Visible Defects – A Glossary of Potential Visible Defects in Plastics Parts Processing
Blister: A raised area on the surface of a plastic part caused by the pressure of gases inside the part expanding, but not breaking the surface.
- Bleed: A plastic part gives up color when it comes in contact with water or a solvent. Created by an undesired movement of certain materials within the plastic (e.g. plasticizers in vinyl). The color bleeds to the surface of the finished part or into an adjacent part’s incompletely hardened surface.
- Bloom: A visible exudation or efflorescence on the surface of a plastic part.
- Chalking: A powdery residue on the surface of a plastic part often resulting from degradation of the plastic material.
- Clarity: The level of clearness in plastic parts.
- Decomposition: A part decays or decomposes, typically at an undesirable rate.
- Discoloration: Any change from the original color, often caused by overheating, light exposure, irradiation, or chemical attack.
- Gloss: The degree of shine or luster of the surface of a material.
- Splay: A defect in the surface of a molded part that is usually small in size, shiny, and may have the appearance of lightly tinted or silver streaks.
Performance Deficiencies – A Glossary of Potential Part Failures in Plastics Processing
Cosmetic defects often forewarn a part’s ability to do its job as it was designed. Improper drying can not only ruin a part’s looks but make it fail in use.
- Aging: The change of a plastic part or piece with time under defined environmental conditions, leading to the continuation or deterioration of properties.
- Crazing: Fine cracks that may extend in a network on or under the surface or through a layer of a plastic part.
- Creep: The dimensional change with time of a plastic part under load, following the initial instantaneous elastic deformation. Creep at room temperature is sometimes called Cold Flow.
- Ductility: The extent to which a solid plastic part can be drawn into a thinner cross-section.
- Environmental Stress Cracking: The susceptibility of a thermoplastic article to crack or suffer craze formation under the influence of certain chemicals and stress.
- Hardness: The resistance of a plastic article to compression and indentation. Among the most common methods of testing this property is the Brinell hardness test procedure.
- Hydrolysis: Chemical decomposition of a plastic article through the addition of water.
- Impact Strength: The ability of a part to withstand shock loading or the work done in fracturing, under shock loading, a specified test specimen in a specified manner.
- Intrinsic Viscosity: The intrinsic viscosity of a polymer is the limiting value of infinite dilution of the ratio at the specific viscosity of the polymer solution to its concentrate on moles per liter.
- Melt Flow: The flow rate obtained from the extrusion of a molten resin through a die of specified length and diameter under prescribed conditions of time, temperature and load as set forth in ASTM D1238.
- Moisture Vapor Transmission: The rate at which water vapor permeates through a plastic film or wall at a specified temperature and relative humidity.
- Notch Sensitivity: The extent to which the sensitivity to fracture of a part is increased by the presence of a surface in homogeneity such as a face notch, a sudden change in section, a crack, or a scratch. Low-notch sensitivity is usually associated with ductile materials, and high-notch sensitivity with brittle materials. This is an important element in complex part design.
- Oxidation: The change occurring in a part by the addition of oxygen or the reduction of hydrogen.
- Permeability: The passage or diffusion of a gas, vapor, liquid, or solid through a barrier without physically or chemically affecting it and the rate of such passage.
- Resiliency: Ability to quickly regain an original shape after being strained or distorted.
- Voids: In a solid plastic, an unfilled space of such size that it scatters radiant energy such as light or more importantly, a cavity unintentionally formed in a molding.
Tolerance/Size Issues in Plastics Part Processing
Poor drying can often compromise a part’s physical characteristics, typically defeating its ultimate benefit in final use or application.
- Concentricity: For a round container part for instance, the shape in which various cross sections have a common center.
- Tolerance: A specified allowance for deviations in weight, dimensions measuring, etc.
- Warpage: Dimensional distortion in a plastic object after molding, particularly common and critical on flat surfaces.
- Wrinkle: An imperfection in reinforced plastics that has the appearance of a wave molded into one or more plies of fabric or other reinforcing material.
