Introduction

This tutorial is going to cover a method for generating accurate metal parts using 3d printed molds, as well as a functional part I fabricated using this method.

I built a camera arm to document projects a few months ago. The joints are clamped down with rubber pads between them to prevent slipping, but I noticed the joint at the base kept creeping down over long periods of time (like while capturing time lapses). I decided to fix it with a locking plate like you'd find on C-stands, but wanted to avoid using a printed part that would chew itself up after a few months of use.

I felt that a metal locking plate would be ideal, and I'd been fiddling around with stovetop casting alloys anyway. The locking plate assembly is just two copies of the same part that have teeth that mesh together to prevent them from slipping past one another once clamped down.

If you're familiar with some of my other casting projects, you might notice that this metal casting process is similar to the one I used to generate the waxes that are part of the Glaucus build.

The broad strokes of the method are that you design the part you'd like at the end in CAD, design a floor under your part with walls around it (I call this a bathtub), print the bathtub mold you designed, cast the mold using 2-part silicone (making sure it's nice and level), and cast your final material into that mold. Once you've got the knack of replicating parts using 1-part molds, you can get fancier: adding vent holes for letting air escape or labels for your parts or building multiple parts for your molds for even more precise geometry.

Theory and Terminology

First, I'd like to cover some mold making basics to help you generate working molds right off the bat. I use SolidWorks to build CAD, but this method should be easy to replicate in Fusion360, OpenSCAD, or any other program designed for making mechanical components.

The simplest mold I know of is the 1-part open mold. This type of mold mold (example below) is an open cavity with the negative impression of the part you'd like at the end embedded in it. When you pour a liquid material into a mold, it wants to find its level just like water would, so your part is going to have a flat surface wherever the liquid finds that level. If you have cavities inside your mold that trap air, your casting material will be prevented from filling that area just like the s-bend in a toilet prevents the water in the bowl from running down into the sewer until it is flushed.

The large top-hat-shaped mass in the middle of the metal casting pictured below is a well, and the branches leading from it to the stair-stepped ring are called runners. When dealing with a material that you heat up to a liquid state to pour, and then wait for it to cool and freeze to unmold (like wax, metal, chocolate, or Jello), it's important to manage exactly how it flows.

If your casting material cools before it's had a chance to push all the air out of your mold, it will freeze in place and prevent any additional hot material you're pouring in from pushing past it. This is why having a well in your mold (a place where there's a big thermal mass where everything flows out from) helps materials like metal form a good casting. Since there's such a sharp transition between liquid and solid metal, it's helpful to have it as hot as possible for as long as possible while it fills your mold.

However, you wouldn't want to connect your well directly to your part, since you'd have to cut or grind the well away from it at the end of casting. This is the purpose of runners. Runners take hot material from the well and distribute it evenly to the part. Runners are designed to be thick enough to prevent material from freezing or restricting as it passes through, but thin enough to be easily separated from the part when it's being finished.

After trying some experiments with a 1-part mold, I decided to optimize the design a bit more. The challenge was that the metal on the top surface of the mold wasn't ending up flat and even like I'd hoped. I could definitely make these parts work for my design by flattening the back with some sandpaper and snipping off the excess material with clippers, but I thought it would be even better to design a 2-part mold to save anyone replicating the project the trouble of doing all that hand labor themselves.

Below, you'll see a picture of the final design. This mold has a few additional elements besides the well and runners. I extended the well upwards through the new second mold half, forming a hole in which the metal gets poured. This entrance hole is called a sprue. I also added small vent holes at the four corners of the part to allow air to escape as the mold is being filled (remember that the metal being poured in will need to displace the air inside the cavity now that this 2-part mold is totally enclosed). These small vents let air out, but when the metal hits them, it cools quickly. This makes for a neat little mechanical action where the metal pushes out all the air, but can't escape, itself.

This updated mold also has keys (the troughs going around the metal part in the picture below, which are mirrored in the silicone part on the left). Mold keys make sure the mold components are aligned during casting. Since this design is rotationally symmetrical, it's alright that these keys are also symmetrical, but if your molds need to lock in in a very specific orientation, make sure you design your keys to only allow it to fit together one way (I've been burned by that one before).

I also designed labels right into the mold - engraving a mirror image of the text I wanted to appear on the silicone molds into the 3d printed parts. This helps keep version numbers straight. Given that printing your molds allows for iterative prototyping, it's easy to generate a bunch of molds that look a lot like one another and it's helpful to add in features that help prevent mixing up mold components or casting a deprecated design.

