Mold Star™ 40T vs RTV-4140 Silicone Comparison

1. Technical Assessment

Mold Star™ 40T and RTV-4140 are both Shore A40 platinum-cure silicones, placing them in a higher-hardness range intended for molds that need more rigidity and resistance to deformation than softer mold rubbers. This makes them relevant for larger, more rigid or repeatedly handled molds where maintaining shape during casting and demolding is important.

For this type of application, hardness is only part of the decision. Published mechanical data show a different balance between the two materials. RTV-4140 reports higher tensile strength and tear strength, while Mold Star™ 40T reports substantially higher elongation. These differences may matter when the mold must resist handling and tearing, or when additional flexibility is useful during demolding. Because the manufacturers' testing conditions and data presentation are not identical in every respect, the figures should be treated as published reference values rather than as a strict laboratory ranking.

Processing time is another significant difference. Mold Star™ 40T has a published 20-minute pot life and approximately 3-hour cure time, while RTV-4140 has a 35-minute working time and approximately 6-hour cure time under its stated conditions. Mold Star™ 40T therefore provides less working time but a substantially faster published turnaround, whereas RTV-4140 allows more time for mixing and pouring before requiring a longer cure cycle.

Viscosity requires careful interpretation in this comparison. Smooth-On reports a mixed viscosity of 11,000 cps for Mold Star™ 40T, while MinghuiLink reports approximately 4,800 mPa·s for Part A of RTV-4140. Since these values refer to different measurement points, they should not be treated as a direct like-for-like viscosity test. Actual flow, detail reproduction and air release should be evaluated with the intended mold geometry and pouring method.

Mold Star™ 40T also has several application-specific characteristics that may influence material selection. Smooth-On identifies the material for translucent molds, low-temperature melt metal casting, and certain dry-food and aqueous-food applications after proper curing. RTV-4140 is positioned by MinghuiLink for rigid molds used in rapid prototyping, resin casting, thin-shell casting and concrete or brick applications. The final selection should therefore consider not only hardness and strength, but also the intended casting material, required turnaround time and any specific regulatory or application requirements.

2. Technical Comparison

Technical Parameter	Smooth-On Mold Star™ 40T	MinghuiLink RTV-4140
Silicone Type	Platinum-cure silicone	Platinum-cure silicone
Color	Translucent	Translucent
Specific Gravity	1.12 g/cc (1.12 g/cm³)	1.08–1.10 g/cm³
Mix Ratio	1A:1B by volume or weight	1A:1B by weight or volume
Shore Hardness	40 Shore A	40 Shore A
Viscosity	11,000 cps (mixed)	4,800 mPa·s* (Part A)
Working / Pot Life	20 min	35 min
Cure Time	3 hours	6 hours
Tensile Strength	656 psi (~4.52 MPa)	5.3 MPa
Tear Strength	140 pli (~24.5 N/mm)	32.0 N/mm
Elongation	500%	220%
Shrinkage	<0.001 in./in. (<0.1%)	≤0.1%
Heat Resistance	232°C / 450°F	250°C / 482°F
Food Contact	Complies with 21 CFR 177.2600 for dry food and aqueous food; suitable for water/ice	Passed US FDA 21 CFR 177.2600 total extractives testing
Technical Data Sources	Mold Star™ 40T	RTV-4140

Competitor specifications are based on publicly available manufacturer documentation, with links to the original sources where available. Viscosity values are reported according to the manufacturers' stated measurement points and should not be treated as a direct like-for-like comparison where test bases differ. Missing values are shown as “Not publicly listed” and are not estimated or inferred. Specifications may change, so always check the latest manufacturer documentation and conduct application-specific testing before production use.

3. Key Differences

Key Factor	Smooth-On Mold Star™ 40T	MinghuiLink RTV-4140	What It Means
Hardness	40 Shore A	40 Shore A	Both target the same relatively rigid mold hardness.
Mix Ratio	1A:1B by volume or weight	1A:1B by weight or volume	Both use a straightforward 1:1 mixing ratio.
Processing Window	20 min pot life	35 min working time	RTV-4140 provides more published working time for mixing and pouring.
Turnaround Time	3 hours cure	6 hours cure	Mold Star™ 40T has the shorter published cure cycle.
Mechanical Profile	656 psi tensile; 140 pli tear; 500% elongation	5.3 MPa tensile; 32.0 N/mm tear; 220% elongation	RTV-4140 reports higher published tensile and tear strength, while Mold Star™ 40T reports substantially higher elongation.
Temperature Resistance	232°C / 450°F	250°C / 482°F	RTV-4140 has the higher published temperature-resistance value.
Special Application Information	Specified for certain dry-food and aqueous-food applications and water/ice; suitable for low-temperature melt metal alloys	Positioned for rapid prototyping, resin casting, thin-shell casting and concrete/brick molds	The published application focus is not identical, so project requirements should determine which material is more appropriate.

