Hardener - A substance or mixture of substances added to plastic composition, or an adhesive to promote or control the curing reaction by taking part in it. The term is also used to designate a substance added to control the degree of hardness of the cured film. See also Catalyst.

Hardness - The resistance of a plastic material to compression and indentation. Among the most important methods of testing this property are Brinell hardness, Rockwell hardness and Shore hardness.

Haze - The degree of cloudiness in a plastic material.

Head - The end section of a blow molding machine (in a general extruder) in which the melt is transformed into a hollow parison.

Head Space - The space between the fill level of a container and the sealing plane.

Heat Chamber - In injection molding, that part of the machine in which the cold feed is reduced to a hot melt. Also Heating Cylinder.

Heat Deflection Temperature - The temperature at which a standard test bar (ASTM D648) deflects 0.010 in. under a stated load of either 66 or 264 psi.

Heat Sealing - A method of joining plastic films by simultaneous application of heat and pressure to areas in contact. Heat may be supplied conductively or dielectrically.

Heat-Distortion Point - The temperature at which a standard test bar (ASTM D648) deflects 0.010 in. under a stated load of either 66 or 264 psi.

Hob - A master model in hardened steel used to sink the shape of a mold into a steel block.

Hobbing - Forming multiple mold cavities by forcing a hob into soft steel (for beryllium-copper) cavity blanks.

Homopolymer - A polymer, consisting of (neglecting the ends, branch junctions, and other minor irregularities) a single type of repeating unit.

Honeycomb - Manufactured product consisting of sheet metal or a resin impregnated sheet material (paper, fibrous, glass, etc.) which has been formed into hexagonal-shaped cells. Used as core material for sandwich constructions.

Hoop Stress - The force per unit area in the wall of the pipe in the circumferential orientation due to internal hydrostatic pressure.

Hopper - Conical feed reservoir into which molding powder is loaded and from which it falls into a molding machine or extruder, sometimes through a metering device.

Hopper Dryer - A combination feeding and drying device for extrusion and injection molding of thermoplastics. Hot air flows upward through the hopper containing the feed pellets.

Hopper Loader - A curved pipe through which molding powders are pneumatically conveyed from shipping drums to machine hoppers.

Hot Gas Welding - A technique of joining thermoplastic materials (usually sheet) whereby the materials are softened by a jet of hot air from a welding torch, and joined together at the softened points. Generally a thin rod of the same material is used to fill and consolidate the gap.

Hot-runner mold - A mold in which the runners are insulated from the chilled cavities and are kept hot. Parting line is at gate of cavity, runners are in separate plate(s), so they are not, as is the case usually, ejected with the piece.

Hot-stamping - Engraving operation for marketing plastics in which roll leaf is stamped with heated metal dies onto the face of the plastics. Ink compounds can also be used. By means of felt rolls, ink is applied to type and by means of heat and pressure, type is impressed into the material, leaving the marking compound in the indentation.

Hydraulic - A system in which energy is transferred from one place to another by means of compression and flow of a fluid (e.g., water, oil).

Hydrocarbon Plastics - Plastics based on resins made by the polymerization of monomers composed of carbon and hydrogen only.

Hydrogenation - Chemical process whereby hydrogen is introduced into a compound.

Hydrolysis - Chemical decomposition of a substance involving the addition of water.

Hygroscopic - Tending to absorb moisture.