I had a pretty detailed Tutorial a few montsh back but my store site was closed down and I no longer have the page active, so I will be reconstructing as much of that info here as I can. I'll be covering the basics of mold building and some more advanced stuff like pressure casting.
I will add stuff in pieces over time, I'm going to approach this thread as a living text so I'll add in new finds and information as I come across it. My knowledge is nowhere near complete and I'm very interested in hearing what techniques you've used sucessful or not so feel free to add you comments. Most everything I've learned so far has been from internet searches and talking to more experienced casters, hopefully you'll find it helpful and you can use it to avoid some of the mistakes and pitfalls I had to fumble through as part of learning.
The first and most critical thing to remember as you start venturing into the world of casting is that a lot of mistakes will be made. Accept that you'll screw up and that the early projects may not come out well. Sucessful casting means constantly experimenting and refining your work, so don't get frustrated if the first attempts fail. Figure out what did work correctly and figure out ways to correct the things that didn't work.
I wasted a lot of material when I initally started casting and wrote it off as a complete loss and something I'd never do correctly, but a year later I picked up a new starter kit and resolved to give it another try and with that second attempt I finally made some sucessful casts. I discovered that a large part of my inital failures were due to using the wrong materials, and most of all in-experience. I hadn't really read up much and didn't investigate techniques as well as I could have. But if you are reading this then you're already ahead of where I was when I started.
The first thing you need to do is pick up a casting kit, this will include a number of items most importantly the resin and silicon as well as release agents. I highly recommend using the starter kit from Micro Mark. I've also used a kit from Aluminite but had some problems with the resin as it's thicker and has a short working time, so it's not the kit I'd recommend for a beginner. The Micro Mark stuff is excellent the resin is very thin and has a longer working life menaing it's much more forgiving.
The most basic type of mold is a single sided mold, this works well for flat parts with no details on the rear. Armor plates, capes, purity seals, doors, walls, or miniature bases.
Press molding makes use of a crude single sided mold. Press molding is done by using a piece of putty, greenstuff, wax or other material that will take the shape or relief of whatever object you press into it. You remove the piece and let the putty harden, then you can press a piece of greenstuff or putty into the form you created and it should form a duplicate.
Press molding is quick and can be used to to create parts and details to fit curved surfaces as the piece is flexible until it cures. The advatage of press molding is that you won't need any materials beyond greenstuff.
Casting resin ina single sided mold works a bit differantly but is still a fairly simple process. You place the object on a thick piece of styrene, wood or other solid material which will serve as the bottom of your mold form. You'll need to glue of pin the piece down so that it doesn't lift or float up. You'll probably want to use a glue that you can remove later. White glue works great, gives enough grip to keep it from moving but comes off easily later. White glue takes a while to dry on plastic so make sure you give it plenty of time to dry.
Once your piece is attached you build the walls of the form, this can be done with styrene or any other solid material. You want to keep the form walls at least a 1/2 of a inch away from the part, if it is closer the mold may be too weak and tear. If you are casting very small parts like purity seals you can get away with using walls that are about a 1/4 inch away but anything larger than a purity seal you'll want to stick to the 1/2 inch buffer.
Here is a really great link that shows the basic process:
A lot of people use wooden forms that they cut to the correct size, this works well but requires additional wood working tools. You can use styrene which works well, you can get stryene sheets quite cheap by picking up for sale signs at the local hardware stores, overhead lighting panels also work great for this as they provide a huge amount of material compaired to the stuff you get at most hobby stores. You just want to make sure it's stiff enough, the super thin stuff may be too flexible and weak. Styrene is pretty much a one shot use for mold forms as it'll be ruined by teraing or warping when you take the form apart.
Legos work incredibly well for making mold forms, they fit together tighly and be constructed into all sorts of shapes. I use legos for almost all of my molds.
You can also use rings for PVC pipe for smaller mold forms
Once you have the form walls constructed you need to make sure it sits flush on the bottom piece of the form so there's no gaps. (legos have a nice straight edge which is another reason I like them) Make sure the form walls are glued down securely, or held in place with a seam of non-hardening modelling clay. (the clay coems with most start up kits) This prevents the liquid silicon from leaking out of the form. Seal any cracks and you're ready to pour the silcon.
Make sure the area you're working on is covered with newspaper or a workcloth as the silicon can get very messy and it'll discolor a lot of surfaces if it spills. Also make sure the work surface is level, so that the silicon will set up level in the mold.
Mix the silicon by the directions, preferrably in a disposable cup. Make sure you mix it thoroughly by scraping along the sides and bottom whenever possible, try not to mix too quickly as you want to avoid mixing in air bubbles. An excess of air bubbles in the mold weakens it and can lead to tearing or causing the surface of the cast to look bumpy.