4. Application Comparison

Application	Mold Star™ 40T	RTV-4140	Key Consideration
Resin Casting	Suitable	Suitable	Both can be used for resin mold making; consider working time, cure cycle, mold rigidity and repeated-use requirements.
Concrete / Brick Molds	Suitable	Suitable	Higher hardness can help control mold deformation when casting heavier materials. Mold design and required flexibility should still be evaluated.
Rapid Prototyping	Suitable	Suitable	RTV-4140 provides a longer published working time, while Mold Star™ 40T provides a shorter published cure cycle.
Low-Temperature Melt Metal	Suitable	Suitable	Smooth-On specifically lists Mold Star™ 40T for low-temperature melt metal alloys. RTV-4140 should be validated for any metal casting application before use.
Food / Ice Molds	Specified for dry food, aqueous food and water/ice	Suitable	Mold Star™ 40T has specific published food-contact information. Do not assume equivalent food-contact suitability without applicable documentation.
Detailed or Complex Molds	Suitable	Suitable	Both are relatively rigid at 40 Shore A; demolding requirements, undercuts and the need for elongation should be evaluated with the actual mold geometry.
Repeated Production Molds	Suitable	Suitable	Compare tear strength, elongation, expected casting material and production handling before selecting the material.

Source basis: Application suitability is based on the manufacturers' published application information and general material characteristics. Food-contact and low-temperature metal-casting claims are listed only where supported by manufacturer documentation. Actual compatibility should be validated with the intended casting material, mold design and process.

5. FAQ

1. Is RTV-4140 a direct replacement for Mold Star™ 40T?

RTV-4140 can be evaluated as an alternative to Mold Star™ 40T for similar mold-making and casting applications because both are Shore A40 platinum-cure silicones with a 1:1 mix ratio. However, their processing window, cure time and published mechanical properties differ, so application-specific testing is recommended before switching.

2. Why choose a Shore A40 silicone for mold making?

Shore A40 provides more mold rigidity than softer silicone grades, which can help reduce deformation during casting and handling. It may be useful for larger molds, heavier casting materials or applications where dimensional stability is important. The trade-off is reduced flexibility compared with softer grades, so complex undercuts and difficult demolding conditions should be evaluated carefully.

3. Do I need to vacuum degas Mold Star™ 40T?

Usually not for most applications. Smooth-On states that vacuum degassing is not necessary for the Mold Star™ T Series, although vacuum can still help remove entrapped air when a highly detailed or bubble-free mold is required. Pouring the silicone slowly from a low point can also help reduce trapped air.

4. Why is Mold Star™ 40T sticky or uncured against my model?

Platinum-cure silicone can be inhibited by certain contaminants. Smooth-On specifically warns about latex, sulfur-containing clay, certain wood surfaces, newly cast polyester, epoxy, tin-cure silicone and urethane materials. Mold Star™ 40T will not cure against sulfur-containing surfaces even when sealed. A small compatibility test should be performed before molding an important master.

5. Can Mold Star™ 40T be used with 3D-printed masters?

Yes, but 3D-printed masters should be thoroughly cleaned, fully cured where applicable and tested for compatibility before making the complete mold. UV resin and other contaminants can inhibit platinum-cure silicone, so a small test area is recommended. Acrylic sealing may also be used to isolate a compatible master surface when appropriate.

6. Is Mold Star™ 40T food safe?

Properly cured Mold Star™ 40T complies with 21 CFR 177.2600 for repeated use with dry food and aqueous food and is identified by Smooth-On as suitable for water and ice. It is not intended for repeated contact with fatty foods, so it should not be treated as a general-purpose food-contact or baking silicone.

7. Which has more working time, Mold Star™ 40T or RTV-4140?

RTV-4140 has the longer published working time at 35 minutes, compared with a 20-minute pot life for Mold Star™ 40T. Mold Star™ 40T, however, has the shorter published cure time at approximately 3 hours versus 6 hours for RTV-4140.

8. Which is better for demanding mold applications: Mold Star™ 40T or RTV-4140?

There is no universal winner. RTV-4140 reports higher published tensile and tear strength, while Mold Star™ 40T reports much higher elongation and has a shorter published cure cycle. The better choice depends on whether the project prioritizes mold strength, flexibility during demolding, production turnaround, temperature requirements or a specific application such as food or low-temperature metal casting.

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Disclaimer

Competitor specifications are based on publicly available manufacturer documentation, with links to the original sources where available. Missing values are shown as “Not publicly listed” and are not estimated or inferred. Specifications may change, so always check the latest manufacturer documentation and conduct application-specific testing before production use. Product names and trademarks belong to their respective owners. This comparison is provided for technical and informational purposes only and is not endorsed by the brands mentioned.