Plastic Technology FAQ

Plastic is a family of materials based on high-molecular-weight polymers. The polymer establishes the basic thermal and mechanical behaviour, while molecular architecture, additives, fillers, reinforcement and processing history determine the performance of the finished component.

A monomer is a relatively small reactive molecule. Polymerisation joins many monomers into long chains or networks called polymers. Repeating-unit chemistry, chain length, branching and molecular-weight distribution affect melt viscosity, toughness, strength and processability.

Addition polymerisation joins reactive monomers without releasing a small-molecule by-product. Condensation polymerisation produces the polymer through step reactions that may release water or another small molecule. Copolymerisation combines different monomers to tailor the balance of properties.

Thermoplastics have linear or branched chains and can soften repeatedly when heated, provided degradation is avoided. Thermosets form a permanent three-dimensional network during curing and cannot be remelted. Elastomers have widely spaced crosslinks that permit large reversible deformation.

Amorphous polymers have a largely disordered structure and soften around the glass-transition region. Semi-crystalline polymers contain ordered crystallites within an amorphous phase and also have a melting range. Morphology affects transparency, shrinkage, chemical resistance, stiffness and dimensional stability.

Spherulites are approximately spherical crystalline structures that grow from nuclei as a semi-crystalline polymer cools. Their size and number depend on polymer structure, nucleating agents and cooling rate and may influence appearance, toughness and dimensional behaviour.

Additives protect the polymer, improve processing or create a function. Examples include heat and UV stabilisers, pigments, plasticisers, lubricants, flame retardants, antistatic agents, fillers and fibres. The complete compound must be evaluated because additives may improve one property while reducing another.

Temperature changes polymer-chain mobility. At higher temperature, most plastics lose stiffness and creep faster; at low temperature, some become brittle. Design values should represent the actual service temperature and load duration rather than room-temperature short-term data.

The glass-transition temperature, Tg, is the region in which the amorphous phase changes from hard and glass-like to softer and more rubber-like behaviour. Tg is not a melting point and is important for amorphous plastics and the amorphous phase of semi-crystalline plastics.

Creep is time-dependent deformation under sustained load. It accelerates with stress, temperature and time and can occur below the short-term yield stress. Long-term creep curves or time-dependent modulus data should be used for continuously loaded components.

Stress relaxation is the gradual loss of stress under fixed deformation. It can reduce clamping or sealing force in snap fits, press fits, seals and bolted joints. Material, temperature, strain level, geometry and required life must be considered together.

Resistance depends on polymer grade, chemical identity, concentration, temperature, exposure time and mechanical stress. Effects may include swelling, dissolution, loss of strength or environmental stress cracking. Supplier data is useful for screening, but critical applications require representative testing.

Environmental stress cracking is brittle failure caused by the combined action of tensile stress and a chemical environment. Residual moulding stress, sharp corners and assembly strain can increase sensitivity even when the chemical causes little visible effect on an unstressed sample.

Datasheet values are measured on standard specimens under defined conditions. Real parts differ in thickness, orientation, weld lines, moisture, temperature and loading history. Final decisions should combine current grade data, calculations, prototypes and validation testing.

PE offers low density, chemical resistance and electrical insulation. LDPE is generally more flexible, while HDPE is stiffer and more crystalline. Molecular architecture strongly affects toughness, permeability, shrinkage and melt behaviour.

PP combines low density, chemical resistance and useful heat resistance. Homopolymers are generally stiffer, while impact copolymers improve toughness. UV exposure, low-temperature impact and long-term load must be evaluated for the grade.

General-purpose PS is stiff, clear and easy to process but brittle. HIPS contains a rubber phase for impact improvement. SAN improves chemical and thermal performance, while ABS combines useful impact strength, stiffness and surface quality.

PVC can be rigid or flexible depending on formulation and plasticiser content. It is used for pipe, profiles, sheet, film and cable products. Thermal stability, formulation, corrosion-resistant equipment and controlled melt temperature are important in processing.

PMMA is an amorphous transparent plastic with high optical clarity and good weather resistance. It is relatively notch-sensitive. Part design, surface quality, drying and chemical compatibility should be checked for the selected grade.

PA grades provide strength, fatigue resistance and wear performance. They absorb moisture, which changes dimensions, stiffness, toughness and electrical properties. Drying before processing and conditioning or service humidity must be included in tolerances and testing.

POM is a semi-crystalline engineering plastic with high stiffness, low friction and good dimensional stability. Homopolymer and copolymer grades differ. Excessive melt temperature or residence time must be avoided because degradation can produce hazardous gases.

PET and PBT are thermoplastic polyesters used in packaging and engineering applications. Reinforced grades can provide strength, dimensional stability and electrical performance. Moisture must normally be removed before melt processing to prevent hydrolytic chain degradation.

PC is an amorphous engineering plastic with high impact strength, transparency and useful heat resistance. Effective drying and controlled processing are normally required. Chemical compatibility and environmental stress cracking should be assessed.

