Plastics and Their Processing
Introduction to Plastics
Plastic is fundamentally defined as any material that is capable of being modelled or moulded into a specified shape. In modern engineering, the term plastic refers to a broad group of synthetic or semi-synthetic organic materials which, when subjected to heat and pressure, can be formed into useful finished articles through processes such as moulding, casting, or extrusion. A plastic is a polymeric material that can be shaped or moulded into a desired form, usually by applying heat, pressure, or both.
Plastics are predominantly organic materials, commonly referred to as resins in their base form. The raw or base material used to manufacture finished plastic items is generally designated as resin. The industrial process follows the chemical sequence where raw resin combined with processing additives produces the plastic moulding material, which is subsequently converted into the final plastic product. For industrial manufacturing, raw plastic materials are supplied in several physical forms: pellets or granules (which are very common for thermoplastics), fine powders, liquid resins, flakes, and preforms or compounds.
Plastic materials soften or become formable upon heating, allowing them to flow into and take the shape of a mould cavity. They are ubiquitous in modern engineering applications because they are low-cost, lightweight, and easy to form into complex shapes. Consequently, plastics can effectively replace metals in many light-duty structural and mechanical applications.
Plastics offer a wide range of engineering advantages: light weight, low cost, excellent moisture resistance, high strength-to-weight ratio, high chemical resistance, toughness, high wear and abrasion resistance, attractive physical appearance with unlimited built-in colourability, excellent thermal insulation, excellent electrical insulation, high formability, and good machinability.
Despite these benefits, plastics possess specific engineering limitations: lower overall mechanical strength compared to structural metals, low stiffness, poor high-temperature resistance, poor dimensional stability, high thermal expansion coefficients, brittleness or water absorption in specific formulations, and an absolute inability to reshape or re-use thermosetting plastics once cured.
Polymer Chemistry and Molecular Structure
A polymer is a large macromolecule formed by joining many small recurring chemical units together. The term originates from the Greek words "poly" meaning many, and "mer" meaning a unit or part. A monomer represents the basic structural building block of a plastic material. It consists of a molecule built around carbon atoms, with other atoms such as hydrogen, chlorine, or fluorine attached as lateral ribs.
Polymerization is the specific chemical reaction by which monomers join end-to-end to build long chain-like molecules called polymers. Most plastics are composed primarily of carbon, hydrogen, oxygen, and nitrogen, occasionally incorporating other chemical elements. During polymerization, small monomer units link sequentially end-to-end. As these chains lengthen, they become heavy enough to exhibit the mechanical characteristics of a solid plastic.
The lateral atoms (ribs) attached to the main carbon backbone dictate the primary chemical and mechanical properties of the resulting plastic, such as flexibility, stiffness, and mechanical strength. For example, when hydrogen atoms are attached as side atoms along the carbon backbone, they form polyethylene, which is lightweight and flexible. Substituting chlorine atoms yields polyvinyl chloride (PVC), whereas attaching fluorine atoms produces polytetrafluoroethylene (PTFE).
An example of monomer conversion is ethylene (), which serves as the fundamental monomer for synthesizing polyethylene. The general chemical reaction scheme for polymerization can be represented as:
In this scheme, represents individual monomer units, and the resulting chain represents the macromolecular polymer.
The Glass Transition Temperature () is a critical thermal parameter defined as the specific temperature at which an amorphous polymer transitions from a hard, rigid, brittle, glass-like state into a soft, flexible, rubbery state.
Classification of Plastics by Thermal Behaviour
Based on their thermal characteristics and response to heating, plastics are categorized into two primary engineering classes: Thermoplastics and Thermosetting Plastics (Thermosets).
Thermoplastics are linear or branched polymers whose molecules are structured as long, independent thread-like chains without covalent cross-links between them. Thermoplastics undergo physical softening upon heating and harden upon cooling. This thermal cycle can be repeated indefinitely. Because no permanent chemical cross-linking occurs during moulding, the physical softening is reversible, and thermoplastic scrap can be reprocessed and re-used.
