SILASTIC™ RTV-4251-S2 and RTV-4120 are both Shore A20-class addition-cure silicone systems for reusable mold making. Dow lists 20 Shore A for RTV-4251-S2, while MinghuiLink lists 20±1 Shore A for RTV-4120. The hardness is therefore broadly comparable, with both offering a balance of flexibility and mold support.
Dow positions RTV-4251-S2 as a high-strength, highly elastic mold-making silicone with outstanding release. It is specifically intended for detailed reproduction and easier removal of complex replica parts, and Dow also lists plaster, cement, rigid polyurethane and polyester applications.
Working time is one of the clearest differences. RTV-4251-S2 provides approximately 60 minutes, compared with approximately 35 minutes for RTV-4120. The longer window gives more time for mixing, deaeration and controlled pouring, which can be useful for detailed or larger molds.
RTV-4120 has the simpler mixing system. It uses a 1:1 ratio by weight or volume, while RTV-4251-S2 uses a 10:1 ratio by weight. For frequent manual batching and small production runs, the equal-parts system is easier to measure and less sensitive to small weighing errors.
Viscosity also favors RTV-4120 for routine handling. Dow lists approximately 9,000 cP for the mixed RTV-4251-S2, while MinghuiLink publishes approximately 4,700±200 mPa·s for Part A and 4,200±200 mPa·s for Part B. Because the reported material states and measurement points differ, these values are not a strict like-for-like comparison, but the published data indicate different flow and handling characteristics.
The mechanical profile is more balanced. RTV-4251-S2 publishes approximately 6.27 MPa tensile strength, 22.8 N/mm tear strength and 600% elongation. RTV-4120 publishes 5.0±0.5 MPa tensile strength, 27±0.5 N/mm tear strength and 550±50% elongation. RTV-4251-S2 therefore publishes higher tensile strength and slightly higher elongation, while RTV-4120 publishes the higher tear-strength value.
For difficult demolding, RTV-4251-S2 has the more distinctive published profile. Its high elongation and Dow's positioning for complex replica removal make it attractive where the mold must stretch around detailed or undercut geometry. RTV-4120 may be preferable when tear resistance, simpler batching and routine production efficiency are more important.
For large or thick molds, both materials require appropriate mold design and support. Shore A20 silicone can deform under casting loads, so a support shell or mother mold may be necessary. Dow also highlights fast thick-section curing for RTV-4251-S2, while RTV-4120 has the shorter overall published turnaround of approximately 4–5 hours.
Overall, RTV-4251-S2 is best aligned with detailed molds, complex demolding, longer working time and higher tensile performance. RTV-4120 is better aligned with routine production where simple 1:1 mixing, lower published component viscosity, higher published tear strength and faster turnaround are the stronger priorities.