Pewter Casting Homework Helper

How to Make Metal Miniatures With 3D Printing

in Tutorials

Ever wanted to turn your digital sculpture into a figurine made out of metal? As much as we love resin 3D prints, metal has a unique weight and cold, smooth surface that is hard to match. With recent developments in temperature resistant 3D printing materials (like High Temp Resin for the Formlabs Form 2), it’s possible to 3D print molds for direct casting.

Compared to directly printing metal, the direct pewter casting process offers significantly better detail and surface finish at a small fraction of the cost. Compared to wax casting, directly printing a mold saves you steps and effort, while preserving the most possible detail. If something goes wrong with your casting and you need to re-try, or if you get tired of a model, just melt it down and use the metal again and again!

In this tutorial, we’ll walk through a process for making fully metal objects out of pewter for decorative applications such as detailed miniatures, jewelry, scale models, and replicas of antiques. We teamed up with Hero Forge to make a mini figure from their online customizer.

What You’ll Need

Step 1: Design or Find the Right Model

This guide assumes you’re starting with an STL or OBJ file, which is the format for most miniatures and models from ZBrush. If you’re using a solid model from SolidWorks or Rhino, the workflow will be a bit different.

We started with a mini figure model from Hero Forge, where you can download your own custom configured characters.

Design Tip: Most models will be castable at some scale. The thinnest part of the model should be at least 1.5 mm, and around 15 mm at the thickest. If features are too thin, the metal part will be too fragile. If features are too thick, the casting won’t develop a smooth surface due to shrinkage. Small features under 1.5 mm are fine.

Step 2: Prepare the Mold Design

You can prepare the mold design using either a sacrificial mold or a pull-apart mold.

Option A: Sacrificial Mold

Most models aren’t designed from the get-go to be moldable. Luckily, you can still create almost anything in metal with a shell mold designed to be broken off later. You can make a shell in Meshmixer or 3D editing software like Blender or Maya. Learn how to make a shell mold from an STL in Autodesk's video.

For larger models (2 - 10 cm in tall), making a uniform shell will save a lot of resin, print time, and money. If you are making a shell, it should be uniformly 3 mm thick to withstand the shrinkage and expansion forces from the molten pewter.

For small models (1 - 2 cm tall), designing a uniform thickness shell may not be worth the effort. Simply subtracting your model from a rectangular volume can save design time.

Whatever the size of the model you’re working with, make sure the inside of the mold can be reached by light during post-curing. We split our mold in half to make post-curing simpler, then glued and clamped it back together before pouring.

Option B: Pull-Apart Mold

Did you design your mold in CAD to pull apart in halves? A conventionally designed two-sided mold, with draft and no undercuts, will work well and can even be reused. However, this method puts limits on your design, and most organic models aren’t designed with molding in mind. If you have a mold already designed, move on to step 3.

Moldmaking with 3D Prints

Download our white paper to learn techniques for prototyping and production and the benefits of different moldmaking processes.

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Air Vents

No matter what type of mold you use, air vents are a must. Metal will flow downwards and air will bubble up, so make sure to add air vents wherever there is an isolated high point in the model (when oriented as it will be filled). Good venting will always improve how well a model fills with pewter.

Step 3: Print the Mold

Print your model in High Temp Resin. Try to orient your model in PreForm such that it doesn’t require internal supports. This is very simple with a two-sided, pull-apart mold: just point the cavity upwards.

With a shell, this can be more challenging, but usually you can find an orientation that does not need interior support. Splitting the shell in half for gluing later is very helpful if you are having trouble finding an orientation that doesn’t require unremovable supports, and also enables uniform post-curing for the interior of the mold.

Step 4: Wash and Post-Cure

In order for High Temp Resin to reach its required thermal properties, it must be fully post-cured. The print will transition from nearly clear to orange as it cures.

Form Cure is designed to make post-curing as fast and easy as possible. Your mold can also be cured in a UV nail spa, which will require several hours of curing time. Heat also accelerates part curing. Learn more about how post-curing works.

Make sure parts are fully dry from washing before use. Moisture will cause air bubbles in your mold and lower the surface quality of your casting.

Tip: Dust the cavity of your mold with graphite powder lubricant. Graphite powder acts as an effective mold release.

Step 5: Clamp the Mold and Pour

Fix your mold in a vise or clamp. You can also partially embed it in a bucket of sand for extra safety, but make sure your air vents are not blocked.

Safety Warning: When casting pewter, make sure no skin is exposed in case of splashing. Wear long pants, closed-toe shoes, long sleeves, and leather gloves. Eye protection is essential. Always cast pewter outdoors or in a well-ventilated area.

