Among quality indicators of injection molding raw materials, several terms are frequently encountered. Accurately understanding their meanings helps in better grasping performance of raw materials. Here are some commonly used terms:
(1) Density: Density refers to mass contained in a unit volume of a substance; simply put, it is the ratio of mass to volume. Its unit is kilograms per cubic meter (kg/m³) or grams per cubic centimeter (g/cm³).
(2) Viscosity: Viscosity is internal frictional resistance generated by a fluid (liquid or gas) during flow. Its magnitude is determined by factors such as type of substance, temperature, and concentration. It is generally an abbreviation for dynamic viscosity, and its unit is Pascal-second (Pa·s) or millipascal-second (mPa·s). Viscosity is divided into dynamic viscosity, kinematic viscosity, and relative viscosity; these three are distinct and should not be confused. Viscosity can also be measured using a Forecast-4 or Forecast-1 cup, with unit being seconds (s).
(3) Water absorption: A measure of degree to which a substance absorbs water. It refers to percentage increase in mass of a substance when immersed in water for a certain period of time at a given temperature.
(4) Relative humidity: A method of expressing humidity, it is ratio of absolute humidity to saturated absolute humidity under same conditions (temperature and pressure). In other words, it is ratio of actual mass of water vapor in the air (or other gas) to saturated mass of water vapor under same conditions. It is usually expressed as a percentage.
(5) Degree of polymerization: Number of chain segments that make up a polymer molecule is called degree of polymerization, denoted by n or DP. It can be used as a measure of molecular weight of polymer.
(6) Molecular weight distribution: Due to their varying sizes, polymers, in addition to statistical characteristics of molecular weight, also exhibit polydispersity, i.e., molecular weight distribution. Same average molecular weight can have different molecular weight distributions, exhibiting different properties.
(7) Homopolymer: A polymer formed by polymerization of same monomer into a single repeating chain segment is called a homopolymer.
(8) Copolymer: A polymer formed by polymerization of two or more monomers or monomers with a polymer is called a copolymer. It is classified into block copolymers, random copolymers, regular copolymers, graft copolymers, etc.
(9) Graft Copolymer: A copolymer in which side chains of polymer segments with different chemical structures from main chain are attached to certain atoms of polymer backbone is called a graft copolymer, such as grafted chloroprene rubber and SBS graft copolymer.
(10) Glass Transition Temperature: Approximate midpoint of narrow temperature range at which an amorphous or semi-crystalline polymer transitions from a viscous or elastic state to a glassy state (or vice versa) is called glass transition temperature, usually expressed as Tg, and is an indicator of heat resistance.

 

(11) Brittle Temperature: A measure of low-temperature performance of a polymer. When a sample is impacted by a hammer with a certain energy, probability of cracking reaches 50% is called brittle temperature, also known as brittle-break point.
(12) Heat Deflection Temperature (HDT): A measure of heat resistance of a polymer. It is measured by immersing a polymer sample in a suitable heat transfer medium with a constant heating rate under a static bending load (simply supported beam). Temperature at which sample's bending deformation reaches a specified value is the HDT.
(13) Softening Point: Softening point is temperature at which a polymer sample, subjected to a specific load and heated at a specified rate, reaches a specified deformation value.
(14) Marten's Test: A method for evaluating high-temperature deformation tendency of a material. In a furnace, a sample is subjected to a specific bending stress and heated at a specific rate. Temperature at which free end of sample exhibits a specified deflection is called Marten temperature.
(15) Vicat Softening Point Test: A method for evaluating high-temperature deformation tendency of thermoplastics. Vicat softening temperature is measured under constant rate of heating conditions. A flat-topped needle with a specified load and a cross-sectional area of 1 mm² is placed on sample. Temperature at which needle penetrates 1 mm into sample is measured Vicat softening temperature.
(16) Melt Index: Melt index (MI) is an indicator reflecting melt flow characteristics and molecular weight of thermoplastic resins. It is mass of melt passing through a standard capillary tube in 10 minutes under a certain temperature and load, expressed in g/10min.
(17) Stress Relaxation: Stress relaxation is phenomenon where stress decreases with increasing time while deformation remains constant.
(18) Creep: Creep is phenomenon where deformation changes over time when stress remains constant.
(19) Shrinkage Ratio: Defined as percentage of shrinkage to original size. Shrinkage is difference between original and original sizes.
(20) Internal Stress: Stress generated within adhesive layer (material) in absence of external force due to defects, temperature changes, solvent effects, etc.
(21) Tensile Strength: Tensile strength is maximum tensile stress at which a specimen breaks. This commonly used term has historically been inconsistent, referred to as breaking force, tensile strength, tensile strength, or simply "strength." According to GB 6039-85, it is uniformly called tensile strength, with unit MPa.
(22) Shear Strength: Formerly known as shear strength, it refers to maximum load that a unit bond area can withstand parallel to bond area. Commonly used unit is MPa.
(23) Elongation: Increase in length of a specimen under tensile force, expressed as a percentage of its original length.
(24) Vulcanization: Vulcanization is process by which rubber reacts with sulfur and accelerators under specific temperature and pressure conditions, causing rubber macromolecular chains to crosslink. In other words, it transforms plastic rubber into elastic or hard rubber. Broadly speaking, vulcanization refers to process by which rubber compounds are treated chemically or physically, causing rubber macromolecules to transform from a linear to a network structure through crosslinking, thereby improving physical, mechanical, and chemical properties of rubber.
(25) Crosslinking: Refers to chemical bonding between main chains of linear polymer molecules.
(26) Oil resistance: Ability of a material to resist swelling, dissolution, cracking, deformation, or reduction in physical properties caused by oils.
(27) Solvent resistance: Ability to resist swelling, dissolution, cracking, or deformation caused by solvents.
(28) Chemical resistance: Ability to resist acids, alkalis, salts, solvents, and other chemical substances.
(29) Water resistance: Ability of a material to retain its physicochemical properties after exposure to water or moisture.
(30) Flame resistance: Ability of a material to resist combustion when in contact with a flame or to prevent further combustion after flame is removed.
(31) Weatherability: Ability of a material to withstand exposure to climatic conditions such as sunlight, heat, cold, wind, and rain.
(32) Aging: During processing, storage, and use, a series of physical or chemical changes occur due to external factors (heat, light, oxygen, water, radiation, mechanical force, and chemical media, etc.), causing polymer materials to become cross-linked and brittle, crack and become sticky, discolor and crack, become rough and blister, powder on the surface, delaminate and peel off, and gradually deteriorate in performance, eventually losing their mechanical properties and becoming unusable. This phenomenon is called aging.