Tag Archive for: silicone rubber manufacturing

Tensile strength and tear resistance are two of the most important mechanical properties in custom silicone product manufacturing. They directly affect whether a silicone part can withstand stretching, assembly force, repeated deformation, sealing pressure, pulling, twisting, and long-term use.

For products such as silicone seals, gaskets, membranes, tubes, keypads, baby-care parts, medical components, wearable parts, and industrial rubber parts, poor tensile strength or low tear resistance can lead to cracking, edge tearing, hole enlargement, broken thin walls, premature leakage, or product failure during assembly.

Improving these properties is not achieved by one single adjustment. It requires a controlled combination of material selection, formulation, filler reinforcement, curing, mold design, processing conditions, and final testing.

This article explains how to improve the tensile strength and tear resistance of silicone products, with practical manufacturing methods, real data, and engineering examples.

Understand the Difference Between Tensile Strength and Tear Resistance

Tensile strength measures how much pulling force a material can withstand before breaking. In silicone rubber testing, it is usually measured using dumbbell or ring specimens under a defined test method such as ASTM D412 or ISO 37. ASTM D412 is used to evaluate tensile properties of vulcanized thermoset rubbers and thermoplastic elastomers, but ASTM also notes that tensile properties alone may not directly represent the total end-use performance of a product. Test conditions such as extension rate, temperature, humidity, specimen geometry, and preconditioning can affect results.

Tear resistance measures how well a rubber material resists the initiation and propagation of a tear. This is especially important for silicone parts with holes, slots, sharp corners, thin walls, undercuts, or repeated flexing areas. ASTM D624 states that vulcanized rubber and TPE parts often fail because of tear generation and propagation, and it also notes that tear strength can be influenced by stress distribution, strain rate, specimen size, and mechanical anisotropy.

In simple terms, tensile strength tells you how well the silicone resists being pulled apart. Tear resistance tells you how well it resists a crack or cut from spreading. A silicone product can have acceptable tensile strength but still fail from tearing if the geometry has sharp notches or the material has poor notch resistance.

Start With the Right Silicone Grade

The fastest way to improve tensile strength and tear resistance is to choose a silicone grade designed for high mechanical performance. Standard general-purpose silicone may be enough for simple pads, plugs, or low-stress parts, but it may not be suitable for thin membranes, flexible tubes, baby nipples, medical valves, dynamic seals, or parts exposed to repeated assembly force.

Typical silicone rubber properties vary widely. WACKER lists a typical silicone rubber mechanical property range of 5–11 N/mm² tensile strength, 100–1,100% elongation at break, and 5–55 N/mm tear strength under ASTM D624, depending on the grade and test method.

This range shows why material selection matters. Two silicone compounds may look similar, have the same color, and even have similar hardness, but their tear resistance may be completely different.

A public product data comparison makes this clear:

Dow’s SILASTIC™ DY 32-542 U is listed with 4.2 MPa tensile strength and 16 N/mm tear strength, while Dow’s SILASTIC™ SE 1643 U is listed with 9.5 MPa tensile strength, 1,150% elongation, and up to 48 N/mm tear strength depending on the tear test geometry. WACKER’s ELASTOSIL® LR 3003/60 US A/B lists 10.6 MPa tensile strength, 420% elongation, and 31 kN/m tear strength after press curing and post-curing.

The practical lesson is clear: do not specify only “50 Shore A silicone” or “60 Shore A silicone.” Hardness is not enough. For high-strength applications, specify tensile strength, elongation, tear strength, compression set, curing system, and test method.

Use Reinforcing Fillers Correctly

Unreinforced silicone rubber has limited mechanical strength. Reinforcing fillers are commonly used to improve tensile strength, tear resistance, hardness, dimensional stability, and processing behavior.

Fumed silica is one of the most important reinforcing fillers in silicone rubber. A Rubber News technical article describes fumed silica as a vital component in silicone rubber and high-consistency rubber products, noting that it is widely used to increase hardness, tensile strength, and tear strength in cured rubber. It also states that fumed silica provides reinforcement due to properties such as high purity, small particle size, large surface area, and superior dispersion.

For silicone rubber, the filler effect depends on several factors:

Filler type: fumed silica generally provides stronger reinforcement than many non-reinforcing fillers.

Filler surface area: smaller particle size and higher surface area usually increase reinforcement, but they can also raise viscosity and make processing more difficult.