There are numerous pewter alloys with different melt temperatures. We use Alloy R98 from Rotometals. R98 is a sturdy pewter alloy which melts around 260 °C.

We melted our pewter ingot on a dedicated small frying pan atop a Primus portable stove, which allowed us to cast outside. As the pewter melts, it will form an oxide layer on top of the puddle. Before pouring, scrape off the oxide with a spoon. Go slowly, and fill your mold in one continuous pour.

It can be tricky to tell when the pewter is at the correct temperature. A low-tech but effective method is to test the molten pewter with a piece of dry wood. If the wood starts smoking, the pewter is too hot and you should let it cool for a few seconds before pouring.

Step 6: Demolding

Wait for your part to completely cool. This can take up to 20 minutes for models with large or thick sections, or only a minute or two for small models. If you are using a sacrificial shell mold, running cold water over the model will help it cool faster.

Finally, demold the part. For a sacrificial shell mold, a small screwdriver works very well as a chisel to break the shell away from the model. Don’t use too much force, or you risk chiselling into your part.

The Power of Pewter

Pewter pieces have their own unique color and patina. The relatively soft alloy can be buffed and polished by hand or with a rotary tool to achieve a high gloss finish. The parts can be easily electroplated in precious metals to simulate gold or silver for beautiful jewelry, or nickel plated for higher mechanical strength. Create replica keys and hood ornaments, the possibilities are endless.

Plunk down your beautiful mini character with pride! Try making your own heroes in pewter, and show us your results in the forums.

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High Temp Resin has an HDT of 289 °C @ 0.45 MPa, ideal for static applications that will undergo higher temperatures.

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Unformatted text preview: Homework 4: Metal Casting 1. (a) Sketch and label the relationship between volume of a pure metal casting and temperature for the range from room temperature to the pouring temperature. Thermal contraction of liquid Solidification shrinkage Thermal contraction of solid R.T. TM Temperature T P (b) A mold is being designed that includes a rectangular mold cavity that is 30 cm x 30 cm in section and 20 cm high. The proposed riser is a cylindrical cavity that is 20 cm in diameter and 10 cm high. If the mold constant is 3 min/cm2 and the mold exponent is 2, determine whether this riser will prevent a defect in the casting. 30 cm 30 cm 20 cm 20 cm 10 cm Casting Riser Mold volume = (30)(30)(20) = 18,000 cm3 Mold surface area = (2)[(30)(30) + (30)(20) + (20)(30)] = 4200 cm2 (V/A) = 18000/4200 = 4.28 cm TTS = Cm (V/A)n = (3 min/cm2)(4.28 cm)2 = 55 min Riser volume = pD2h/4 = p(20)2(10)/4 = 3,140 cm3 Riser surface area = 2 (pD2/4) + pDh = (2)(p)(20)2/4 + (p)(20)(10) = 400 p = 1260 cm2 (V/A) = 3140/1260 = 2.49 cm TTS = Cm (V/A)n = (3 min/cm2)(2.49 cm)2 = 19 min Since the riser solidifies before the casting does, it will not prevent a defect in the casting. 2. (a) If you are provided with the melting point of a pure metal and the superheat used in the casting process, list the other material properties that would be required to estimate the energy required to heat a given volume of the metal to the pouring temperature. (1) Density (2) Heat capacity of solid (3) Heat capacity of liquid (4) Heat of fusion. (b) An alloy in the Cu-Ni system is to be cast using a pouring temperature of 1400°C. If the composition is 80%Cu, what is the value of the superheat? Use the Cu-Ni phase diagram in the PowerPoint presentation on metal casting to answer this question. For 80% Cu - 20% Ni alloy, liquidus temperature = 1200°C. Superheat = Pouring temperature - Liquidus temperature = 1400°C - 1200°C = 200°C 3. (a) Sketch the typical relationship between temperature and time for cooling an alloy from the pouring temperature to room temperature during casting. T P Solidification of alloy Tliq Tsol Time (b) List three variables that can be used to control the fluidity of liquid metals in the pouring stage of metal casting. (1) Composition (2) Superheat (3) Heat transfer. 4. (a) Investment casting of 200 turbine blades is to be undertaken with a nickel alloy. Draw a flow diagram that describes the process. Fabricate wax patterns Connect to tree Coat with refractory Drain wax Preheat mold Melt metal Pour metal Solidification and cooling Break out of mold Cleaning and inspection Finished casting (b) Explain why sand casting of the 200 turbine blades in part (a) would require fewer patterns than investment casting. For sand casting, the same pattern can be reused a number of times, so fewer than 200 patterns are needed to cast 200 turbine blades. For investment casting, the wax pattern is melted before the casting is made, so a new pattern is required for each casting. For 200 turbine blades, 200 patterns are needed. ...
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