Processing of polymers, ceramics and composites

Shaping routes for thermoplastics, thermosets, ceramics, glass and fibre composites, with rule-of-mixtures, clamp-force, mould-shrinkage and extruder-output calculations.

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Why it matters

Most products now mix metals with plastics, ceramics and fibre composites — bumpers, pipes, spark-plug insulators, cutting inserts, wind-turbine blades, aircraft panels. Each class needs its own shaping route, and the choice of route fixes cost, cycle time, tolerances and the defects you must design around (shrinkage, warpage, porosity, fibre misalignment).

Key ideas

Polymers.

  • Thermoplastics (PE, PP, PVC, PS, nylon, PC) soften on heating and harden on cooling reversibly, so they are melted, shaped and cooled; scrap can be reground. Thermosets (epoxy, phenolic, polyester, urea-formaldehyde) cross-link irreversibly during curing, so they are shaped first and cured in the mould by heat or a chemical hardener. Elastomers are lightly cross-linked and are vulcanised.
  • Polymer melts are very viscous and shear-thinning, so they are processed at high pressure (tens to over a hundred MPa in injection moulding) and moulds need cooling time. Thermoplastics shrink on cooling (roughly 0.5–3 % linear depending on the polymer; take the value from your data sheet), so mould cavities are made oversize.
  • Extrusion: a rotating screw in a heated barrel (feed, compression and metering zones) conveys, melts and pressurises the polymer and pushes it through a die — pipes, profiles, sheet, film, wire coating. Output is the drag flow of the screw minus the back-pressure flow from the die.
  • Injection moulding: a reciprocating screw plasticises a shot, injects it into a closed, cooled mould, holds pressure to pack out shrinkage, then the part cools and is ejected. Highest-volume process for complex discrete parts. Clamp force must resist the cavity pressure acting on the projected area of all cavities and runners. Defects: short shots, flash, sink marks, weld lines, warpage.
  • Blow moulding (bottles, tanks: an extruded or injected parison is inflated in a mould), thermoforming (heated sheet drawn onto a mould by vacuum or pressure), rotational moulding (large hollow parts from powder), compression and transfer moulding (mainly thermosets), calendering (sheet between rolls).

Ceramics. Ceramics are brittle and have very high melting points, so they are not cast or forged as metals are; they are shaped from powder or slurry and then fired.

  • Traditional (clay-based): clay + water forms a plastic mass shaped by slip casting (slurry poured into a porous plaster mould that draws water out), jiggering, pressing or extrusion; then drying (large shrinkage, must be slow to avoid cracks) and firing, where a glassy phase forms (vitrification) and bonds the grains.
  • Advanced ceramics (alumina, zirconia, silicon nitride, carbides): fine powders are dry pressed, isostatically pressed, injection moulded or tape cast, then sintered at a high fraction of the absolute melting temperature, often with hot pressing or HIP to remove porosity.
  • Glass is melted and shaped hot: pressing, blowing, drawing, and the float process for flat glass; annealing removes residual stresses, tempering puts the surface into compression.
  • Shrinkage on drying and firing is large and porosity controls strength, so density and porosity checks are routine.

Composites. A reinforcing phase (glass, carbon or aramid fibres; ceramic particles) in a matrix (polymer, metal or ceramic). Fibres carry load; the matrix holds them in place, transfers load by shear and protects them. Properties depend on fibre volume fraction, length and orientation.

  • Open-mould: hand lay-up and spray-up (boats, tanks), prepreg lay-up with autoclave cure (aerospace).
  • Closed-mould: compression moulding of SMC/BMC, resin transfer moulding (RTM), injection moulding of short-fibre compounds.
  • Continuous: filament winding (pressure vessels, pipes) and pultrusion (constant-section profiles — fibres pulled through a resin bath and a heated die).
  • Metal-matrix composites are made by stir casting, squeeze casting or P/M; ceramic-matrix composites by infiltration.

Formulas

E_L = E_f·V_f + E_m·V_m (longitudinal, iso-strain) 1 / E_T = V_f / E_f + V_m / E_m (transverse, iso-stress) ρ_c = ρ_f·V_f + ρ_m·V_m E = Young's modulus (Pa), V = volume fraction (V_f + V_m = 1, voids neglected), ρ = density (kg/m³), subscripts f = fibre, m = matrix. Continuous, aligned, perfectly bonded fibres.

