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S Glass
A family of magnesium-alumina-silicate glasses with high mechanical strength.

SAN
Styrene Acrylonitrile (Thermoplastic Resin).

Sandwich Construction
A composite composed of lightweight core material (usually honeycomb or foamed plastic) to which two relatively thin, dense, high strength, functional, or decorative skins (also called faces) are adhered.

Scrim
A low-cost reinforcing fabric made from continuous filament yarn in an open-mesh construction. Used in the processing of tape or other B-stage material to facilitate handling. Also used as a carrier of adhesive, to be used in secondary bonding.

Scrimp
Seamann Composite Resin Infusion Molding Process.

Secondary Bonding
The joining together, by the process of adhesive bonding, of two or more already cured composite parts, during which the only chemical or thermal reaction occurring is the curing of the adhesive itself.

Separator
A permeable layer that also acts as a release film. Porous Teflon-coated fiberglass is an example. Often placed between lay-up and bleeder to facilitate bleeder system removal from laminate after cure.

Set Up
To harden, as in curing of a polymer resin.

Shear
An action or stress resulting from applied forces which causes or tends to cause two contiguous parts of a body to slide relative to each other in a direction parallel to their plane of contact. Inter laminar Shear (ILS). The plane of contact is composed of resin only.

Shear Edge
The cut-off edge of the mold.

Shear Strength
The maximum shear stress that a material is capable of sustaining. Shear strength is calculated from the maximum load during a shear or torsion test and is based on the original cross-sectional area of the specimen.

Sheet Molding Compound (SMC)
A ready-to-mold glass fiber reinforced polyester material primarily used in compression molding.

Shelf Life
The length of time a material, substance, product, or reagent can be stored under specified environmental conditions and continue to meet all applicable specification requirements and/or remain suitable for its intended function.

Short Beam Shear (SBS)
A flexural test of a specimen having a low test span-to-thickness ratio (for example, 4:1), such that failure is primarily in shear.

Shrinkage
The relative change in dimension between the length measured on the mold when it is cold and the length on the molded object 24 hours after it has been taken out of the mold.

Silicon Carbide Fiber
A reinforcing fiber with high strength and modulus; density is equal to that of aluminum. It is used in organic metal-matrix composites.

Single Yarn
The simplest strand of textile material suitable for operations such as weaving, knitting, etc.

Sink Mark
A shallow depression or dimple on the surface of an injection molded part due to collapsing of the surface following local internal shrinkage after the gate seals; an incipient short shot.

Size
The treatment applied to the glass fiber to allow the resin and glass to adhere to one another. Also allows glass fiber to be conveniently handled.

Skin
A layer of relatively dense material used in a sandwich construction of the surface of the core.

Soft Glass
A roving product whose sizing is moderately soluble in acetone or styrene, which results in the tendency of the bundles to open readily or filamentize the matrix resin. The size is generally between 50% and 80% soluble in acetone.

Specimen
An individual piece or portion of a sample used to make a specific test; of specific shape and dimensions.

Splice
The joining of two ends of yarn by intertwining, knotting, overlapping or adhering them together.

Split Mold
A mold in which the cavity is formed of two or more components held together by an outer chase. The components are known as splits.

Spray Pattern
In connection with the spray-up process (see below), it refers to the width and uniformity of the fan of resin and glass while it is travelling between the gun and the mold.
See also - Spray-up

Spray-Up
Techniques in which a spray gun is used as the processing tool. In reinforced plastics, for example, fibrous glass and resin can be simultaneously deposited in a mold. In essence, roving is fed through a chopper and ejected into a resin stream, which is directed at the mold by either of two spray systems. In foamed plastics, very fast-reacting urethane foams or epoxy foams are fed in liquid streams to the gun and sprayed on the surface. On contact, the liquid starts to foam.

Stacking Sequence
A description of a laminate that details the ply orientations and their sequence in the laminate.

Staple
Filaments produced in short lengths from the bushing (usually less than 17 inches), to be gathered into strands or sliver. (See Continuous Filament)

Starved Area
An area in a plastic part that has an insufficient amount of resin to wet out the reinforcement completely. This condition may be due to improper wetting or impregnation or excessive molding pressure.

Storage Life
The period of time during which a liquid resin or packaged adhesive can be stored under specified temperature conditions and remain suitable for use. (Also 'shelf life.')
See also - Shelf life

Strand
Normally an untwisted bundle or assembly of continuous filaments used as a unit, including slivers, tows, ends, yarn, and so forth. Sometimes a single fiber or filament is called a strand.

Strand Count
The number of strands in a plied yarn; the number of strands in a roving. U.S. Yardage System; the length, in hundreds of yards, of a single strand having a mass of one pound. European TEX System; the mass, in grams, of a strand 1000 meters in length.

Strand Integrity
Relating to the ability of the size to keep all the filaments of a bundle stuck together during chopping. Good strand integrity is required for good flow in or wet-through and wet-out on the mold.

Structural Adhesive
Adhesive used for transferring required loads between adherends exposed to service environments typical for the structure involved.

Structural Bond
A bond that joins basic load-bearing parts of an assembly. The load may be either static or dynamic.

Styrene Monomer
A water-thin liquid monomer used to thin polyester resins and act as the crosslinking agent.

Surface Preparation
Physical and/or chemical preparation of an adherend to make it suitable for adhesive bonding.

Surfacing Mat
A very thin mat, usually 7 to 20 mils thick, of highly filamentized fiber glass used primarily to produce a smooth surface on a reinforced plastic laminate.