Molding and Casting Kit Parts for the Y-Wing Project

DaveG

Master Member
RPF PREMIUM MEMBER
MOLDING AND CASTING KIT PARTS

At the recent Southern California Prop Party I had several inquiries about the kit part molds and castings for the Y-Wing project and questions about how they were made. I thought it might be a good idea to outline the methods I use here on the forum. For many of you, making molds is old hat, and there are certainly other methods that can yield good results. But for those of you that are new to the hobby this information should save you a lot of trial and error.

I’ve started a new thread for this since the subject matter is pretty much independent of the Y-Wing build project. This is a pretty lengthy post so grab a cold one, sit back and kick your shoes off.

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MAKING THE MOLD

Equipment and materials needed:

¼” Clear acrylic sheet
Double-stick paper tape
.040” thick styrene sheet
Solvent cement and applicator needle bottle
Clay – optional
Short grain white rice
Disposable mixing bucket, plastic or paper
Paint stir stick
Digital gram scale
Two-part silicone mold rubber
Nitrile rubber gloves
Vacuum chamber
Leveling board and level (see text)
Kit parts to mold

A note on silicone mold rubber and vacuum chambers – As will be described, a vacuum chamber is used to de-air, or pull all the air bubble out of, the mixed silicone rubber. This has two advantages, first, it eliminated larger bubbles that might get trapped against the surface of a parts and second, it eliminates the very tiny air bubbles in the mix that can later expand due to heat generated by the curing resin causing indentation dimples in the cast part. There are some silicone rubbers that are very thin and are less likely to trap air bubbles. These rubbers a designed to be used without a vacuum chamber, which can be an expensive piece of equipment. I do not have any experience with these silicones as I have a chamber and prefer to stick with what works for me.

A NOTE ABOUT WORKING SAFELY – Molding and casting is a messy process and some of the chemicals involved, particularly the urethane resin, are hazardous. Wear appropriate safety equipment: gloves, goggles, apron, etc. and work away from pets and children. THIS IS NOT A JOB FOR THE KITCHEN TABLE!

The following method for making a mold box is one I’ve developed over time. It’s simple and reliable. It works for small molds like these and can be scaled up as needed (with appropriately thicker materials). I like to use acrylic and styrene sheet for mold boxes, as they are easy to work with and glue together, and I’ve got plenty of scraps of both lying around the shop. However, if these materials are not available you can substitute ¼” masonite or particleboard for the mold base and cardboard or foam core for the sides (along with an appropriate adhesive). The important thing is that the base be absolutely flat.

The first step is to cut the ¼” acrylic to the exact overall size of the mold. This can be based either on the size of the parts to be molded, or other considerations. I have settled on a standard base size for most of my kit parts molds of 3 ½” square. I’ve found this to be a reasonably optimum size for most kit part molds. Not too small as to be difficult to handle and not so large or filled with too many pieces to pour efficiently. Of course for parts that are two large to fit on a mold this size the size can be increased

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Once the acrylic is cut to the desired size, remove the paper backing from both sides and cover one side completely with double stick tape. Be careful to apply the tape tightly edge to edge but without any overlaps.

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Remove the paper liner from the tape exposing the adhesive surface. Press the kit parts firmly onto the tape leaving a ¼” border around the outside and approximately 1/8” between parts. Severe undercuts on parts can be filled with clay. Large hollow parts should also be filled with clay if the mold is to be placed in a vacuum chamber.

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Cut four pieces of 040” thick styrene sheet for the sides of the mold box. They should be about ¼: wider than the mold base and about an inch taller than the anticipated depth of the mold. Exact size is not important, only that the corners are square.

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Glue one of the pieces of styrene to the edge of the acrylic mold base with solvent cement. The styrene should be flush to one edge of the base and overhang the other.

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Glue the remaining sides to the base, butting each in turn into the overhanging end of the previous side to form the mold box. Be sure to glue the styrene panels to each other at the vertical corner joints to prevent silicone from leaking out.

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Mark the sides of the mold box to the desired depth of the mold. I usually make the mold about ¼” deeper than the tallest part.

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To determine the amount of silicone rubber needed for the mold fill the mold box with the white rice up to the marks on the sides of the mold box. Pour the rice from the mold box into a disposable mixing bucket making sure no pieces are trapped by the kit parts or stuck to the double stick tape. The mixing bucket should be at least four times the volume of the rice. Mark the sides of the bucket at the height of the rice then pour out the rice.

