Let’s share how to rotocast with clear resins! They are notoriously more troublesome than opaque resins for a number of reasons, so I thought it would be helpful to the model-making community to have a thread for this stuff. If a thread like this already exists please forgive me, point me in the right direction, and delete this thread.
I’ve had a heck of a time over the past couple years getting good reliable pulls on my clear helmets. I’ve tried a few different resins, and many different methods while prepping and casting, and below is what I find to be the best method. I’m hoping that someone out there can either learn from this or share an even better method so I can improve! =)
Put simply, rotocasting (or slush casting) with clear = a constant battle against bubbles! If you want a clear part, it’s probably because you need to see through it (duh). So, any bubbles suspended in the cast are going to be undesirable. Everything you do is meant to prevent or eliminate the formation of these suckers.
MATERIALS:
Resin: I found the best results using BJB Enterprises WC-783. You can get different working times. I started with their 15-minute resin, but found it took me ~3hrs to cast a helmet (4 layers of resin). Plus with a longer gel stage, you’re constantly moving the resin around in the mold creating more opportunities to pick up air. Now I’m using the 5-minute resin. I cut my casting time almost in half, and am picking up fewer bubbles. All that said, I haven’t used Smooth-on’s clear. If they offered it in a smaller amount, I would give it a try.
Pigment: both BJB and Smooth-on (aka "so-strong") have a good range of color pigments for use with polyurethane. I use a total of 8 drops of the black to get the level of tint you see in my helmet.
Anti-foam: BJB’s AF-4 is an anti-surfactant. It aids in de-airing the resin by lowering the surface tension of the resin thereby allowing the bubbles to break easier. I use 8-10 drops per helmet. I find the more drops added increases the working time. So use sparingly.
Release Agent: Ease Release 400. Not shown on Smooth-on’s website, but this stuff is the only stuff I have found to actually work well on glossy surfaces. The other more commonly used Mann products leave a hazy or textured finish on your clear cast part. You can order online at Blick art materials.
TOOLS:
Vacuum Pump: Cheap-o Harbor freight pump, or equivalent. I am using the Pittsburgh Automotive 2.5 CFM pump.
Vacuum Chamber: I don’t know what mine is, but it’s very similar to the 3 gallon VAC-it pro chamber. Metal can, clear acrylic top with gasket and threaded hole for line to pump/ vac gauge. Mine has a hole which I cover with a piece of used silicone. This is my ‘valve’ for gently getting rid of vac. when I’m done. You wanna let the air back in slowly so as not to blow over your cup of resin!
Misc stuff: use plastic cups and plastic spoons for all of your material handling. These do not collect moisture. Moisture = bubbles in your resin.
PREP:
I prep the mold with a quick spraying of the mold release. Nothing too heavy. Prior to this spray, when the mold is inside out and free of the mother mold, I will carefully wipe down the mold surface with mold release and microfiber cloth, removing as much dust and debris as possible.
CASTING:
I cast my helmets in 4 ‘batches’. The smaller the amounts, the less drastic the exothermic reaction, and in turn you can minimize large globs of resin as the slush gels up. I start by pouring out my 4 batches of part B into 4 cups (marked with a line to show how much to add), and 1 batch of part A into another cup. Each part B cup gets 2 drops of pigment and mixed thoroughly. All five cups go into the vac chamber to degas for about 3-4 minutes. This is mostly an extra step, but I find it helps shorten the degassing process when A and B parts are mixed (and the clock is ticking against you). It pulls out air from the pigment mixing stage, and any dust from the cups or the resin.
Once these are done, I’ll pull them out and with a spoon cut a skim coat off the top of each cup removing dust or microscopic bubbles at the surface that didn’t get popped. Now I have clean parts ready to mix. Sure, I’ll introduce more air when mixing A and B, but I feel better. =)
Put ~2 drops of AF-4 into B cup, then pour A into B cup. Refill A cup. Start the clock. Start mixing. I mix for about 50 seconds, scraping walls, floor, and middle of cup. The mixed cup and the second batch of part A cup get put back into the vac. chamber. I degas for 3 minutes. Before I pour the resin into the mold, I will do another surface skim on the cup to remove any bubbles that made it to the top but didn’t pop.
