90A Elastomer Vacuum Casting: Material, Design and Testing Guide

90A elastomer vacuum casting is used to make firm rubber-like prototypes and low-volume components without investing in production elastomer tooling. A flexible silicone mold reproduces a master pattern, while a two-part polyurethane elastomer is mixed, degassed and cast under vacuum. The finished part can simulate grips, seals, protective covers, bumpers, wheels, boots and overmolded product features.

Shore A 90 is near the hard end of the Shore A scale. It feels much firmer than a soft gasket and may resemble a hard rubber wheel, durable boot or stiff overmold. Hardness alone does not define performance. Tensile strength, tear resistance, elongation, compression set, rebound, temperature, color and bond strength may be equally important.

This guide explains how to specify a 90A vacuum-cast elastomer, design stable geometry, plan overmolding, control dimensions and prepare meaningful tests before production.

What Is Shore A 90?

Shore hardness measures resistance to indentation using a defined durometer method. The Shore A scale is commonly used for flexible elastomers. A higher number generally indicates a harder material, but the scale is not linear and does not directly predict stiffness, strength or durability in service.

A nominal 90 Shore A polyurethane is relatively firm. Thin features can still flex, while thick sections may feel almost rigid. Perceived flexibility therefore depends on both hardness and geometry.

The drawing should specify:

• Target hardness and acceptable range.
• Shore scale, normally Shore A for this application.
• Conditioning time before measurement.
• Test thickness or approved specimen method.
• Required color and appearance.
• Additional mechanical or environmental properties.

How 90A Elastomer Vacuum Casting Works

1. Review CAD data, drawing, quantity and performance targets.
2. Produce and finish an SLA or CNC master pattern.
3. Plan mold split, gates, vents, inserts and overmold location.
4. Cast liquid silicone around the approved master.
5. Cure and cut the flexible silicone mold.
6. Measure, mix and degas the polyurethane elastomer.
7. Fill the tool under controlled vacuum and pressure conditions.
8. Cure, demold and post-cure the casting as required.
9. Trim gates and flash without damaging functional edges.
10. Inspect hardness, dimensions, appearance and function.

The silicone tool supports short production runs but gradually wears. Severe undercuts, thin mold sections and repeated stretching can reduce usable mold life.

90A Polyurethane Is Not the Same as Silicone Rubber

Silicone vacuum casting describes the mold material; the cast part is often polyurethane. Buyers sometimes request a “silicone part” when they actually need a silicone-molded polyurethane simulation. The distinction matters because the two material families have different chemical, temperature, weathering and regulatory behavior.

If the final product requires true silicone rubber, medical-grade material, food-contact approval or long-term outdoor durability, specify that need explicitly. A prototype polyurethane may be excellent for form, fit and short-term function but should not be presented as certified production silicone.

When to Use a 90A Vacuum-Cast Elastomer

• Firm grips and protective product corners.
• Robot feet, stops and vibration-isolation features.
• Hard rubber wheels and rollers for functional trials.
• Connector boots, cable strain reliefs and grommets.
• Seals that require firmness rather than soft compression.
• Wear pads, bumpers and impact-protection components.
• Overmolded housings and handheld product prototypes.
• Short-run samples for customer and assembly testing.
• Bridge quantities while injection or compression tooling is prepared.

The process is less suitable when the program needs thousands of identical pieces, production-certified elastomer, continuous high-temperature exposure or extremely tight dimensions on a flexible geometry.

Selecting a 90A Elastomer System

Hardness tolerance

A nominal Shore A value normally needs an agreed acceptance band. Very tight hardness limits can increase material control and test requirements. The supplier should state the expected range after the specified cure and conditioning period.

Tensile strength

Tensile strength describes the stress a test specimen can resist before breaking. It helps compare materials but does not fully predict the behavior of a real part with corners, holes and molded-in stress.

Elongation

Elongation at break indicates how much a material can stretch in a standardized test. A high value can help undercuts and flexible joints survive, but thickness and notch geometry remain important.

Tear resistance

Tear behavior is critical around slots, lips, thin edges and demolding undercuts. Generous radii and adequate material around holes reduce stress concentration.

Compression set

Compression set describes how much permanent deformation remains after a material is compressed for a defined time and condition. It matters for seals, pads and parts held under constant load.

Rebound and damping

Two 90A materials can return energy differently. Wheels, impact stops and vibration parts may need rebound or damping tests in addition to hardness.

Temperature and chemical exposure

Prototype polyurethane behavior can change with heat, cold, moisture, oils, cleaners and UV exposure. Define the duration and severity of the real test instead of asking only for a generic “chemical-resistant” grade.

Part Geometry Changes the Feel of 90A

A 1 mm membrane and a 10 mm block made from the same resin do not feel equally flexible. Bending stiffness changes strongly with thickness, while ribs, curves, holes and support conditions also affect response.

When comparing a vacuum-cast prototype to a future molded product, match the actual wall structure and load path. A hardness coupon alone cannot validate button force, grip comfort or seal compression.