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Weigh the empty bucket on a gram scale and make a note of the weight. I like to write the weight on the side of the bucket for convenience. Pour silicone into the bucket up to the mark and weigh again. Subtract the weight of the bucket to determine the actual weight of the silicone. Multiply that weight by .1 to determine the amount of catalyst needed. (The silicone I use is mixed 10:1 by weight, for silicones with different ratios adjust accordingly.) In this case the bucket with silicone weighed 151 grams, minus the 18 grams for the bucket made for 133 grams of rubber. Multiplied by .1, the amount of catalyst needed would be 13.3 grams.

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Zero (or Tare) the scale and add the required amount of catalyst. Mix the catalyst thoroughly, scraping the sides of the bucket, until the mixture is of a smooth consistent color with no visible streaks of catalyst or unmixed rubber.

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Place the mixed rubber into the vacuum chamber and de-air. During the process the bubbles in the mixture will expand causing it to foam up to approximately four times it’s original volume. The bubbles will burst and the mixture collapse back down. Leave under vacuum for an additional five minutes. There will still be bubbles visible on the surface after five minutes but that’s okay. Release the vacuum.

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Pour the de-aired rubber into the mold box scraping down the side of the bucket to get as much material out as possible. At this point I like to put the mold box back into the vacuum chamber and de-air for a second time for five minutes or so to make sure there are no air bubbles trapped in the deep crannies of the parts. This step is not absolutely necessary but as long as you’ve got a vacuum chamber, it’s a good idea. Another option is to make the styrene mold box at least four times deeper than the amount of rubber and to just de-air the silicone once actually in the mold, letting it foam up and collapse. I use to do it this way all the time but there's no real advantage and it uses up a lot more styrene.

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Remove the mold box from the chamber and place on a level surface to cure. Having a mold where the front and back surfaces are perfectly parallel (from the mold curing on a level surface) is very helpful later when casting. Most work tables are not perfectly level so I’ve made a small leveling board, just one foot square piece of particle board with three leveling screws and Tee nuts inserted into the bottom.

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Allow the mold to cure completely, the rubber I use calls for a 24-hour cure time. Once fully cured, break off the styrene sides of the mold box and carefully peel the mold away from the mold base and kit parts. Sometimes there may be some flashing of rubber that has crept under a few of the parts; this can be trimmed away with a sharp knife. Trim the meniscus of rubber off the bottom edges of the mold with a knife so it will lie flat. The mold is now ready for casting!

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POURING AN ARCHIVE CASTING PLATE

Equipment and Materials Needed:

Silicone Mold
¼” Acrylic sheet
.040” styrene sheet
Solvent cement and needle applicator bottle
Baby powder
Soft bristle brush
Nitrile rubber gloves
Urethane resin
Gram scale
Disposable mixing cup
Mixing stick, wooden coffee stir stick or tongue depressor.

Silicone molds don’t last forever. The rubber will start to break down after a couple dozen sets of castings and little bits will start to break off and stick to the resin eventually rendering the mold useless. Molds stored for many years can also become un-usable. When this happens, you’ll need to make a new mold. Easy enough if you still have all the kit parts, but as many of the parts I am molding are borrowed I like to pour an archive casting plate. From this plate I can easily make additional molds.

The archive plate is the very first thing I cast from the virgin mold, before pouring any parts. To do this I place the mold on top of a piece of ¼” acrylic, this time larger than the mold base used before. Cut four pieces of .040” styrene sheet and glue in place tightly around the mold (this time on top of the acrylic rather than against it’s side) forming a mold box much like was done before.

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Dust the surface of the mold lightly with baby powder using a soft brush to work into all the recesses. Dump out the excess baby powder and blow off the mold gently. The amount of powder needed to aide in molding is almost invisible. The baby powder helps to break down surface tension on the mold and help the resin flow into all the recesses, eliminating bubbles.

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Most urethane castings resins are mixed 50/50 by weight. Using the gram scale, weigh out enough urethane resin to fill the mold box to a height of about ¼” to 3/8” over the top surface. I use the rice again to help determine how much resin I’ll need. Mix the two-part resin thoroughly then pour into the mold box. Place the mold into a pressure chamber and allow to cure complete under 40psi to 60psi. I’ll talk a bit more about using a pressure chamber in the next section. Once the resin is fully cured break off the styrene mold box sides and peel the mold away from the plate. The plate can now be used to make additional molds, as well as being a handy reference for parts you have molds of.