Filler surface treatment: hydrophobic or treated silica can improve compatibility, dispersion, and storage stability.

Filler loading: too little filler may not provide enough strength; too much filler may reduce elasticity, flowability, transparency, or processing stability.

Dispersion quality: poorly dispersed filler creates weak points, surface defects, and inconsistent mechanical performance.

Rubber News explains that smaller particle size can increase surface area and structure, improving reinforcement, strength, and hardness, while filler amount and dispersion also affect vulcanizate properties.

For manufacturers, filler reinforcement is not just about adding more silica. The goal is to build a stable polymer-filler network without making the silicone too stiff, too viscous, too difficult to mold, or too brittle.

Optimize Polymer-Filler Interaction

High tensile strength and high tear resistance depend heavily on the interaction between silicone polymer chains and reinforcing fillers. Good interaction allows stress to be distributed through the material instead of concentrating at one weak point.

WACKER notes that silicone rubber can achieve good mechanical properties through effective polymer-filler interaction. It also states that specialty silicone grades can have very high tear resistance and are suitable for demanding products such as bottle nipples and pacifiers.

This is important for products that experience repeated stretching or biting, such as baby-care silicone parts, medical flexible components, and soft consumer products. In these applications, the material must combine softness, elasticity, and tear resistance. A hard material may resist deformation, but it may not provide the flexibility or comfort required by the product.

The formulation target is usually a balance:

High tear strength without excessive hardness.

High elongation without poor dimensional stability.

Good tensile strength without difficult molding.

Good transparency without poor filler dispersion.

Good compliance without restricted additives or unsafe ingredients.

This is why high-performance silicone products often require custom compounding rather than simply using a general-purpose silicone rubber.

Control Crosslink Density

Crosslink density has a major effect on silicone strength, elongation, elasticity, and tear behavior. If the crosslink density is too low, the product may feel weak, tacky, or under-cured. If the crosslink density is too high, the product may become hard, brittle, and easier to crack under dynamic stress.

The right crosslink structure helps the silicone recover after deformation while still resisting rupture. For platinum-cured LSR, peroxide-cured HCR, or addition-cure systems, the curing chemistry must be matched to the final application, production process, compliance requirements, and mechanical performance target.

A practical example is WACKER’s ELASTOSIL® LR 3003/60 US A/B. Its listed properties are obtained after mixing Part A and Part B at a 1:1 ratio, press curing for 5 minutes at 166°C, and post-curing for 4 hours at 200°C. Under those conditions, the data sheet lists 59 Shore A hardness, 10.6 MPa tensile strength, 420% elongation, and 31 kN/m tear strength.

That data point matters because it shows mechanical properties are tied to curing conditions. Changing mold temperature, cure time, post-cure time, mixing ratio, or contamination level can change the final performance.

Avoid Under-Curing and Over-Curing

Curing control is one of the most common causes of poor tensile strength and tear resistance in silicone products.

Under-cured silicone may show:

Low tensile strength.

Low tear resistance.

Sticky or tacky surface.

Poor elastic recovery.

Higher deformation after stretching.

Weak parting-line strength.

Over-cured or heat-aged silicone may show:

Increased hardness.

Reduced elongation.

Brittleness.

Cracking under repeated flexing.

Lower tear resistance in thin sections.

WACKER notes that prolonged high-temperature exposure above 200°C can increase hardness and reduce tensile strength and elongation at break because of heat aging and embrittlement.

For production, this means cure settings should not be guessed. They should be validated by part thickness, mold temperature, material grade, cavity layout, and required performance. A thick silicone part and a thin silicone membrane do not cure in exactly the same way.

Improve Mold Design to Reduce Tear Initiation

Many silicone parts fail not because the material is weak, but because the part design creates stress concentration.

Common tear-starting features include:

Sharp internal corners.

Thin wall transitions.

Small holes too close to the edge.

Deep undercuts.

Poorly designed parting lines.

Rough trimming edges.

Gate marks in high-stress areas.

Air traps or weld lines near flexing zones.

For better tear resistance, use larger radii, smoother transitions, uniform wall thickness, reinforced edges, and rounded holes. Avoid placing gates, vents, or parting lines in areas that will be stretched, bent, assembled, or repeatedly compressed.

