KE-1329/CAT-1329SF vs RTV-4140 Silicone Comparison

KE-1329/CAT-1329SF and RTV-4140 are both addition-cure silicone systems for mold making, but they address different production needs. KE-1329 is specifically designed for polyurethane casting, with excellent urethane resin resistance, high transparency and oil-bleed properties that support easier demolding and repeated mold use. RTV-4140 uses a simpler 1:1 mixing system, lower published component viscosity and a shorter cure cycle for more straightforward mold production.

KE-1329/CAT-1329SF and RTV-4140 are both firm addition-cure mold silicones in a similar Shore A range, but they are designed around different production priorities. KE-1329 is a 42 Shore A silicone specifically positioned by Shin-Etsu for polyurethane resin resistance, repeated mold use, high transparency and easier demolding through its oil-bleed properties. RTV-4140 is a 40 Shore A platinum-cure silicone focused on straightforward processing, with a simple 1:1 mixing system and a shorter published cure cycle.

The most important difference is their intended use with polyurethane resins. Shin-Etsu specifically states that KE-1329 offers excellent resistance to urethane resins and is suitable for applications requiring multiple polyurethane parts from the same mold. This makes KE-1329 particularly relevant when mold life, consistent release performance and repeated production are more important than simply producing an initial mold. For users making a large number of polyurethane castings, the ability of the mold to retain its physical properties and release characteristics over repeated cycles can be more important than a small difference in hardness or cure time.

KE-1329 also has documented oil-bleeding properties that promote easier demolding. This feature is especially relevant in repeated casting operations, where release performance may become increasingly important as the same mold is used again and again. The oil-bleed characteristic is therefore part of KE-1329's service-oriented positioning rather than simply a processing feature. RTV-4140 is suitable for mold making and casting applications, but its current published documentation does not identify an equivalent oil-bleed release feature.

Processing is more straightforward with RTV-4140. KE-1329/CAT-1329SF uses a 100:10 mixing ratio by weight, while RTV-4140 uses a 1:1 ratio by weight or volume. The 1:1 system is easier to measure and batch manually, particularly for small or frequently adjusted quantities. The 100:10 system requires more precise measurement of the smaller component, but this difference is mainly a processing consideration rather than an indication of final mold performance.

The published working times are relatively close, at approximately 40 minutes for KE-1329 and approximately 35 minutes for RTV-4140, so working time alone is not a major reason to choose one material over the other. Viscosity shows a more noticeable processing difference, although the published values are not directly like-for-like. Shin-Etsu reports approximately 38 Pa·s (38,000 cP) for mixed KE-1329, while MinghuiLink reports approximately 4,800±200 mPa·s for Part A of RTV-4140. Because the material states differ, these values should not be used as a strict viscosity ranking. However, RTV-4140 has the substantially lower published Part A viscosity, which is generally favorable for routine measuring, mixing and pouring operations.

Cure-cycle differences may also matter for production scheduling. KE-1329 has a published demolding time of approximately 8 hours at 23°C and a standard cure of 24 hours, while RTV-4140 is published with a cure time of approximately 6 hours. For users prioritizing faster mold turnaround, RTV-4140 therefore has a processing advantage. For users whose main concern is repeated polyurethane casting and maintaining release performance over multiple production cycles, the longer KE-1329 cure cycle may be an acceptable trade-off for its more specialized material positioning.

The published mechanical data do not establish a universal performance winner. KE-1329 reports 5.0 MPa tensile strength, 22 kN/m tear strength and 260% elongation, while RTV-4140 reports 5.3 MPa tensile strength, 32 N/mm tear strength and 220% elongation. The published tensile values are broadly similar, while RTV-4140 reports a higher published tear-strength value and KE-1329 reports higher elongation. Because manufacturers may use different test methods and conditions, these figures are best treated as published reference data rather than a strict head-to-head mechanical ranking.

High transparency is another documented feature of KE-1329 and can be useful when visual inspection of the master, cavity or mold-cutting position is important. However, transparency is secondary to its main production advantage. Overall, KE-1329 is better aligned with repeated polyurethane casting where urethane resistance, oil-bleed release and retention of mold performance are priorities, while RTV-4140 is better aligned with users who value simple 1:1 processing, lower published component viscosity and faster mold turnaround.