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"Venting areas that are far from the plane of the miniature" Topic

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one hundred eyes31 Mar 2020 5:44 a.m. PST

I am working mostly with 9" natural (black) rubber moulds and Pewter 92, and have been for a few years now, only part time and home workshop based.

Still finding it very difficult to successfully vent areas that sit high or low in the mould. They are too far and inaccessible from the split line in the mould. Most of the time these top and bottom vents seem not to be needed, but when they are, they are.

I have tried to push various sharp pins, nails etc through to create a vent but the rubber has a tendency to "heal" itself.

Thanks in advance for any insight from more experienced mould makers and casters.

HMS Exeter31 Mar 2020 6:11 a.m. PST

I've never done any molding, but using a pin to pierce rubber will displace the rubber, but not deform it. That's why, once retracted, the rubber "heals."

Depending on the depth of rubber to penetrate, consider using a micro drill bit to drill down th the mold void. In this case you're not trying to force open a channel, you're removing rubber to create a channel. Not really sure what drill diameter to suggest. Start super small, then increase if needed.

Hope this helps.

Personal logo StoneMtnMinis Supporting Member of TMP31 Mar 2020 7:34 a.m. PST

After many years(decades actually) I learned really quick that vent holes throught the top or bottom of a mould will only result in the tearing and destruction of the mould cavity rendering the mould useless.

Odd cavities can be addressed by changing the angle of the figure in relation to the metal inflow gate. Sometimes changing the location of the inlet can also remedy the problem.

Or, you might just need to re-design the master figure.


khanscom31 Mar 2020 9:06 a.m. PST

At Archive Miniatures we sometimes used a very fine drill bit to vent the topside of a mold-- most usually to allow corners of figure bases to fully fill. The channel didn't reseal and usually left only a slight "nipple" on the casting at the site of the penetration. It probably would work OK for you.

Personal logo Steve Roper Supporting Member of TMP31 Mar 2020 11:37 a.m. PST

I've used a thin, sharpened brass tube to cut a core out. A piece of wire cleans the tube. Sometimes I cut a vent channel at the top of the mold to take the air all the way to the edge.

one hundred eyes01 Apr 2020 3:28 a.m. PST

Thank you all for your help.

Master Caster02 Apr 2020 5:22 a.m. PST

I vent only where and when it's needed to fill an area with metal. Rule is: metal in, air out. Sometimes I have to drill thru the mold to the other side and then cut a channel all the way to the edge.
For bits I use three types; .025 (#72), .021 (#75) or I cut longer pieces of piano wire (same thicknesses as the two previously listed bits) but I cut a nice sharp chisel bit at the end with fine wire cutters.
Mold makers in the jewelry industry have used this same method for stubborn void areas.
You will find it necessary to redrill and clean out the drilled holes. Casting metal and casting powder clogs the holes and the rubber does tend to expand and seal a bit over time the rehealing term is appropriate I guess. The chisel ended piano wire can often be a superior method when done correctly. The chisel end tends to cut a cleaner hole whereas a regular drill bit can leave rubber residue in the hole.
Redrilling vent holes will be a recurring task. I have molds which I have to redrill and clean out the drilled vent holes each time I use the mold in a session. Three weeks later I may need to cast more from it again and I have to redrill and clean out the holes again.
Venting channels cut into the interior surface of the mold are always preferable because it is easy to see when metal and powder clog it up and easier to free up. But drilling thru the mold and venting on the obverse side is very often necessary.
Toby Barrett
Thoroughbred Figures

Master Caster02 Apr 2020 5:30 a.m. PST

PS – chisel ended piano wire is sometimes the only way to bore thru thick molds when the small drill bit sizes are not long enough to penetrate all the way to the other side. Some of my molds are 2 inches thick and the smaller drill bits can't cut all the way thru. You will also discover that if you use the wire method place the chisel end at the point where you want the vent and THEN hit the start button. You'll see why.

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