Moulding, Forming and Coating of Polymers

This chapter opens the second half of Section XE5.7 of XE5 Polymer Science and Engineering, one of the nine optional sections of the Engineering Sciences (XE) paper, any two of which a candidate chooses in the exam. The syllabus line it answers is: “Spin coating; … Film casting; Compression molding; Transfer molding; Injection molding; Blow molding; Reaction injection molding; … Calendaring; Rotational molding; Thermoforming; Powder coating.” It teaches how each process shapes a polymer, the machine zones and cycle, and the link between process parameters and defects. The numericals are clamp force from projected area and pressure, shot weight, blown-wall thickness from the blow-up ratio, thermoformed wall thinning and the spin-coating thickness law.

1. Compression and transfer moulding (thermosetting resins)

In compression moulding a weighed charge (powder, preform or sheet) is placed in the open cavity of a heated two-part mould, the press closes, and heat and pressure make the charge flow, fill the cavity and cure. The part is ejected hot, because a thermoset needs no cooling to become rigid; its crosslinking, not cooling, sets the cycle. Excess charge squeezes out at the parting line as flash. The process suits phenolic and melamine articles, rubber parts and large flat composite panels, with low flow-induced orientation and low tooling cost.

In transfer moulding the charge is first softened in a heated pot and then forced by a plunger through a sprue and runners into a closed cavity. The resin is already hot and nearly fluid when it enters, so cure time is shorter, flash is thin, dimensions are more accurate, and delicate inserts (metal pins, electronic leads) can be encapsulated, as in epoxy encapsulation of chips. The cost is a cured slug (the cull) and runner waste that cannot be remelted.

Compression versus transfer moulding
FeatureCompressionTransfer
Charge goesDirectly in open cavityVia pot, sprue and runners into closed cavity
FlashHeavy, needs trimmingThin
Inserts, thin sectionsPoorGood
WasteSmallCull and runners, not remeltable

2. Injection moulding: machine, cycle, gate and clamp force

The injection unit is a reciprocating screw in a heated barrel with three zones: feed (deep channel, conveys pellets), compression (channel shallows, pellets melt by shear and barrel heat) and metering (shallow channel, homogenises the melt). The cycle is: close and clamp the mould; the screw advances as a ram and injects the melt (injection stage); pressure is held while the polymer shrinks (packing/holding); once the gate, the small entry to the cavity, freezes, packing no longer feeds the part; the screw rotates and retreats to accumulate the next shot (plasticising) while the part cools; the mould opens and ejector pins push the part out. Cooling is usually the longest step, and it scales roughly with the square of the thickest wall.

Shot size (shot weight) is the mass injected per cycle: (volume of all cavities + runner system) × density. It must fit the machine's capacity, usually 20% to 80% of it. Clamp force must exceed the force that cavity pressure exerts to push the mould halves apart: F = p × A_proj, where A_proj is the area of cavities and runners projected on the parting plane (not the surface area) and p is the mean cavity pressure. Only the projected area matters because pressure acts normal to every surface and the opening components of the sidewall forces cancel.

⚠️ Worked example: clamp force and shot weight
A two-cavity mould has a projected area of 90 cm² per cavity plus 20 cm² for runners; mean cavity pressure is 35 MPa. A = 200 cm² = 0.0200 m². F = 35 × 10⁶ × 0.0200 = 7.0 × 10⁵ N = 700 kN (about 71 tonne-force). Each part is 45 cm³ and the runners 12 cm³; with density 0.90 g/cm³ the shot weight is (2 × 45 + 12) × 0.90 = 91.8 g. The usual slip is using the surface area, or forgetting that runners add projected area and shot weight.

3. Blow moulding, reaction injection and rotational moulding

Extrusion blow moulding: an extruder produces a hollow tube, the parison, which a split mould pinches at the bottom; air inflates it against the cooled mould wall. It makes HDPE bottles and drums; the pinch-off leaves flash and a weld seam, and wall thickness varies. Injection blow moulding: a preform with the finished neck is injection moulded on a core rod, then transferred and inflated: no flash, precise neck threads, but small, costly tooling. Injection stretch blow moulding (PET bottles): a preform at a temperature just above Tg is stretched axially by a rod and blown radially, giving biaxial orientation, which raises strength, clarity and barrier properties. The blow-up ratio is BUR = final diameter / parison (or preform) diameter.