If the model has a lot of deep details or undercuts you may want to give the model a coating of mold release agent, this helps the model pop out of the cured mold a lot easier and helps prevent tearing. Just make sure it's a thin coat and it doesn't distort any of the details. I use the spray on release from micromark. Every company makes it's own brand so you'll want to go with that, if you use a differant brand of release it can have odd reactions with the silicon.
Once everything is mixed you're ready to pour, but don't just dump the silicon into the mold. There's a tech to pouring which helps you minimize the airbubbles. You need to pour from at least 10 inches above the mold and use a very thin stream of silicon. It'll take you a couple minutes to pour if you are doing it properly. The reason you pour with such a thin stream is that it forces a lot of the air out as it passes over the lip of the cup and stretches downwards. Because the stream is very thin it'll be a bit difficult to control and any breeze or drafts may blow it around which is why you want your work area properly covered, even witha very careful pour you will ened up getting some outside the mold.
Additionally you don't want to pour directly on top of the part, you want to pour at the corners of the mold, that way the material is slowly flowing towards the model as it builds up, this helps avoid collecting bubbles on the models surface and the silicon slowly fills up the mold form. The silicon is self leveling so just keeping pouring from the corner and it'll flow to the rest of the form. Make sure that the model is completely covered and you want to make sure there's enough material between the top of the model and the top of the mold. So if you're casting a 1/2 inch tall model you want a mold that is at least 1 inch thick.
Once everything is poured you simply wait until everything cures, this can take anywhere from 4 hours to 24 hours depending on the brand of the silicon. The micro mark brand silicon in their starter kit sets in about 4 hours, but it's slightly weaker than other brands that set in 24 hours.
Once the mold is cured you remove it from the form. If it's difficult to remove go slow and work differant areas until it comes free. You want to avoid using too much pressure as it can tear a mold. Remove the model piece from the silicon and now you'll have a completed mold. It'll be flat on one side and have the impression of the model on the other side, hence the name single sided mold.
Now you can use it to cast reproductions of your part with resin or plaster.
Make sure to give the mold a coat of mold release before each casting.
Mix the resin or plaster according to the direction. Resins have a working life which is the amount of time you have before the resin transforms into a solid, some brands have a very short working life other have a long one. You should be able to mix your resin within 10-15 seconds of stirring. Make sure it's mixed thoroughly scraping the sides and bottom. Like mixing the silicon you want to avoid mixing in air, so stir slowly don't whip up bubbles.
Once mixed properly pour it into the mold until it is flush with the molds surface. (this is why using a level work surface is important) You don't need to poor the resin from high up so you can just pour right into the mold. It's a good idea to give the mold a few gentle taps to help dislodge any bubbles and make sure the resin seeps into all of the rescessed areas.
Let the resin cure fully before removing it. The resin will also get quite hot as the chemicals fuse, do not add any additional heat from lamps, as this will cause the resin to boil and ruin the cast. As resin cures it will shrink down a bit, which will give the back surface a tiny bit of a lip but it should be even all the way around the parts edge (if the work surface is level). Once it's fully cured pop it out of the mold, don't yank at it or pull with force if it fights just work it free a little bit at a time by flexing the mold carefully. File the part smooth once it's had time to completely harden up and your done.
Most resins will have a demold time of 15 minutes to an hour depending on the brand that's the amount of time the resin needs to set before it's hard enough to remove from the mold. Sometimes the resin will still be soft and flexible even if the demold time has passed, just give it some more time before you take it out. The thickness of the parts will effect how long it needs ot harden up, thinner parts will take longer than a thick part as they don't build up the heat needed to cure as fast as a large part. As you work with more castings you'll get a feel for if the demold time is accurate or not.
While it will be hard enough to remove from teh mold and handle the resin will still be a bit soft for a while and will be difficult to work with. Most resin takes about 24 hours to completely cure, for the best results let it sit for a day before you remove any sprues or do any cutting and filing. Cold inhibits the curing process, and excess heat will cause it to cure unevenly so just let it sit at normal room tempature and do it's thing.
A two part mold allows for you to cast a model that has detail on all sides and won’t have a flat back piece like parts made in a single sided mold. In it’s most basic form a two part mold if like having two separate single side molds placed face to face that way each mold forms half of the object. In practice it’s a bit more complex than that but that’s the basic idea behind how a 2 sided mold works.
There are two separate techniques to build a two sided mold, the first is a hanging mold and the second is with a clay base. Both techniques are used to place the sprue and model in the center of the mold, but each has a slightly different process.
