in the near future I intend on casting a gun barrel (sim. to a modern tank). I always try to provide a relief channel at the "bottom" of my molds to allow the resin to work its way through and for trapped air to escape.
I know this is a good practice, but if I'm pressure casting is this necessary?
Its difficult for my to imagine the resin seeping all the way down a long thin cylinder channel (roughly 3.5" long and 1/8" wide)..and filling in all the detail..without the trapped air below going someplace.
I would like to add a relief channel, but if I add it where it is needed (the very bottom of a very detailed barrel, then the clean up of cured sprue stub is more problematic.
thoughts?
(thnx in advanced)
Resin Casting without Relief
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Joseph Osborn
- Posts: 1323
- Joined: Thu Oct 06, 2005 9:22 pm
- Location: Alabamastan
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Are you intending a one-part mold or a two-part mold? For a conical shape like this, I use a one-part mold with a nice big casting block at the base of the piece to act as a resin reservoir and to aid the demolding process. Does your barrel have a big gas evacuator in the center or a big muzzle brake? Those features may create undercuts that are bad enough to preclude the use of a one-part mold. If you have to make a two-part mold, then it's simply a matter of making a pour sprue next to the barrel and a vent at the top of the barrel. The resin will fill from the bottom of the part and push all the air up and out the vent. Pressurizing really has no bearing on how you design your mold, because you still have to pour the resin into the mold before you pressurize. Pressurizing will not eliminate the big voids that are caused by air trapped in undercuts. So, short answer is to make your mold the same way if you intend to pressurize it or not. The pressure just make for a nice, solid part and helps to push the resin into little nooks and crannies. Hope this helps, and the mechs on your website look really good.
<i>Fireball Modelworks</i>
i think you're missing my point a little.
here's a pic of the barrel i would like to cast
http://www.dormoy.ca/3d/StarwarsTX130.jpg
i want to do a two part mold. and i should add (that i didn't earlier) that i'm making this as a kit to sell, not just for my own usage.
for this exercise i would only want to cast the "tip" of the barrel and the cylinder of the barrel as one piece (the rest could be cast separately).
in my mind's eye, the barrel would be point "down" (tip down) while pouring "into" the butt of the barrel. you recommend a vent, but this vent line would be at the tip and would make finishing the gun barrel more difficult due to its smaller more detailed nature.
THAT is the crux my question.
hope that cleared things up.
thx
here's a pic of the barrel i would like to cast
http://www.dormoy.ca/3d/StarwarsTX130.jpg
i want to do a two part mold. and i should add (that i didn't earlier) that i'm making this as a kit to sell, not just for my own usage.
for this exercise i would only want to cast the "tip" of the barrel and the cylinder of the barrel as one piece (the rest could be cast separately).
in my mind's eye, the barrel would be point "down" (tip down) while pouring "into" the butt of the barrel. you recommend a vent, but this vent line would be at the tip and would make finishing the gun barrel more difficult due to its smaller more detailed nature.
THAT is the crux my question.
hope that cleared things up.
thx
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Joseph Osborn
- Posts: 1323
- Joined: Thu Oct 06, 2005 9:22 pm
- Location: Alabamastan
- Contact:
Hmmm okay; that's not at all like a modern tank gun but it's good to see the illustration.
#1 Is there any taper to either the barrel or the flash suppressor-thingy on the end? The taper in the part dictates the orientation in the mold to allow air to move efficiently as the mold is filled.
#1A If there is no taper to the barrel, have you thought about using brass or aluminum rod and then adding the resin-cast flash suppressor-thingy?
#2 A two-part mold of a cylinder is going to show seams on both sides especially after the mold gets several pulls out of it. Try to make this as a one-part mold if at all possible. Whenever possible, I don't want my customers to see seams or any other artifact from the molding process on a finished surface. If you absolutely have to make this part in a 2-part mold, try this little trick: bed the part and pour the 1st side as normal, then when you prepare the mold & master for the other side, put mold release only on one half of the RTV so that the 2nd pour bonds to half of the first mold and "butterflies" open on the other half to release the master. Doing this is the same as molding in one piece and then cutting the mold open, but this way you have more control and can put proper registration keys into the open side. You'll only have one seam line on the finished part, and you might be able to avoid having a vent going off the muzzle.
If it were me, I'd use rod stock for the barrel and cast the flash suppressor in a one-part mold. Simple mold and consistent quality.
#1 Is there any taper to either the barrel or the flash suppressor-thingy on the end? The taper in the part dictates the orientation in the mold to allow air to move efficiently as the mold is filled.
#1A If there is no taper to the barrel, have you thought about using brass or aluminum rod and then adding the resin-cast flash suppressor-thingy?
#2 A two-part mold of a cylinder is going to show seams on both sides especially after the mold gets several pulls out of it. Try to make this as a one-part mold if at all possible. Whenever possible, I don't want my customers to see seams or any other artifact from the molding process on a finished surface. If you absolutely have to make this part in a 2-part mold, try this little trick: bed the part and pour the 1st side as normal, then when you prepare the mold & master for the other side, put mold release only on one half of the RTV so that the 2nd pour bonds to half of the first mold and "butterflies" open on the other half to release the master. Doing this is the same as molding in one piece and then cutting the mold open, but this way you have more control and can put proper registration keys into the open side. You'll only have one seam line on the finished part, and you might be able to avoid having a vent going off the muzzle.
If it were me, I'd use rod stock for the barrel and cast the flash suppressor in a one-part mold. Simple mold and consistent quality.
<i>Fireball Modelworks</i>