12. Molding & Casting

Overview

This week I worked through a three stage molding and casting process. The goal was to design a form, mill it into wax to create a positive mold, cast silicone from that to create a negative mold, and finally pour plaster into the silicone to produce the final object. Each stage feeds directly into the next — a mistake at any point carries through to the end.

Safety — Materials and Data Sheets

Before working with any molding or casting material it is essential to review the safety data sheet for each product. The three materials used in this process each carry different risks:

• Milling wax — low risk, no significant fumes, but dust should not be inhaled. Use the machine’s dust extraction where possible.

• Silicone — the two-part silicone used for mold making contains a base and a catalyst. Avoid skin contact, work in a ventilated space, and follow the manufacturer’s mix ratio precisely. Incorrect ratios result in silicone that never fully cures.

• Plaster — generates heat during curing (exothermic reaction). Do not pour into a closed mold without a vent. Dust is a respiratory irritant — avoid inhaling during mixing.

Design — Ring Form in Rhino

The object I chose to cast was a ring form — an oval shaped ring extruding from a flat base, designed so that the height of the ring varies as it goes around, giving it a more dynamic profile under the finger rather than a flat uniform band.

The concept connects directly to my final project. I’m exploring whether this cast plaster ring could serve as a substitute for the capacitive touch ring I’m envisioning — and specifically whether it’s possible to electroplate the surface to make it conductive. Plaster is non-conductive and porous, which makes electroplating a challenge, but there are methods worth investigating.

Modeling in Rhino

• Step 1 — Model the wax block Start by modeling the exact dimensions of the wax block you will be milling. This gives you a physical boundary to design within and ensures nothing in your model exceeds the available material.

• Step 2 — Design for demolding Before drawing anything, keep two rules in mind: No undercuts — the walls of the mold must be slanted outward slightly so the silicone can be poured in and pulled out cleanly. A vertical or inward-leaning wall will trap the silicone.

• Leave headroom — leave enough wax above the highest point of your model to give the silicone a solid base when poured. If the ring is too close to the top surface of the wax, the silicone mold will be too thin and fragile at that point.

• Step 3 — Model the ring Draw an oval profile and extrude it upward. Vary the height around the ring using control point editing or a loft between cross sections — this gives the surface its dynamic quality. Keep the form clean and smooth, as any surface imperfection in the Rhino model will transfer through to the silicone and then into the final plaster piece.

• Step 4 — Export Once the model is complete, export it as an STL file — this is the format Modela 4 requires.

Toolpath — Modela 4

Modela 2 uses two separate operations to mill the wax — a roughing pass that removes the bulk of the material quickly, and a finishing pass that refines the surface detail. Each operation uses a different bit.

Machine and Tool Setup

• Step 1 — Set the machine Go to File > Set Machine and select SRM-20.

• Step 2 — Set the roughing tool Go to Options > Set Tools and configure the first tool:

  • Tool name: ⅛ inch flat end mill
  • Diameter: 3.18mm
  • Cutting material: Molding Wax
  • Adjust the cutting parameters and press Register to save.

Roughing Operation

• Step 3 — Create a new roughing operation Go to New Operation > Roughing. The default settings are mostly fine. The one value worth adjusting is the cut depth per pass — set this to 1.2mm for wax. This keeps each pass shallow enough to produce a clean cut without overloading the bit.

• Review the toolpath preview, then save the operation.

Finishing Operation

• Step 4 — Set the finishing tool Go back to Options > Set Tools and configure the second tool:

  • Tool name: ⅛ inch ball nose end mill
  • Diameter: 3.18mm
  • Same material setting: Molding Wax
  • Configure and Register.

• Step 5 — Create a new finishing operation Go to New Operation > Finishing. The ball nose bit follows the surface contours more precisely than the flat end mill, picking up the curved detail of the ring. Use the default settings.

• Review the toolpath preview, then save the operation.

Running the Job

• Step 6 — Send to the mill Press Cut. Modela 4 will queue both operations — roughing first, finishing second. You will run them sequentially on the machine, changing the bit between operations.

Milling the Wax — Roland SRM-20

Machine Setup

• Step 1 — Fix the wax block Secure the wax block firmly to the machine bed using double-sided tape. Press down across the entire base to avoid any flex or movement during cutting.

• Step 2 — Install the roughing bit Insert the ⅛ inch flat end mill into the collet and tighten securely.

• Step 3 — Zero the coordinates Jog the machine to align the bit with the bottom-left corner of the wax block and zero X and Y

• Move the bit to the center of the block, lower it until it just touches the surface, and zero Z

**Problem — Misaligned Toolpath

When I ran the first pass it was immediately clear something was wrong. The cutting path was misaligned with the block — off by 90 degrees. The tool was cutting in completely the wrong orientation relative to the wax. My first instinct was to rotate the wax block and re-zero. But the real problem was upstream — the STL file imported into Modela was not oriented to match the physical orientation of the wax block on the machine bed

The fix:

• Go back to the Rhino file and reorient the model so its axis matches the orientation of the wax block on the bed

• Re-export as STL

• Re-import into Modela 4 and redo both the roughing and finishing operations from scratch

• Return to the machine, re-zero all coordinates, and restart

It was time consuming but an important lesson — always check the orientation of your model relative to your stock before running the job. A quick check at the Modela preview stage would have caught this before any material was touched.

