Vacuum Casting Resin: Material Selection and Properties Guide
Compare rigid PU, flexible PU, silicone, epoxy, and transparent casting resins. Select by hardness, temperature resistance, transparency, and cost to match your production material — with property data and casting parameters from FIRMFG.
Vacuum Casting Resin: Material Selection and Properties Guide
Selecting the right vacuum casting resin is the single most important decision in the casting process. The resin determines mechanical strength, flexibility, temperature resistance, optical clarity, and surface finish of every cast part. At FIRMFG, we offer five primary resin families — rigid PU, flexible PU, silicone, epoxy, and transparent PU — covering a hardness range from Shore 10A (soft gel) to Shore 90D (hard plastic).
Each resin family is engineered to simulate specific production plastics. Rigid PU replicates ABS and PC for cosmetic housings; flexible PU matches TPU and rubber for overmolds; silicone provides heat resistance to 250°C for seals and gaskets; epoxy delivers maximum structural strength; and transparent PU achieves optical clarity comparable to PMMA. With over 20 material variants available, we can match 90% of production material properties.
This guide compares all five resin types across hardness, density, tensile strength, HDT, transparency, and cost — plus casting parameters and design guidelines specific to each material. Ready to get started? Request a quote and our engineers will recommend the optimal resin for your application.
Vacuum Casting Resin Quick Specifications
Key material parameters spanning all vacuum casting resin types at FIRMFG.
| Specification | Value |
|---|---|
| Viscosity Range | 200 – 3000 mPa·s |
| Cure Time | 30 min – 24 h (resin-dependent) |
| Shrinkage | 0.1 – 0.5% |
| Mix Ratio | 1:1 to 100:100 (by weight or volume) |
| Pot Life | 3 – 10 min |
| Shelf Life | 6 – 12 months (sealed, cool storage) |
Vacuum Casting Resin Types Comparison
Five primary resin families cover the full spectrum of production material requirements. Each card summarizes key properties and typical applications.
Rigid PU Resin
Shore 60D – 85D
High-strength polyurethane that simulates engineering plastics like ABS, PC, and PMMA. Excellent dimensional stability and impact resistance for structural prototypes and cosmetic housings.
Key properties: Impact-resistant, paintable, low shrinkage
Simulates: ABS, PC, PMMA
Flexible PU Resin
Shore 20A – 90A
Elastomeric polyurethane that replicates TPU, rubber, and TPE materials. Adjustable hardness from soft gel to firm rubber for gaskets, overmolds, and flexible components.
Key properties: Elastomeric, tear-resistant, high elongation
Simulates: TPU, Rubber, TPE
Silicone Casting Resin
Shore 10A – 60A
Heat-cured silicone resin for parts requiring high temperature resistance, biocompatibility, or chemical inertness. Ideal for seals, gaskets, and medical-grade components.
Key properties: Heat resistant to 250°C, biocompatible, UV stable
Simulates: LSR, RTV silicone
Epoxy Casting Resin
Shore 75D – 90D
Two-component epoxy with exceptional mechanical strength and minimal shrinkage. Used for high-strength structural parts, tooling, and applications requiring chemical resistance.
Key properties: High strength, low shrinkage, chemical resistant
Simulates: Engineering thermosets
Transparent PU Resin
Shore 70D – 85D
Crystal-clear polyurethane for optical-quality transparent parts. Simulates PC clear and PMMA with high light transmission and minimal yellowing for lenses and display windows.
Key properties: Optical clarity, UV-stable, low yellowing
Simulates: PC clear, PMMA, glass-like
Vacuum Casting Resin Properties Matrix
Comprehensive mechanical and thermal property data for all five resin types. Use this matrix to compare materials side-by-side before making a selection.
| Resin Type | Hardness | Density | Tensile Str. | Flexural Str. | Impact Str. | Elongation | HDT | Transparency | Cost |
|---|---|---|---|---|---|---|---|---|---|
| Rigid PU | 60D – 85D | 1.05 – 1.25 g/cm³ | 40 – 75 MPa | 60 – 120 MPa | 8 – 25 kJ/m² | 3 – 15% | 55 – 90°C | Opaque | $$ |
| Flexible PU | 20A – 90A | 1.00 – 1.20 g/cm³ | 5 – 30 MPa | — | No break | 100 – 600% | — | Opaque | $$ |
| Silicone | 10A – 60A | 1.10 – 1.30 g/cm³ | 4 – 12 MPa | — | No break | 150 – 700% | 200 – 250°C | Translucent | $$$ |
| Epoxy | 75D – 90D | 1.10 – 1.40 g/cm³ | 50 – 90 MPa | 80 – 150 MPa | 5 – 15 kJ/m² | 2 – 8% | 80 – 140°C | Clear / Opaque | $$$$ |
| Transparent PU | 70D – 85D | 1.10 – 1.20 g/cm³ | 45 – 70 MPa | 70 – 110 MPa | 10 – 20 kJ/m² | 4 – 12% | 60 – 85°C | Transparent | $$$ |
Values represent typical ranges across available grades. Actual properties depend on specific resin grade, cure conditions, and post-cure protocol. Cost index: $ = low, $$ = moderate, $$$ = high, $$$$ = premium.
