BISCO HT-840 Extra Firm Cellular Silicone is a very firm grade silicone foam that offers superior sealing capabilities. HT-840 silicone provides a higher tear and tensile strength than our lighter grade foams. HT-840 silicone is designed for use in enclosures requiring a more durable, high closure force gasket. This material is also ideal for use in press pads requiring greater conformability and even pressure distribution at high temperatures. HT-840 silicone is a great choice for sealing and protecting outdoor communication, lighting and electronic enclosures from small dust particles, wind-driven rain and fire.


  • Compact cell structure
  • Available in thicknesses from 0.063 to 0.250 in.
  • Typical density of 28 PCF
  • CFD range of 16-26 PSI
  • Resistant to UV, ozone, extreme temperatures and flame


  • Provides enhanced durability and sealing performance
  • Offers consistent performance in all environments
  • Reduces maintenance costs associated with gasket failures due to compression set and softening



Explore our calculators, conversion tools, technical papers and more.

  • BISCO® Silicones Product Properties Guide

    Filter BISCO product information by various criteria, including property groups, product category or units, to find the best material for the application.

  • Application Design Tool

    Determine the best BISCO® silicone or PORON® polyurethane material for specific design application.

  • Compression Force Deflection Curve Tool

    Identify design-appropriate BISCO silicone, PORON polyurethane and ARLON silicone materials using stress-strain data.

  • Impact Prediction Tool

    Select the best BISCO silicone or PORON polyurethane for energy absorbing applications using impact data.

  • Vibration Isolation Tool

    The Vibration Isolation Tool recommends the proper BISCO® silicone and PORON® polyurethane materials for your vibration mitigation applications.

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