Custom injection molding or sprues

I would recommend trying again on the resin molding.  Even with modern cad/cam the cost of the molds for injection molding would run into tens of grand.

For resin molds the secret is to learn to think like an air bubble, and how you would escape a mold as the resin enters.  Biggest suggestion is to use J-shaped pour sprue.  Resin should enter mold cavity from bottom, not top, so it can easily push air out vent tubes.  There should be a seperate vent tube for every local high spot in the mold cavity.

I've never had any luck with pour sprues.  It seems my resin starts setting too fast, and gets too thick to actually pour into the channels.  I lay my molds on their side and pour a lump of resin into the bottom half and then slap the top half of the mold down and squeeze the resin into the upper half.  It also seems to squeeze any small bubbles out, but I try to pour slow enough to not generate many bubbles.  Depending on the shape of the part you want to mold, this may not be a good method, though.

Gene Beaird,
Pearland, Texas

Injection moulding is an expensive game - tens if not hundreds of thousands!

Have you considered the 3D printing route - its far more suited to prototype & small run work............

I make my own molds using Micro Marc two part silicone mold making kit and have had great success with some exceptions. Once the mold is done I use the I use the Micro Marc color cast resin which takes between 7 and 10 minutes to set up and is fully cured in about 1/2 hour.  If you can not pour in a 7 minute time frame I would make the piece in several parts and then glue them together.

Here is a few pointers that I hope you will find helpful, first very small pieces such as a sprue do not pour well without a vacuum chamber, this is not too hard to make, you can even just use certain pool or aquarium canister filters with the insides taken out, and any pump that sucks air rather then blows air like a shop vac.  Not the best situation but it does help with thin lines of resin.

Next, while I don't agree with the "J" shaped spout (everything I have read and people who have helped me learn) a cone shape works really well.  Make sure the opening is big enough to handle the pour, a very small opening makes it hard to control the resin unless you use a funnel.  

You have to vent every small part where air can be trapped-no exceptions! even a small piece that is high on the model will trap a small bubble and ruin the cast.  I use florist wire for my vents, it is small and easy to bend so it makes perfect vents for the air.  Small amounts of resin will go up it creating a tab but you just need to clip these off when you are ready to paint.

Here is a link to making molds that I have found to be very helpful:

http://www.mold-making.com/

I have also used several different foundries that make molds and they make a much better and longer lasting mold then I can make with the Micro Marc.  If I do it myself the mold begins to lose detail by 10 or 12 copies while a professionally done one will easily so 200 or more before any noticeable deterioration.  You have to have a complete figure to have them make a mold, they can not do it from a drawing but if you have the figure just look up local foundries in your area.  The molds are not cheap, but if you are making 200 or more the cost of each piece becomes very affordable.

CrashTestDummy

I've never had any luck with pour sprues.  It seems my resin starts setting too fast, and gets too thick to actually pour into the channels.  I lay my molds on their side and pour a lump of resin into the bottom half and then slap the top half of the mold down and squeeze the resin into the upper half.  It also seems to squeeze any small bubbles out, but I try to pour slow enough to not generate many bubbles.  Depending on the shape of the part you want to mold, this may not be a good method, though.

Gene Beaird,
Pearland, Texas

 

I find it better to make molds tall.  Use a reasonable length pour sprue, to get some hydraulic head on the resin. I have even added styrene tubing to extend pressure head without using too much expensive RTV mold material.  Now, I know some folks say, hey, the density of resin is pretty low, compared to heavy metals like lead alloys.  The pressure head is minimal.  I say, if you haven't tried it, give it a try.  Any head helps.  Doubling height of mold doubles pressure head!

Could it be that the channels are too narrow and impeding flow?  I find the brands of casting resin I use have pretty low viscosity, and pour much like water, so setting too fast is not a problem. I do not use the quick set stuff, more for fear of it setting in pot.  But the regular certainly pours fast enough for the working life specified.

Don,  brands, please.  All I seem to find around here is Alumalite, and it's viscous and really starts to set while being mixed.  I have had reasonable luck with it, but it does set to a non-pourable state quickly, even when refrigerated before using.  When the part is about 1/4X1/2 inch, it's kind of hard to have big pour channels.  Thanks.

Gene Beaird,
Pearland, Texas

CrashTestDummy

Don,  brands, please.  All I seem to find around here is Alumalite, and it's viscous and really starts to set while being mixed.  I have had reasonable luck with it, but it does set to a non-pourable state quickly, even when refrigerated before using.  When the part is about 1/4X1/2 inch, it's kind of hard to have big pour channels.  Thanks.

Gene Beaird,
Pearland, Texas

 

I have used the stuff from Ace Resin, and thought it was fine.  However, lately I have been using the stuff from Micro Mark (forget the brand).  I find it works fine, is a fair price, and since I order other stuff from them it is convenient to order the resin (and mold material) as part of a larger order.

While speaking of resin, one big caution.  I thought a lot of mine was going bad too quickly.  In discussing it in a forum somewhere, I found the solution!  I had been agitating by turning bottle upside down and back up.  People told me that was not nearly enough.  When resin has been sitting for days or longer, it must be turned over and back at least 10 times, or equivalent agitation.  Once I started doing this, I found resin lasted much longer and I got great pours.  This does not refer to the mixing of the resin and hardener, but to shaking or agitating the resin before pouring out to do the mixing.

Thank you all for the replies! I like the idea of 3D printing to create a prototype. Any idea the average cost of such a thing for a 1/32 scale bomb would run? it would be 4.406" long and .415" dia. The detail would need to be ultra fine (recessed panel lines, recessed  bolts, breakaway connectors) down to .001”.  Sound feasible?

Beak

Thank you all for the replies! I like the idea of 3D printing to create a prototype. Any idea the average cost of such a thing for a 1/32 scale bomb would run? it would be 4.406" long and .415" dia. The detail would need to be ultra fine (recessed panel lines, recessed  bolts, breakaway connectors) down to .001”.  Sound feasible?

I'm not certain about the current capabilities, but here are a couple of links for you - I don't know if these guys do bespoke or not, but if they don't they should be able to point you in the right direction. One thing though, some of the 3D printing outfits will charge an arm & a leg because they have "rapid turnaround" - but that's unique to the process used, not the company......

It will also reduce your costs considerably if you can provide a 3D plan in a compatible file format for them.

http://www.click2detail.com/

http://www.live-resin.com/

I would definitely try to improve your resin skills .The last time I helped create a set of mold heads for an injection molder it was for model car engines .When we got to finalizing developemental costs for the product ,we would,ve had to sell over 500,000 units just to cover engraving costs !! The total would,ve been close to $$675.000 just for all the mold heads needed. Needless to say being as how we only had the one machine we chose to go back to making plastic pellets for others to use .This was in 1967 dollars by the way .           tankerbuilder