Design Guidelines for 90A Elastomer Parts

Use consistent wall thickness

Large transitions between thin and thick sections can create uneven cure, sink, trapped heat and dimensional movement. Use smooth transitions and hollow thick masses when the design allows.

Add generous radii

Sharp internal corners concentrate stress and can initiate tears. Radii improve resin flow, reduce notch sensitivity and help the silicone mold release the part.

Protect thin lips

Thin sealing lips and fins are vulnerable during filling, trimming and demolding. Increase root thickness, add a radius and define a safe gate location away from the functional edge.

Review deep holes

Long flexible mold cores can bend, trap air or tear. Removable cores, post-drilling or a design change may produce more reliable results.

Plan undercuts

Silicone tools can release undercuts that would require actions in a production mold. The part and tool still experience stretch during demolding. Extreme reverse geometry shortens mold life and may distort dimensions.

Define edge breaks

A nominally sharp edge may become rounded by master finishing, mold behavior and trimming. Use realistic edge radii and identify any edge that controls sealing or appearance.

90A Elastomer Overmolding

Vacuum casting can simulate a soft or firm elastomer over a rigid plastic or metal substrate. The rigid element may be machined, printed, molded or cast first, then located in a second mold for the elastomer stage.

Mechanical retention

Holes, slots, grooves, dovetails and wraparound geometry can lock the elastomer to the substrate. Mechanical features are often more reliable than depending only on chemical adhesion during a prototype program.

Chemical adhesion

Bond performance depends on the substrate, resin chemistry, surface preparation, primer, cleanliness and cure. Adhesion should be verified on actual material coupons before the complete batch is made.

Insert positioning

The substrate must locate repeatably without floating or moving during casting. Fixture contact marks and allowable shutoff lines should be reviewed in the DFM stage.

Overmold thickness

Very thin elastomer layers can be difficult to fill and may reveal the texture beneath them. Very thick layers can increase material use, cure time and shrinkage. Use only the thickness needed for function.

Bond-line design

A gradual, protected transition reduces peeling risk. Avoid placing a highly loaded exposed edge where users or assembly tools can lift the elastomer away from the substrate.

Color, Texture and Cosmetic Control

Pigment can be mixed into the polyurethane to create colored parts. State a Pantone, RAL or approved sample, but expect wall thickness and lighting to affect perceived color. A physical limit sample gives the supplier a more practical acceptance target.

Texture comes from the master and the silicone cavity. Fine grain, logos and lettering can reproduce well, but deep texture increases demolding stress and can trap bubbles. Matte blasting, painted surfaces and molded texture should not be treated as identical finishes.

Cosmetic drawings should identify primary, secondary and hidden zones. Define acceptable parting lines, gate scars, vents, bubbles, color variation and trim marks.

Tolerances for Firm Elastomer Castings

A 90A part is firmer and easier to fixture than a soft 30A gasket, but it can still deflect under measuring force. Dimensional variation comes from the master, silicone tool, resin cure, post-cure, mold alignment and inspection method.

Apply tight tolerances only to features that control assembly or performance. Large envelopes, thin free walls, flexible lips and dimensions crossing the parting line are more variable than short, supported local features.

Define measurement condition

State whether the part is measured free, supported or installed on a mating component. A flexible ring measured on a table may not match the same ring held on its intended housing.

Use functional gauges

Go/no-go fixtures and mating parts often provide a better acceptance method than many direct dimensions. They reproduce actual fit while reducing variation caused by contact force.

Control critical interfaces

Dowel locations, insert positions, sealing lands and overmold shutoffs should use stable datums. If an interface requires metal-like precision, consider a rigid insert or secondary machining.

See the detailed guide to vacuum casting tolerances for drawing and inspection planning.

Hardness Measurement on a Prototype

A durometer reading depends on specimen thickness, support, surface condition, temperature and time after contact. Curved or thin product features may not provide a valid test area. In that case, cast a separate flat coupon using the same resin and cure cycle.

The inspection plan should specify when hardness is checked because polyurethane properties can continue to stabilize after demolding and post-cure. Customer and supplier should use comparable instruments and conditioning.

Functional Testing for 90A Components

Compression test

Measure load at defined compression levels and record recovery after release. This is more useful for a bumper or seal than hardness alone.

Tear and pull test

Test thin lips, cable exits, tabs and overmold edges using realistic loading directions. Note whether failure begins in the elastomer, at the bond line or in the rigid substrate.

Insertion and retention test

For boots, grommets and snap-on covers, measure assembly force and pull-off force across several samples. Lubrication and mating-part tolerance should match intended use.

Wheel or roller test

Evaluate load, rolling resistance, flat spotting, abrasion, noise and rebound over a representative cycle. A short visual test cannot predict long-term tread life.

Environmental exposure

Condition samples at relevant temperature, humidity, UV or chemical exposure, then repeat the functional test. Appearance and hardness changes should be recorded.

Overmold adhesion test

Use peel, pull or torque tests suited to the product geometry. Define the minimum force and acceptable failure mode before the parts are made.