Most thermoplastics experience severe structural softening and become unusable when subjected to temperatures within the range of 66\,\text{^\circ C} to 315\,\text{^\circ C}. Thermoplastics constitute the majority of plastic materials manufactured and consumed globally today.
The fundamental thermoplastics, their monomers, and their basic chemical structural units include:
Polyethylene (PE), derived from the ethylene monomer ().
Polypropylene (PP), derived from the propylene monomer ().
Polyvinyl Chloride (PVC), derived from the vinyl chloride monomer ().
Polystyrene (PS), derived from the styrene monomer ().
Polytetrafluoroethylene (PTFE), derived from the tetrafluoroethylene monomer ().
Polymethyl Methacrylate (PMMA), derived from the methyl methacrylate monomer ().
Polycarbonate (PC), featuring the characteristic carbonate group ().
Polyacetal or Polyoxymethylene (POM), featuring the repeating acetal unit ().
PTFE exhibits outstanding chemical inertness and resistance. Flexible plasticized PVC represents one of the structurally weakest thermoplastics, showing poor performance in compressive strength and machinability.
Detailed Characteristics and Applications of Common Thermoplastics
Polyethylene (PE) leads all plastics globally in annual production volume. It is characterized as a lightweight, tough, chemical-resistant polymer with excellent electrical insulation properties and very low moisture absorption. Polyethylene is manufactured in three primary density classes: Low-Density Polyethylene (LDPE), Intermediate-Density Polyethylene, and High-Density Polyethylene (HDPE). PE is widely processed using blow moulding, injection moulding, and extrusion. Common products include bowls, plates, dishpans, paintbrush handles, flexible tubing, vegetable packaging bags, hardware containers, and squeeze bottles.
Low-Density Polyethylene (LDPE) possesses a highly branched molecular structure, making it soft and flexible. It is primarily used for carry bags, thin films, squeeze bottles, and electrical cable insulation.
High-Density Polyethylene (HDPE) exhibits a more linear structure, rendering it harder, stronger, and stiffer than LDPE. Typical applications include high-pressure pipes, blow-moulded bottles, buckets, storage tanks, and heavy-duty industrial crates.
Polypropylene (PP) is an exceptionally lightweight and rigid polymer characterized by outstanding mechanical fatigue resistance. Due to its fatigue resistance, thin sections of polypropylene can be flexed repeatedly without failing, making it the standard material for living hinges, hinged lids, bottle caps, reusable food containers, and buckets. Polypropylene also offers unlimited colourability.
Polyvinyl Chloride (PVC) offers excellent chemical resistance, corrosion resistance, water resistance, and electrical insulation. Unplasticized Polyvinyl Chloride (uPVC) contains no plasticizing additives, resulting in a hard, rigid material used for structural pressure pipes, pipe fittings, window frames, and electrical conduits. Plasticized PVC contains added plasticizers that convert it into a soft, flexible material ideal for flexible electrical cable insulation, hoses, raincoats, and artificial leather. Coating fabrics for tents and tarpaulins represents another major application of PVC.
Polystyrene (PS) ranks second in total worldwide resin volume. It is a crystal-clear, odourless, tasteless, high-gloss thermoplastic that is inherently hard, rigid, and brittle. Unless chemically modified, polystyrene retains static electricity. It possesses high tensile strength but is thermally limited to operating environments between 66\,\text{^\circ C} and 90\,\text{^\circ C}. Standard PS is used for rigid packaging, disposable articles, and product housings.
Expanded Polystyrene (EPS), commonly known as Thermocol, is produced in beads or slabs containing gas cells. EPS provides extreme light weight, shock-absorbing cushioning, and thermal insulation, making it ideal for packaging delicate electronics and thermal insulation panels.
High-Impact Polystyrene (HIPS) is modified with rubber compounds to yield significantly higher toughness than standard PS, making it suitable for appliance liners, packaging, and durable housings.
