Dash-Pot - A device used in hydraulic systems for damping down vibration. It consists of a piston attached to the part to be damped and fitted into a vessel containing fluid or air. It absorbs shocks by reducing the rate of change in the momentum of moving parts of machinery.

Daylight Opening - Clearance between two platens of a press in the open position.

Deckle Rod - A small rod, or similar device, inserted at each end of the extrusion coating die which is used to adjust the length of the die opening.

Decomposition Product - The constituent elements or simpler compounds formed when a substance decays or decomposes.

Decorative Sheet - A laminated plastics sheet used for decorative purposes in which the color and/or surface pattern is an integral part of the sheet.

Deflashing - Covers the range of finishing techniques used to remove the flash (excess, unwanted material) on a plastic molding.

Degassing - See Breathing.

Degree of Polymerization - The number of structural units or mers in the "average" polymer molecule in a particle sample. In most plastics the DP must reach several thousand if worthwhile physical properties are to be had.

Delamination - The separation of the layers in a laminate caused by the failure of the adhesive.

Deliquescent - Capable of attracting moisture from the air.

Density - Weight per unit volume of a substance, expressed in grams per cubic centimeter, pounds per cubic foot, etc.

Desiccant - Substance which can be used for drying purposes because of its affinity for water.

Die Gap - The distance between the metal faces forming the die opening.

Die Lines - Vertical marks on the parison caused by damage of die parts or contamination.

Die Swell Ratio - The ratio of the outer parison diameter (or parison thickness) to the outer diameter of the die (or die gap). Die swell ratio is influenced by polymer type, head construction, land length, extrusion speed, and temperature.

Dielectric - Insulating material. In radio frequency preheating, dielectric may refer specifically to the material being heated.

Dielectric Constant - Normally the relative dielectric constant; for practical purposes, the ratio of the capacitance of an assembly of two electrodes separated solely by a plastic's insulating material to its capacitance when the electrodes are separated by air (ASTM D150-59T).

Dielectric Heating (Electronic Heating) - The plastic to be heated forms the dielectric of a condenser to which is applied a high-frequency (20 to 80 mc.) voltage. Dielectric loss in the material is the basis. Process used for sealing vinyl films and preheating thermoset molding compounds.

Dielectric Strength - The electric voltage gradient at which an insulating material is broken down or "arced through," in volts per mil of thickness.

Dimensional Stability - Ability of a plastic part to retain the precise shape in which it was molded, fabricated, or cast.

Dip Coating - Applying a plastic coating by dipping the article to be coated into a tank of melted resin or plastisol, then chilling the adhering melt.

Discoloration - Any change from the original color, often caused by overheating, light exposure, irradiation, or chemical attack.

Dispersion - Finely divided particles of a material in suspension in another substance.

Dissipation Factor - See Power Factor.

Divergent Die - A die in which the internal channels leading to the orifice are diverging (applicable only to dies for hollow bodies).

Dome - In reinforced plastics, an end of a filament wound cylindrical container.

Double-shot molding - A means of turning out two-color parts in thermoplastics materials by successive molding operations.

Draft - The degree of taper of a side wall or the angle of clearance designed to facilitate removal of parts from a mold.

Drape Assist Frame - In sheet thermoforming, a frame (made up of anything from thin wires to thick bars) shaped to the peripheries of the depressed areas of the mold and suspended above the sheet to be formed. During forming, the assist frame drops down, drawing the sheet tightly into the mold and thereby preventing webbing between high areas of the mold and permitting closer spacing in multiple molds.

Drape Forming - Method of forming thermoplastic sheet in which the sheet is clamped into a movable frame, heated, and draped over high points of a male mold. Vacuum is then pulled to complete the forming operation.

Draw Down Ratio - The ratio of the thickness of the die opening to the final thickness of the product.

Drawing - The process of stretching a thermoplastic sheet or rod to reduce its cross-sectional area.

Dry - Refers to data that is obtained from a sample of material with equivalent moisture content as when it was molded (typically <0.2%). Learn more.

Dry Coloring - Method commonly used by fabricators for coloring plastics by tumble blending uncolored particles of the plastic material with selected dyes and pigments.

Dry Strength - The strength of an adhesive joint determined immediately after drying under specified conditions or after a period of conditioning in the standard laboratory atmosphere. See Wet Strength.

Dry-blend - A free-flowing dry compound prepared without fluxing or addition of solvent. Also called powder blend.

Ductility - The extent to which a solid material can be drawn into a thinner cross section.

Dwell - A pause in the application of pressure to a mold, made just before the mold is completely closed, to allow the escape of gas from the molding material.

Dyes - Synthetic or natural organic chemicals that are soluble in most common solvents. Characterized by good transparency, high tinctorial strength, and low specific gravity.