Best way to go from shape to resin casting

I've been planning to create a master for a set of resin casts (I actually need 12), which I'd like to create from a Shapeways original. Which raises 2 questions:

1) Which is the best material for making a casting?

2) Should I create a "positive" (i.e. the shape I want) or a "negative" (the mold)?

Obviously it would save a step to directly create the mold, and I can't see any reason why that wouldn't work, since it's just as easy to create a concave shape as a convex one with this technique. But I haven't done this before, so perhaps there's a reason I'm overlooking.

I'm planning for this to be a single-sided pour mold (simplest kind). This makes sense since the object is essentially a decorative cabinet handle, and the back can simply be flat. (more or less). A core will be suspended in the casting to form the back part of the handle with the threaded part.

Any other considerations from anyone else who's tried this would be welcome too.

Hello,

Start with the cheapest/smoother SW material. Then, use a silicon to pour your mold. And that will go right.

Schuss.

DONT USE White Strong & Flexible! WSF does not take silicon well. use transparent high detail to make a silicon mold, and then pour resin in that. You'll always have the original for when the silicon wears out over time.... Silicon will grab the pores of WSF material and you wont be able to remove it

Baltimore's reply makes sense!
Thanks for the tip, I'll remember that.

OT: Baltimore, do you stay in Baltimore, USA? :laughing:

QX

Thanks for the responses!
Summary: "Transparent Detail" positive -> silicone negative -> resin result.

Right for the summary.
My opinion is that with transparent detail you will have printing horizontal lines. WSF is sandblasted so surface is softer. If you brush WSF with petroleum jelly, silicon won't penetrate the model surface. Believe me, I do it like this.
Of course, it's sure that Transparent detail material is better in definition.
See you guys.

yes i'm in baltimore, md, usa

I can confirm "transparent detail" masters work fine for creating RTV silicone molds. I've done several little molds using master parts printed by Shapeways. My parts are always quite intricate (scale model parts). I cast PU resin in the molds.

For me I have found just simply dipping WSF parts into Johnsons Klear for a few minuets then letting it dry is all that is needed to prevent RTV from sticking. This keeps the original look of the part.

For a smooth finish I also sand my parts then paint them with filler primer. I repeat this process a few times and finish by buffing. This may not be practical if your models have a lot of detail.

This is a cast from one of my painted/polished parts. I've not taken a photo of the Johnsons Klear dipped cast but I can if anyone is interested.

FX2 - Do you have any images or links to a site where we could see some results of your molding and casting of Shapeways parts?

Is there any reason (other than price) to go with Transparent Detail as opposed to black or white detail?