Glossary

Plastics Glossary - S

Safety pawl A peg or wedge that is activated in between the notches on a rack type safety bar, or which is dropped in front of the end of the safety bar to stop its movement and therefore inhibit accidental mold closure.

Salting Out Precipitation of a material from its dispersion.

Sampling The trial procedures used to determine the initial molding parameters for a new mold.

Saponification The process of hydrolysis of fats or oils by an alkali of a fluid to form soap.

Saybolt Universal Seconds (SUS) Kinematic viscosity of a fluid as determined by the time in seconds required for 60 cm3 of liquid to flow through a standard orifice.

Saybolt Viscometer Transpiration type of viscometer primarily used to determine viscosity of petroleum products.

Scattering Deflection of light rays by particles of suspended matter.

Scrap Parts that do not meet quality standards.

Screw A helical flight shaft that rotates within a barrel to mechanically process and advance a material being pre-pared for extrusion or injection molding.

Screw flight The helical metal thread of a screw in an extruder or injection molding machine.

Screw pull back Another name for melt decompression where the screw is pulled back inside the barrel to suck the plastic back up inside the nozzle.

Screw recovery The backward motion of the screw as it rotates and pushes plastic forward in the barrel.

Screw speed The revolutions per minute (rpm) of an extruder or injection molding machine screw.

Screw tip The tip of the reciprocating screw is the ram face that pushes the melt into the mold and it contains the shut-off valve (non-return valve) which prevents the melt from sliding backward along the flights of the screw.

Screw In extrusion, the shaft provided with helical grooves which conveys the material from the hopper outlet through the barrel and forces it out through the die.

Scripting Using three-dimensional automation to produce decorative fonts by dispensing a precise amount of material to give the lettering a raised effect.

Sealing Closing or securing a part or assembly with a fluid-tight, air-tight adhesive

Second stage timer The cycle timer that controls the amount of time that the low volume pump is used to hold the injection ram forward.

Secondary Operation Any finishing that must be performed on a molded piece part before it can be considered usable for its designed purpose.

Sedimentation Spontaneous separation of two or more materials (liquids, solids or gases).

Segregation Separation of the constituents of a mixture, e.g. by electrostatic charges.

Seizing Metallic freezing together of materials under load.

Semiautomatic operation The machine runs through one cycle and then stops when the mold is opened.

Series circuit A circuit where baffles or bubblers are placed one after another along the same tube.

Servo-valve An adjustable valve that can continuously alter the speed or pressure of a system when used in conjunction with a transducer and control unit.

Setup sheet The data sheet on which all of the molding parameters that were used to run the mold successfully have been recorded.

Setup The planned downtime for installing a new mold and setting the new parameters.

Shake Flask Test Screening test for the determination of biodegradability, proposed by the Soap and Detergent Association.

Shear heating The frictional heat produced in the plastic melt as the polymers slide along each other inside the plasticating chamber of the processing machine.

Shear modulus The ratio of the shear stress to the corresponding shear strain for shear stresses below the proportional limit in shear of the material.

Shear Rate The relative sliding velocity of any two shearing planes divided by the distance between them.

Shear Stability Ability of a grease to withstand repeated working with minimum change in structure or consistency.

Shear Strength The maximum load required to shear the specimen in such a manner that the moving portion has completely cleared the stationary portion.

Shear Stress The force per unit area required to cause two adjacent planes to move in relationship to each other.

Shear Viscometer Instrument for measuring viscosities of complex fluids at different shear rates.

Sheet Any material manufactured in sheet form and cut to suit in processing. In extrusion, the extrudate is considered sheet if it is more than 10 mils thick.

Sheet molding compound A composite of glass fibers, polyester resins, and pigments, fillers, and other additives which have been compounded and processed into sheet form to facilitate handling in the molding operation.

Shelf life The time during which a molding compound, adhesive, etc. can be stored without losing its suitable physical properties.

Shock Chilling Addition of a cold fluid to a hot fluid in order to obtain rapid cooling of the blend to a given temperature.

Shore hardness A test to determine a plastic's hardness using an indentation durometer or scieroscope.

Short Shot In injection molding, failure to fill the mold completely.

Shot Capacity The maximum weight of material that can be delivered to an injection mold by one stroke of the ram. See barrel capacity.

Shot size The amount of plastic injected into a mold that will fill the cavity, runner and sprue. Should be between 30-75% of the barrel capacity. The maximum amount of plastic that can be injected from the barrel at a time.