Resin Selection Guide by Application
Match your application requirements to the recommended resin type. This decision matrix covers the most common use cases in vacuum casting.
| Application | Recommended Resin | Key Property | Shore Hardness |
|---|---|---|---|
| Appearance & cosmetic housings | Rigid PU | Surface finish fidelity, paintable | Shore 70D – 85D |
| Seals & gaskets | Silicone | Heat resistance, chemical inertness | Shore 20A – 60A |
| Optical parts (lenses, light guides) | Transparent PU | Optical clarity, low yellowing | Shore 70D – 85D |
| High-strength structural parts | Epoxy | Maximum tensile & flexural strength | Shore 80D – 90D |
| Flexible parts (overmolds, boots) | Flexible PU | Elongation, tear resistance | Shore 20A – 90A |
For applications spanning multiple requirements — such as a transparent, heat-resistant part — consult our engineers for a custom resin recommendation.
Casting Parameters by Resin Type
Each resin family requires specific mixing ratios, degas times, and cure protocols. Following these parameters ensures optimal mechanical properties and surface finish.
| Resin Type | Mix Ratio | Pot Life | Degas Time | Cure Temp | Cure Time | Post-Cure |
|---|---|---|---|---|---|---|
| Rigid PU | 100:60 – 100:100 | 3 – 6 min | 2 – 3 min | 60 – 70°C | 2 – 4 h | Optional, 70°C / 2 h |
| Flexible PU | 100:50 – 100:100 | 4 – 8 min | 2 – 3 min | 60 – 70°C | 2 – 4 h | Optional, 70°C / 2 h |
| Silicone | 1:1 or 10:1 | 5 – 10 min | 3 – 5 min | RT or 60 – 100°C | 30 min – 4 h | Recommended, 100°C / 1 h |
| Epoxy | 2:1 – 4:1 | 6 – 10 min | 3 – 5 min | 60 – 80°C | 4 – 12 h | Required, 80 – 120°C / 2 – 4 h |
| Transparent PU | 100:80 – 100:100 | 3 – 5 min | 3 – 5 min | 60 – 70°C | 2 – 4 h | Required, 70°C / 2 – 4 h |
Casting Parameter Workflow
Step 1: Resin Preparation
Weigh each component precisely according to the specified mix ratio. Pre-heat resin in cold conditions to reduce viscosity and improve flow. Ensure containers are dry and clean to prevent moisture contamination.
Key detail: Use a precision scale with 0.1g accuracy for small batches
Step 2: Mixing & Degas
Combine components A and B, stirring slowly to minimize air entrainment. Place the mixed resin in a vacuum chamber at –0.1 MPa for 2–5 minutes until bubbles cease to rise. Degas time depends on viscosity and pot life.
Key detail: Vacuum degassing at –0.1 MPa for 2 – 5 minutes
Step 3: Pouring Under Vacuum
Pour the degassed resin into the silicone mold inside the vacuum chamber. Vacuum-assisted pouring ensures complete filling of fine details, thin walls, and complex geometries without trapped air pockets.
Key detail: Maintain vacuum during pour for bubble-free casting
Step 4: Curing & Post-Cure
Transfer the filled mold to a curing oven at the resin-specific temperature. After initial cure, demold carefully and apply post-cure if required to achieve full mechanical properties and dimensional stability.
Key detail: Oven cure at 60 – 80°C, post-cure for engineering-grade resins
Design Guidelines for Vacuum Casting Resins
Resin-specific DFM rules for minimum wall thickness, shrinkage compensation, and insert compatibility. Follow these guidelines to achieve defect-free cast parts.
| Design Feature | Recommended | Minimum |
|---|---|---|
| Min Wall Thickness — Rigid PU | 1.0 mm (0.040") | 0.8 mm (0.031") |
| Min Wall Thickness — Flexible PU | 1.2 mm (0.047") | 1.0 mm (0.040") |
| Min Wall Thickness — Silicone | 1.2 mm (0.047") | 1.0 mm (0.040") |
| Min Wall Thickness — Epoxy | 1.8 mm (0.071") | 1.5 mm (0.059") |
| Shrinkage Compensation | Add 0.3% to master dimensions | Resin-specific (0.1 – 0.5%) |
| Insert Molding Compatibility | Metal inserts: Rigid PU, Epoxy | Silicone & Flexible PU: limited |
| Draft Angles | 2° – 3° on all vertical walls | 1° (rigid), 3° (flexible/silicone) |
| Boss & Rib Thickness | 60% of nominal wall | 50% of nominal wall |
DFM Tips for Resin Selection
- Account for resin-specific shrinkage (0.1–0.5%) by scaling the master pattern slightly oversized to achieve final dimensional accuracy.
- Use rigid PU or epoxy for insert-molded metal components. Flexible and silicone resins do not bond reliably to metal inserts.
