Back Pressure - The viscosity resistance of a material to continued flow when a mold is closing. In extrusion, the resistance to the forward flow of molten material.

Backing Plate - In injection molding, a plate used as a support for the cavity blocks, guide pins, bushings, etc.

Baffle - A device used to restrict or divert the passage of fluid through a pipe line or channel. In hydraulic systems the device, which often consists of a disc with a small central perforation, restricts the flow of hydraulic fluid in a high pressure line. A common location for the disc is in a joint in the line. When applied to molds, the term is indicative of a plug or similar device located in a steam or water channel in the mold and designed to divert and restrict the flow to a desired path.

Bag molding - A method of applying pressure during bonding or molding, in which a flexible cover, usually in connection with a rigid die or mold, exerts pressure on the material being molded, through the application of air pressure or drawing of a vacuum.

Bakelite - The proprietary name for phenolic and other plastics materials produced by Bakelite Limited, but often used indiscriminately to describe any phenolic molding material or molding. The name is derived from that of Dr. Leo Hendrik Baekeland (1863-1944), a Belgian who, through his work on synthesis of phenolic resins and their commercial development in the early 1900's, is generally considered to be the "father" of the plastic industry.

Benzene Ring - The basic structure of benzene, the most important aromatic chemical. It is an unsaturated, resonant 6-carbon ring having three double bonds. One or more of the 6 hydrogen atoms of benzene may be replaced by other atoms or groups.

Binder - In a reinforced plastic, the continuous phase which holds together the reinforcement.

Bleed - To give up color when in contact with water or a solvent; undesired movement of certain materials in a plastic (e.g. plasticizers in vinyl) to the surface of the finished article or into an adjacent material. Also called Migration.

Blister - A raised area on the surface of a molding caused by the pressure of gases inside it on its incompletely hardened surface.

Block Copolymer - An essentially linear copolymer in which there are repeated sequences of polymeric segments of different chemical structure.

Blocking - An undesired adhesion between touching layers of a material, such as occurs under moderate pressure during storage or use.

Bloom - A visible exudation or efflorescence on the surface of a material.

Blow molding - A method of fabrication in which a parison (hollow tube) is forced into the shape of the mold cavity by internal air pressure.

Blow Pressure - The air pressure used to form a hollow part by blow molding.

Blow Rate - The speed at which the air enters the parison during the blow loading cycle.

Blowing Agents - See Foaming Agents.

Blown Tubing - A thermoplastic film which is produced by extruding a tube, applying a slight internal pressure to the tube to expand it while still molten and subsequent cooling to set the tube. The tube is then flattened through guides and wound up flat on rolls. The size of blown tubing is determined by the flat width in inches as wound rather than by the diameter as in the case of rigid types of tubing.

Blowup Ratio - In blow molding, the ratio of the mold cavity diameter to the parison diameter. In blown tubing (film), the ratio of the final tube diameter (before gusseting, if any) to the original die diameter.

Boss - Protuberance on a plastic part designed to add strength, to facilitate alignment during assembly, to provide for fastening, etc.

Boston Round - A particular shape of container; cross section as well as shoulders are round.

Bottom Blow - A specific type of blow molding machine which forms hollow articles by injecting the blowing air into the parison from the bottom of the mold.

Bottom Plate - Part of the mold which contains the heel radius and the push-up.

Branched - In molecular structure of polymers (as opposed to Linear), refers to side chains attached to the main chain. Side chains may be long or short.

Breakdown Voltage - The voltage required, under specific conditions, to cause the failure of an insulating material. See Dielectric Strength.

Breaker Plate - A perforated plate located at the rear end of an extruder head. It often supports the screens that prevent foreign particles from entering the die.

Breathing - The opening and closing of a mold to allow gases to escape early in the molding cycle. Also called Degassing. When referring to plastic sheeting, "breathing" indicates permeability to air.

B-stage - An intermediate stage in the reaction of a thermosetting resin in which the material softens when heated and swells in contact with certain liquids but does not entirely fuse or dissolve. Resins in thermosetting molding compounds are usually in this stage. See also A-Stage and C-Stage.

Bubbler Mold Cooling (injection molding) - A method of cooling an injection mold in which a stream of cooling liquid flows continuously into a cooling outlet normally positioned at the end opposite the inlet. Uniform cooling can be achieved in this manner.

Bulk Density - The mass per unit volume of a molding powder as determined in a reasonably large volume. The recommended test method is ASTM D1182-54.

Bulk Factor - Ratio of the volume of loose molding powder to the volume of the same weight of resin after molding.

Bushing (extrusion) - The outer ring of any type of a circular tubing or pipe die which forms the outer surface of the tube or pipe.

Butadiene - A gas, insoluble in water but soluble in alcohol and ether, obtained from the cracking of petroleum, from coal tar benzene or from acetylene produced from coke and lime. It is widely used in the formation of copolymers with styrene, acrylonitrile, vinyl chloride and other monomeric substances, where it imparts flexibility to the subsequent moldings.

Butadiene Styrene Plastics - A synthetic resin derived from the copolymerization of butadiene gas and styrene liquids.

Buttress Thread - A type of threading in which the thread sides terminate abruptly in threading gradually tapering down to the neck finish. Designed to withstand maximum force in one direction only. Cross section of thread is triangular.

Butylene Plastics - Plastics based on resins made by the polymerization of butene or copolymerization by butene with one or more unsaturated compounds, the butene being in greatest amount by weight.