Polymethyl Methacrylate (PMMA), widely known by trade names such as Perspex, Plexiglas, Lucite, and Acrylite, is an acrylic polymer with optical clarity and transparency comparable to glass. PMMA offers superior weatherability and resistance to ultraviolet sunlight, but its surface is susceptible to scratching. Applications include aircraft canopies and windows, optical lenses, outdoor illuminated signboards, historic hard contact lenses, brush backs, transparent bowls, and liquid dispensers.
Polycarbonate (PC), commercially available under trade names such as Lexan, Merlon, and Polycarbafil, exhibits exceptionally high impact strength, earning it the designation of bulletproof glass. Polycarbonate can be moulded, extruded, machined, nailed, or riveted without structural cracking. It retains its toughness and mechanical strength over a temperature range from -138\,\text{^\circ C} to 120\,\text{^\circ C}. Major applications include safety goggles, motorcycle helmet visors, machine guards, bullet-resistant architectural glazing, and optical discs (CDs and DVDs).
Polyamide (PA), universally known as Nylon, is marketed under trade names including Zytel, Nylatron, Nylafil, Plaskon, Rilsan, and Ultramid. Polyamides feature a low coefficient of friction, high mechanical strength, toughness, light weight, and resistance to heat, abrasion, and chemicals. Nylon components run without lubrication, making them suitable for small unlubricated mixer gears, industrial bearings, bushes, drawer slides, and rollers. However, nylon is not recommended for environments exposing it to ultraviolet light, hot water, or alcohols. At high ambient humidity, nylon absorbs up to water by weight, leading to dimensional expansion.
Nylon-6 (Polyamide-6) exhibits high toughness and wear resistance, making it suitable for synthetic fibres, gears, and bearings.
Nylon-6,6 (Polyamide-6,6) provides elevated mechanical strength and exceptional wear resistance for heavy-duty engineering parts, gears, and structural components.
Polytetrafluoroethylene (PTFE), known commercially as Teflon, features an extremely low coefficient of friction, complete chemical inertness, high temperature stability, and electrical insulation properties. Applications include non-stick cookware coatings, chemical valve seals, low-friction sleeve bearings, chemical processing equipment linings, and high-temperature electrical insulation.
Polyethylene Terephthalate (PET) is a strong, transparent polymer with low permeability to gases, making it the standard resin for soft drink and water bottles, food packaging films, and synthetic polyester fibres.
Polybutylene Terephthalate (PBT) offers good electrical insulation and dimensional stability for electrical connectors and automotive components.
Acrylonitrile Butadiene Styrene (ABS) is a tough, impact-resistant styrenic terpolymer used for protective safety helmets, automotive trim, appliance housings, and children's building toys.
Polyoxymethylene (POM), also called Polyacetal or Acetal and sold under trade names Delrin and Celcon, exhibits low friction, high wear resistance, and excellent dimensional stability for precision mechanical gears, bearings, and bushings.
Polyphenylene Oxide/Ether (PPO/PPE) delivers thermal resistance and dimensional stability for electrical enclosures and automotive parts.
Polyphenylene Sulfide (PPS) possesses high heat and chemical resistance for automotive engine components and electrical switchgear.
Polyether Ether Ketone (PEEK) retains its mechanical properties at elevated temperatures and under aggressive chemical exposure, servicing aerospace, medical, and high-performance engineering sectors.
Polyetherimide (PEI) is an amorphous high-temperature engineering thermoplastic utilized in aerospace and electrical hardware.
Thermoplastic Polyurethane (TPU) combines flexibility, toughness, and abrasion resistance for footwear soles, industrial hoses, caster wheels, and heavy cable jacketing.
Ethylene Vinyl Acetate (EVA) is a soft, shock-absorbing polymer used in shoe insoles, flexible foam sheets, and protective packaging.
Styrene Acrylonitrile (SAN) is a rigid, transparent, chemical-resistant polymer used for rigid food containers and kitchen appliance components.
