of another segment? I couldn't figure out how I would ever cast that piece whole, but that was before I knew about vacuum and split mold casting.

Am I on the right track, or is there zero purpose to cutting them in half and having to glue them together after casting?

[Blindhorizon]

If the pieces aren't made to be separate why cast them separately?

[lastspartacus]

Thinking back, the person advising me was doing so on the assumption that I would be two part casting, and that 'hollow' end would be impossible to do right as one piece.

[mangozac]

[lastspartacus]

[mangozac]

I don't know if anybody has tried it but I am very doubtful it would work.

[Blindhorizon]