Poly 74 vs. Poly 75: Understanding the Differences
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Poly 74 vs. Poly 75: Understanding the Differences
A sculptor's guide to Polytek's two polyurethane mold rubber series — when to use soft, flexible 74 Series vs. firm, abrasion-resistant 75 Series, and how Shore hardness determines your mold's performance.
If you're new to polyurethane mold making, the Poly 74 and Poly 75 product names look almost identical — and that's exactly where the confusion starts. Both are Polytek polyurethane liquid mold rubbers. Both come in multiple hardness grades. Both produce excellent molds. But they're designed for fundamentally different applications, and choosing the wrong series can mean a mold that tears on the first pull or one that's so rigid you can't demold your casting.
This guide explains the key differences, walks through every hardness grade in both series, and includes an interactive Shore hardness selector to help you pick the exact product for your project.
At a Glance: Two Series, Different Jobs
Poly 74 Series
Soft-to-medium hardness (Shore A20–A45) polyurethane rubbers designed for intricate molds that need to flex and stretch during demolding. The sculptor's and foundry's choice.
- Shore A20, A30, A30 Clear, A45 hardness options
- Easy 1:1 mix ratio (by weight)
- Excellent for plaster and wax — no release agent needed
- Captures finest detail from any model
- Pourable as-is or thickened for brush-on
Poly 75 Series
Mid-range to firm hardness (Shore A60–A80) polyurethane rubbers designed for high-volume concrete casting, architectural formliners, stamping tools, and applications demanding maximum abrasion resistance.
- Shore A60, A65, A70, A80 hardness options
- 1:1 or 2:1 mix ratios (varies by grade)
- Optimized for concrete, GFRC, and cast stone
- Superior abrasion resistance for high-volume use
- Makes durable stamps, formliners, and rubber tools
The simple rule: if your casting material is plaster or wax, start with 74 Series. If your casting material is concrete, start with 75 Series. The Shore hardness grade fine-tunes from there.
Sculpture Depot — Mold Making NotesWhat Shore Hardness Means
Shore A hardness is a standardized scale that measures how soft or firm a cured rubber is. Lower numbers mean softer, more flexible rubber; higher numbers mean firmer, more rigid rubber. For mold making, hardness directly determines two critical performance characteristics:
Flexibility (demolding ease): Softer rubbers flex and stretch more, making it easier to peel the mold away from complex shapes with undercuts, deep textures, and fine projections. A Shore A20 mold bends like a thick rubber glove; a Shore A80 mold feels more like a hard shoe sole.
Durability (mold life): Firmer rubbers resist abrasion and tearing better, producing molds that survive more casting cycles before wearing out. A Shore A80 concrete stamping mat might survive 1,000+ pours; a Shore A20 mold might handle 50–100 plaster casts before showing wear.
The trade-off is always the same: more flexibility = easier demolding but shorter mold life. More firmness = longer mold life but harder to demold. The art of mold making is choosing the hardness that balances these two factors for your specific project.
Poly 74 Series Part C is an additive that can be mixed into either 74 or 75 Series rubbers to reduce Shore hardness and lower mixed viscosity. This means you can fine-tune any formulation softer than its stock grade — a powerful option when you need a hardness between the standard offerings.
Pick Your Hardness Grade
Click any hardness grade to see full specs, working times, best applications, and our recommendation.
Full Specs Comparison
| Spec | 74 Series (Soft) | 75 Series (Firm) |
|---|---|---|
| Shore Hardness Range | A20 – A45 | A60 – A80 |
| Primary Casting Material | Plaster, wax, resin | Concrete, GFRC, cast stone |
| Mix Ratio | 1A:1B (all grades) | 1:1 or 2A:1B (varies) |
| Pour / Working Time | 15–20 min typical | 35–45 min typical |
| Demold Time | 16–24 hrs | 16–24 hrs (75-60: hours) |
| Release Agent Needed? | Not for plaster/wax; yes for resins & cement | Yes — Pol-Ease for all casting |
| Flexibility | High — peels from undercuts | Low-moderate — structural rigidity |
| Abrasion Resistance | Good | Excellent — high-volume rated |
| Transparency Option | Yes — 74-30 Clear | No |
| Part C Compatible? | Yes — softens further | Yes — softens to mid-range |
| Brush-On Capable? | Yes — with PolyFiber II thickener | Yes — with PolyFiber II thickener |
| Best For | Foundry wax, sculpture, fine detail | Concrete pavers, stamps, formliners |
Every hardness grade from A20 to A80, plus Part C softener, 74/75 X accelerator, PolyFiber II thickener, and accessories.
Pol-Ease 2300 aerosol release, Pol-Ease 2650 silicone-free release, PolyCoat sealer — required for proper mold making and casting.
