Glossary

Plastics Glossary - H

Hardness The resistance to surface indentation usually measured by the depth of penetration of a blunt point under a given load using a particular instrument according to a prescribed procedure.

Haze The cloudy or turbid aspect or appearance of an otherwise transparent specimen caused by light scattered from within the specimen or from its surfaces. ASTM D1003; haze is the percentage of transmitted light that, in passing through the specimen, deviates from the incident beam through forward scatter more than 2.5 degrees on average.

Heat capacity (C_p) The energy required to heat (in calories) one gram of material one degree centigrade.

Heat capacity trace Provides a blueprint for polymer heating performance by indicating: 1) at what temperatures the material characteristics change, 2) at what temperature the material can be processed, and 3) the amount of heat required to process the material.

Heat deflection temperature (HDT) See Deflection temperature under load (DTUL).

Heat exchanger A device full of cooling tubes in which water flows through to keep the hydraulic oil from getting too hot.

Heat guns Electrically heated guns for softening, curing, drying, preheating, and welding plastics, coatings, and compounds as well as shrinking of heat shrinkable plastic tubing and plastic films.

Heat of crystallization The amount of energy that is given off as the polymer builds crystallinity upon cooling.

Heat of fusion The amount of heat needed to melt a crystalline structure (calories per gram of BTUs per pound). Also called latent heat of fusion.

Heat pipe A device that transfers localized heat to a heat sink (water line).

Heat soaking The extra time used to heat the barrel or mold completely after the setpoints have been reached.

Heating circuit Grooves or channels, cut into core plates, cavity plates, and inserts, that are designed to transfer heat to the mold, keeping it warm.

Hegman Gage Arbitrary scale for comparing fineness of grind, expressed in "mesh ."

Helmholtz Double Layer The sharp and continuous decrease into the bulk of the fluid of the electrical potential of a single layer of oriented, ionized surfactant molecules at the interface. This effect can exist on both sides of the interface in an emulsion.

Hesitation effect The phenomena that occurs when a melt front "hesitates" momentarily at a thin wall section because there is an alternate path of less resistance to flow.

High impact polystyrene A copolymer of styrene and butadiene known for its good dimensional stability, impact strength, rigidity, and especially for its ease of processing. Amorphous material.

HLB Number (Hydrophile-Lipophile Balance) System for the selection of emulsifying agents. Lipophilic (nonpolar) emulsifiers have HLB numbers below 9; hydrophilic (polar) have HLB numbers above 11. The HLB of an emulsifier (or blend of emulsifiers) may be related to its performance in an emulsion system.

Hob Master steel punch used to sink or form a mating shape (mold cavity) into mild steel block.

Hobbing The process of forming a mold by forcing a master metal shape into a soft steel block.

Hoist rings Eye bolts with a rotating ring. Hoist rings can handle more sideways force than an ordinary eye bolt.

Holding pressure Another name for the second stage pressure that is used to hold the plastic under pressure until the gate is frozen.

Holding time Another name for the second stage timer that is used to control the amount of time that the low volume pump holds the ejection ram forward.

Holdup Amount of liquid retained by a piece of equipment when the equipment is emptied.

Homogeneous Fluid Substance that is identical throughout.

Homogenization The process of making incompatible or immiscible components into a stabilized uniform suspension in a liquid medium.

Homogenizer Mixing machine used for the preparation of emulsions of fine particle size. The emulsion is forced at high pressure through the annular space between an adjustable valve and its seat.

Homopolymer A polymer that is formed by the polymerization of a single monomer.

Hoop stress The circumferential stress in a material of cylindrical form subjected to internal or external pressure.

Hopper blend Mixes multiple materials such as virgin resin, regrind, blowing agents, fillers, and colorants. Materials to be blended are metered in ratio to a mixing chamber and then discharged into the feed throat of the processing machine.

Hopper blender Mixes multiple materials such as virgin resin, regrind, blowing agents, fillers, and colorants. Materials to be blended are metered in ratio to a mixing chamber and then discharged into the feed throat of the processing machine.

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 In injection molding, the container holding a supply molding material to be fed to the screw.

Horn Pin A hardened steel pin fitted to one mold plate, on an angle, for the purpose of operating slides and side cores.

Hot air dryer A type of dryer that operates by heating the air, thus decreasing the relative humidity; it does not lower the dew point.

Hot melt adhesive Thermoplastic adhesives which set by cooling, rather than by absorption or evaporation of water or a solvent.

Hot runner molds Molds where the runners are heated. In these molds, the runners do not solidify and thus do not have to be ejected with the part and trimmed off.

Hot sprue bushing Mold element that contains a heating element to keep the resin melt hot within the bushing. The bushing is inserted into the mold to provide a hot channel between the molding machine nozzle and the mold cavity. Sprue bushings which typically contain some form of heating device so that the plastic in the sprue is always molten.

Hot stamping Process in which a decorative image is transferred from a carrier to the part via pressure exerted onto a heated die by the hot stamping machine.

Hot-runner mold A mold in which the runners are kept hot and insulated from the chilled cavities. Plastic freezeoff occurs at gate of cavity; runners are in a separate plate so they are not, as is the case usually, ejected with the piece.

Humidity The moisture in the air.

Hydrate Crystallized substance that contains water of hydration.

Hydration Attachment of water molecules to ions or molecules.

Hydraulic accumulator A large hydraulic cylinder capable of providing large volumes of oil at high flow rates.

Hydraulic clamp Used in variety of molding and forming machines. A hydraulic clamp basically consists of a high-speed, variable hydraulic pump, valving, a fast-acting cylinder, and a high-pressure cylinder. Cylinders can be single or combination units. The clamp closes the mold halves to form the part.

Hydraulic core pull The hydraulic system used to slide cores in and out of the mold.

Hydraulic valve A valve used to divert hydraulic fluid to various hydraulic cylinders or motors to control their motion.

Hydrogel Colloidal gel in which water is the solvent.

Hydrolysis The splitting of a molecule with the addition of water in the presence of heat and pressure, as in processing. Decomposition of a substance by reaction with water.

Hydromechanical press Molding machine by which the clamp forces are created partly by a mechanical system and partly by a hydraulic system.

Hydrometer Densimeter Device used for the measurement of specific gravities.

Hydrophilic Attracted to water; water-soluble.

Hydrophobic Capable of repelling water.

Hydrotopy Effect of large additions of various substances to water in increasing the solubility of other substances therein.

Hygroscopic Capable of absorbing and retaining environmental moisture. A material's tendency to absorb water, even from the atmosphere.