Injection Moulding Glossary: 130 Terms Every Buyer and Engineer Should Know

Injection moulding has its own vocabulary across tooling, materials, processing and quality. This glossary defines 130+ terms in plain language, grouped alphabetically. It is written for buyers and engineers who specify, quote or troubleshoot moulded parts — the same vocabulary Nordmould uses in its DFM reviews and quotes.

A

Term	Definition
A-side (cavity side)	The mould half forming the part's outer, usually cosmetic, surface.
Additive	Substance compounded into resin — colourant, UV stabiliser, flame retardant, lubricant — to modify properties.
Amorphous polymer	Resin with disordered molecular structure (ABS, PC, PMMA, PS); low shrinkage, transparent grades possible.
Annealing	Controlled post-mould heating to relieve internal stress and stabilise dimensions.
Anisotropic shrinkage	Direction-dependent shrinkage, typical of glass-filled grades: less along flow, more across it.

B

Term	Definition
B-side (core side)	The mould half forming the part's inner features; usually houses the ejection system.
Back pressure	Resistance applied to the screw during recovery, improving melt mixing and colour dispersion.
Barrel	The heated cylinder in which the screw melts and conveys resin toward the mould.
Black specks	Defect: degraded resin particles appearing as dark spots, usually from barrel or hot-runner dead spots.
Blister	Defect: raised surface bubble from trapped gas or volatiles in the melt.
Boss	Cylindrical feature receiving screws or inserts; sized ~2× fastener diameter, wall 40-60% of nominal.
Bridge tooling	Interim aluminium tooling for low-to-mid volumes between prototype and production steel.
Bump-off	Ejecting a shallow undercut by letting the part flex over the core; works only with flexible resins.
Burn marks (dieseling)	Defect: scorched resin from air compressed at end of fill; a venting problem.

C

Term	Definition
Cavitation	The number of part impressions in a mould; multiplies output per cycle.
Clamp force	Force holding the mould closed against injection pressure, rated in tonnes; ~0.3-0.8 t per cm² of projected area.
Cold runner	Unheated channel system whose melt solidifies and is ejected with the parts each cycle.
Collapsible core	Segmented core that collapses inward to release internal undercuts such as threads.
Colour streaks (marbling)	Defect: swirled colour from poor masterbatch dispersion.
Cooling time	Cycle phase in which the part solidifies; scales roughly with wall thickness squared.
Core	The mould steel forming internal part geometry; also, to core out = to hollow a thick section.
Crazing	Fine surface cracks from stress, chemicals or degradation.
Creep	Slow permanent deformation of plastic under sustained load.
Cushion	Small melt reserve ahead of the screw after filling, enabling effective packing.
Cycle time	Total time per moulding cycle: close, inject, pack, cool, open, eject.

D

Term	Definition
Delamination	Defect: surface layers flaking off, usually from contamination or incompatible materials.
DFM	Design for Manufacturability — pre-tooling geometry review; Nordmould provides it free with quotes.
Dieseling	See burn marks.
Draft angle	Taper on faces parallel to mould opening so the part releases; baseline 1° per side.
Drag marks	Defect: scratches in the ejection direction caused by insufficient draft or rough steel.
Drying	Removing moisture from hygroscopic resin (PA, PC, PBT, PET, POM, TPU) before processing.
Dwell	See hold time.

E

Term	Definition
Edge gate	Common cold-runner gate cut into the parting line feeding the part edge; leaves a trimmed mark.
Ejection	Removal of the part from the mould, normally by ejector pins, sleeves or a stripper plate.
Ejector pin marks	Witness dents or whitening where pins pushed the part; minimised by cooling, draft and pin layout.
EDM	Electrical discharge machining — sinker, wire and small-hole — used to shape tooling features; available in Nordmould's network.
Engineering resin	Higher-performance polymer (PA, POM, PBT, PC, PPS, PEEK) specified for mechanical or thermal duty.

F

Term	Definition
Family mould	One tool producing different parts of an assembly per shot; volumes of the parts stay locked together.
FAI	First Article Inspection — full dimensional report verifying the first production-representative parts.
Fan gate	Wide cold-runner gate spreading flow evenly across wide flat parts.
Fill time	Phase in which the cavity fills with melt, typically 0.5-3 seconds for small parts.
Flash	Defect: thin excess material escaping at parting lines, pins or shut-offs.
Flow length ratio	Ratio of melt travel distance to wall thickness; predicts whether a thin section will fill.
Flow lines	Defect: visible rings or streaks recording the melt front's movement.