Shot All material injected into the mold during the molding cycle, including flash, runner, sprue, etc.

Shot One complete cycle of a molding machine.

Shrink fixture (cooling fixture) A jig or block to hold the shape of a molded part after it is removed from the mold until it is cool enough to retain its shape.

Shrinkage Allowance The dimensional allowance which must be made in molds to compensate for shrinkage of the plastic compound on cooling.

Shrinkage The difference between the room temperature plastic article dimensions and the room temperature dimensions of the mold. A volume reduction in polymers that occurs during cooling due to a reduction in space between the molecules.

Shut height The amount of space available between the platens in which to clamp the mold.

SI Abbreviation for Le Systeme International d'Unites.

Side bars Loose pieces used to carry one or more molding pins and operated from outside the mold.

Side draw pins Projection used to core a hole in a direction other than the line of closing of a mold and that must be withdrawn before the part is ejected from the mold.

Sil Viscometer Instrument used for the determination of viscosities of Newtonian fluids in the 0.6-10,000 cS range.

Silicone fluid Silicone additives are clear polydimethylsiloxane liquids that perform two basic types of functions. At low concentrations, silicone additives are used as a process improver, increasing mold flow and providing self-release characteristics. At higher concentrations, they are used to form self-lubricating plastics for bearings and other mechanical applications.

Silicone molding compounds Thermosetting material that may resemble expoxies, phenolics, rubbers and other thermosets. However, they have unique heat and chemical stability and purity.

Simple Liquid Liquid in which the rate of shear is proportional to the shearing stress. The constant ratio of shearing stress to rate of shear of a simple liquid is the viscosity of the liquid.

Single-cavity mold A mold having one cavity and producing only one finished part per cycle.

Single-phase blend A type of polymer modification in which two parent homopolymers are soluble in each other. This is a physical combination.

Sink mark A shallow depression or dimple on the surface of the plastic article due to shrinkage. Sink marks often are caused by thick wall sections in the part design. Sink marks are also caused by insufficient packing pressure/time and from localized hot spots in the mold.

Sintering The welding together of powdered particles at temperatures below the melting or fusion point. Particles are fused together to form a mass, but the mass as a whole does not melt.

Slides Moveable steel sections of the mold containing part of the cavity that are slid out of the way to allow the part to be removed.

Slip agent Provides surface lubrication during and immediately following processing plastics. Compounded into the plastic, the additive acts as an internal lubricant that gradually migrates to the surface.

Slip Displacement loss in a pump that is affected by the viscosity of the liquid being pumped and the pressure differential.

Slip plane Marks evident in or on finished parts due to poor welding or shrinkage upon cooling.

Soap Metallic salt of a fatty acid.

Software A name given to instructions, programs, mathematical formulas, and the like utilized in the computer system, in contrast to the actual physical hardware of the system.

Solid state controls The standard type of electronic controls used to run the molding machine.

Solids Active ingredients of a mixture. In the emulsion the "solids" includes the internal phase and the emulsifiers, stabilizers etc., all of the system except the continuous phase or solvent .

Solubility of an Emulsion Dispersibility of an emulsion as determined by the nature of the continuous phase.

Solubility The tendency or capacity of a solvent (or polymer) to dissolve a substance under specific conditions.

Solute Substance that is dissolved in a solvent; the substance in lesser concentration in a solution.

Solution Homogeneous mixture of two or more substances. It is characterized by an ionic or molecular-level subdivision of the components that will not become heterogeneous upon standing.

Solvation Adsorption of molecules of a dispersing liquid by the dispersed particles.

Solvent bonding The process of joining articles made of thermoplastic resins by applying a solvent. Solvent softens the surfaces to be joined. Then surfaces are pressed together. Adhesion comes from evaporation of solvent, absorption of solvent into the material surfaces and/or polymerization of the solvent cement.

Solvent Constituent of a solution present in the larger amount.

Solvent molding A process for forming thermoplastic articles by dipping a male mold into a solution or dispersion of the resin and drawing off the solvent to leave a layer of plastics film adhering to the mold.

Solvent resistance Ability of a plastic to resist swelling and dissolving in a solvent.

Solvents Substances with the ability to dissolve other substances.

Space Velocity Product of the feed rate times specific volume of feed divided by volume of vessel used in a converting operation, expressed in volume processed per unit time. It is the reciprocal of hypothetical retention time.