F_f / F_c = E_f·V_f / (E_f·V_f + E_m·V_m) — fraction of a longitudinal load carried by the fibres.

F_clamp = p_c · A_p p_c = cavity pressure (Pa), A_p = total projected area of cavities and runners on the parting plane (m²).

D_c = D_p / (1 − S) D_c = mould cavity dimension, D_p = required part dimension, S = linear shrinkage (fraction) of the polymer.

Q_d = 0.5 · π² · D² · N · h · sin A · cos A Drag flow of a single-screw extruder (m³/s): D = screw diameter (m), N = speed (rev/s), h = channel depth (m), A = helix angle. The actual output is lower by the pressure (back) flow from the die.

P = 1 − ρ / ρ_th — porosity of a fired ceramic, as for P/M parts.

Worked examples

Example 1 — GFRP lamina (standard). Continuous E-glass fibres (E_f = 72 GPa, ρ_f = 2.54 g/cm³) in epoxy (E_m = 3.5 GPa, ρ_m = 1.20 g/cm³), V_f = 0.60.

  1. E_L = E_f·V_f + E_m·V_m = 72 × 0.6 + 3.5 × 0.4 = 43.2 + 1.4 = 44.6 GPa.
  2. 1/E_T = V_f/E_f + V_m/E_m = 0.6/72 + 0.4/3.5 = 0.00833 + 0.11429 = 0.12262 GPa⁻¹, so E_T = 8.16 GPa — five times lower than E_L, which is why fibre orientation matters.
  3. ρ_c = 2.54 × 0.6 + 1.20 × 0.4 = 2.00 g/cm³.
  4. Fibre load share = 43.2/44.6 = 0.969 — the fibres carry about 97 % of a longitudinal load.

Example 2 — injection mould (GATE level). A two-cavity mould makes a part whose projected area is 120 mm × 80 mm (runners negligible). Average cavity pressure is 40 MPa. The part must be 100.0 mm long; the polymer's linear shrinkage is 2 %.

  1. A_p = 2 × 120 × 80 = 19 200 mm².
  2. F_clamp = p_c · A_p = 40 N/mm² × 19 200 mm² = 768 000 N = 768 kN (choose a machine of about 80 t or more with margin).
  3. D_c = D_p / (1 − S) = 100/(1 − 0.02) = 102.04 mm cavity length.

Example 3 — extruder output. D = 50 mm, N = 60 rpm = 1 rev/s, h = 3 mm, A = 17.7° (pitch ≈ D).

  1. sin A · cos A = 0.3040 × 0.9527 = 0.2896.
  2. Q_d = 0.5 × π² × (0.05)² × 1 × 0.003 × 0.2896 = 1.07 × 10⁻⁵ m³/s = 10.7 cm³/s (upper limit; die back-pressure lowers it).

Common mistakes

  • Using the iso-strain (parallel) rule for transverse modulus; transverse stiffness follows the inverse rule and is matrix-dominated.
  • Using mass fractions instead of volume fractions in the rule of mixtures.
  • Basing clamp force on part surface area or volume instead of projected area of all cavities and runners.
  • Making the cavity exactly the part size, forgetting shrinkage.
  • Comparing sintering or firing temperature with melting point in °C; homologous temperature ratios use kelvin.
  • Thinking thermosets can be reground and remoulded like thermoplastics.

For GATE PI

Expect MCQs matching products to processes (bottle–blow moulding, pipe–extrusion, pressure vessel–filament winding, constant profile–pultrusion, sanitaryware–slip casting), thermoplastic vs thermoset behaviour, and roles of fibre and matrix. Numericals use the rule of mixtures (modulus, density, load share), clamp force from projected area, mould shrinkage allowance, extruder drag flow and ceramic porosity.

Quick check

  1. Which process makes a CFRP pressure vessel?
  2. V_f = 0.5, E_f = 230 GPa, E_m = 3 GPa. Longitudinal modulus?
  3. Why are moulds for plastic parts made oversize?
  4. Which process forms a hollow PET bottle?
  5. Why must ceramic green ware be dried slowly?

