ForMatter/Processes/formative/Injection Molding
proc_injection_molding

Injection Molding

formative · injection moulding, thermoplastic injection molding, plastic molding

Melted plastic shot at high pressure into a metal mold, cooled, ejected. The reason almost every plastic object in the room exists. Tooling is expensive — tens of thousands of dollars — but per-part cost drops to pennies once you're running.

High-pressure injection of molten thermoplastic into a closed steel or aluminum tool. Cycle times 10–60 seconds. Tools amortize over runs of 10,000 to millions. Part design constrained by draft angles, uniform wall thickness, ejector access, and gate location.

Scale & Tolerance

  • scale (mm)1 – 2000
  • tolerance (mm)0.1
  • skilladvanced — process tuning, mold design, polymer rheology all matter
  • min skilladvanced
  • whereprofessionalindustrial
  • costvery low per part at volume; capital cost very high (tooling)

Equipment

  • school_shoprare — desktop injection units (Galomb, Morgan, Babyplast) appear in some product-design programs
  • professionalArburg, Engel, Sumitomo Demag from 50 to 500 tons clamp
  • industrialHusky and Engel up to 6,500 tons for automotive and crate molding

Environmental

  • energy_usemoderate to high
  • waste_streamsprue and runners — regrindable in-house; flash and rejects
  • consumablespurge compound, mold release

Cost over volume

1101001k10k100k1M1101001000units (log scale)total cost (relative, log scale)

Numbers are relative ratios, not dollars. The crossover point matters more than the magnitude. Anchored to injection molding + ABS = 1.0.

Trade-offs

constraints · what is lost · what is gained
ABS (Acrylonitrile Butadiene Styrene)
  • constraints
    • min wall thickness ~0.8 mm without short shots
    • draft angle ≥1° on all vertical faces; ≥3° on textured faces
    • no undercuts without side-action core (doubles tooling cost)
    • uniform wall thickness — thick-thin transitions sink and warp
    • max unsupported span ~150 mm before sag in cooling
  • what is lost
    • surface variation across parts becomes near-zero — every unit reads identical
    • tool-mark register absent (no chasing, no file evidence, no annealing color)
    • parting line and gate vestige are visible artifacts of the process, not design choices
  • what is gained
    • dimensional repeatability ±0.1 mm across runs of 10,000+
    • SPI mold finish 'free' from the cavity — no post-processing required for surface
    • internal ribs and bosses unreachable by hand or CNC become routine
    • snap-fit features moulded in without secondary operations
PLA (Polylactic Acid)
  • constraints
    • min wall thickness ~1.0 mm — PLA is more brittle than ABS at thin sections
    • lower melt-flow than ABS limits long thin-walled flow paths
    • draft angle ≥1° on vertical faces
  • what is lost
    • no opaque-color uniformity at high gloss; bio-derived feedstock varies batch to batch
    • post-mold thermal stability is poor (Tg ~55 °C) — limits dishwasher / hot-fill applications
  • what is gained
    • biopolymer feedstock — Cradle-to-Cradle and biodegradability story available
    • lower processing temperatures than ABS reduce energy per part
    • approved for direct-food-contact in many jurisdictions
PETG (Glycol-Modified PET)
  • constraints
    • min wall thickness ~1.0 mm
    • draft angle ≥1° vertical, ≥3° textured
    • needs slow cooling on thick sections to avoid stress whitening
  • what is lost
    • loses the optical clarity of cast PMMA — slight haze on thicker sections
  • what is gained
    • near-PMMA optical clarity at injection-grade economics
    • tougher than PMMA — drop-resistant where acrylic would shatter
    • FDA-approved for food contact

Plain language. Neutral framing — perfection is contextual, defined by use. Cf. Winchester, The Perfectionists (HarperCollins, 2018).

Roland Barthes (dead — channeled)

To watch the press close on the molten polymer is to watch transformation itself made plain. Raw matter on one side, finished form on the other, and between them a brief and silent violence — heat, pressure, geometry. The machine teaches what Aristotle taught: that matter desires form, and that form, given the chance, will accept any matter offered.

Channeled within the philosophy of Roland Barthes, *Mythologies* (1957), 'Plastic.'
Penny Sparke (living — quote)

While metal had developed as a replacement for wood in the move towards large-scale production, plastics were developed as an even cheaper alternative. As such, they gradually replaced metal, particularly as the material for the body-shells of technological products. As early as 1929 Raymond Loewy had used bakelite as the material for his restyled Gestetner duplicating machine, and quite quickly pressed metal was replaced by this new material, which was cheaper and easier to manufacture in bulk.

Sparke, *Design in Context* (Bloomsbury, 1991 [first published Quarto, 1987]), Chapter 5, 'Industry, Technology and Design,' on the canonical 1929 Loewy / Gestetner pressed-metal-to-bakelite shift that opened the postwar product-design route through compression-molded then injection-molded thermoplastics. Penny Sparke verified living 2026-04-29.

Second life

reversibilitylow — once a thermoplastic part is moulded, the part itself is fixed. The polymer can be re-melted (regrind) and re-moulded into new parts.
output recyclabilityyes
waste streams
  • sprue and runners — regrindable in-house and re-pelletized
  • flash and rejects — same recovery path
  • purge compound between color / material changes — disposed
repair compatible withproc_adhesive_bonding

Editorial pass 2026-04-28; Lefteri *Making It* injection-molding chapter.

Citations

Further reading