Reaction injection moulding (RIM): two low-viscosity reactive liquids (for polyurethane, a polyol and an isocyanate) are metered, mixed by high-pressure impingement in a mixing head and flow into a closed mould at low pressure, where they polymerise. Because the viscosity is low, clamp force and tooling cost are small and large parts (car bumpers, fascias) are practical; the cycle is limited by cure. Rotational moulding: a measured charge of powder is placed in a hollow mould that is heated in an oven while rotating about two perpendicular axes, so the powder melts and coats the wall uniformly; the mould is then cooled, still rotating. No pressure is applied, so parts are nearly stress-free with uniform wall thickness and no weld lines (tanks, kayaks, bins), but cycles are long and mostly polyethylene is used.

🧠 Thin-wall estimate for a blown bottle
For a thin cylindrical wall whose length does not change, the material cross-section π·D·t is conserved, so t_bottle = t_parison × D_parison / D_bottle = t_parison / BUR. A 30 mm parison of 3 mm wall blown to a 90 mm bottle (BUR = 3) gives 1.0 mm. Corners and the base, which stretch more, are thinner.

4. Thermoforming, calendering, film casting, powder coating and spin coating

Thermoforming: a thermoplastic sheet is clamped, heated until soft (above Tg for amorphous polymers), then pulled onto a one-sided mould by vacuum, pressure or a plug assist (drape forming lays it over a male mould), cooled, and trimmed. It makes trays, cups and refrigerator liners cheaply; tooling is one-sided and low-cost, but trimmed scrap is large and the wall thins where stretching is greatest (corners, deep draws). Since volume is conserved, average thickness after forming ≈ t₀ / (areal stretch ratio). A 2 mm sheet stretched to four times its area averages about 0.5 mm.

Calendering squeezes a heated, plasticised melt (mainly PVC) through the narrow gaps of a train of counter-rotating heated rolls to make film and sheet of accurate thickness; roll-gap, roll temperature and speed ratio set the gauge and surface, and the rolls can also coat fabric or emboss. Film casting: the melt leaves a flat slit die and is quenched on a polished chill roll; the quick cooling gives small crystals, high clarity and gloss with little orientation (cast PP, PE film). A solution version spreads a polymer solution and evaporates the solvent. Powder coating: finely ground thermoset powder (epoxy, polyester) is charged by a corona gun, sticks electrostatically to an earthed metal part, and is cured in an oven into a continuous film; no solvent is used and overspray is recovered.

Spin coating: a drop of polymer solution is placed at the centre of a substrate, which is spun at angular speed ω. Centrifugal flow throws off the excess and solvent evaporates, leaving a uniform film. Thickness falls as the inverse square root of spin speed, h ∝ ω^(−1/2); it rises with solution concentration and viscosity. It gives films from tens of nanometres to micrometres on flat wafers (photoresists, thin-film devices) but wastes most of the solution and is limited to small flat substrates.

🧠 Spin-coating scaling
Doubling the speed does not halve the thickness: it multiplies it by 1/√2 = 0.707. To halve a film made at 1000 rpm the speed must be quadrupled to 4000 rpm.

5. Process parameter versus defect

Most moulding defects are one of two things: too little material or pressure (the cavity is not filled or compensated), or too much (the mould is forced open or the polymer degrades). Work from the symptom to the parameter that moved.

Common defects and their causes (injection moulding unless stated)
DefectAppearanceTypical causes and cures
Short shotCavity not filledLow melt temperature or injection pressure, small shot, frozen gate; raise them
FlashThin fin at parting lineClamp force below p × A, worn mould, excess pressure or melt temperature
Sink mark, voidDimple or bubble over a thick sectionPacking pressure or time too low, gate froze early, thick rib; pack longer
Weld lineSeam where flow fronts meetCold fronts; raise melt and mould temperature, move gate
WarpageDistorted partUneven cooling or orientation, differential shrinkage
Burn mark, degradationDark streaks at flow endTrapped air compressing, excessive melt temperature or residence time
Thin spots in thermoform or blow partCorners thinHigh stretch ratio, uneven sheet or parison temperature; plug assist or parison programming
⚠️ Packing and clamp force pull in different directions
Raising packing pressure removes sink marks but raises cavity pressure and so the clamp force needed, and may cause flash. A higher mould temperature improves surface finish and weld lines but lengthens cooling. A tick for one defect often means a cross for another.

6. Choosing a process for a product

Exam questions often ask which process makes a given article. Reason from three things: the polymer class (thermoset, thermoplastic, liquid reactants or powder), the shape (hollow, flat, sheet, or solid with fine detail) and the volume of production, which decides whether expensive steel tooling pays off.