For example, a silicone pull tab with a sharp rectangular slot may tear after repeated use. Changing the slot to a rounded oval shape, increasing the edge distance, and selecting a high-tear silicone grade can significantly improve durability. This is a common manufacturability improvement in silicone straps, protective sleeves, medical pull tabs, and wearable parts.

ASTM D624 specifically notes that tear strength is affected by stress distribution and specimen size, which is why geometry and test conditions must be considered rather than relying only on material data-sheet values.

Improve Mixing, Degassing, and Material Handling

Air bubbles, contamination, poor pigment dispersion, and inconsistent mixing can all reduce silicone tensile strength and tear resistance. Even a high-performance silicone compound can fail if the production process introduces defects.

For HCR silicone, mixing quality affects filler dispersion, pigment distribution, curing agent uniformity, and final mechanical consistency. For LSR, A/B ratio control, static mixing quality, shot consistency, and contamination control are critical.

Process problems that reduce mechanical performance include:

Poor filler dispersion.

Wrong A/B ratio in LSR.

Expired or contaminated material.

Moisture or oil contamination.

Trapped air.

Pigment overdose.

Incompatible additives.

Insufficient degassing in casting or RTV systems.

Over-shearing or overheating during mixing.

Rubber News notes that fumed silica dispersion and surface properties affect silicone reinforcement and processing behavior. Poor dispersion creates localized weak points where cracks can begin.

For high-strength silicone parts, material preparation should be treated as part of quality control, not just as a pre-production step.

Select the Right Manufacturing Process

Different silicone manufacturing processes affect mechanical performance differently.

LSR injection molding is suitable for precision parts, thin-wall parts, high-volume production, medical parts, baby products, seals, and complex geometries. It offers accurate dosing, automated production, and clean processing when properly controlled.

Compression molding is suitable for HCR silicone parts, gaskets, pads, keypads, seals, and medium-volume custom parts. However, flash control, venting, pressure, cure time, and material placement have a strong effect on final quality.

Extrusion is suitable for silicone tubes, hoses, cords, strips, and sealing profiles. For extruded parts, tensile strength and tear resistance depend on compound selection, extrusion temperature, die design, curing tunnel conditions, and pulling speed.

Dow’s SILASTIC™ SE 1643 U is an example of an extrusion-grade silicone rubber used for tubes, hoses, tapes, and cables, with listed mechanical properties of 9.5 MPa tensile strength and 1,150% elongation. Dow’s SILASTIC™ HV 1510-40 LSR is a 40 Shore A liquid silicone rubber designed for high-voltage cable accessories, terminations, and cold-shrink applications, with listed elongation of 800%, tear strength of 200 ppi, and tensile strength of 1305 psi.

The material and process must be selected together. A high-tear material will not perform well if the process creates bubbles, knit lines, under-cure, over-cure, or rough edges.

Control Surface Defects and Trimming Quality

Tear failure often starts at the edge of the part. A small nick, rough trimming mark, die-cut defect, or flash-removal scratch can become the starting point for a larger tear.

This is especially important for:

Thin silicone membranes.

Medical valves.

Baby nipples.

Silicone tubes.

Wearable straps.

Keypads.

Silicone seals with thin lips.

Custom gaskets with cut holes.

Manual trimming can create inconsistent edges if operators are not trained. Cryogenic deflashing can improve efficiency for some parts, but the process must be validated because overly aggressive deflashing may damage delicate edges. Die cutting must also be controlled, especially for soft silicone sheets and gaskets.

For high-tear applications, the edge quality should be inspected under magnification, not only by visual checking. A silicone part may pass appearance inspection but still fail under repeated stretching if the edge contains micro-cuts.

Be Careful With Pigments and Additives

Pigments, flame retardants, conductive fillers, thermal fillers, anti-static additives, and processing aids can change silicone mechanical properties. Some additives improve function but reduce elongation or tear resistance.

For example:

High pigment loading can affect curing and reduce transparency.

Conductive fillers can increase stiffness and reduce elongation.

Thermal fillers can improve heat transfer but make the material less flexible.

Flame-retardant systems can change tensile strength, tear resistance, and surface quality.

Processing aids may affect bonding, printing, or coating.

WACKER notes that silicone rubber properties can be adjusted electrically from insulating to semiconducting and that silicone can be pigmented, but these adjustments must be balanced with processing and mechanical requirements.

For critical parts, every additive should be evaluated as part of the complete compound, not as an isolated material decision.