Cellulosic Plastics are derived from natural wood pulp or cotton fibers through chemical processing. Treatment of cellulose yields materials such as Cellophane film. Cellulose Acetate (CA) is the most easily worked plastic material. In sheet form, cellulose acetate is known as Cellophane and is used for electrical insulating tape and packaging films; solid formulations are processed into optical spectacle frames, cutlery handles, toys, and cigarette filter fibres. Cellulose Nitrate (CN), trade-named Celluloid when plasticized with camphor, offers toughness and varied colourability but is highly flammable; historically used for photographic film, combs, and decorative coatings. Cellulose Acetate Butyrate (CAB) is a weather-resistant cellulosic used for tool handles and exterior coatings.
Thermosetting Plastics and Curing Mechanisms
Thermosetting plastics consist of polymer chains that are chemically cross-linked during processing to form an irreversible, three-dimensional molecular network. Once the cross-linked network is established, thermosets do not soften upon reheating; exposure to excessive thermal energy causes them to char or decompose thermally.
Because of their rigid 3D molecular network, thermosets are fireproof, insoluble in organic solvents, and generally hard and brittle. A consequence of this cross-linked structure is that thermosetting scrap cannot be remelted, reshaped, or re-used, rendering scrap material without thermal reprocessing value. Thermosets are primarily processed using compression moulding and transfer moulding.
Curing is defined as the chemical process of permanently hardening a thermosetting resin by forming covalent chemical cross-links between adjacent polymer chains.
Epoxy resins are typically supplied as a two-part system: Part A consisting of the epoxy resin, and Part B consisting of the hardener or curing agent. Polymerization and cross-linking occur upon mixing these components. Cured epoxies exhibit chemical resistance, high electrical insulation, and strong adhesive bonding capabilities. Epoxies filled with mineral or fibrous additives operate continuously at temperatures ranging from 150\,\text{^\circ C} to 260\,\text{^\circ C}. Epoxy serves as the structural matrix in Glass Fibre Reinforced Plastics (GFRP) and Carbon Fibre Reinforced Plastics (CFRP), where the high-strength reinforcement fibres carry structural loads while the epoxy matrix holds the fibres in alignment, transfers stress, and protects them from environmental damage. Epoxies are used in structural adhesives, electrical equipment castings, printed circuit boards (PCBs), and marine vessel hulls.
Amino Resins comprise two principal polymer families: Urea-Formaldehyde and Melamine-Formaldehyde. Urea-Formaldehyde compounds serve as wood adhesives for manufacturing plywood, particle board, and Medium-Density Fibreboard (MDF); they provide strong electrical insulation but possess lower water resistance than melamine. Melamine-Formaldehyde exhibits high hardness and scratch resistance, making it suitable for moulding dinnerware, plates, business machine housings, electrical switch cover plates, and radio cabinets. When laminated with paper or cloth, melamine produces durable countertop and tabletop surfaces.
Phenolics (Phenol-Formaldehyde), historically known by the trade name Bakelite, are produced by reacting phenol with formaldehyde in the presence of a catalyst. Phenolics are hard, rigid materials with heat resistance and electrical insulation. Phenolic resins are used for electrical switches, plugs, sockets, switchboards, heat-resistant handles for cooking utensils and kitchen appliances, laminated industrial sheets, friction binders for automotive brake linings and clutch plates, and historical telephone enclosures.
Silicones represent a chemical hybrid polymer structure combining organic carbon groups with inorganic silicon-oxygen backbones. This hybrid structure produces fluids, greases, resins, and elastomeric compounds. Silicone fluids are utilized in cosmetics, polishes, and lubricants. Silicone resins are processed by extrusion, transfer moulding, compression moulding, and casting to produce high-temperature seals, gaskets, and flexible moulds.
Comprehensive Comparison: Thermoplastics vs Thermosetting Plastics
Regarding molecular structure, thermoplastics consist of linear or branched long thread-like polymer chains without cross-links, whereas thermosets consist of a cross-linked three-dimensional network.
Under the effect of heat, thermoplastics soften reversibly upon heating and harden upon cooling, whereas thermosets set permanently during initial curing and do not soften upon reheating.
Regarding chemical changes during moulding, thermoplastics undergo purely physical softening with no chemical reaction, while thermosets undergo permanent chemical polymerization and cross-linking (curing).
Regarding reshaping capabilities, thermoplastics can be resoftened and reshaped repeatedly, whereas thermosets cannot be reshaped after initial cure.