Article and the usual process
ArticleProcessReason
Phenolic switch housingCompression mouldingThermoset, simple shape, hot ejection
Chip encapsulationTransfer mouldingFragile leads need low-pressure fluid charge
Bottle cap, gear, syringe barrelInjection mouldingFine detail, millions of parts
Milk bottle, drumExtrusion blow mouldingHollow, low cost, flash acceptable
Carbonated drink bottleInjection stretch blow mouldingBiaxial orientation, gas barrier
Water storage tankRotational mouldingLarge hollow, stress-free, low tooling cost
Food tray, cupThermoformingThin sheet, one-sided cheap tooling
Vinyl flooring sheetCalenderingWide PVC sheet of controlled gauge
🎯 Why the tooling is so different
Injection and compression moulds are machined steel to withstand tens of MPa and cost a great deal; thermoforming and rotational moulds see little or no pressure and can be aluminium or even cast. The process choice is therefore partly an economic choice of run length.

Key takeaways

  • Thermosets are shaped by compression or transfer moulding and ejected hot, since cure, not cooling, sets the cycle; transfer moulding gives thin flash and allows inserts.
  • Injection moulding: feed, compression and metering zones; cycle of injection, packing, cooling, plasticising, ejection; clamp force F = p × A_proj with runners included.
  • Blow moulding: extrusion (flash, HDPE), injection (precise neck) and stretch (biaxial orientation, PET); wall thickness ≈ parison thickness / BUR for a thin cylinder.
  • RIM mixes two reactive liquids by impingement at low mould pressure; rotational moulding uses powder, heat and biaxial rotation without pressure.
  • Thermoforming thins walls as 1/areal stretch ratio; calendering suits PVC; film casting on a chill roll gives clarity; powder coating is solvent-free; spin-coating thickness ∝ ω^(−1/2).
  • Diagnose defects from the symptom: short shot and flash are opposite faces of pressure, sink marks need packing, weld lines need heat.

Practice questions (16)

Attempt each one before opening the answer. Every explanation names the tempting wrong option as well as the right one, because that is where marks are lost.

  1. Why can a thermosetting moulding be ejected from a compression mould while still hot, whereas a thermoplastic part is normally cooled first?

    1. Thermosets have such a high thermal conductivity that they cool to room temperature the moment the mould opens
    2. Rigidity comes from the crosslinked network formed by curing, so the part is stiff at the cure temperature
    3. Thermosets are always highly crystalline, so they set by crystallising quickly once at the mould temperature
    4. The mould is water-cooled briefly during ejection, so the heat is removed through the ejector pins
    Show answer

    Answer: B — Rigidity comes from the crosslinked network formed by curing, so the part is stiff at the cure temperature

    A thermoplastic stiffens only on cooling below Tg or Tm. A thermoset, once cured, is a permanent network that stays rigid when hot, so the cycle is set by cure time. The conductivity and crystallinity explanations are wrong: thermosets are amorphous networks of low conductivity.

  2. In injection moulding, the clamp force needed to keep the mould closed is estimated as the product of the mean cavity pressure and the

    1. area of cavities and runners projected on the parting plane
    2. total wetted surface area of all the cavity walls and the runner channels
    3. cross-sectional area of the barrel bore through which the screw moves
    4. area of the gate opening through which the melt enters the cavity
    Show answer

    Answer: A — area of cavities and runners projected on the parting plane

    Cavity pressure pushes the two mould halves apart only through the area projected on the parting plane, so F = p × A_proj, with runners included. The wetted surface area overstates the force because side-wall components cancel; the barrel and gate areas are unrelated to the opening force.

  3. A moulded part shows sink marks over a thick rib. The gate freezes after 3 s, holding time is set to 8 s and cooling time is 20 s. Which change most directly cures the sinks?

    1. Lengthen the holding time from 8 s to 15 s so more melt is pushed into the part
    2. Lower the mould temperature so that the skin freezes earlier and holds its shape
    3. Enlarge the gate or move it into the thick section so packing continues
    4. Raise the clamp force so the mould halves are held more tightly shut
    Show answer

    Answer: C — Enlarge the gate or move it into the thick section so packing continues

    Packing can feed the part only while the gate is open. Once the gate froze at 3 s, extra holding time (the tempting fix) achieves nothing, so the gate must be enlarged or relocated. A colder mould makes the thick section shrink away from the skin more, and clamp force does not feed material into the cavity.