Use Proper Testing and Compare Results Correctly

A common mistake is comparing tear strength values from different test methods as if they were the same.

WACKER notes that tear strength depends on the test standard used. Its material guide states that typical values under ASTM D624 B are 5–55 N/mm, while values measured by ISO 34-1 Method B-b can be up to 30% lower, and ISO 34-1 Method A can yield values about 50% lower.

This matters when comparing supplier data sheets. One supplier may report ASTM D624 Die B, another may report ISO 34-1 trouser tear, and another may report die-angle tear. The numbers may not be directly comparable.

For reliable evaluation, define:

Test standard: ASTM D412, ISO 37, ASTM D624, ISO 34-1, or customer-specific method.

Specimen type and thickness.

Test speed.

Curing and post-curing conditions.

Aging conditions before testing.

Test temperature.

Acceptance criteria.

Production sampling frequency.

ASTM D412 also warns that tensile properties depend on test conditions and that materials should only be compared under the same conditions.

Practical Manufacturing Examples

Example 1: Thin Silicone Membrane Tearing Around a Hole

Problem: A thin silicone membrane tears around a small hole during assembly.

Likely causes: The hole is too close to the edge, the wall is too thin, the hole has a sharp cut edge, or the silicone grade has insufficient tear resistance.

Improvement plan: Increase the hole radius, improve edge distance, polish the mold core pin, use a high-tear silicone grade, avoid gate marks near the hole, and test the finished part using both tensile and tear-related methods.

Example 2: Silicone Tube Splitting During Stretching

Problem: A silicone tube splits when expanded over a connector.

Likely causes: Low elongation material, poor extrusion curing, uneven wall thickness, rough cut edge, or unsuitable compound.

Improvement plan: Use an extrusion-grade silicone with higher elongation and tear strength, control tube wall tolerance, improve cutting quality, and validate expansion force during assembly. Dow’s SE 1643 U data shows how extrusion-grade silicone can combine high elongation and strong tear resistance, with 1,150% elongation and up to 48 N/mm tear strength depending on test geometry.

Example 3: Molded Seal Cracking at the Lip

Problem: A silicone seal lip cracks after repeated compression and installation.

Likely causes: Sharp lip root, poor compression design, under-curing, excessive post-cure, unsuitable hardness, or low tear compound.

Improvement plan: Add radius at the lip root, optimize compression ratio, choose a silicone grade with better tear resistance and compression set, confirm curing conditions, and perform aging tests under the actual temperature and compression load.

Example 4: Soft Consumer Product Feels Good but Tears Easily

Problem: A soft-touch silicone product has a good hand feel but tears at the pull area.

Likely causes: Material is too soft for the force required, low filler reinforcement, poor notch design, or rough trimming.

Improvement plan: Keep the soft-touch area soft, but locally increase thickness or add ribs at the pull area. Select a high-tear material rather than simply increasing hardness across the whole product. This approach preserves user comfort while improving durability.

Checklist for Improving Silicone Tensile Strength and Tear Resistance

Use this checklist during product development:

Specify tensile strength, elongation, and tear strength, not only hardness.

Choose a high-tear silicone grade for thin, flexible, or dynamic parts.

Use reinforcing fillers such as fumed silica with proper surface treatment and dispersion.

Control crosslink density through correct curing chemistry and processing conditions.

Avoid under-curing and over-curing.

Add radii to corners, holes, slots, and thin-wall transitions.

Avoid gates, vents, and parting lines in high-stress areas.

Improve trimming, cutting, and deflashing quality.

Control pigment and additive loading.

Validate the actual finished part, not only raw material data.

Compare test results only under the same test standard and conditions.

Perform aging, assembly, flexing, and real-use testing before mass production.

Conclusion

Improving the tensile strength and tear resistance of silicone products requires more than selecting a harder material. The most effective approach is to combine high-performance silicone grades, proper filler reinforcement, optimized polymer-filler interaction, controlled curing, good mold design, clean processing, and reliable testing.

For custom silicone products, the best solution depends on the product’s function. A silicone tube, baby nipple, medical valve, keypad, gasket, wearable strap, and automotive seal may all need different material strategies.

As a silicone manufacturer, we help customers review drawings, select suitable silicone materials, optimize mold design, improve tear-prone structures, and validate parts before mass production. For products that require high tensile strength, high elongation, and strong tear resistance, early engineering review is the best way to reduce failure risk and improve long-term product performance.