- Increase draft angles to 3° for flexible and silicone resins, as their high elongation can cause parts to grip the mold during demolding.
- For transparent PU parts, design walls at least 1.5 mm thick to avoid flow marks and ensure optical clarity throughout the part.
- Specify the exact Shore hardness in your drawings. PU resin hardness ranges from Shore 20A (soft rubber) to Shore 90D (hard plastic).
- Epoxy resin requires thicker walls (1.5 mm minimum) due to higher viscosity and exothermic cure. Design gates and vents to accommodate slower flow.
Vacuum Casting Resin Cost Factors
Per-part cost varies significantly by resin type. Material cost, processing complexity, and post-curing requirements all influence the final price of cast parts.
Rigid PU
$15 – $50 / part
Most economical, widest application range
Flexible PU
$20 – $60 / part
Elastomeric grades, moderate cost
Silicone
$30 – $80 / part
Higher material cost, heat-resistant
Epoxy
$40 – $120 / part
Premium material, highest strength
Transparent PU
$25 – $70 / part
Optical grade, requires pressure casting
Per-Part Cost at Different Volumes
| Order Quantity | Rigid PU | Silicone | Epoxy | Notes |
|---|---|---|---|---|
| 1 unit (first part) | $180 | $220 | $300 | Includes master + mold + 1 cast |
| 10 units | $35 | $55 | $85 | Mold cost amortized across parts |
| 25 units | $25 | $45 | $70 | Full mold life, maximum efficiency |
Example based on a 100 × 60 × 40 mm part with matte finish. Flexible PU and Transparent PU costs fall between Rigid PU and Silicone. Actual quotes vary with geometry, material grade, and finishing requirements.
Vacuum Casting Resin FAQ
Answers to the most common questions about vacuum casting resin selection and properties at FIRMFG.
QWhat level of transparency can vacuum casting resin achieve?
Transparent PU resin achieves light transmission above 90%, comparable to polished PMMA and clear PC. With vacuum degassing and optional pressure casting at 4 bar, parts are bubble-free and optically clear. Slight yellowing may occur over time, but UV-stable grades minimize this effect. For applications requiring sustained optical clarity, post-curing and UV-resistant topcoats are recommended.
QWhat temperature resistance do vacuum casting resins offer?
Temperature resistance varies by resin type. Rigid PU and Flexible PU typically withstand 55–90°C continuous use. Silicone casting resins tolerate up to 200–250°C, making them suitable for under-hood automotive and high-heat applications. Epoxy resins offer HDT values of 80–140°C. For applications above 120°C, silicone or specialized high-temperature PU grades are the recommended choice.
QAre FDA-compliant or food-grade vacuum casting resins available?
Yes, select silicone casting resins are available in FDA-compliant and biocompatible grades suitable for food contact and medical applications. These platinum-cured silicones meet USP Class VI and ISO 10993 standards. Food-grade PU resins are also available for limited-contact applications. Always specify your regulatory requirements when requesting a quote so we can match the correct certified material.
QCan vacuum casting resin be color-matched to specific Pantone or RAL colors?
Yes, vacuum casting resins can be pigmented to match Pantone, RAL, or custom color samples. Pigment is added to component A before mixing, and color matching achieves 90–95% accuracy. For critical cosmetic parts, we recommend providing a physical color sample. Solid colors are most reliable; metallic and pearlescent finishes can be achieved but may require a clear topcoat for full effect.
QWhat is the full hardness range of vacuum casting resins?
Vacuum casting resins span the full hardness spectrum from Shore 10A (soft gel, comparable to silicone erasers) to Shore 90D (rigid, comparable to hard hats). Flexible PU covers Shore 20A–90A, rigid PU covers Shore 60D–85D, silicone covers Shore 10A–60A, and epoxy covers Shore 75D–90D. This range lets us simulate everything from soft rubber seals to hard plastic enclosures within a single mold system.
QHow should vacuum casting resin be stored, and what is the shelf life?
Vacuum casting resins should be stored in sealed containers at 15–25°C in a dry, dark environment. Shelf life ranges from 6 to 12 months depending on resin type — silicone and epoxy typically last 12 months, while PU resins last 6–9 months. Moisture exposure causes PU resins to react and degrade, so containers must be resealed immediately after dispensing. Expired resin should never be used as it will produce parts with poor mechanical properties.
Vacuum Casting Resin Applications
From optical lenses to heat-resistant seals, vacuum casting resins serve a wide range of functional and cosmetic applications across industries.
Optical Components
Lenses, light guides, display windows, and transparent covers with PMMA-like clarity
Seals & Gaskets
O-rings, custom gaskets, and sealing components in heat-resistant silicone
Structural Parts
Brackets, housings, and load-bearing components in high-strength epoxy
Cosmetic Housings
Consumer product enclosures with painted or textured finishes in rigid PU
Heat-Resistant Parts
Under-hood automotive and industrial components rated to 250°C
Clear Prototypes
Transparent prototypes for design validation, flow visualization, and inspection
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