With respect to scrap reusability, thermoplastic scrap and runners can be reground and re-used, whereas thermoset scrap cannot be thermally reprocessed or re-used.
Regarding mechanical hardness, thermoplastics are soft and tough, while thermosets are hard and brittle.
Regarding chemical solubility, thermoplastics dissolve in selected organic solvents, whereas thermosets are insoluble.
In fire behaviour, many thermoplastics are flammable and melt, whereas thermosets are fireproof and char rather than melt.
Under room-temperature mechanical creep, thermoplastics may exhibit molecular flow under continuous loads, whereas cross-linked thermosets resist continuous load flow.
Regarding optimal processing operations, thermoplastics are predominantly shaped by injection moulding, extrusion, blow moulding, and thermoforming, whereas thermosets are processed via compression moulding and transfer moulding.
Elastomers and Vulcanization
Elastomers are high-molecular-weight polymers whose constituent molecular chains are loosely bound together, exhibiting high elasticity. They can withstand elastic deformations of to (stretching 5 to 10 times their original unstressed length) and return immediately to their original dimensions upon release of the applied load.
Structurally, elastomers remain non-crystalline at room temperature. They occupy an intermediate structural position between the linear un-crosslinked chains of thermoplastics and the densely cross-linked 3D networks of thermosetting plastics.
Natural rubber is the most widely known elastomer, derived from latex, a viscous milky fluid containing a linear polymer of polyisoprene. Synthetic elastomers include silicone rubber, polyurethane elastomers, and chlorinated polyethylene.
Most elastomers are shaped and then permanently cross-linked. Vulcanization is the chemical process of heating raw rubber with elemental sulphur at elevated temperatures to form cross-linking bridges between parallel polyisoprene chains. This chemical cross-linking enhances mechanical strength, elasticity, and thermal stability. Elastomers are utilized for industrial gaskets, flexible moulds, elastomeric foams, mattresses, and electrical cable insulation.
Additives for Plastic Processing
Raw polymeric resins are combined, compounded, or chemically formulated with additive ingredients to tailor their mechanical, physical, and processing characteristics. Eight functional classes of additives are utilized in plastic compounding:
Plasticizers function as internal molecular lubricants. They consist of organic solvents, resins, or water that reduce intermolecular forces between polymer chains, preventing crystallization and keeping chains separated. This increases chain mobility, melt flow, flexibility, and impact toughness. For example, adding plasticizers converts hard, brittle unplasticized vinyl into soft, flexible PVC.
Fillers are inorganic or organic powders and fibres added in high volume proportions to improve mechanical strength, dimensional stability, and heat resistance while reducing overall material costs. Common fillers include wood flour, asbestos fibre, glass fibre, cloth fibre, mica powder, slate powder, calcium carbonate, talc, and clay.
Catalysts, also termed accelerators or hardeners, accelerate chemical polymerization rate and ensure complete cross-linking in thermosetting resins.
Initiators are reactive chemicals used to start the chain polymerization reaction by reacting with monomer active sites. They stabilize the chain ends during synthesis. Common initiators include Hydrogen Peroxide () and Benzoyl Peroxide.
Dyes and Pigments impart visual colour. They must disperse uniformly throughout the viscous polymer melt without thermally decomposing at processing temperatures. Typical inorganic colorants include Titanium Dioxide () for opacity and whiteness, Carbon Black for black pigmentation and UV stabilization, and various organic dyes.
Blowing Agents are physical or chemical additives used to create a cellular porous structure (plastic foam). Physical blowing agents involve injecting an inert gas such as nitrogen or argon under pressure into the polymer melt prior to mould entry, which expands to produce a porous interior. Examples include disposable foam drinking cups and Expanded Polystyrene (EPS).
Modifiers are chemical additives incorporated to enhance processing behavior or specific mechanical properties of the base polymer resin.
Reinforcements consist of high-strength structural fibres such as glass fibres or carbon fibres added to increase tensile strength and flexural stiffness.
