Resin Casting Production Jigs: From Prototype to 50 Units

You've got a prototype. Maybe it's 3D printed, maybe it's hand-sculpted, maybe it's a modified off-the-shelf part. Now you need 20, 50, or 100 of them — and injection molding wants a $5,000 mold fee with a 1,000-piece minimum order.

Here's the secret the contract manufacturers don't want you to know: for runs under 200 units, silicone mold casting produces parts that are visually and mechanically indistinguishable from injection-molded plastic. And you can do the whole thing in your garage for under $150 in tooling.

When Casting Beats Injection Molding

What You Need

Tooling total: ~$155 — and the silicone mold lasts 25–50 pours, so your per-unit cost drops fast.

Step 1: Prepare the Master

Your master pattern is what the mold copies. Quality here determines quality of every cast:

• 3D printed master: Sand to 400 grit, then spray with filler primer (2-3 coats). Sand again to 600 grit. The smoother the master, the less finishing on each cast.
• Machined master: Usually good as-is. Light polish with Novus #2 if needed.
• Found object: Clean thoroughly, fill any unwanted features with body filler.

Step 2: Build the Mold Box

For a two-part mold (most enclosure-type parts):

1. Hot-glue the master to a flat piece of foamcore, parting line facing up
2. Build walls around it with foamcore or LEGO bricks — leave 15mm clearance on all sides
3. Seal all joints with hot glue or clay (silicone will find any gap)
4. Add registration keys — press marbles or clay hemispheres into the first half before pouring. These create alignment divots for the second half.

Step 3: Mix and Pour Silicone

Platinum-cure silicone (Mold Star 30) is mixed 1:1 by volume:

1. Measure Part A and Part B into separate cups
2. Combine and stir thoroughly for 3 minutes — scrape the sides and bottom
3. Vacuum degas for 2–4 minutes until the mixture rises, falls, and stops bubbling
4. Pour in a thin stream from 12" height into the lowest corner of the mold box
5. Let silicone flow over the master naturally — don't dump it on top

Cure time: 6 hours at room temperature. Don't rush it.

Step 4: Casting Parts

Once your mold is cured and demolded:

1. Spray mold release into both halves — light even coat, let flash off 5 minutes
2. Mix casting resin per instructions (Smooth-Cast 65D is 1:1 by volume, 5-minute pot life)
3. Vacuum degas the mixed resin for 60 seconds (you have limited pot life — work fast)
4. Pour into mold through the pour spout — thin stream, let gravity fill
5. Close mold halves, clamp with rubber bands or C-clamps
6. Demold in 30 minutes — resin is handleable but not fully cured. Full cure at 24hrs.

Step 5: Production Workflow

For batch production (making 10+ parts in a session):

┌─────────────────────────────────────────┐
│           Batch Casting Cycle            │
│                                         │
│  Prep (spray release)         2 min     │
│  Mix resin                    1 min     │
│  Degas                        1 min     │
│  Pour + close mold            2 min     │
│  Cure (while prepping next)  30 min     │
│  Demold + inspect             2 min     │
│                                         │
│  Cycle time per unit: ~5 min labor      │
│  Parallel molds: 6 units/hour easy      │
└─────────────────────────────────────────┘

With 3 molds running in rotation, you can produce 6 parts per hour with one person. That's 50 units in a single day.

Quality Control

Inspect each part for:

• Bubbles: Surface bubbles mean inadequate degas or mold release pooling. Fix: longer degas, thinner release coat.
• Short shots: Mold not fully filled. Fix: add vent channels cut into the mold parting line with a hobby knife.
• Flash: Thin film at parting line. Normal — trim with a hobby knife or deburring tool.

Cost Per Unit at Scale

Compare that to a contract manufacturer's $5,000 tooling + $3/unit at 1,000 MOQ. For anything under 200 units, you're saving thousands by casting in-house.