Slowly, and with the cup close to the surface of the mold, pour the resin into the mold. Slow = good as a quickly flowing resin tends to pick up or trap air on the surface of your mold. This is especially true when the resin is gelling up. In fact, I try not to let the resin flow to new areas in the mold when it’s thickening up. I think this is partly why I am getting pin holes in the outer surface of the casts (I’ve heard another reason could be too much mold release).
As you rotate the mold, try not to let the resin drip to another part in the mold. The impact of the drip traps air.
As I’m rotating, I’m constantly looking for bubbles inside. I have a plastic fork (with only 1 prong) ready to scoop out the bubble.
When the resin is well into the gel stage, scooping out bubbles is difficult. So it’s best to scoop out bubbles early.
After about 20-25 minutes, the resin isn’t really moving much anymore, so put the mold down and mix up the next batch. While you’re waiting for the degas to finish, take a look in the mold and make sure it isn’t pooling. Go grab a snack or use the bathroom. =)
CURING:
I let the helmet cure for 24hrs at room temp, then it needs another 16hrs of post cure heat at 140-160F (your resin may have different recommendations here). You need to leave the cast in the mold during this time, as it will need support. BJB website explains well why a heated post-cure is necessary. I’ll summarize by saying that it helps harden the part quicker and prevents distortion of the part when exposed to any amount of heat. Before I employed this curing method, I used to have to let the cast cure for ~5 days in the mold at room temp! With the heated post cure, I can demold in less than 2 days, and the part is stronger and more resistant to deformation under mild heat.
I prop my mold, open side down, on an old cookie sheet. The sheet has a hole cut into the middle of it where a 60 watt light bulb rests. The bulb generates enough heat to gently heat my casting. Possibly because the shape of the helmet and distance from bulb helps to trap the right amount of heat. But really, the temp needed for post cure is fairly wide: 140-180F (again, check your resin supplier's recommendations).
If I think of anything else, I’ll post it up, but please feel free to add your own experiences here!
I’ve had a heck of a time over the past couple years getting good reliable pulls on my clear helmets. I’ve tried a few different resins, and many different methods while prepping and casting, and below is what I find to be the best method. I’m hoping that someone out there can either learn from this or share an even better method so I can improve! =)
Put simply, rotocasting (or slush casting) with clear = a constant battle against bubbles! If you want a clear part, it’s probably because you need to see through it (duh). So, any bubbles suspended in the cast are going to be undesirable. Everything you do is meant to prevent or eliminate the formation of these suckers.
MATERIALS:
Resin: I found the best results using BJB Enterprises WC-783. You can get different working times. I started with their 15-minute resin, but found it took me ~3hrs to cast a helmet (4 layers of resin). Plus with a longer gel stage, you’re constantly moving the resin around in the mold creating more opportunities to pick up air. Now I’m using the 5-minute resin. I cut my casting time almost in half, and am picking up fewer bubbles. All that said, I haven’t used Smooth-on’s clear. If they offered it in a smaller amount, I would give it a try.
Pigment: both BJB and Smooth-on (aka "so-strong") have a good range of color pigments for use with polyurethane. I use a total of 8 drops of the black to get the level of tint you see in my helmet.
Anti-foam: BJB’s AF-4 is an anti-surfactant. It aids in de-airing the resin by lowering the surface tension of the resin thereby allowing the bubbles to break easier. I use 8-10 drops per helmet. I find the more drops added increases the working time. So use sparingly.
Release Agent: Ease Release 400. Not shown on Smooth-on’s website, but this stuff is the only stuff I have found to actually work well on glossy surfaces. The other more commonly used Mann products leave a hazy or textured finish on your clear cast part. You can order online at Blick art materials.
TOOLS:
Vacuum Pump: Cheap-o Harbor freight pump, or equivalent. I am using the Pittsburgh Automotive 2.5 CFM pump.