Bubble molding sheets, flat molding sheets, mold keys, rubber bands, mixing containers — everything for the mold-making workflow.
When to Use Each Series
Use Poly 74 Series When…
You're a sculptor making molds for wax or plaster casting. This is the 74 Series' core purpose. The soft, flexible rubber captures fine detail from your clay original, releases plaster and wax castings without a release agent, and bends enough to demold complex shapes with deep undercuts. Foundries across the country use 74-30 as their standard wax mold rubber. The 74-30 Clear variant is popular for cut molds where you need to see the model through the rubber to plan your cut lines.
You need maximum detail capture. Softer rubbers flow more completely into fine surface texture, capturing fingerprint-level detail that firmer rubbers may bridge over. For sculpture reproduction where surface quality is paramount, 74-20 (the softest grade) provides the ultimate detail capture.
Your model has significant undercuts. Deeply carved relief, extended fingers, wrapped fabric folds — any geometry where the mold must stretch and flex to pull free — demands the flexibility of a 74 Series rubber. Using a 75 Series rubber on a deeply undercut model risks tearing the mold during demolding.
Use Poly 75 Series When…
You're casting concrete, GFRC, or cast stone. Concrete is abrasive — every pour grinds the mold surface with aggregate particles. The 75 Series' superior abrasion resistance means your mold survives dramatically more casting cycles than a 74 Series mold would. For commercial concrete operations producing hundreds of pavers, blocks, or panels, the 75 Series pays for itself in mold longevity.
You're making stamping tools or formliners. Concrete stamping mats and architectural formliners need to be rigid enough to hold their shape under the weight of wet concrete while being flexible enough to peel off the cured surface. The 75-60 to 75-70 range hits this balance perfectly.
You need maximum mold life for high-volume production. Any application where a mold will see 200+ casting cycles benefits from the 75 Series' toughness. Even for plaster casting, if you're producing a high-volume edition (hundreds of identical pieces), the 75 Series' longer mold life may justify its slightly firmer handling characteristics.
Both series have approximately 6-month shelf life when stored at room temperature (~75°F). The biggest enemy is moisture — humidity reacts with the isocyanate component and causes gassing and curing problems. Use PolyPurge (a heavier-than-air dry gas) every time you open a container to create a moisture barrier. Reseal immediately after pouring. Store components in a climate-controlled space, never in an unheated garage or outdoor shed.
Frequently Asked Questions
You can — with a proper release agent (Pol-Ease 2650 is recommended). However, the softer rubber will wear faster than 75 Series when exposed to concrete's abrasive aggregate. For limited-run concrete projects (under 50 pours), 74-45 works adequately. For production volumes, the 75 Series' abrasion resistance is worth the trade-off in flexibility.
Not for plaster and wax — 74 Series rubber releases these materials cleanly without any release agent. For casting resins (polyurethane, polyester, epoxy) or concrete into a 74 Series mold, you do need release agent. Pol-Ease 2300 (aerosol) or Pol-Ease 2650 (silicone-free) are the recommended options.
Same Shore hardness (A30), same performance, same mix ratio. The difference is transparency: 74-30 Clear cures to a translucent amber rather than the standard opaque color. This is specifically useful for cut molds — when you need to see the model through the cured rubber to plan where to make your cut lines for a two-part mold. If you're making a poured block mold where you don't need to see through the rubber, standard 74-30 is identical in performance and typically less expensive.
Both series are supplied as pourable liquids, but can be converted to brush-on consistency by mixing with PolyFiber II thickener or fumed silica. This is essential for large models where a poured block mold would use impractical amounts of rubber. Brush-on (blanket) molds use a fraction of the rubber by coating the model in layers, then backing with a plaster or fiberglass mother mold.
Silicone rubber (our PlatSil and TinSil lines) is the premium alternative to polyurethane for mold making. Silicone offers superior chemical resistance, longer shelf life, and doesn't require release agents for most casting materials. The trade-off is cost — silicone is significantly more expensive per pound than polyurethane. For foundry and sculpture work where budget allows, silicone is often the preferred choice. For high-volume concrete and architectural work, polyurethane's lower cost and adequate performance make the 74/75 Series the standard.
For sculpture mold making (plaster or wax castings), start with Poly 74-30. It's the most popular grade for a reason: Shore A30 provides an excellent balance of flexibility and durability, the 1:1 mix ratio is easy to work with, and it releases plaster and wax without a release agent. Buy a small kit (available in 4 lb, 16 lb, and larger sizes) along with Pol-Ease 2300 release and mold-making accessories to have a complete starter setup.
Find Your Perfect Mold Rubber
Browse the complete Poly 74 & 75 Series, release agents, mold-making tools, and casting supplies. Questions? Call 970-663-5190 — we help sculptors and concrete professionals choose the right grade every day.
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