Antioxidants and UV stabilizers protect the polymer matrix from degradation caused by ultraviolet light radiation and thermal oxidation, while maintaining melt-flow characteristics during moulding operations.
Primary Plastic Processing Techniques
Compression moulding is a forging-type hot-forming process carried out inside a heated metal die cavity. It is the primary production method for processing thermosetting plastics, including phenolic resins (Bakelite), melamine-formaldehyde, urea-formaldehyde, epoxy systems, and thermosetting polyester compounds.
The process sequence involves placing a pre-measured quantity of polymer powder or a pre-compressed tablet called the charge into the open, heated female mould cavity. The mould halves close, and heat and hydraulic pressure are applied using a downward-moving upper die plunger designated as the force, plug, or core. The applied heat and pressure cause the charge to melt, flow, and fill the mould geometry. While held under pressure within the closed heated cavity, a chemical curing reaction occurs, cross-linking the polymer chains to permanently harden the part. Heating is maintained through the cavity walls via internal steam channels or electrical resistance elements.
Compression moulds are designed as positive types, semi-positive types, or flash types. In flash-type compression moulds, a small excess of material escapes along the moving die perimeter over a narrow cut-off land area, forming a thin web of flash or fin that is subsequently trimmed off. Flash moulds provide close dimensional tolerances and are economical to manufacture.
Transfer moulding is an advanced variation of compression moulding designed for producing complex thermosetting components, parts with delicate embedded metal inserts, or thin-walled sections. The thermosetting charge (typically a preheated preform) is loaded into a separate heated transfer pot (chamber). Once the polymer softens into a fluid state, a hydraulic plunger forces the melted resin through a sprue channel into the closed mould cavity. The material undergoes its final cross-linking cure within the heated mould, after which ejector pins eject the hardened component. Transfer moulding operates at pressures to higher than standard compression moulding, yielding higher structural density, sharper detail, and superior mechanical strength around metal pins and inserts.
Injection moulding is the primary manufacturing process for thermoplastics, though modified systems can process thermosetting compounds. Softened thermoplastic resin is injected under high pressure into a closed, temperature-controlled split die.
The operation functions similarly to hot-chamber metal die casting. Solid polymer pellets or powders are fed from a hopper into a heated barrel containing a rotating reciprocating screw. As the screw rotates, it conveys the resin forward past heater bands that melt the plastic. Melted plastic collects in front of the nozzle end, taking 10 seconds to 5 or 6 minutes per shot depending on material volume. The reciprocating screw is then driven forward hydraulically like a ram, injecting the shot of molten polymer under high pressure into the closed split mould cavity.
The ram maintains holding pressure for several seconds to compensate for thermal shrinkage as the component solidifies against the cooled mould walls. The screw then retracts, the split mould opens, and mechanical knock-out pins eject the finished part. Sprues and runner systems are subsequently trimmed off in a secondary press operation. Fully automated injection moulding machines achieve production rates of 300 to 400 shots per hour.
Injection moulding machine configurations include the reciprocating screw type (the standard design), the conventional single-stage plunger type, and the two-stage plunger or screw-plasticizer type.
Jet moulding is a modified injection moulding variation where heated plastic resin is forced at high velocity through a restricted nozzle jet orifice directly into the mould cavity.
Reaction Injection Moulding (RIM) is a process in which two low-viscosity liquid monomers or reactive resins are metered and mixed together in a high-pressure mixhead immediately prior to cavity injection. A chemical polymerization reaction occurs at low temperatures inside the mould cavity, synthesizing the solid polymer in situ. Unlike standard injection moulding, RIM does not require thermal pre-melting of polymer pellets prior to injection. Fibrous reinforcement can be introduced into the liquid resin streams.
Extrusion represents the continuous processing of plastic material forced through a shaped die orifice to manufacture uniform cross-sectional profiles such as pipes, tubes, solid bars, sheets, films, wire insulation coatings, door sections, and synthetic filaments. Extrusion processes thermoplastics including vinyl polymers, cellulose derivatives, nylon, polyethylene, and polypropylene.
