KE-1311/CAT-1310 and RTV-4140 are both Shore A40 addition-cure silicone systems, but they serve different production priorities. KE-1311 is specifically positioned by Shin-Etsu for high-strength mold making with excellent resistance to urethane casting resins and extended mold life, making it particularly relevant to repeated polyurethane production rather than simply general-purpose A40 molding.
The most important difference is therefore the service environment. A silicone may be generally compatible with polyurethane yet still experience changes in surface condition, release performance or physical properties after repeated exposure to reactive casting resins. Shin-Etsu specifically identifies KE-1311 for excellent urethane resistance and applications where multiple polyurethane parts are produced from the same mold. For production users, this makes resin resistance and mold longevity more important selection factors than hardness alone.
The two products also use very different mixing systems. KE-1311/CAT-1310 uses a 100:10 ratio by weight, while RTV-4140 uses a 1:1 ratio by weight or volume. KE-1311 therefore requires more careful component measurement, particularly for small batches, whereas RTV-4140 is simpler to batch for routine manual production. This is primarily a workflow difference rather than a direct indicator of cured performance.
Working time provides another meaningful difference. Shin-Etsu specifies approximately 1.5 hours for KE-1311, compared with approximately 35 minutes for RTV-4140. The longer KE-1311 window can be useful when preparing large molds, performing vacuum deaeration or carefully filling complex cavities. RTV-4140 instead favors a shorter processing cycle and faster progression toward demolding.
Viscosity is also significantly different in the published specifications. Shin-Etsu reports approximately 80,000 cP for the KE-1311 base, while MinghuiLink reports approximately 4,800±200 mPa·s for Part A of RTV-4140. Because these figures refer to different material states, they should not be treated as a strict like-for-like laboratory ranking. The much higher published viscosity of KE-1311 nevertheless means that appropriate mixing, vacuum deaeration and controlled pouring are more important considerations, particularly for detailed molds.
The published mechanical data show a mixed profile rather than a universal winner. KE-1311 reports 850 psi (~5.86 MPa) tensile strength, 22 kN/m tear strength and 350% elongation, while RTV-4140 reports 5.3 MPa tensile strength, 32.0 N/mm tear strength and 220% elongation. KE-1311 therefore reports higher published tensile strength and elongation, while RTV-4140 reports the higher published tear value. Because the manufacturers use different test methods and reporting conditions, these figures should be treated as published reference values rather than a direct laboratory ranking.
For users making polyurethane molds repeatedly, the key advantage of KE-1311 is not simply its Shore A40 hardness or mechanical strength. Its strongest differentiation is the combination of documented urethane resistance and extended mold-life positioning for repeated production. RTV-4140 offers a more straightforward 1:1 mixing system, lower published component viscosity and a shorter cure cycle, making it attractive when easier processing and faster mold turnaround are the main priorities.
