High Pressure Laminates
Industrial Laminates are layers of a substrate (bases) which are impregnated, or coated with a thermosetting resin. The resin, or binder, is like a "glue" which, when both heat and pressure are added to the base material, forms a profile of sheet, rod, tube, or molded shape. Products which require a curing pressure above 100 psi are considered High Pressure Laminates.
The two distinguishing components of High Pressure Laminates are the base material and the resin. The combination of these two components produces products which, although often similar, are unique and can offer a wide range of physical, mechanical and electrical properties.
The resins used in the manufacture of High Pressure Laminates are called Thermosetting Resins. A thermoset is like concrete, once it has been formed, its shape cannot be changed. Heating may soften the structure, but cannot restore flowability nor will it melt. There are four groups of resins used in the manufacture of High Pressure Laminates:
Phenolics: Phenolic resin is the most widely used resin This group has good mechanical and electrical strength properties. They offer resistance to heat, flame, moisture, mild acids and alkalies. Most paper phenolics, X, XX, XXX, and cloth-reinforced C, CE, L, LE are produced with Phenolic resins. On a cost basis, this group is comparatively low.
Melamines: Melamines have excellent resistance to electrical arcing and tracking. Melamines have the highest tensile strength, high mechanical strength; good fungus resistance, good flame and heat resistance; and good resistance to akalies and solvents.
Epoxies: Epoxies have high resistance to acids, alkalies, and solvents, and have extremely low moisture absorption. Additional advantages are dimensional stability, mechanical strength, bond strength, and compatibility with epoxy potting compounds used for encapsulation.
Silicones: Silicones are used primarily with glass cloth, have very high heat resistance, and good mechanical and electrical properties. Silicones can be used to 500 degrees F.
There are four base types of materials which are used in the manufacture of high perssure laminates. The base materials, or plies of sheet material used are:
Paper-Base: Paper-Base Laminates are found in the lowest cost products and are primarily used where electrical properties are the primary concern. Both wood pulp and rag papers aer used. Polymer Plastics offers a special mechanical riser tooling plate with excellent machinability.
Cotton Cloth: When mechanical strength is important, cotton cloth is an effective candidate. Better machinability is achieved when a fine weave cloth such as linen is used.
Glass Fiber: Glass is recommended whenever very low moisture absorption, high mechanical strength, high heat resistance, dimensional stability, and superior electrical properties are required.
Nylon: Nylon fabric based phenolic offers better water-absorption characteristics, good electrical resistance properties in high-humidity areas, and high impact strength. Nylon also has good abrasion resistance, toughness, and excellent resistance to chemical attack. Its major drawbacks are that it lacks dimensional stability and may warp.
In determining which combination of base material and resin to use, overspecification and underspecification can occur if several of the properties specified cannot exist together in the same laminate.
Overspecification: Various methods can be used to avoid overspecification. For example, the water resistance of glass-fabric laminates may be duplicated by a cheapter paper-base laminate with a high resin content or by a slightly thicker paper-base laminate. Frequently a design change permits the use of less expensive material; especially where electrical insulation is important.
Underspecification: A low-cost material should not necessarily be chosen until its effect on production costs is examined. For example, a more expensive material may be more economical provided there is net cost savings because of ease of fabrication.