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CASTING PARTS

Equipment and Materials Needed:

Silicone Mold
Silicone baker mold
¼” Acrylic backing plate
Small weights
Newsprint
Pressure pot
Mold rack to go inside pot
Air Compressor with storage tank (a small airbrush compressor will not work for this)
Baby powder
Soft bristle brush
Nitrile rubber gloves
Urethane casting resin
Gram scale
Disposable mixing cup
Mixing stick, wooden coffee stir stick or tongue depressor

Pressure Casting is a process where resin is left to cure in a chamber under 40 to 80 psi of air pressure. The pressure compresses any bubbles present in the resin down to a size where they are virtually invisible. The pressure also forces resin into all the nooks and crannies of the mold resulting in bubble free castings.

There is a second advantage to useing a pressure chamber for casting. Urethane resin is 'hygroscopic', when means that the uncured resin will start to absorb moisture out of the air once the containers have been opened for the first time. Even when sealed back up, resin left to sit on the shelf for month will gradually get contaminated. This moisture can cause little tiny bubbles to form in the castings as the resin cures. Pressure casting can help prevent these bubbles from forming.

The simplest pressure chamber is a small 2 ½ gallon pressure paint spraying pot. Harbor Freight has one that works perfectly well for around $100. A Few minor modifications need to be made to make the pot work for pressure casting, mainly removing the paint siphon tube from the outlet side and adding ball valves to both the inlet and outlet. It is very important that the regulator that comes with the pot be used to regulate the maximum pressure in the pot, around 40psi. DO NOT by-pass the regulator and run air straight from the compressor tank into the pot. This can over pressurize the chamber.

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IMPORTANT SAFETY NOTE – PAINT POTS ARE NOT INTENDED FOR USE AS PRESSURE CHAMBERS. CARELESS USE AND OVER PRESSURIZATION CAN CAUSE THE POTS TO EXPLODE CAUSING INJURY. USE WITH CAUTION AND AT YOUR OWN RISK. DO NOT PRESSURIZE BEYOND THE RECOMMENDED LIMIT FOR THE POT.

To make it easier to get the filled mold into the tank, I built a mold rack which consists of two disc of ¼” particleboard held together by three aluminum rods. Wooden dowels would also work instead of aluminum. The discs each have a finger hole in the center for lifting.

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To cast a set of parts, first make sure your compressor is turned on and it’s storage tank fully charged. Put two layers of newsprint on the bottom disc of the mold rack. Dust the mold with baby powder and place on top of the paper.

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Use the gram scale to weigh out equal parts A & B of the casting resin. Mix thoroughly in a small paper cup. Carefully pour the resin into the mold, slightly overfilling it. The little bit of overfilling is the key to getting good flat backed castings.

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Place the silicone backer mold (This is just a flat slab of silicone mold rubber, the one I use has a small lip to prevent it from sliding off the filled mold.) on top of the filled mold, topped with a piece of ¼” acrylic and a couple of small weights allowing the excess resin to be forced out. Lightly press down on the backer mold.

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Place the mold rack carefully into the pressure pot, put the lid on top and clamp in place. Open the air valve pressurizing the tank. Do not over tighten the screw clamps; the rubber seal will expand to seal the lid as the internal pressure rises.

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Allow the resin to cure under pressure. The amount of time needed will depend on the setting time of the resin and the thickness of the parts. The resin I use has a pot life of about 5 to 7 minutes before it starts to set and generally small parts are ready to de-mold in 45 minutes to an hour. Thinner parts take longer to cure. Don’t leave the cured resin in the mold any long than necessary as doing so can dramatically shorten mold life.

Remove the mold from the tank and peel off the backer plate. You should have a set of bubble free castings with perfectly flat backs. The tissue thin membrane of resin between the parts is easily broken off or trimmed away with a sharp knife.

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I'm happy to answer any questions about the molding and casting process about anything that was un clear.
 
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Thank you Dave! I've always been bad at making molds and casting, and will have to with the Y project. This helps so much! :thumbsup
 
I will second that on degassing the silicone in the molds. ive been doing it for a couple years and it works perfectly every time.
very nice tutorial !!!
 