An extrusion machine consists of a supply hopper, a heated cylindrical barrel, an internal rotating screw feeding mechanism, a high-pressure nozzle, and a shaped die assembly. Granulated resin fed from the hopper into the heated barrel is conveyed forward by the rotating screw. The combination of external heating and internal friction converts the plastic into a melt that is forced continuously through the heated die orifice. Upon exiting the die, the continuous profile passes through a water cooling bath or air cooling system while supported on a conveyor belt, after which it is cut to length or wound onto spools. Unlike batch-type injection moulding, extrusion is a continuous production process where the die geometry dictates the cross-sectional profile.
Calendering is a continuous process used to produce thin plastic films and heavy sheet goods, such as PVC sheeting, vinyl floor tiles, shower curtains, artificial leather, table covers, and packaging films. A compounded plastic formulation consisting of base resin, functional fillers, plasticizers, and colour pigments is fed into a series of heated counter-rotating precision rollers. The rollers squeeze the plastic compound into a continuous sheet of controlled thickness. Sheet thickness is regulated by adjusting the gap space and relative rotation speeds of the finishing rolls. The hot plastic sheet is then chilled by passing over water-cooled rolls before being wound onto take-up spools.
Thermoforming, or vacuum forming, is the process of heating a pre-extruded thermoplastic sheet (typically ranging in thickness from to ) until it reaches a soft, pliable state, and then forming it over a male or female mould cavity using vacuum suction, compressed air, or mechanical pressure. The plastic sheet is clamped into a structural frame and heated with electric radiant heaters until it sags. A vacuum applied through small evacuation holes in the mould draws the pliable sheet tightly against the contoured cavity surface. After cooling, the frame releases the formed component, which is trimmed in a punch press. Thermoforming is used to manufacture restaurant jelly containers, luggage shells, refrigerator inner door liners, lighting diffusers, protective helmets, and thin packaging trays.
Specialized Moulding, Casting, and Fabrication Techniques
Casting involves pouring liquid resin mixed with a curing catalyst into an unpressurized open or closed mould, where it solidifies without applied pressure. Casting is utilized for short production runs or very large components where the high tooling cost of metal injection or compression dies is not economically justified.
Open Mould Casting involves pouring a liquid resin compound containing hardeners directly into an open cavity to solidify.
Centrifugal Casting utilizes rotating moulds to distribute liquid resins, such as polyesters, against outer mould walls using centrifugal force to form hollow cylindrical shapes.
Shell Moulding uses an unpressurized heated metallic mould filled with dry powdered resin. Heat from the mould wall fuses a thin layer of plastic powder to a controlled wall thickness. Excess unfused powder inside the core is dumped out, leaving a hollow shell component.
Rotational Moulding is a variant of shell moulding where a split mould containing powdered resin is rotated simultaneously along two perpendicular axes inside a heating oven, causing the melting polymer to coat all internal surfaces evenly. Rotational moulding produces large, seamless hollow products such as heavy-duty water storage tanks.
Slush Moulding uses a liquid thermoplastic slurry or suspension (slush) poured into a preheated hollow mould. The thermal energy fuses a viscous layer of plastic onto the interior mould walls. Excess liquid slurry is poured out, and the mould is heated to complete fusion. Both shell and slush moulding produce thin-walled hollow goods such as flexible toys and seamless gloves.
Hand Lay-Up is a manual fabrication technique used to manufacture large Fibre-Reinforced Plastic (FRP) structures such as boat hulls, chemical storage tanks, and architectural panels. A male or female open mould is coated with a release agent and a surface gel coat. Structural reinforcement mats (such as glass fibre fabric) are placed manually over the mould geometry, and liquid thermosetting resin (such as epoxy or unsaturated polyester) is applied over the glass mats using hand rollers or brushes to thoroughly saturate the fibres and remove trapped air pockets. The matrix is allowed to cure at room temperature or under low heat.
Laminating consists of bonding multiple sheets or layers of substrate materials—such as wood veneers, asbestos paper, woven cotton fabric, or glass cloth—impregnated with thermosetting resins under high temperature and pressure. Depending on processing conditions, laminates are categorized as high-pressure or low-pressure laminates. High-pressure decorative and industrial laminates are marketed under trade names such as Formica, Micarta, and Limicoid. Glass fabric represents the primary reinforcing medium.
