Potting of electromagnets in steel mills with dc silicone elastomers

Project Sales Corp, Visakhapatnam

Potting of Electromagnets in Steel Mills with Silicone

Elastomers

Prepared by:Project Sales Corporation

28 Founta Plaza, SuryabaghVisakhapatnam 530020, AP

Phones: 0891 2564393, 5566482


Important Considerations in choosing a potting material

Viscosity of the material Specific gravity Thermal conductivity Reversion Resistance Shore hardness Curing schedule Pot life-Working Time Electrical properties Temperature Stability


Viscosity• The viscosity of the material

defines the ease with which large quantity of material can be poured into the magnet. If the viscosity is too high, the pourability becomes difficult and air pockets may develop in the pot. Lower viscosity self-leveling material gradually spread when poured from one corner releasing air pockets which may be in the coils. Measured in poise. 1 poise=100cps=100mpa.s.

• The viscosity continues to increase with the pot life until it cures to a solid elastomer in 8-24 hours.


Specific Gravity• The specific gravity is expressed in terms of “as supplied”

and “as cured”. The “as supplied” specific gravity determines the weight of the compound you buy against an unit of volume. Higher specific gravity materials fill less space with more volume, hence are not cost-economical for large volume applications like potting applications in lifting magnets.

• The ideal specific gravity should be in the range of 1.1 to 1.3 as supplied.

• The higher the specific gravity, the higher quantity of the material required.


Thermal Conductivity

• The thermal conductivity of the material is expressed as its ability to conduct heat. For potting applications where the material is subjected to a high amount of heat in confined conditions, the heat has to be dissipated quickly. This ensures too much thermal stresses is not encountered by the material or the coil.

• Poor thermal conductive materials require additional filler materials to improve the conductivity as the potting time. Else, when heated under confined conditions or deep sections the materials may depolymerize and revert to a more liquid stage.


Reversion Resistance• Many types of potting materials when heated under

confined conditions or in deep sections, will depolymerize or revert to a more liquid state. This degradation process is referred to as reversion.

• For potting applications, this is an important consideration if instances of depolymerization have occurred.

• The curing mechanism and cross-linking of the product decides its ability to be reversion resistant. Reversion resistant products cure by an “addition reaction” process which produces no cure by-product.


Shore Hardness• The shore hardness of a silicone material should

be such as it allows for the material to provide stability and relief from mechanical shocks and thermal cycling stresses.

• It should cure to a flexible material allowing for easy reparability and removal without damage to the coils.

• The ideal shore hardness for potting application should be in the range of 45-50 Shore A.


Curing Schedule

• A potting material designed for deep section cure in confined conditions should cure within 8-24 hours with the addition of the catalyst irrespective of thickness or depth.

• It is advantageous to have flexible curing schedule with different catalyst (slow curing, fast curing, normal curing).

• Exposure to humidity and heat decreases curing time.


Pot Life- Working Time• The material after mixed with a catalyst gradually

increases in its viscosity to cure to a solid elastomer.

• For large applications like lifting magnet the material should have a pot life of anywhere above 15 minutes at 25 deg C.

• Where base/catalyst mixing ratio is above 20:1 or 10:1, catalyst prices can escalate the cost of potting larger sections.

• Higher pot life but not affecting the curing schedule allows for larger packs to be bought bringing cost savings in material costs. This allows for mixing of larger quantity of materials per single pour cycle.\

• The application time (or the time till which the material is in a liquid state ) is normally half the pot life.


Electrical Properties

The potting material for lifting magnets should have:

•good dielectric strength•as high a volume resistivity as possible


Temperature Stability• Materials designed for deep section cure working in confined

conditions should not melt or appreciably soften even at elevated temperatures >200 deg C.

• Aging tests conducted on the material should show little or no effect on the basic electrical or elastomeric physical properties.

• The overall cured material should handle the thermal cycling experienced in a lifting magnet in a quick time.

• Higher quality materials designed for very high temperatures work even at elevated temperatures >300 deg C for a few hours despite confined conditions and are used for critical applications where reliability is extremely critical.


For Further details

• For further details, contact the Dow Corning distributors for Silicone Elastomers in this region:

Project Sales Corp28 Founta Plaza, Suryabagh,Visakhapatnam 530020, APPhones: 0891 2564393; 5566482Fax: 0891 2590482mailto: [email protected]

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Potting of electromagnets in steel mills with dc silicone elastomers