Scrap: any output of a mold that is not usable as the primary product.
Screw Travel: The distance the screw travels forward when filling the mold cavity.
Shear rate: the rate at which a layer of melt slides over the layer below. Shear rate is velocity-related rather than force- related.
Shear Strength: The maximum load required to shear the specimen in such a manner that the moving portion has completely cleared the stationary portion. Sheet Sheets are distinguished from films in the plastics industry only according to their thickness. In general, sheets have thicknesses greater than .040″.
Shear stress: the shearing force divided by the area. It is always a maximum at the outside of the flow channel. As it is force-related, it depends on the viscosity of the material, which in turn depends on the material and molding conditions. The maximum allowable stress level is usually taken as 1% of the tensile strength of the material. High shear stress is unimportant at gates and in sprues and runners.
Short Shot (also known as Non-Fill): Failure to completely fill the mold or cavities of the mold. Edges may appear melted.
Shot Capacity: Generally based on polystyrene, this is the maximum weight of plastic that can be displaced or injected by a single injection stroke. Generally expressed as ounces of polystyrene.
Shot: One complete cycle of a molding machine.
Shrinkage Allowance: The dimensional allowance which must be made in molds to compensate for shrinkage of the plastic compound on cooling.
Shrinkage: contraction upon cooling of all or areas of the part. Shrinkage occurs less is disorientated material and more across chains of molecules than along their lengths. Lower pack area has lower areas of orientation and shrinkage.
Side-Draw Pins: Projections used to core a hole in a direction other than the line of a closing of a mold and which must be withdrawn before the part is ejected from the mold. See also Retractable Cores.
Sink Mark: an indentation on the surface of the part as a result of the significant local change in wall section. The mark will occur in the thicker area.
Skin: a relatively dense layer at the surface of the material.
Slide: projection in the mold used to form the geometry of the part, which is not in the direction of the closing of the mold and must be withdrawn before the part can be ejected.
Slip Agent: Additive used to provide lubrication during and immediately following the processing of plastics.
Solvents: Substances with the ability to dissolve other substances.
Specific Gravity: The ratio of the density of a material as compared to the density of water at standard atmospheric pressure (1 ATM) and room temperature (73F).
Specific Volume: The volume of a unit of weight of a material; the reciprocal of density.
Spiral Flow: Test performed by injection molding a sample into a spiral mold and used to compare the processability of different resins.
Splay Marks: Scan or surface defects on molded part caused by abnormal racing of the melt in the mold or moisture.
Split-Ring Mold: A mold in which a split cavity block is assembled in a channel to permit the forming of undercuts in a molded piece. These parts are ejected from the mold and then separated from the piece.
Sprue Bushing: A hardened-steel insert in the mold that accepts the nozzle and provides an opening for transferring the melt.
Sprue Gate: A passageway through which melt flows from the nozzle to the mold cavity.
Sprue Lock: The portion of resin retained in the cold-slug well by an undercut. This lock is used to pull the sprue out of the bushing as the mold opens. The sprue lock itself is pushed out of the mold by an ejector pin.
Sprue: The feed opening provided in injection molding between the nozzle and cavity or runner system.
Stabilizer: An agent used in compounding some plastics to assist in maintaining the physical and chemical properties of the compounded materials at suitable values throughout the processing and a service life of the material and/or the parts made therefrom.
Stack Molds: Two or more molds of a similar type that are positioned one behind the other to allow for additional parts to be manufactured during a cycle.
Stationary Platen: The large front plate of an injection molding press to which the front plate of the mold is secured. This platen does not move during normal operation.
Stiffness: The capacity of a material to resist elastic displacement under stress.
Strain: In tensile testing, the ratio of the elongation to the gage length of the test specimen, that is, the change in length per unit of original length.
Stress Concentrators: abrupt changes in the geometry of the part serving as the focus of high stresses. Various means can be devised to relieve the abruptness of the geometric changes and thus the stresses.
Stress Cracking: There are three types of stress cracking:
1. Thermal stress cracking is caused by prolonged exposure of the part to elevated temperatures or sunlight.
2. Physical stress cracking occurs between crystalline and amorphous portions of the part when the part is under an internally or externally induced strain.
3. Chemical stress cracking occurs when a liquid or gas permeates the partís surface. All of these types of stress cracking have the same end result: the splitting or fracturing of the molding.
Stress Relaxation: The decay of stress at a constant strain.
Stress: The force producing or tending to produce deformation in a body measured by the force applied per unit area.
Stress-Crack: External or internal cracks in a plastic caused by tensile stresses less than that of its short-time mechanical strength. Note: The development of such cracks is frequently accelerated by the environment to which the plastic is exposed.
Stress-Strain Curve: The curve plotting the applied stress on a test specimen versus the corresponding strain. Stress can be applied through shear, compression, flexure, or tension.
Striations: Marks evident on the molded-part surfaces that indicate melt flow directions or impingement.
Strings: Strings of material due to poor gate cut off. See pulled gate.
Stripper Plate: A plate that strips a molded piece from core pins or force plugs. The stripper plate is set into operation by the opening of the mold.
Structural Foam Molding: The process of molding thermoplastics articles with a cellular core and integral solid skins in a single operation.
Subgate: entrance to the part from the runner located below the parting line. On ejection, the part breaks away from the sub-gate.
Suck-back: When the pressure on the sprue is not held long enough for the melt to cool before the screw returns. Some of the melt in the cavities or runner system may expand back into the nozzle and cause sinks marks on the finished part.