Silicone vacuum casting produces plastic parts by pouring polyurethane resin into a silicone mold under vacuum. A professional mold yields 20–25 parts at ±0.05 mm tolerance and a surface finish that closely matches injection molding. The vacuum step removes air during resin fill. This prevents bubbles and ensures the resin reaches every detail of the mold cavity.
Most discussions of vacuum casting treat the silicone mold as just one step in the process. That framing misses the point. The silicone mold is the component that determines part accuracy, surface quality, and production capacity. Get the mold right and the parts are right. Cut corners on the mold and every part in the batch pays for it.
This guide covers how silicone molds are made, which silicone type to choose, what Shore hardness means in practice, and how to extend mold life beyond the standard 20–25 cast range. You will have a clear technical framework for your next project by the end.
What Silicone Vacuum Casting Produces
Silicone vacuum casting serves one core purpose: creating small batches of production-accurate plastic parts without the cost or lead time of hard tooling. The silicone mold is the key component that makes this possible.
Standard outputs from a professional silicone vacuum casting process include:
- Surface finish: Ra 1.6–3.2 µm — close to injection-molded quality
- Dimensional tolerance: ±0.05 mm for parts under 100 mm
- Parts per mold: 20–25 copies from a standard RTV platinum-cure mold
- Mold cost: $200–$1,000 per silicone mold
- Lead time: 10–15 days from approved drawing to shipped parts
The vacuum casting market reflects the growing reliance on this process. According to Market Research Future, the global vacuum casting market was valued at USD 3.82 billion in 2024 and is projected to reach USD 6.77 billion by 2035, growing at a 5.33% CAGR.
For a full overview of service capabilities, part size limits, and materials, see Yanmee’s silicone vacuum casting service page.
How the Silicone Mold Is Made
The silicone mold is the most labor-intensive stage of the entire casting process. Errors here affect every part in the batch — not just the first one.
Steps 1–3: Mold Box, Silicone Pour, and Degassing
The process starts with building a mold box from melamine board and securing the master model inside with CA-glue. All gaps between the master and the box base are filled with plastalina clay to create a clean parting line for the first mold half.
Before pouring, the mold maker brushes liquid silicone into all fine detail areas of the master using an acid brush. This step prevents air traps in corners and engraved features. The bulk silicone is then poured into the box and placed in a vacuum chamber at ≤0.1 mbar to remove all bubbles before cure begins.
Degassing the liquid silicone before cure is one of the most critical steps in the process. In our experience, the most common cause of surface defects in cast parts is skipping or rushing this degassing step. Bubbles in the silicone equal defects on every part cast from that mold.
Steps 4–6: Cure, Cut, and Prep for Casting
The first mold half cures in an oven at 40°C for 8–16 hours. After cure, the mold box is disassembled, the first half is flipped, and vents, risers, and a fill port are installed for the second half. A fresh mold box is built, mold release is sprayed, and the second half is poured and degassed the same way.
Once both halves are cured, a skilled technician cuts the mold open along a sine-wave parting line. The sine-wave pattern — rather than a straight cut — creates a keyed joint so the two mold halves lock back together precisely for every subsequent cast.
For a look at how Yanmee manages this process in production, the vacuum casting factory guide covers our mold production workflow and quality checkpoints in detail.
RTV vs. HTV Silicone — Choosing the Right Type
Choosing the wrong silicone type is the most expensive mistake in a vacuum casting project. It affects shrinkage, mold life, and part accuracy on every single cast.
RTV Silicone (Room Temperature Vulcanization)
RTV silicone comes in two chemistry types with very different performance profiles. Tin-cure silicone (condensation-cure) shrinks 1–4% during cure. That level of shrinkage compounds across a part’s dimensions and can push large parts out of tolerance. Tin-cure molds also yield fewer parts — typically 10–15 casts before quality degrades. They cost less, making them acceptable for rough concept models but not for precision validation work.
Platinum-cure silicone shrinks near-zero during cure. This is the professional standard for silicone vacuum casting work. It delivers 20–25 usable parts per mold with consistent accuracy across the full mold lifespan. Translucent platinum-cure grades are preferred because they let the mold maker spot air traps and contaminants inside the mold before casting begins.