Spacer See parallel.

SPE Abbreviation for the Society of Plastics Engineers.

Specific gravity The density (mass per unit volume) of any material divided by that of water at a standard temperature. It provides a more accurate means of comparing material costs because plastic parts are sold by volume, not weight.

Specific heat capacity (CP) The energy in calories required to heat one gram of material one degree centigrade. It is expressed in calories per gram per degree centigrade (cal/gram/ºC).

Specific heat The amount of heat required to raise the temperature of a unit mass of a substance one degree under specific conditions.

Specific Surface Surface area of a substance per unit volume.

Specific Viscosity Relative viscosity of a fluid.

Specific Volume Reciprocal of density; volume per unit mass, as cm3/g, ft3/1b, etc .

Speed profiler An injection speed controller capable of changing the injection rate of the ram throughout the complete injection stroke.

Speed Velocity see Space Velocity.

SPI Society of the Plastics Industry. Headquartered in Waldorf, MD. Phone 1-800-541-0736.

Spin welding Bonds two cylindrical plastic parts together by frictional heat. This method is well suited for joining thermoformed container halves. Injection molded cylindrical parts, etc. Because the actual bond is made through heating and then fusing the mating surfaces, only thermoplastics can be joined by spin welding.

Spiral flow test A method for finding flow properties of either thermoplastic or themosetting resin, formulated by the distance it will flow under specified pressure and temperature and along a spiral runner. Test is usually performed using an injection molding machine and a test mold into which material is fed at the center of the spiral cavity.

Splay marks Marks or tracks caused by gas or liquid present or trapped in a material migrating to the surface of the mold, which then slid over the surface in the direction of flow or toward a vent, leaving tracks. A cosmetic blemish on a plastic part.

Splay Spray or droplet marks on the surface of the part. Also known as silver streaking.

Split-ring mold A mold in which a split cavity block is assembled in a chase to permit the forming of undercuts in a molded piece. These parts are ejected from the mold and then separated from the piece.

Spray metal and/or melt mold Constructed similarly to those used for conventional injection molding. Majority are built from forged aluminum plate, generally 6061-T651 grade.

Spreader A streamlined metal block placed in the flow of the molten resin in the heating cylinder or extruder system to spread the flow into thin layers to facilitate better heat transfer.

Spreading Coefficient (Harkins) Number describing the decrease in the total interfacial energy of an aqueous system by the addition of a specific surface substance.

Sprue break A machine control function where the carriage is retracted during the cycle to pull the nozzle away from the mold sprue bushing.

Sprue bushing The channel or feed opening for the passage of plastic from the nozzle of the injection molding cylinder to the runners of the mold.

Sprue gate The passageway through which the resin melt flows from the nozzle directly to the mold cavity.

Sprue puller The portion of resin retained in the cold slug well by an undercut; used to pull the sprue out of the bushing as the mold opens. The sprue puller itself is pushed out of the mold by an ejector pin.

Sprue In an injection mold, the main feed channel that connects the mold filling orifice with runners leading to each cavity gate. The plastic piece formed in the sprue bushing.

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 service life of the material and/or the parts made therefrom.

Stable Emulsion Emulsion that retains its form and condition and in which the particles of the internal phase do not coalesce.

Stack mold Two level mold (two sets of cores and cavities stacked one behind the other) for molding large area, flat parts. A multilevel mold in which two stacked sets of cavities are filled at the same time.

Stagnation Point Point at which the fluid velocity of a system is zero.

Standard Deviation Statistical measure of the scattering of data from the average; equal to the root mean square of the individual deviations from average.

Stationary mold half The half of the mold that does not move, sometimes called the cavity half of the mold.

Stationary platen In a horizontal injection molding machine, the platen immediately adjacent to the machine nozzle to which the front half of the mold is attached. This platen does not move during mold cycling.

Stationary platen In a horizontal injection molding machine, the platen immediately adjacent to the machine nozzle to which the front half of the mold is attached. This platen does not move during mold cycling.

Statistical process control A parameter charting method used to ensure that any parameter fluctuations are within acceptable limits.

Stem Diffuse Double Layer Presence of a sharp Helmholtz layer of i nic thickness at the interface followed by a Gouy exponential electrical potential drop into the bulk of the fluid. This effect is two-sided; it exists on both sides of the interface in an emulsion.