Yes, being able to de-gas silicone molds will really improve their quality. When I first put together my vacuum chamber I think I spent $1,500 on a pump (a big scientific model) but now looking around the web you can find pump and chamber set ups for under $200, so the barrier to entry is much lower.
 
I've been molding and casting for years, and i never thought of the rice idea.

Thanks for the lesson!

C

Youre welcome, there's a bunch of "second layer" information I didn't include because the text of the post was already getting long. I recommend short grain rice over regular rice as it's got less sharp edges than long grain rice. If you're doing a two part mold where you've clayed up one half, the rounded edges of the rice are less likely to make marks in the clay. Also it's less 'dusty' than long grain rice. Before I settled on rice I tried salt at one point. Huge mistake! I was picking little grains of salt out of the kit parts for hours!

Definition of an Expert - Someone that's made every mistake at least once!
 
This is terrific!

Question: my meager experience is just a few one-piece dump molds, but I did find it easier to just zero out the scale with the empty bucket on it, rather than weighing the bucket and doing math later. What do you think?
 
Treadwell - Yes you can just zero out the scale to weigh the rubber alone. That will work just fine. I do it this way for seveal reasons; First, I'm often pouring rubber out of a 5 gallon bucket and it's really heavy to lift over a bucket on a scale, much easier to lay the the big container of silicone on it's side overhanging the table edge and hold the bucket underneath it, and sometimes the scale will reset or power off before I can get the bucket and rubber back onto it. Second, even if working from a smaller container, this way I can place the mixing bucket on some newsprint to catch spills rather than possibly spill silicone on the scale. Third, I used an "analog" triple beam balance for decades (and still do when the batteries on my digital scale die at an inoportune moment) before getting a digital scale and had to pre-weigh the bucket and subtract it from the gross weight. So a big part is just force of habit. In the end you should use whatever method you feel most comfortable with.
 
Man, you should write a book on that type of subject; very informative and complete as far as I'm concerned:cool
 
That is fantastic. I'll be saving this and making it up into a word document for future reference. Many thanks Dave.


Great idea. Something I've been doing for years , is using PDF my URL site to convert a thread to PDF. It often cuts a picture into two pages, but at least all the info is still there. How many times do I go to a great thread and find all the pics gone. And nobody seems to revisit their threads and replace the missing pics. Some threads on this forum and others are priceless. It's an incredible loss when they disappear.
There are some that I didn't save, and they are gone forever. Disposable world we live in. Oh well at least we will preserve some of the best stuff.
 
Youre welcome, there's a bunch of "second layer" information I didn't include because the text of the post was already getting long. I recommend short grain rice over regular rice as it's got less sharp edges than long grain rice. If you're doing a two part mold where you've clayed up one half, the rounded edges of the rice are less likely to make marks in the clay. Also it's less 'dusty' than long grain rice. Before I settled on rice I tried salt at one point. Huge mistake! I was picking little grains of salt out of the kit parts for hours!

Definition of an Expert - Someone that's made every mistake at least once!

I was wondering why short grain rice. I know you should always use short grain for paella, but wasn't sure on resin casting ;-)
 
Wow. I know the basics but have only done one-piece and jacket molds. I was using a light coat of elmers to stick the parts on. I'm going with your double sided tape idea! Love it!

The rice idea? Never occurred to me! Brilliant!

I have had pretty good luck not getting significant bubbles in silicone or resin without degassing but I'm fairly certain that is blind luck. Most of the time I use Alumimite heavy strength (which is actually very soft, but high tear strength), and a slow pour put on top of my scroll saw for a few minutes to shake the bubbles out (pretty redneck, I know). In the Texas humidity, I'm shocked I don't get that many bubbles with the resin, but I figure the time will come where I have to get a pot!

Thanks for this tutorial! Free knowledge from a pro! I love it!
 
Starting a pour from as high as possible and starting from one corner of the mold box helps too.

This is coming from my phone, expect errors.
 
Great thread. I love the mold rack idea, might have to steal that. I've also used plastic pellets instead of rice.
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When weighing silicone, and trying to beat the scale's auto shut off, I first fill the cup to the level I need. Then I weigh an identical but empty cup, zero out the reading, then replace the empty cup with the filled one.
 

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