Roughing Pass

With the corrected toolpath loaded, I ran the roughing operation. The flat end mill removed the bulk of the wax efficiently, cutting down in 1.2mm passes to rough out the ring form.

Finishing Pass

Once roughing was complete I swapped the flat end mill for the ⅛ inch ball nose bit, re-zeroed Z, and ran the finishing operation. The ball nose follows the curved surface of the ring precisely, smoothing out the stepped texture left by the roughing pass and bringing the surface to a clean finish ready for silicone casting.

[Photos of finishing pass and completed wax mold here]

The surface came off the machine clean and smooth — no post-processing needed before moving to the silicone stage.

Casting the Silicone Mold

Silicone mold making is a two-part process — a base and a catalyst that when combined begin a chemical curing reaction. The mix ratio, mixing technique, and pouring method all directly affect the quality of the final mold.

Mixing

• Step 1 — Measure Measure equal parts of Part A and Part B by weight — a 1:1 ratio. I used approximately 90g of each, giving a total of 180g. Measuring by weight is more accurate than by volume and ensures the chemical reaction cures correctly. An incorrect ratio means the silicone will either never fully cure or cure with a tacky, soft surface.

• Step 2 — Mix Combine both parts in a single container and mix continuously by hand for 10 minutes without stopping. This is longer than it feels. Scrape the sides and bottom of the container regularly throughout — unmixed material clinging to the edges will leave uncured patches in the mold. The mixture is ready when the color and consistency are completely uniform with no streaks.

Removing Bubbles — Vacuum Chamber

• Step 3 — Degas the silicone Pour the mixed silicone into a vacuum chamber. Seal the lid and activate the pump to draw out the air. As the pressure drops, bubbles trapped in the mixture will rise and expand to the surface. When bubbles appear, briefly open the air release valve to break the surface tension, then reseal and re-pressurize the chamber. Repeat this cycle until no new bubbles appear. This step is critical — any bubble left in the silicone will show up as a void on the mold surface and transfer into every cast made from it.

Pouring

• Step 4 — Pour into the wax mold Pour the degassed silicone immediately — do not let it sit. Choose one corner of the wax mold and pour from that single point only, letting the silicone flow naturally across the surface rather than pouring directly onto the model. Pour from a height of at least 20–30cm — the thin stream falling from a distance helps break any remaining bubbles before the silicone hits the mold.

• Continue pouring slowly and steadily until the silicone covers the highest point of the model by at least 5–10mm. This ensures the mold wall is thick enough to be handled without tearing.

• Step 5 — Cure Leave undisturbed on a flat surface. I left mine over the weekend — approximately 48 hours. Do not rush this. Demolding too early produces a mold that is soft, tears easily, and loses fine surface detail.

• Step 6 — Demold Once fully cured, gently flex the silicone and peel it away from the wax starting from the edges. The flexibility of the silicone allows it to release from the form cleanly without damaging the wax — which can be reused if needed.

Casting the Final Piece — Plaster

Measuring

• Step 1 — Calculate the volume needed Based on the size of the silicone mold I estimated I would need approximately 160g of plaster powder. This product uses a ratio of 4 to 4.5 parts plaster to 1 part water by weight, so for 160g of plaster I measured out 40g of water.

• Getting the ratio right matters — too much water produces a weak, crumbly cast. Too little and the mixture sets before it can be poured properly. Mixing

• Step 2 — Add plaster to water Always add the plaster powder to the water — never the other way around. Sift the powder in gradually rather than dumping it all at once. This prevents dry clumps from forming at the bottom of the container.

• Step 3 — Mix continuously Stir immediately and mix continuously without stopping until the mixture is completely smooth and lump free. The consistency should be uniform and pourable — similar to thick cream. Do not let it sit between mixing and pouring as plaster begins to set quickly once the reaction starts.

Pouring and Curing

• Step 4 — Pour into the silicone mold Pour the mixture immediately into the silicone mold from a single corner, letting it flow naturally across the form. Tap the mold gently on the table a few times after pouring to bring any trapped air bubbles to the surface.

• Step 5 — Cure Leave undisturbed for 1 hour. The plaster will generate mild heat as it sets — this is a normal exothermic reaction and a sign the curing is working correctly. Do not attempt to demold before the hour is up.

• Step 6 — Demold Once cured, gently peel back the silicone mold from the edges. The flexibility of the silicone releases the plaster cleanly without chipping or cracking the cast surface.