Fluoropolymers, sulphur-containing polymers and other high-performance thermoplastics are used when standard plastics cannot meet thermal, chemical, friction or electrical requirements. Cost and specialised processing requirements must be considered.

TPEs combine rubber-like service behaviour with thermoplastic melt processing. They can often be injection moulded or extruded without vulcanisation, and clean process scrap may be reusable. TPE families differ substantially in hardness, compression set, thermal range, chemical resistance and adhesion in multi-material parts.

Reactive resin components form a permanent three-dimensional network during curing. Mixing ratio, time, temperature, catalyst or hardener level and heat removal determine conversion and final properties. Once cured, the material cannot be remelted without degradation.

PUR is formed mainly by reaction between isocyanate groups, NCO, and hydroxyl groups, OH, in polyols, producing urethane linkages. By changing polyol type, functionality, chain length, isocyanate chemistry and formulation, PUR can range from flexible elastomers and comfort foams to rigid structural or insulating foams. Isocyanate index, component temperature, viscosity, mixing quality and moisture content directly affect reaction balance and final performance.

A PUR formulation normally contains a polyol component and an isocyanate component. Depending on the product, it may also contain catalysts, chain extenders, crosslinkers, surfactants, pigments, fillers, flame retardants and physical or chemical blowing agents. The polyol controls much of the soft-segment behaviour, while functionality and isocyanate balance influence crosslink density, hardness and heat resistance.

Foam requires gas generation or vaporisation while the polymer network develops. Water can react with isocyanate to generate carbon dioxide, while physical blowing agents may vaporise from reaction heat. Catalysts balance blowing and gelling reactions, and surfactants stabilise the cells until the polymer has sufficient strength. Poor balance can cause collapse, shrinkage, coarse cells, voids or high density.

The isocyanate index expresses the relationship between available NCO groups and the reactive hydrogen groups in the formulation. It is a formulation-control parameter, not a universal quality number. A change can affect cure, crosslink density, brittleness, dimensional stability and residual reactive groups, so the specified ratio and current certificates of analysis should be used.

Critical variables include component ratio, temperature, viscosity, conditioning, moisture, metering accuracy, mixing energy, dispense pattern, mould temperature, fill quantity, venting, cream time, rise time, gel time, demould time and post-cure. Process monitoring should correlate these variables with density, mass, dimensions, hardness, cell structure and mechanical performance.

Incorrect ratio or poor mixing may produce soft or brittle regions and incomplete cure. Moisture or uncontrolled blowing can cause bubbles, voids and density variation. Cold components or moulds may give poor flow or surface reproduction; excessive exotherm can cause scorching or internal damage. Trapped air, weak venting, contamination and premature demoulding are additional causes of reject parts.

Rigid PUR foam can serve as an insulating or structural core between stronger face sheets. The core separates the skins, transfers shear and supports them against buckling, while the skins carry much of the bending stress. Performance depends on foam density, cell structure, adhesion, skin material, edge details, inserts, moisture protection and the actual load case.

Controls may include raw-material identity and temperature, component ratio, cream and gel times, part mass, free-rise or moulded density, dimensions, hardness, cell structure, adhesion and mechanical or thermal tests. Acceptance criteria should be linked to product function and based on the exact formulation and process.

PUR production can involve reactive isocyanates, catalysts, blowing agents and heated equipment. Exposure controls, closed handling where practical, local ventilation, suitable protective equipment, spill procedures, training and medical or regulatory requirements must follow current safety data sheets and local rules. Cutting or sanding cured foam can also generate dust requiring control.

Unsaturated polyester supports many open- and closed-mould composite processes. Epoxy is valued for adhesion, relatively low cure shrinkage and strong composite performance when correctly mixed and cured. Both require controlled formulation, fibre wet-out, cure and safe chemical handling.

Fibres carry load and can greatly raise stiffness and strength. Glass fibre is widely used, carbon fibre provides high specific stiffness, and aramid provides low density and useful tensile and impact properties. Fibre length, orientation, wet-out and interface quality control the benefit.

Fibres provide their greatest effect along their direction, creating anisotropic stiffness, strength, expansion and shrinkage. Laminate stacking sequence or mould-flow orientation should therefore be aligned with the actual load paths.

A sandwich panel combines strong face skins with a lightweight core. Skin separation creates high bending stiffness at low mass, while the core transfers shear. Bonding, concentrated loads, edges, inserts and moisture protection require careful detailing.

Granules are conveyed, melted, mixed and metered by a rotating screw. The screw injects the melt into a closed mould, followed by holding pressure, cooling, mould opening and ejection. Stable production requires alignment of material, machine, mould, settings and part design.

Filling depends on melt and mould temperature, speed, pressure capability, viscosity, wall thickness, gate and runner design and venting. Thin sections and long flow paths increase pressure demand. A controlled velocity profile manages shear and air displacement.

Holding pressure compensates for volumetric contraction until the gate freezes. It affects part mass, sinks, voids, dimensions, stress and flash. A part-weight or gate-freeze study can help determine effective holding time.

