Grade HT

Note: This is a variation of Glastherm-HT and Glastherm-S (Up to 550ºF Fiberglass Reinforced Polyester Composite Boards).

GLASTHERM® brand thermal insulating sheet is an excellent choice for mold insulation applications. Glastherm sheet offers significant advantages when compared with other materials such as mica, asbestos concrete or calcium silicate. Glastherm sheet is an efficient thermal barrier. It is strong in order to resist thcell nameffects of molding pressure and rough handling. It is resistant to oil and water absorption, and is completely asbestos-free.
A major profit constraint in the molding and casting of rubber, plastics and metals continues to be the cost of energy. Managers who are concerned with the current and forecasted price of electricity, natural gas, and oil are focusing on methods which will reduce energy consumption. Installing or upgrading thermal insulation in curing presses and molds is one relatively easy and inexpensive way to achieve important energy savings.
In order to cure or “freeze” a resin or compound, molding equipment is almost always dependent upon heated or cooled zones Glastherm sheet reduces energy consumption by minimizing a wasteful flow of energy between these zones.
In the past, little or no insulation has been used in or around molds. Where it was used, the insulation was often a thin sheet of asbestos-reinforced concrete (e.g. Transited®). Although inexpensive, asbestos-reinforced concrete is a poor thermal barrier and is so brittle that it is easily damaged. It readily absorbs oil and water, further reducing its value as an insulating material. Asbestos concrete also tends to compress after a period of time in use. This can be a problem where parallelism between mold halves is important. Alternatives to asbestos concrete, such as calcium silicate (e.g. Marinite®) and mica, also have drawbacks. Calcium silicate is highly absorbent and has very poor compressive strength. Mica has poor long-term compressive strength and is expensive. Insulating material for today’s needs must have:

  • Low thermal conductivity
  • High compressive strength at process temperatures
  • Excellent control of thickness dimension
  • No asbestos or other toxic materials
  • Ability to withstand rough handling during mold changes

Glastherm sheet provides all of these features. It is a fiberglass reinforced, mineral filled sheet bonded together with heat resistant thermosetting resins. This highly durable composite is then precision finished to close dimensional tolerances. It has less than half the thermal conductivity of asbestos concrete and is rugged enough to provide years of service.
When installed on the base of a mold. Glastherm sheet will reduce heat transfer between mold and press platen. This also results in more even temperatures within the mold. Another benefit, in the case of heated molds, is that less plant ventilation will be needed.


Compressive Modulus ( psi )1.8 x 106
IZOD Impact Strength ( ft lb/in )8.0
Thickness Tolerance ( in )±0.002
Electric Strength Perpendicular at 0.5 in ( vpm )50
Resistance to Lubricant and Hydraulic FluidsNo Effect
Glastherm Sheet Thickness ( in )1
Glastherm Sheet Size ( ft )3 x 6
4 x 8
Density ( lb/ft³ )123
Flexural Strength ( psi )31000
Maximum Service Temperature ( ºF )550
Water Absorption0.2 % by Wt.
Thermal Conductivity ( BTU-in/hr-ft²-ºF )1.9
Coefficient of Thermal Conductivity Across Thickness ( in/in/ºF )6.43 x 10-5
Coefficient of Thermal Conductivity Across Surface ( in/in/ºF )1.24 x 10-5
Compressive Strength Force Applied to Surface at 75ºF ( psi )49000
Compressive Strength Force Applied to Surface at 302ºF ( psi )27000
Compressive Strength Force Applied to Surface at 392ºF ( psi )18000
Compressive Strength Force Applied to Surface at 425ºF ( psi )17500
Compressive Strength Force Applied to Surface at 550ºF ( psi )17000
Compressive Strength Force Applied to Edge at 75ºF ( psi )31000
Compressive Strength Force Applied to Edge at 425ºF ( psi )3600
Rockwell M Hardness98

Grades of Glastherm Sheet

Two grades of Glastherm insulating sheet are currently available. Both otter a combination of high physical strength and low thermal conductivity.
Glastherm Grade HT Sheet
A thermal insulating sheet with superior strength and heat resistance for applications with continuous use temperatures up to 550ºF. Designed for plastics and zinc die cast mold requirements.

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