  4. Part X is a flat plate with a projected area of 200 cm². Part Y, a deep cup of the same volume, has a projected area of 80 cm². Both are moulded at the same mean cavity pressure, runners ignored. Compared with Y, the clamp force needed for X is

    1. 0.4 times, since the cup has more surface area
    2. equal, because the volumes are equal
    3. 1.5 times, from the ratio of volumes to the two-thirds power
    4. 2.5 times, from the ratio of projected areas
    Show answer

    Answer: D — 2.5 times, from the ratio of projected areas

    F = p × A_proj, so at equal pressure the ratio is 200/80 = 2.5. Volume does not enter the clamp force and surface area is not the governing area. (A flat plate can need a larger press than a deeper part of the same volume.)

  5. PET bottle preforms are stretch blow moulded with the preform temperature held

    1. above the melting point of PET so that the polymer flows freely and fills the mould
    2. just above the glass transition temperature, so it stretches and keeps orientation
    3. at room temperature, since PET is ductile and can be cold drawn at any speed
    4. well below the glass transition temperature to avoid any wall thinning at all
    Show answer

    Answer: B — just above the glass transition temperature, so it stretches and keeps orientation

    Just above Tg the chains are mobile enough to stretch, and the induced biaxial orientation is retained on quick cooling, raising strength and barrier. Above Tm the melt would not hold orientation; below Tg the preform is glassy and would crack or whiten.

  6. Large polyurethane car bumpers are made by reaction injection moulding on presses of modest clamp force because

    1. the mixed liquids have very low viscosity, so cavity pressure is small
    2. a molten thermoplastic is injected at very high pressure into a thin-walled steel cavity
    3. the mould is rotated about two axes so no pressure is required
    4. the parts are cured by cooling, which needs only a small press
    Show answer

    Answer: A — the mixed liquids have very low viscosity, so cavity pressure is small

    RIM mixes reactive liquids of low viscosity that fill the mould at low pressure and polymerise there, so clamp force and tooling are light. Rotating the mould describes rotational moulding, and high-pressure melt injection describes conventional injection moulding.

  7. A rotationally moulded polyethylene tank differs from an injection moulded one in that it

    1. is produced in seconds under very high pressure from a pre-melted polymer
    2. always contains weld lines where the two moulds meet
    3. requires a stretching rod to produce biaxial orientation
    4. is formed from powder without applied pressure, giving near stress-free walls of fairly uniform thickness
    Show answer

    Answer: D — is formed from powder without applied pressure, giving near stress-free walls of fairly uniform thickness

    Rotational moulding melts powder against a hot rotating mould wall at essentially zero pressure, so cycles are long but walls are stress-free and even. The other statements describe injection moulding or stretch blow moulding.

  8. Which statement about powder coating is correct?

    1. The powder is dissolved in an organic solvent that evaporates in the oven
    2. The coating is applied to an electrically insulating, unearthed part
    3. Charged thermoset powder sticks electrostatically to an earthed part and is then oven cured, with no solvent
    4. The powder is a thermoplastic that remains uncured and washes off with water
    Show answer

    Answer: C — Charged thermoset powder sticks electrostatically to an earthed part and is then oven cured, with no solvent

    A corona gun charges the powder; it clings to an earthed metal part and the oven melts and cures it into a film, so no solvent is needed and overspray is recovered. An insulating, unearthed part would not attract the charged powder.

  9. A spin-coated film is 500 nm thick when spun at 2000 rpm. Taking h ∝ ω^(−1/2) for the same solution, which spin speed should give about 250 nm?

    1. 4000 rpm, since thickness is inversely proportional to speed
    2. 8000 rpm, since halving thickness needs four times the speed
    3. 2828 rpm, since speed scales with the square root of thickness
    4. 16000 rpm, since thickness varies with the cube root of speed
    Show answer

    Answer: B — 8000 rpm, since halving thickness needs four times the speed

    h₂/h₁ = (ω₁/ω₂)^(1/2) = 0.5 gives ω₂/ω₁ = 4, so ω₂ = 8000 rpm. Doubling speed (4000 rpm) only gives 500/√2 ≈ 354 nm, which is the usual trap from assuming inverse proportionality.