The transfer point changes control from velocity to pressure. Late transfer may overpack the cavity; early transfer can cause short shots or inconsistent packing. Transfer should correspond to a repeatable fill condition.

Causes include insufficient shot size, early transfer, restricted flow, low temperature, insufficient speed or pressure, blocked vents and trapped air. The cause should be isolated systematically rather than by changing many settings together.

Both are associated with shrinkage in thick regions, ribs or bosses. Inadequate packing, early gate freeze and poor cooling can worsen them. Correction often combines uniform walls, suitable rib proportions, effective gating and balanced packing.

Weld lines form where melt fronts meet and depend on temperature, pressure, venting and material. Burn marks may result from compressed trapped air, shear or material degradation. Location and process data help distinguish the mechanisms.

Warpage results from non-uniform shrinkage caused by wall thickness, cooling, pressure distribution, crystallisation or fibre orientation. Effective correction may require coordinated changes to design, gate, cooling, tooling and process.

Absorbed moisture can split polymer chains at processing temperature, reducing molecular weight and performance. Drying temperature, time, airflow and dew point should follow current grade-specific supplier instructions.

A rotating screw conveys, compresses, melts and mixes the polymer and generates stable flow through a die. The product is calibrated, cooled, pulled and cut or wound. Stability depends on feeding, screw and die design, temperature control and downstream equipment.

Extrusion is used for pipe, profiles, sheet, film, monofilament, cable insulation and compounding. It also produces the parison for extrusion blow moulding.

Possible causes include irregular feeding, bridging, moisture, temperature drift, wear, screen blockage, unstable speed and downstream pull variation. Pressure, load, temperatures, speeds and product dimensions should be trended together.

Coextrusion combines multiple molten materials into a multilayer product. It can combine structural, barrier, colour, sealing, recycled-content or surface functions. Viscosity matching, layer stability and adhesion are important.

A hot tubular parison is extruded, captured in a mould and inflated against the cooled cavity. Parison programming can redistribute material where stretching would otherwise create thin walls.

A preform is reheated, stretched axially and expanded radially. Biaxial orientation can improve strength and barrier performance. Temperature distribution, stretch ratio, timing and pressure control wall distribution.

A thermoplastic sheet is heated and shaped against a tool using vacuum, pressure and sometimes a plug. It is cooled, released and trimmed. Sheet-temperature uniformity and controlled stretching are central to repeatable thickness.

Different areas experience different stretch ratios. Zoned heating, plug assistance, pre-stretching, tool geometry and process sequence can improve material distribution.

Consider draw ratio, radii, draft, undercuts, trimming, sheet orientation, shrinkage and support during cooling. Positive and negative tools distribute material differently.

Polymer powder is placed in a hollow mould that rotates about two axes while heated. Melt coats the internal surface and cools while rotating. The process suits large hollow parts and lower volumes but has relatively long cycles.

Uniform walls improve flow, cooling and shrinkage. Thick masses increase cycle time and can cause sinks, voids, residual stress and warpage. Necessary thickness changes should normally be gradual.

Ribs and bosses add stiffness or fastening features without thickening the whole wall. Thickness, height, root radius, draft and wall connection affect filling, sink and stress.

Radii reduce stress concentration and improve flow. Draft supports reliable release with lower friction and ejector load. Requirements depend on depth, texture, material shrinkage and tool construction.

The snap must flex during assembly without excessive strain and retain enough force in service. Beam length, thickness, root radius, engagement, creep and assembly frequency are important.

Options include welding, adhesives, screws, rivets, snap fits and insert moulding. Selection depends on compatibility, load, sealing, appearance, volume, environment and disassembly needs.

The materials must be compatible and the joint must support controlled heat and pressure. Methods include hot-plate, hot-gas, extrusion, ultrasonic, vibration, spin and laser welding.

Low surface energy, contamination and release agents can prevent adhesion. Cleaning, abrasion, plasma, flame treatment or primer may be required. Joints should favour distributed shear or compression rather than peel.

Plastics can be turned, milled, drilled, threaded and sawn. Low thermal conductivity, elastic recovery, moisture and thermal expansion affect heat and dimensions. Sharp tools, clearance, controlled feed and chip removal are important.

Define the defect and frequency, retain samples and compare actual data with the approved process. Check material, machine, tool, auxiliaries and environment, then change one controlled factor at a time and document the result.

Regrind should be identified, clean, homogeneous and correctly dried. Particle size, contamination, heat history and mixing ratio affect feeding and properties. The permitted proportion should be validated against product requirements.

Many thermoplastics can be sorted, cleaned, reduced and remelted, although contamination and heat history may reduce quality. Crosslinked thermosets cannot be remelted in the same way and require other recovery routes.

Hazards include hot material, pressure, stored energy, moving machinery, fumes, dust, reactive chemicals and fire. Guarding, ventilation, lockout, training and protective equipment must be based on current risk assessment and safety data.

Avoid unnecessary mass, rejects and energy use. Longer life, repairability, recycled content, fewer incompatible materials and design for disassembly can improve resource efficiency when consistent with performance and safety.