Packing: The filling of the mold cavity or cavities as full as possible without causing undue stress on the molds or causing flash to appear on the finished parts. Over-or under-packing results in less than optimum fill.

Part Picker: An auxiliary unit usually mounted on the fixed platen, which reaches into the open mold to grab parts and remove them prior to next molding cycle. Also called a robot, the device is used when you do not want to drop parts from mold upon ejection.

Partially-balanced runner system: composed of both naturally and artificially balanced runners.

Parting line: mark on the part indicating where the two halves of the mold met in closing.

Peeling: An open blister.

Pellets: Tablets or granules of uniform size, consisting of resins or mixtures of resins with compounding additives which have been prepared for molding operations by extrusion and chopping into short segments.

Photodegradation: Degradation of plastics due to the action of light.
Pigment (also know as Colorant, Color Concentrate): A plastic compound which contains a high percentage of pigment, to be blended in appropriate amounts with the base resin so that the correct final color is achieved.

Pinpoint Gate: A restricted gate of 0.030 in or less in diameter, this gate is common on hot-runner molds.

Pit: An imperfection, a small crater in the surface of the plastic.

Plastic Deformation: A change in dimensions of an object under load that is not recovered when the load is removed.

Plastic: A material that contains as an essential ingredient one or more organic polymeric substances of large molecular weight is solid in its finished state, and, at some stage in its manufacture or processing into finished articles, can be shaped by flow.

Plasticity: The ability of a material to withstand continuous and permanent deformation by stresses exceeding the yield value of the material without rupture.

Plasticize: To render a material softer, more flexible and/or more moldable by the addition of a plasticizer.

Plasticizer: A substance or material incorporated in a material (usually a plastic or an elastomer) to increase its flexibility, workability or extensibility.

Platens: The mounting plates of a press on which the mold halves are attached.

Plate-Out: An objectionable coating gradually formed on metal surfaces of molds during processing of plastics due to extraction and deposition of some ingredient such as pigment, lubricant, stabilizer or plasticizer.

Plots: graphical representations of analysis results.

Poisson’s Ratio: The constant relating the changes in dimensions which occur when a material is stretched. It is obtained by dividing the change in width per unit length by the change in length per unit length.

Polymer Structure: A general term referring to the relative positions, the arrangement in space, and freedom of motion of atoms in a polymer molecule.

Polymer: High-molecular-weight organic compound, natural or synthetic, whose structure can be represented by a repeated small unit, e.g. polyethylene, rubber, cellulose. If two or more monomers are involved, a copolymer is obtained.

Polymerization: A chemical reaction in which the molecules of a simple substance (monomer) are linked together to form large molecules whose molecular weight is a multiple of that of the monomer.

Preform: A plastic pre-shaped part produced by injection molding systems in the first step of a two-stage injection molding and blow molding process used to produce bottles or containers. The preform is subsequently reheated and stretches blown through a blow molding process into the final container shape.

Pressure Pads: Reinforcements of hardened steel distributed around the dead areas in the faces of a mold to help the land absorb the final pressure of closing without collapsing.

Processing Aids: Additives specifically used to improve the injection process.

Proportional Limit: The greatest stress which a material is capable of sustaining without deviation from proportionality of stress and strain. (Hooke’s Law).

Prototype Tool: A preliminary mold built upon which the final mold will be based.

Pulled Gate: Area where the part was connected to the sprue or runner that has been drawn out or stretched from the surface.

Purging: In extrusion or injection molding, the cleaning of one color or type of material from the machine by forcing it out with the new color or material to be used in subsequent production, or with another compatible purging material.

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