Materials and Tools

You can download all the 3d files to generate build your own locking plates here.

That page also includes the SolidWorks files so you can go into the CAD and see into my decision making process a bit more. The locking plates are part of the Flat-Pack Camera Arm project.

To build a set of locking plates for yourself you'll need:

Materials

  • Plywood
  • ~1lb Tin-Bismuth Alloy (I got mine from RotoMetals - Alloy #281)
  • 1 Pint or more 2-Part Silicone Mold Rubber (I used Smooth-On MoldStar 15, but just about any silicone intended for mold making will hold up to these temperatures - check the material's datasheet to confirm)
  • Baby Powder
  • Mixing Cup
  • Stir Sticks or Tongue Depressors
  • Nitrile Gloves
  • 8x 0.5" long Drywall or Wood Screws

Tools

  • Hand Drill
  • 0.5" Drill Bit
  • Hot Plate
  • Cheap Cast Iron Skillet
  • Chip Brush
  • Temperature Gun
  • Pliers
  • Hammer
  • Heat Gun (optional)
  • 1' x 1' Melamine Board
  • Spirit Level
  • Digital Scale
  • Pilot Drill for Screws
  • Heat-Proof Gloves

Molding silicones can tolerate high temperatures (typically 400° F or above). This makes them ideal for low-temp metal casting alloys like the tin-bismuth I'm using here. There are special mixes that can handle much higher temps, but normally silicones won't be able to cast higher temperature melting metals like Aluminum.

I'm using a Tin-Bismuth alloy that is 58% Bismuth and 42% Tin, which gives the metal a melting temperature of 281° F. There are other alloys that will work well with silicones going by the names "stovetop casting alloy", "bullet casting alloy", "low melt fusible alloy", and "fishing tackle weight alloy". Just make certain they melt below your silicone's rated max temp (compare datasheets for your metal and your silicone) and stay away from alloys containing lead. Although lead is relatively benign in most cases, breathing its casting fumes or long term contact that may lead to ingestion is a serious health hazard.

The heat gun is optional in this case because most molds take a few tries to pull a successful casting from. Pouring a hot casting material into a cold mold usually results in a sub-par casting, but if you know you're going to run the mold a few times, there's little harm in taking a crack at a casting to get your eye in. Casting a metal part or two into this mold will heat it up to an ideal temperature for subsequent castings, however you can improve your odds of a great first time casting by using a heat gun to preheat the silicone mold to between 100-115° F before pouring.

Print

Printing molds isn't tough, though you do have to dial in your printer settings to get parts that are up for the job. The most important thing to remember is that flaws in your printed parts will show up as flaws in your silicone castings.

The parts pictured above are the ones you'll need to print to replicate the locking plate. These are "LOCKING-PLATE_pattern_bathtub_mold_02.STL" and "LOCKING-PLATE_sprue_bathtub_mold_02.STL" in the "Locking Plate for Flat Pack Camera Arm" Thingiverse project.

The things to look out for are stringing across the print, cracks along the layer lines, and underfilling. Your printed part needs to hold silicone in without leaking for a few hours until it cures, which means the printed layers need to be watertight. I find that taking it slow and printing with at least a 1.2mm wall thickness gets me dependable parts on my Ultimaker 2+. You'll also want to set things up to prevent warping, whether that's using a raft to keep your part's corners from lifting or enclosing your printer to avoid drafts.

Once your print is complete, clean up any support material or stray wisps of filament. A light blast with a heat gun can melt and clean up fine filament hairs.

Silicone Casting

Since these molds are designed to be filled right up to the top with silicone, you need to make sure things are level before casting. I do this by taking a sheet of wood (melamine works great because its plastic surface is easy to clean) and shimming its corners with paper or tongue depressors until it is level in both the X and Y axes.

Next, you'll need to mix equal parts of your silicone A and B by weight. This mold requires 8.5 fl oz of liquid silicone. Since you're going to lose a bit in the cup when you pour, it's good to have some excess, so mix together 5 fl oz of part A and 5 fl oz of part B to bring the combined total to 10 fl oz.

Put on your nitrile gloves and place your cup on to your digital scale, tare the scale to cancel out the weight of your cup, and pour in your silicone. When you've got the right amount, mix it thoroughly with a stir stick or tongue depressor. Stir for at least a minute, making sure to scrape the sides and bottom of the cup to ensure there are not unmixed areas that could end up as uncured goo in your finished mold.

Pour your silicone into your mold halves slowly, in a thin stream, holding the cup about 6" or more above the mold. This thin stream will help get bubbles that were introduced to the liquid while stirring pop before landing in your casting. Keep pouring until the liquid is level with the lip of your prints.