Answers: 1. Filament winding. 2. 0.5 × 230 + 0.5 × 3 = 116.5 GPa. 3. To allow for thermal shrinkage of the polymer on cooling. 4. (Stretch) blow moulding. 5. Fast drying shrinks the surface faster than the core and cracks or warps the part.

Interview questions

All Casting, Forming and Joining interview questions

Try answering each one aloud before you open it.

  1. 1.What is polymer processing and why is it important in industrial applications?Concept

    Polymer processing refers to the methods used to shape and form polymers into useful products. It is important because it allows for the mass production of plastic parts with specific properties, such as strength, flexibility, and resistance to chemicals. This is crucial in industries like automotive, packaging, and electronics, where polymers are used extensively.

  2. 2.Explain the difference between thermoplastics and thermosetting plastics.Concept

    Thermoplastics are polymers that become soft when heated and harden upon cooling, allowing them to be reshaped multiple times. Thermosetting plastics, on the other hand, undergo a chemical change when heated, forming a rigid structure that cannot be remolded. This makes thermosets ideal for high-heat applications, while thermoplastics are used where recyclability and reshaping are important.

  3. 3.What are the main methods of ceramic processing?Concept

    The main methods of ceramic processing include powder processing, slip casting, tape casting, and extrusion. Powder processing involves compacting ceramic powders into a desired shape and then sintering them. Slip casting uses a liquid clay mixture poured into molds. Tape casting creates thin ceramic sheets, and extrusion forces ceramic material through a die to create long shapes.

  4. 4.How are composite materials processed, and what are their advantages?Concept

    Composite materials are processed using methods like lay-up, filament winding, and pultrusion. These methods involve combining two or more materials to create a composite with superior properties. The advantages of composites include high strength-to-weight ratio, corrosion resistance, and the ability to tailor properties to specific applications, making them ideal for aerospace, automotive, and sports equipment.

  5. 5.Why is injection molding commonly used for polymer processing?Application

    Injection molding is commonly used because it allows for high-volume production of complex shapes with excellent surface finish and dimensional accuracy. It is efficient and cost-effective for producing large quantities of parts, making it ideal for industries like automotive and consumer goods.

  6. 6.What happens if the sintering temperature is too high during ceramic processing?Application

    If the sintering temperature is too high, it can lead to excessive grain growth, which may weaken the ceramic material. It can also cause deformation or melting of the ceramic, resulting in defects and reduced mechanical properties. Therefore, controlling the sintering temperature is crucial for achieving the desired material characteristics.

  7. 7.Why are composites preferred over metals in certain aerospace applications?Application

    Composites are preferred over metals in aerospace applications because they offer a high strength-to-weight ratio, which is crucial for fuel efficiency and performance. They also provide better corrosion resistance and can be engineered to have specific directional properties, enhancing the performance of aerospace components.

  8. 8.A moulded polypropylene part must measure 150 mm × 100 mm × 50 mm after cooling. The polymer's linear shrinkage is 2 %. What cavity dimensions should the mould have?Numerical

    Each cavity dimension is D_c = D_p/(1 − S) with S = 0.02: 150/0.98 = 153.1 mm, 100/0.98 = 102.0 mm and 50/0.98 = 51.0 mm. In practice shrinkage is not perfectly uniform — it differs with flow direction, wall thickness and packing pressure — so critical dimensions are often trimmed after trial shots.

  9. 9.A ceramic part has a porosity of 10%. If the theoretical density of the ceramic is 3.5 g/cm³, what is the actual density?Numerical

    The actual density can be calculated using the formula: Actual Density = Theoretical Density × (1 - Porosity). Here, Actual Density = 3.5 g/cm³ × (1 - 0.10) = 3.5 g/cm³ × 0.90 = 3.15 g/cm³.

  10. 10.Explain the role of additives in polymer processing.Concept

    Additives in polymer processing are used to enhance the properties of the base polymer. They can improve flexibility, strength, UV resistance, and flame retardancy, among other properties. Additives allow manufacturers to tailor polymers for specific applications, making them more versatile and functional.

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