Anyone that has ever built a plastic model kit or resin is familiar with sprues, they are the extra material that comes attached to the parts. Sprues are important because you will need them as a way of feeding the resin into the form and also allowing air to escape as the resin flows in. Before we can discuss the mold making we need to look at how to construct the sprue as it will determine which type of mold to make and also the dimensions and design of the mold.
Sprues and the attached model form a cavity inside the mold, it is a negative space in the shape of the parts. When resin or other liquids enter the mold it will take the shape of that cavity and then it cures in that shape. Once it’s hardened you have a duplicate of the original part. The shape and design of the sprue determines how the material enters and fills the cavities.
1. Gate
2. Feed Channel
3. Part Cavity
4. Vent Channel
The first part of the sprue or mold is called the gate this is where resin enters the mold, usually it is funnel shaped to help resin flow into the smaller branches. These branches are the second part and called “feed channels”, they are like pipes for the resin to flow through, these pipes or channels will run from the gate to each part you want to cast. This supplies the resin to the cavity.
The third part is the cavity, which is the hollow space in the shape of the Master part. The channels attach to the cavity which feed resin into it. In order for the resin to displace the air inside the cavity air needs a point of exit, so a second set of channels is added to form the exit path or “vent channels”. The vent channels lead from the part to the outside edge of the mold allowing air to escape the mold as the resin fills the internal cavities.
Gravity is what causes the resin to flow down into the mold, resin like water is a self leveling fluid so it will keep draining down into the mold until it has reached a point where it is level with the resin in the gate. Because of this the gate and the vent channels must be located on the top of the mold otherwise if they are on the side or bottom of the mold the resin will simply run out of the mold.
It takes a bit for the resin to drain down into the mold completely so the gate will act a bit like a resevoir and provides the push to force the resin through the channels, that why it's important to have the part cavities lower than the gate whenever possible.
Here's shots of an actual mold showing how I set up the sprue:
Black: The gate and feed channels
Grey: The cavity body
White Vent channels
As the resin is poured it'll flow from the black to the grey areas and finally the white. It's important to make sure the resin is always moving in one direction. Basically you want the resin to enter the body of the cavity from the bottom and rise up to the top that way it forces the air up and out of the mold. If you try and have the resin enter from the top of a cavity or the side it may trap air below the resin and cause bubbles to form.
You'll note on the mold to the right the fusion blaster has a separate feed going to the battery/ammo clip that sticks out of the side, this is because resin was not flowing down into it from the blasters body and it was trapping air so it required it's own feed to keep bubbles from being trapped.
The position of the parts is also important as it will help control where air pools and collects, causing trap points which results in bubbles in the part. Air will always rise to the highest point in the cavity; this is where you will want to attach the vent channels so that the air will leave through the channel rather than being trapped in the cavity. Also it is generally a good idea to place the wider or thicker points of the model at the bottom as it'll force the air up into the thinner area where you'll only need a single vent rather than 2 or more that a wide piece would need.
If the fusion blaster had been placed tip up instead I would need vents at both barrel ends and another vent at the ammo clip as all of those areas would trap air pockets. By analyzing the key trap points ahead of time I could position it to cut down on the vents I'd need. The number of vent channels or feed channels doesn't really matter, but if you use fewer channels it'll mean less clean up time as there's fewer sprue contact points.
Here's another example:
On each of the parts air will rise to the top and trap in the upper surface of the cavity, so we attach vent channels to those areas. On the left and rightmost triangles all of the air rises up into the upper tip and then out through the vent. The middle triangle however has a large flat edge at the top of the piece which can cause air to trap all along that edge, you can add a vent at each corner but air may still trap in the middle of that edge, so you want to avoid positioning parts like that. By paying attention to the angles and position of the piece you can save a lot of headache over trapped air bubbles.
Also the thickness of the feeds and vents will also impact how bubbles react. The more easily the resin flows through the part the less likely that air will remain trapped. If the resin moves through the part freely and quickly it'll sweep the air out as it flows through. If the channels are small and the resin moves slowly it may not have enough push to displace all of the air.
Here are some stormtrooper heads I made, which came out with virtually bubbles free. The only areas that have any trapped air were in the arm of the sprue which is waste material anyway. In comparison to the size of the heads the channels are quite wide so the resin flows through it quickly and bubbles don't trap in the fine face details. If the channels were small bubbles might trap around the eye or face area.
The reason the air bubbles trapped in the sprue arm was because they are horizontal to the base of the mold, if I had given them a slight upwards slope at the end of the arm all of the air would rise towards the elevated end and not leave bubbles. If you have large flat parts you want to make sure they are at a slight angle on one end so that the air rises towards that area and out through a vent.
To be continued.