Allow the silicone to cure. This will vary between products, so refer to your datasheet on that. The one I'm using, MoldMax 15, takes 4 hours to cure at room temp. If you're casting in a hot place, that time will be a bit shorter, and it will be the opposite in a cold environment.

After it's cured, you can pull the mold. I designed these ones with pull tabs to make the process easier. However, silicone is a forgiving material, and if you design a mold without pull tabs, you should be able to use a tongue depressor or blunt-edged thin metal tool like a stiff-blade putty knife to pry the casting out of the mold. 

Metal Casting

Cut a piece of plywood about 0.5" larger than the mold in X and Y dimensions. Drill a 0.5" hole in the center of the piece. This will allow you to add weight to the mold to ensure the liquid metal doesn't push the two halves apart. I used a couple of bins of screws (about a pound in total) to weigh down the mold.

If you're prepping to pour your first casting of the day, dust insides of the mold with baby powder and brush off any excess. Then, preheat the mold with a heat gun to around 115° F (double check with the temp gun).

Heat up your metal in a cast iron skillet on a hot plate. I like the hot plates with sealed heating elements so that spills are easy to clean and don't risk pouring down into the plate's innards. When the metal is melted and evenly heated through (check that its temp is above the material's listed melting temp with your heat gun) you're ready to pour.

Turn off the hot plate, put on your heat-proof gloves, position the skillet over the sprue of your mold, and pour in a constant, even stream into your mold.

It takes some practice to get a feel for how much metal you'll need to use to fill the mold. Since the metal takes a few seconds to flow through the gates and fill the mold, the feedback you get from watching the metal level in the sprue isn't a great indicator of how close the mold is to full. That's something you get a feel for after you work with the mold a few times.

Sometimes metal will pour out the sides of the mold or out of the gates, but this isn't usually a problem. Just keep filling the sprue with metal, keeping it as close to the top of the silicone mold as possible, until the leak stops.

Once your mold is filled, set the skillet on a heat-proof surface, and wait at least 5 minutes for the mold to cool. For larger metal castings cooling will take longer, but for something small like this 5 minutes is a good safe cooling time. After that, it's fine to open up the mold to speed up cooling.

Once the metal has cooled off enough to be safe to handle (~100° F), take it out of the mold and allow it to come down to room temp.

Finishing

Make sure to allow part to cool to room temperature. There is a transition temp where the metal will act a little like clay instead of a rigid metal, and you risk warping it if you try cleaning up the part before it's fully cool.

Once it's cooled, you can use a hammer to knock out the sprue/well and clippers snip off any flashing or vent artifacts. Although the inner ring where the vents connected with the sprue doesn't need to be perfectly clean and sanded for this component to function, you should inspect it to make sure the vents broke off cleanly and file it to shape up those edges.

Remember recycle all your casting scraps back into the skillet. They will re-melt just like the original ingot.

Check the fit between castings once you have a couple of copies. Check for errors like warping or squashed geometry due to too much weight. Take notes on what's working for your setup so it's easy to dial in to a perfect casting.

Installation

To install the clamping plates on to the Flat-Pack Camera Arm, lay one of the plates on the camera arm so that the ring is centered on the hinge of the camera arm. Mark out the four holes on its corners, and drill them to a depth of 0.5" with the appropriate pilot drill for some small wood or drywall screws (in my case it was a 0.125" drill).

Screw the locking plate down, and repeat the process with a second locking plate on the other side of the hinge.

Once your locking plates are installed, you're ready to assemble the arm. Operating the mechanism is simple - by backing out the hand knob that keeps pressure on the plate a couple of turns, you give the teeth enough space to clear one another. This will allow you to freely reposition the arm before tightening the knob and securing it back together again.

Hopefully you've now got your own ideas brewing about what you could do with the ability to get metal parts out of your 3D printer. This method could easily be modified for casting metal figurines, geometric designs in chocolate, plastic cosplay props, or LED diffusers. Not having to rely on a master to make a mold around really frees you up to design your whole mold making process and iterate on it to dial in some really spectacular parts.

Now that I've got this locking plate, my camera arm is proving to be a really great tool for documenting projects. It's especially useful since I'm almost through with writing a book of soft robotics projects for MAKE along with my co-author Kari Love.

I'd love to see what kinds of projects you come up with using this method. You can see a little more detail on this project, along with more of my work here.

This guide was first published on Feb 20, 2018. It was last updated on Sep 05, 2018.