HTV Silicone (High Temperature Vulcanization)
HTV silicone requires processing at 150–200°C, which means it is not suitable for DIY or low-investment setups. For production environments, it delivers mold life of 300–500 casts from the same mold — far beyond the RTV range.
The breakeven point for HTV typically comes when a project needs more than 50 parts from one master model. At that volume, the higher mold cost is recovered through the extended casting capacity.
| Factor | RTV Tin-Cure | RTV Platinum-Cure | HTV Silicone |
|---|---|---|---|
| Shrinkage | 1–4% | Near-zero | <0.1% |
| Mold life | 10–15 parts | 20–25 parts | 300–500 parts |
| Cost | Low | Medium | Higher |
| Processing temp | Room temp | Room temp | 150–200°C |
| Best for | Rough concept models | Precision prototypes | High-volume runs |
Based on thousands of mold production runs, our team consistently recommends platinum-cure RTV for any project where part accuracy or assembly validation matters. Tin-cure is only appropriate for early-stage appearance models where dimension is not critical.
Shore Hardness — Why It Determines Demolding Success
Shore hardness controls how the silicone mold behaves when you pull a finished part from it. Choose too soft and the mold collapses under the weight of large pours. Choose too hard and the mold tears on deep undercuts during demold.
Selecting Shore Hardness by Part Geometry
Shore A 40–50 is the proven professional range for vacuum casting molds. At this hardness, the mold flexes enough to release parts with moderate undercuts cleanly, while remaining stiff enough to hold dimensional shape across 20+ cast cycles.
For parts with deep undercuts, thin walls, or flexible features, Shore A 35–40 gives extra flex. The trade-off is shorter mold life under repeated pour pressure. For parts with flat, simple geometry and no undercuts, Shore A 50–55 adds durability without demolding risk.
Molds below Shore A 30 work well for very soft or flexible part geometries — wristbands, gaskets, grip covers — but they sag under heavy resin pours and are only practical for small, lightweight parts. Molds above Shore A 60 are rigid enough to damage parts during demold in any geometry with a reentrant angle.
What Controls Silicone Mold Life
A standard platinum-cure RTV mold yields 20–25 parts, but that number is not guaranteed. Three variables determine whether you reach 25 clean parts or lose the mold at part 8.
Factors That Shorten Mold Life
The single biggest cause of premature mold failure is skipping mold release spray before each cast. Without fresh release, the resin bonds chemically to the silicone surface during cure. The first demold tears the mold. The damage is not always visible immediately — it shows up as surface texture breakdown by part 5 or 6.
Sharp internal corners in the part geometry concentrate stress on the silicone at every demold pull. Over time, micro-tears form at these stress points and widen across the mold lifespan. Casting resin at temperatures above 80°C accelerates this degradation. Incomplete degassing of the silicone during mold making leaves micro-voids that collapse under repeated thermal cycling.
How to Extend Silicone Mold Life
Following these steps consistently extends mold life toward the upper end of the 20–25 range:
- Spray fresh mold release before every cast — not just the first one
- Preheat polyurethane resin to 40°C before pour to reduce pour viscosity and cavity stress
- Use a sine-wave parting line, not a straight cut — straight cuts create high-stress seam lines
- Keep oven cure temperature at the resin manufacturer’s minimum — over-curing hardens the resin and increases demold force
- On multi-gate molds, rotate the fill point between casts to distribute wear evenly
For projects that require more than 25 parts from one master, the move to low-volume production with multiple molds — or a switch to HTV silicone or aluminum soft tooling — is the right planning decision.
Silicone Mold Design Rules for Clean Parts
The master model’s design determines how the silicone mold performs. These seven rules apply before the first gram of silicone is mixed.
Follow these design checks before submitting:
- Minimum wall thickness: 1.5–2.5 mm — walls below this collapse or fail to fill completely
- Draft angles: ≥0.5° on all vertical faces — without draft, the resin grips the mold wall on demold
- Gate at the lowest point; vents at the highest — this lets resin fill from bottom up and air escape cleanly
- Fill all gaps on the master before silicone pour — any gap creates a flash fin in the silicone
- Install alignment pins in the mold box — ensures two-half registration stays precise across all 20+ casts
- Avoid 90-degree internal corners — round them to ≥3 mm radius to reduce stress concentration
- Engraved text: ≥0.25 mm depth — shallower than this fills with silicone and becomes unreadable
After reviewing hundreds of failed first-run silicone molds, misplaced gates and skipped vents account for the majority of incomplete-fill defects — and both are preventable at the design stage.