First-Article Approval Plan

1. Inspect the master pattern and critical features.
2. Approve the target resin, pigment and hardness range.
3. Review parting line, gate, vent and insert locations.
4. Cast the first article using the intended cure cycle.
5. Condition the part before hardness and dimensional checks.
6. Test assembly with production-intent mating components.
7. Run compression, pull, tear or adhesion tests relevant to function.
8. Approve appearance under agreed lighting and viewing conditions.
9. Record any accepted deviation before completing the batch.
10. Retain an approved sample for later comparison.

Common Failure Modes

• Bubbles: caused by trapped air, moisture, poor venting or uncontrolled mixing.
• Incomplete fill: caused by narrow sections, unsuitable gates or resin viscosity.
• Hardness variation: caused by ratio, mixing, temperature or cure differences.
• Tearing: caused by sharp corners, thin lips, severe undercuts or trim damage.
• Warpage: caused by uneven wall thickness, cure stress or unsupported geometry.
• Weak overmold bond: caused by contamination, poor surface preparation or unsuitable materials.
• Visible flow or color variation: caused by mixing, fill pattern, pigment or thickness changes.
• Insert movement: caused by weak locating fixtures or casting pressure.
• Sticky surface: caused by incorrect mix, inhibition, cure or material condition.

How Mold Life Affects the Batch

Every demolding cycle stretches the silicone tool. Fine features can tear, split lines can wear and texture can degrade. Firm 90A parts may place more stress on the mold during release than very soft castings, especially around deep undercuts.

For larger quantities, the supplier may use multiple molds. Approve samples from each tool when appearance, hardness or dimensions are critical. The quotation should state the planned cavities, assumed yield and response to mold damage.

90A Vacuum Casting vs Other Processes

Vacuum casting vs direct 3D printing

Flexible 3D printing can produce one-off parts and design variations quickly. Vacuum casting is often chosen when a short batch needs consistent molded appearance, pigment, fine texture or a broader polyurethane property range.

Vacuum casting vs CNC machining

Machining a flexible material can provide production-grade stock but is limited by geometry and fixturing. Vacuum casting reproduces molded shapes, undercuts and overmold features more naturally.

Vacuum casting vs injection or compression molding

Production molding supports certified materials, repeatability and higher volume. Tooling costs more and the design should be stable. Vacuum casting helps validate geometry and function before committing to hard tooling.

Cost and Lead-Time Drivers

• Master-pattern process and manual finishing.
• Part size and silicone mold volume.
• Undercuts, loose cores and mold-split complexity.
• Special 90A resin, color and material availability.
• Rigid inserts, primers and overmolding fixtures.
• Quantity, expected yield and number of molds.
• Hardness coupons and mechanical testing.
• Dimensional reports and functional gauges.
• Painting, marking, assembly and cosmetic inspection.
• Protective packaging and shipping requirements.

Lead time includes data review, master production, master approval, mold preparation, first article, customer approval and batch casting. Fast feedback during approval is often the easiest way to protect the schedule.

RFQ Checklist for 90A Elastomer Vacuum Casting

• STEP model and revision-controlled drawing.
• Total quantity and first-article quantity.
• Nominal 90 Shore A hardness and acceptable range.
• Required tensile, tear, elongation, rebound or compression behavior.
• Service temperature and chemical exposure.
• Color, texture, gloss and cosmetic zones.
• Critical dimensions and measurement condition.
• Parting-line, gate, vent and trim restrictions.
• Insert material and locating requirements.
• Overmold substrate and adhesion test.
• Functional tests and report format.
• Packaging, delivery location and schedule.

Frequently Asked Questions

Is Shore A 90 soft or hard?

It is a firm elastomer near the hard end of the Shore A scale. Thin sections can flex, but thick sections feel much more rigid.

Can vacuum casting make a true 90A silicone part?

Vacuum casting commonly uses a silicone mold and polyurethane casting resin. True silicone parts require a compatible silicone material and process. Confirm the required chemistry, not only hardness.

How accurate are 90A cast parts?

Firm elastomers can be measured more consistently than very soft parts, but geometry, mold behavior, cure and inspection force still matter. Tight features require individual review.

Can 90A be bonded to aluminum or plastic?

Yes, using mechanical locks, suitable surface preparation, primer or a compatible chemical bond. Adhesion should be tested on the actual substrate and finish.

Will a 90A prototype behave exactly like production TPE or rubber?

No. Similar hardness does not guarantee identical modulus, tear, friction, fatigue, compression set or chemical resistance. Revalidate performance in the final production material.

Should hardness be checked on the finished part?

Check the part when it provides a suitable flat, thick test area. Otherwise use a dedicated coupon cast and cured with the same batch.

Request a 90A Elastomer Prototype Review

A useful 90A elastomer vacuum casting project begins with more than a hardness number. Geometry, wall thickness, tear resistance, compression, color, overmold retention and inspection method must match the intended test.

Review Aluprototype's vacuum casting capability and OEM silicone vacuum casting guide. Send your CAD model, drawing, quantity, hardness range and functional test requirements through the request-a-quote page for a DFM review and quotation.