  10. Select ALL correct statements about the injection moulding cycle and machine.

    1. Cooling is usually the longest part of the cycle for thick parts
    2. The reciprocating screw melts and meters the next shot while the part is cooling
    3. Packing continues to compensate shrinkage even after the gate has frozen
    4. Runners add to both the shot weight and the projected area for clamp force
    Show answer

    Answer: A — Cooling is usually the longest part of the cycle for thick parts; B — The reciprocating screw melts and meters the next shot while the part is cooling; D — Runners add to both the shot weight and the projected area for clamp force

    Cooling time scales with the square of the thickest wall; plasticising overlaps with cooling; runners are part of the shot and of the projected area. Packing cannot feed the cavity once the gate has frozen, so that statement is false.

  11. Select ALL correct statements comparing transfer and compression moulding of thermosets.

    1. Transfer moulding allows delicate inserts to be encapsulated
    2. Transfer moulding leaves a cured cull and runners that cannot be remelted
    3. A thermoset compression mould must be cooled below Tg before the part can be ejected
    4. Compression moulding usually gives heavier flash than transfer moulding
    Show answer

    Answer: A — Transfer moulding allows delicate inserts to be encapsulated; B — Transfer moulding leaves a cured cull and runners that cannot be remelted; D — Compression moulding usually gives heavier flash than transfer moulding

    The charge in transfer moulding is hot and fluid, so inserts survive, flash is thin and a cull is left. The cooling statement is false because cured thermosets are ejected hot.

  12. Select ALL correct statements about blow moulding.

    1. Extrusion blow moulding leaves pinch-off flash at the base of the container
    2. Injection blow moulding gives an accurately moulded neck finish without flash
    3. Stretch blow moulding of PET is done above the melting point of PET
    4. For a thin wall of fixed length, a higher blow-up ratio gives a thinner wall
    Show answer

    Answer: A — Extrusion blow moulding leaves pinch-off flash at the base of the container; B — Injection blow moulding gives an accurately moulded neck finish without flash; D — For a thin wall of fixed length, a higher blow-up ratio gives a thinner wall

    Pinch-off flash, the injection-moulded neck and the t ∝ 1/BUR thinning are all correct. Stretch blowing is done just above Tg, far below the melting point of PET.

  13. Select ALL correct statements about forming and coating processes.

    1. Thermoformed articles thin most where the sheet is stretched most, such as corners
    2. Spin-coated film thickness falls as spin speed is increased
    3. Powder coating needs an organic solvent to carry the powder to the part
    4. Calendering is mainly used to make PVC film and sheet of accurate gauge
    Show answer

    Answer: A — Thermoformed articles thin most where the sheet is stretched most, such as corners; B — Spin-coated film thickness falls as spin speed is increased; D — Calendering is mainly used to make PVC film and sheet of accurate gauge

    Corner thinning, the ω^(−1/2) law and PVC calendering are standard facts. Powder coating is solvent-free: the powder is carried by air and held by electrostatic charge.

  14. A three-cavity injection mould has a projected area of 60 cm² per cavity and 25 cm² for the runner system. The mean cavity pressure is 40 MPa. Taking clamp force = pressure × total projected area, with no safety factor, the minimum clamp force is ____ kN (integer).

    Numerical answer — type the value.

    Show answer

    Answer: 820

    Total A = 3 × 60 + 25 = 205 cm² = 0.0205 m². F = 40 × 10⁶ Pa × 0.0205 m² = 8.2 × 10⁵ N = 820 kN. Leaving out the runner gives 720 kN, the usual slip.

  15. A spin-coated film is 450 nm thick at 1600 rpm. Assuming h ∝ ω^(−1/2) for the same solution, the thickness at 2500 rpm is ____ nm (integer).

    Numerical answer — type the value.

    Show answer

    Answer: 360

    h = 450 × (1600/2500)^(1/2) = 450 × 0.8 = 360 nm. Using a linear inverse law would give 288 nm.

  16. A flat circular sheet portion of 120 mm diameter and 3.0 mm thickness is thermoformed into an open cylindrical cup with a 60 mm diameter flat base and straight walls 60 mm high. Assuming volume is conserved and thickness is uniform over the formed area, the wall thickness is ____ mm (one decimal place).

    Numerical answer — type the value.

    Show answer

    Answer: 2.4

    Sheet area = π(60)² = 11 310 mm². Cup area = base π(30)² = 2 827 mm² + wall π × 60 × 60 = 11 310 mm², total 14 137 mm². Areal ratio = 14 137 / 11 310 = 1.25. t = 3.0 / 1.25 = 2.4 mm.