Stiffness The capacity of a material to resist elastic displacement under stress.

Stone Unit of kinematic viscosity, symbol S.

Stone’s Law A small sphere falling under the action of gravity through a viscous medium reaches a constant velocity that is a function of the radius of the sphere, its density, the density of the medium, and its viscosity coefficient.

Stop Pins Multiple islands limiting travel of the ejector mechanism when ejecting and returning to molding position.

Stormer Viscometer Viscometer is which the fluid is placed between two cylinders. The inner cylinder rotates by the action of a falling weight. The rotational speed is determined by measuring the time required for a definite number of revolutions of the inner cylinder. The torque is determined from the weight and radius of the pulley. The viscosity of the fluid is a function of the rotational speed and torque.

STP Standard Temperature and Pressure: 0˚C, 760 mm pressure.

Strain Elastic deformation due to stress. Measured as the change in length per unit of length in a given direction, and expressed in percentage or inches per inch, etc.

Stran Deformation Change in the dimensions of a body under stress.

Stratification Formation of layers in a fluid mixture.

Streamline Flow Flow where fluid particles move along the streamlines.

Streamline Line that gives the velocity direction of a fluid (number of particles) at each point along the line.

Strength The maximum load a material withstands before breaking or yielding.

Stress Relaxation The decay of stress at a constant strain.

Stress The unit force or component of force at a point in a body acting on a plane through the point. Expressed in psi.

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 resin flow directions or impingement.

Stringing This effect occurs between the finished part and the sprue or gate in hot tip molds. When the mold opens, the resin in this area has not cooled sufficiently.

Stripper Bolt Bolt attached to the stationary half of the mold to limit the travel of the cavity plate. Also called an anti-bounce bolt.

Stripper plate A plate in a mold which removes a molded piece from the core pins or plunger. Also called X-Plate.

Structural Color Color due to the physical structure of the mass and not dependent upon the electronic configuration of the substance.

Structural Foam A term originally used for cellular thermoplastic articles with integral solid skins having high strength-to-weight ratios, but now sometimes also used for high density cellular plastics which are strong enough for structural applications.

Styrene-acrylonitrile (SAN) resin Co-polymers are thermoplastic resins which impart strength, rigidity, transparency, excellent dimensional stability, and high chemical resistance to finished products.

Submarine gate Submarine gate breaks the molded item from the runner system upon ejection from the mold. The gate is located where the opening from runner to the mold is below parting line or mold surface (in conventional edge gating, the opening is machined into the mold surface).

Substance Any form of matter that is homogeneous throughout, i.e. solid, liquid, gas.

Suck Back 1) When the pressure on the sprue is not held long enough for the resin to cool before the screw returns, some of the hot resin in the cavities or runner system may expand back into the nozzle and cause sink marks on the finished part. 2) Also called melt decompression and screw pull-back. A processing technique where the screw or ram is drawn rearward to relieve the plastic pressure in the nozzle. Used to control drool, stringing and to improve sliding ring nonreturn valve performance. Note: This technique often causes splay when air is drawn into the polymer melt as the screw or ram is drawn rearward.

Supersaturated Solution One that contains more solute in solution than is present in a saturated solution of the same substance at the same temperature and pressure. A supersaturated solution is usually formed by reducing the temperature or removing some of the solvent from a saturated solution.

Support Pillars Typically cylinder metal pieces housed between the support plate or the B-plate and the bottom clamp plate and through the ejector plates. Located in areas that require additional support against clamping and injection pressures.

Support Plate A plate that is placed behind the B-plate to hold the core inserts in place and provide support against the forces of clamping and injection.

Surface pins/Return pins Also called ejector return pins. Projections that push the ejector assembly back as the mold closes.

Surface-Active Agent Substance that affects the surface tension of a liquid. They include emulsifying agents, detergents, suspending agents, wetting agents, etc.

Surfactant Chemicals which modify the surface properties of plastics to influence the wetting and flow properties of liquids allowing formation of emulsion or intimate mixtures of normally incompatible substances.

Surging An unstable pressure build-up leading to variable output and wavy extrudate surface. Surging may cause flow to stop for a moment at intervals.

Suspensoid Colloidal suspension in which there is little attraction between the dispersed substance and the dispersing liquid; a lyophobic colloid.

Syneresis Exuding of small amounts of liquids from gels by contraction.