Vacuum Chamber: I don’t know what mine is, but it’s very similar to the 3 gallon VAC-it pro chamber. Metal can, clear acrylic top with gasket and threaded hole for line to pump/ vac gauge. Mine has a hole which I cover with a piece of used silicone. This is my ‘valve’ for gently getting rid of vac. when I’m done. You wanna let the air back in slowly so as not to blow over your cup of resin!
Misc stuff: use plastic cups and plastic spoons for all of your material handling. These do not collect moisture. Moisture = bubbles in your resin.
PREP:
I prep the mold with a quick spraying of the mold release. Nothing too heavy. Prior to this spray, when the mold is inside out and free of the mother mold, I will carefully wipe down the mold surface with mold release and microfiber cloth, removing as much dust and debris as possible.
CASTING:
I cast my helmets in 4 ‘batches’. The smaller the amounts, the less drastic the exothermic reaction, and in turn you can minimize large globs of resin as the slush gels up. I start by pouring out my 4 batches of part B into 4 cups (marked with a line to show how much to add), and 1 batch of part A into another cup. Each part B cup gets 2 drops of pigment and mixed thoroughly. All five cups go into the vac chamber to degas for about 3-4 minutes. This is mostly an extra step, but I find it helps shorten the degassing process when A and B parts are mixed (and the clock is ticking against you). It pulls out air from the pigment mixing stage, and any dust from the cups or the resin.
Once these are done, I’ll pull them out and with a spoon cut a skim coat off the top of each cup removing dust or microscopic bubbles at the surface that didn’t get popped. Now I have clean parts ready to mix. Sure, I’ll introduce more air when mixing A and B, but I feel better. =)
Put ~2 drops of AF-4 into B cup, then pour A into B cup. Refill A cup. Start the clock. Start mixing. I mix for about 50 seconds, scraping walls, floor, and middle of cup. The mixed cup and the second batch of part A cup get put back into the vac. chamber. I degas for 3 minutes. Before I pour the resin into the mold, I will do another surface skim on the cup to remove any bubbles that made it to the top but didn’t pop.
Slowly, and with the cup close to the surface of the mold, pour the resin into the mold. Slow = good as a quickly flowing resin tends to pick up or trap air on the surface of your mold. This is especially true when the resin is gelling up. In fact, I try not to let the resin flow to new areas in the mold when it’s thickening up. I think this is partly why I am getting pin holes in the outer surface of the casts (I’ve heard another reason could be too much mold release).
As you rotate the mold, try not to let the resin drip to another part in the mold. The impact of the drip traps air.
As I’m rotating, I’m constantly looking for bubbles inside. I have a plastic fork (with only 1 prong) ready to scoop out the bubble.
When the resin is well into the gel stage, scooping out bubbles is difficult. So it’s best to scoop out bubbles early.
After about 20-25 minutes, the resin isn’t really moving much anymore, so put the mold down and mix up the next batch. While you’re waiting for the degas to finish, take a look in the mold and make sure it isn’t pooling. Go grab a snack or use the bathroom. =)
CURING:
I let the helmet cure for 24hrs at room temp, then it needs another 16hrs of post cure heat at 140-160F (your resin may have different recommendations here). You need to leave the cast in the mold during this time, as it will need support. BJB website explains well why a heated post-cure is necessary. I’ll summarize by saying that it helps harden the part quicker and prevents distortion of the part when exposed to any amount of heat. Before I employed this curing method, I used to have to let the cast cure for ~5 days in the mold at room temp! With the heated post cure, I can demold in less than 2 days, and the part is stronger and more resistant to deformation under mild heat.
I prop my mold, open side down, on an old cookie sheet. The sheet has a hole cut into the middle of it where a 60 watt light bulb rests. The bulb generates enough heat to gently heat my casting. Possibly because the shape of the helmet and distance from bulb helps to trap the right amount of heat. But really, the temp needed for post cure is fairly wide: 140-180F (again, check your resin supplier's recommendations).
If I think of anything else, I’ll post it up, but please feel free to add your own experiences here!
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