Making hollow casting & filling with shot for weight?

[Greg in MO]

I've been reading through the past posts, but have not found a discussion dealing with this.

I need to make replicas for a training program, but they need to have approximately the same weight as the metal originals. I can't say what they are replicas of, but if you picture a common handgun you will be close enough (handheld tool, made of metal, partly hollow).

These replica tools need to be rigid and able to withstand being dropped on the floor occasionally.

So our thought was to do the following:
1) make a silicone mold of the tool
2) do a slush or rotational cast using some rigid urethane
3) drill a hole in the end and pour in enough lead or steel shot to reach the correct weight
4) caulk the hole closed

I would really appreciate hearing your thoughts before we jump into this.

Is it a bad idea? Is there a better way? Any suggestions on a casting resin that is rigid but not too brittle? What rigid resins slush cast well?

Is it perhaps possible to mix metal shot with a resin and just do a solid pour? This would be great for simplicity, but I picture all the shot sinking to the bottom of the pour.

I really appreciate your help. It was great to find such a talented bunch willing to help with other people's projects. Thanks!

 

[Locry]

Can you give us an idea what the shape will be like? If there significant protrusions I can tell you right away that they will break off. Off-hand I'll suggest laying up a fiberglass layer, but then again it will still depend on the shape. And it still won't be 100% damage-proof. A rubbery compound would be best for rough dummy use applications. Like those blue and red dummy guns. I just have not used those kind of compounds. I would experiment with a fiberglass outer shell, then just fill up with whatever. I will stress though that it will greatly depend on the actual shape.

Oh btw, mixing the resin with your weight bits is a bad idea... it pretty much guarantees breakage. You'd need some kind of matrix to secure everything....fiberglass.

 

[mslz22]

Smooth-on makes semi rigid resin. I have used the 65d for hollow castings and bounced them off the concrete with minimal damage. Protrusions could be a problem but depends on how much of a beating you expect them to go thru. As for the added weight, you can use buckshot etc if you can get the "inside" nice and hollow with little to no stalactites, which could prevent filling, which means you may want to use a rotocast machine rather than hand slush filling which tends to result in an inconsistent thickness inside the part. You can use apoxy scupt to fill the drill hole and smooth over if you want it to look good.



Smooth-Cast® 45D & 60D/61D/65D/66D Product Information | Smooth-On

 

[Greg in MO]

Thanks Locry. There are no protrusions. I looked up the "blue and red dummy guns" you mentioned and they are comparable to what we need to make.

Fiberglass would be strong enough, but I'm having a hard time picturing how to hollow mold a small handgun-shaped item with it. Perhaps lay up part of each half, then assemble the two mold halves together and slush cast with more liquid resin?

As I need to make dozens of a few different sizes of the tools, it sounds easier to just hollow cast with liquid resin IF a strong rigid resin is available. But I'm just guessing here and definitely open to all suggestions. :confused

 

[Greg in MO]

Thanks mslz22, I really appreciate hearing about a material that has survived the concrete bounce!

I went and read the Smooth-Cast page you linked me to. Looks the right kind of stuff. It even says it was designed for rotational molding.

I do have a molding and a curing question:
1) Does a urethane like this work well in a silicone mold? About how many casts before the mold needs replaced?
2) Cure times for these Smooth-Cast blends vary from 15 minutes to 60 minutes. Why would a long cure time be better than a short one in slush or rotational molding? Does it have to do with mold life or wall consistency or heat build up or something else?

 

[mslz22]

These work fine in silicone. The longer the pot life generally the longer the cure time. If you are hand slush casting i would go with the short pot life so you don't have to sit there for 10 minutes rotating the mold. The 65d is 3-4 minutes i think. I would say, depending on mass, a mold would be good for 30 castings.

 

[Orbital Drydock]

If your doing tactical training or simulation. Don't mess around, use the blue & red training simulators. Hollow resin casts are very brittle & will be sharp when they break. If it comes apart during a disarmament sim, if someone rolls or falls on shards people could be injured badly. There's a reason why the training sims are made with high density rubber coatings.

 

[Orbital Drydock]

If your dead set on having something custom, or the simulators aren't available.
You can do a double casting. Cast your ballast in a solid resin blank. Then the detail in a high shore silicone.
The blank should be the basic size & shape, but slightly smaller. Don't use lead shot, use larger fishing weights. They can be attached to a string & suspended in the blank mold, so they don't sink or weaken the structure.
Then suspend the finished blank in a second 2 part detail mold. Use gypsum for the mold material. If you use silicone, the mold & finished piece will stick together. Then use a very high shore silicone for the detail. like 80-90+. All materials mold & piece except the gypsum must be de-aired in a vacuum chamber, or you will definitely get bubbles. Use materials with a longer pot life, 30-40min. Iit will give you the extra time to de-air.
Then it's just a little math. Make a test pour in resin to figure out the volume of materials needed for each mold. Subtract the volume of the blank form the volume of the detail to get the volume of silicone needed for the detail portion. Then weigh each material at the needed volume to figure the approximate weight of lead needed. Then drop the lead weight needed in a milliliter water filled graduated measuring cup to get the volume of lead. Subtract the volume of lead needed from the blank volume needed & that number becomes the new volume of blank material needed. Then just play with the numbers until you get the proper weight/volume ratios
Hope this helps

 

[NormanF]

How about casting it in pewter? It melts at a relatively low temperature and SmoothOn shows pictures of one of their rubbers being used to mold a pewter coin. As an added benefit if the simulator gets broken you can melt it down and cast it again.

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