Foamed Plastics are cellular polymers containing discrete or interconnected gas voids created by blowing agents. Closed-cell (discrete) porosity seals gas bubbles within isolated cell walls, yielding high buoyancy, high elasticity, and thermal, acoustic, and electrical insulation. Open-cell (interconnected) porosity features open, continuous gas channels, rendering the foam absorbent and permeable to fluids.
Technical Definitions and Trade Name Glossary
Plastic: A polymeric material capable of being shaped or modelled into articles via moulding, casting, or extrusion when subjected to heat and pressure.
Monomer: The basic low-molecular-weight structural unit of a polymer, consisting of carbon atoms bonded with lateral elements.
Polymer: A macromolecule formed by joining many recurring monomer units end-to-end.
Polymerization: The chemical reaction linking monomers together to form long chain polymers.
Thermoplastic: A linear or branched polymer that softens upon heating and hardens upon cooling in a reversible physical process.
Thermosetting Plastic: A cross-linked polymer featuring a three-dimensional network structure that cures permanently and cannot be resoftened upon reheating.
Elastomer: A loosely cross-linked polymer capable of undergoing high elastic deformation ( to ) and returning to its original dimensions upon load removal.
Vulcanization: The chemical process of cross-linking rubber molecules using elemental sulphur at elevated temperatures.
Charge: The pre-measured quantity of polymer powder, granules, or preformed tablets placed into a compression mould cavity.
Force / Plug / Core: The moving upper die section of a compression moulding press that applies pressure to the charge.
Flash: Excess polymer material squeezed out between closing mould mating surfaces along the land perimeter, removed during final trimming.
Sprue: The primary channel or orifice through which molten plastic enters a closed moulding die from the injection nozzle or transfer pot.
Shot: The total volumetric quantity of plastic material injected into a moulding cavity during one complete machine processing cycle.
Parison: A hollow, extruded thermoplastic tube sealed at one end, clamped inside a blow moulding die to be inflated with compressed air.
Extrusion: The continuous conversion of solid plastic into a profile by forcing it through a shaped die orifice.
Calendering: The production of continuous plastic sheet or film by passing a compounded melt through a series of heated counter-rotating rollers.
Thermoforming: The process of forming a heated, pliable thermoplastic sheet over a mould contour using vacuum, air pressure, or mechanical forces.
Laminate: A composite material formed by bonding impregnated substrate layers together under heat and pressure.
Foaming Agent: A chemical or physical additive incorporated into polymers to produce gas bubbles resulting in a cellular foam structure.
Plasticizer: An internal molecular lubricating additive incorporated into rigid resins to increase flexibility, flowability, and impact toughness.
Glass Transition Temperature (): The temperature at which an amorphous polymer transitions from a rigid, glass-like state to a flexible, rubbery state.
Curing: The chemical cross-linking process that permanently hardens a thermosetting resin system.
The commercial trade name mappings for key engineering plastics are:
Cellulose Acetate (CA) sheet is commercially known as Cellophane.
Cellulose Nitrate (CN) plasticized with camphor is commercially known as Celluloid.
Polyamide (PA / Nylon) resins are commercially designated under trade names Zytel, Nylatron, Nylafil, Plaskon, Rilsan, and Ultramid.
Polycarbonate (PC) is commercially designated under trade names Lexan, Merlon, Polycarbafil, and referred to as Bulletproof Glass.
Polymethyl Methacrylate (PMMA / Acrylic) is commercially designated under trade names Perspex, Plexiglas, Lucite, and Acrylite.
Polyoxymethylene (POM / Polyacetal) is commercially designated under trade names Delrin and Celcon.
Polyvinyl Chloride (PVC) compounds are broadly designated as Vinyl.
Polytetrafluoroethylene (PTFE) is commercially known as Teflon.
High-pressure laminates are commercially designated under trade names Formica, Micarta, and Limicoid.