For master models that require tighter dimensional control or complex internal features, CNC-machined master models for precision silicone molds give better base accuracy than SLA for geometries above 200 mm.
Silicone Vacuum Casting vs. Other Soft Tooling Methods
Silicone is the dominant material for vacuum casting molds, but it is not the only option. Understanding where silicone performs best clarifies when to choose a different approach.
| Factor | Silicone Mold | 3D-Printed Mold | Aluminum Soft Tool |
|---|---|---|---|
| Detail capture | Excellent | Good | Very Good |
| Mold life | 20–25 (RTV) / 300–500 (HTV) | 50–200 | 1,000–5,000 |
| Undercut handling | Excellent | Poor–Moderate | Poor |
| Lead time | 1–2 days | Hours | 2–5 weeks |
| Cost per mold | $200–$1,000 | <$100 | $3,000–$15,000 |
For most appliance and electronics prototyping applications, silicone tooling gives the best balance of detail accuracy, lead time, and tooling cost. The exception is when quantities exceed 100 parts per design — at that scale, aluminum soft tooling or injection mold tooling becomes more economical per part.
For silicone vacuum casting in appliance prototype design, the combination of surface replication quality and short mold turnaround consistently makes silicone the correct tooling choice at the EVT and DVT stages.
Frequently Asked Questions
Q1: What type of silicone is used in vacuum casting?
Vacuum casting molds use RTV (Room Temperature Vulcanization) silicone in most professional applications. Platinum-cure RTV is the standard because it has near-zero shrinkage and yields 20–25 accurate parts per mold. Tin-cure RTV is cheaper but shrinks 1–4% during cure, which reduces part accuracy. HTV (High Temperature Vulcanization) silicone is used for high-volume runs requiring 300–500 casts per mold.
Q2: How long does a silicone mold last in vacuum casting?
A standard platinum-cure RTV silicone mold lasts 20–25 casting cycles before surface quality and dimensional accuracy begin to degrade. HTV silicone molds last 300–500 casts. Mold life is shortened by skipping mold release, sharp part geometry, and casting at temperatures above 80°C. With correct mold release application and proper resin temperature, most platinum-cure molds reach the full 20–25 cycle range.
Q3: What is the difference between RTV and HTV silicone molds?
RTV silicone cures at room temperature and costs less — it is the standard choice for most prototype and small-batch vacuum casting projects. HTV silicone requires processing at 150–200°C and costs more upfront, but yields 300–500 casts per mold versus 20–25 for RTV. Choose RTV for prototyping and small runs; choose HTV when your project needs more than 50 parts from one master and you want to minimize mold replacement cost.
Q4: What Shore hardness should a vacuum casting silicone mold be?
Shore A 40–50 is the professional standard for vacuum casting molds. This hardness range is flexible enough to release parts with moderate undercuts during demold, while stiff enough to maintain dimensional accuracy across 20+ cast cycles. For parts with deep undercuts, Shore A 35–40 gives extra flex. For simple flat geometry, Shore A 50–55 adds durability.
Q5: Why does my silicone mold produce bubbles in cast parts?
Bubbles in cast parts almost always trace back to one of three causes: incomplete degassing of the liquid silicone during mold making, skipped mold release spray before casting, or resin that was not degassed before pour. The fix for all three is process discipline — vacuum degas the silicone at ≤0.1 mbar before cure, apply fresh mold release before every cast, and degas the mixed resin inside the vacuum chamber for 2–3 minutes before pouring.
The Mold Decision Comes First
Silicone vacuum casting produces parts as good as the mold it uses. Three decisions drive the outcome: silicone cure type (platinum-cure for accuracy), Shore hardness (40–50 A for most applications), and mold preparation discipline (degassing, gate placement, mold release). Get these three right and every cast in the batch will be consistent.
Standard RTV platinum-cure silicone is the right starting point for most prototype and validation projects. When your batch grows beyond 25 parts, plan for a second mold from the same master — or discuss the switch to HTV silicone or injection tooling.
To get a free DFM review and silicone mold specification recommendation for your project, visit Yanmee’s silicone vacuum casting service page. The engineering team responds within 24 hours.