G

Term	Definition
Gate	The opening through which melt enters the cavity; type and location control fill and the witness mark.
Gate blush	Hazy or frosted surface near the gate from high shear or turbulence.
Gate freeze	The moment the gate solidifies, ending packing; sets final part weight and dimensions.
Glass-filled (GF)	Resin reinforced with glass fibre (e.g. PA66-GF30); stronger, stiffer, more abrasive, anisotropic shrinkage.
Gloss variation	Defect: patchy surface shine from uneven mould temperature, packing or polish.

H

Term	Definition
Hardened steel	Tool steel through-hardened to ~48-52 HRC (H13, S136) for long runs and abrasive resins.
Hold (packing) pressure	Pressure maintained after filling to compensate shrinkage before gate freeze.
Hot runner	Heated manifold-and-nozzle system keeping melt liquid inside the mould; no runner scrap.
Hot tip gate	Hot-runner gate leaving a small ~0.5-1.5 mm dot on the part.
Hygroscopic	Describes resins that absorb atmospheric moisture and must be dried before processing.

I

Term	Definition
Insert moulding	Moulding plastic around a pre-placed component, typically a metal threaded insert.
ISO 20457	International standard defining tolerance classes for moulded plastic parts (formerly DIN 16742).

J

Term	Definition
Jetting	Defect: worm-like flow pattern when a small gate fires into open space instead of landing on a wall.

K

Term	Definition
Knit line	See weld line.

L

Term	Definition
Lifter	Angled ejector that forms and releases an internal undercut during ejection.
Living hinge	Thin integral flexure (0.2-0.35 mm) joining two rigid sections; PP and PE are the classic materials.
LSR	Liquid silicone rubber — two-component platinum-cured elastomer moulded on dedicated equipment.

M

Term	Definition
Masterbatch	Concentrated pigment or additive carrier dosed into natural resin for colouring.
Melt temperature	Actual temperature of the molten resin at injection; distinct from barrel setpoints.
MFI / MFR	Melt flow index/rate — grams extruded in 10 minutes under standard load; a flow-ability ranking.
Mould (mold)	The precision tool — cavity and core — that shapes the part; European spelling: mould.
Mould flow analysis	Simulation of filling, packing and cooling used to predict weld lines, pressure and warpage.
Mould shrinkage	Percentage a part contracts from cavity size as it cools; ~0.2% filled to 2.5% unfilled crystalline.
MUD insert	Master Unit Die — a shared mould frame accepting interchangeable cavity inserts, cutting tooling cost.
Multi-cavity mould	Tool with several identical impressions, multiplying output per cycle.

N

Term	Definition
Nominal wall	The designed standard wall thickness of a part, against which ribs and bosses are proportioned.
Nozzle	The heated tip connecting the machine barrel to the mould's sprue or hot runner.

O

Term	Definition
Overmoulding	Moulding a second material over a substrate — soft TPE grips over rigid housings are typical.
Over-packing	Excessive hold pressure or time, causing flash, stress and ejection difficulty.

P

Term	Definition
Packing	See hold pressure.
Parting line	The plane where mould halves meet; leaves a witness line and must be placed deliberately.
Pin gate	Small three-plate-tool gate leaving a tiny dot; enables balanced multi-cavity layouts.
PPAP	Production Part Approval Process — automotive documentation package proving production readiness.
Pre-hardened steel	Tool steel supplied at ~30-36 HRC (P20/718); the medium-volume production standard.
Projected area	The part's shadow area in the opening direction; multiplied by cavity pressure to size clamp force.
Purging	Cleaning the barrel or hot runner with purge compound or a compatible resin between materials/colours.

R

Term	Definition
Regrind	Recycled in-house scrap (runners, rejects) ground and blended back with virgin resin, typically 10-30%.
Residence time	How long resin sits molten in the barrel/hot runner; excess causes degradation and black specks.
Rib	Thin reinforcing wall, sized 40-60% of nominal wall thickness, height ≤ ~3× wall.
Runner	The channel network delivering melt from nozzle to gates; cold or hot.

S

Term	Definition
Semi-crystalline polymer	Resin partly ordering into crystals on cooling (PP, PA, POM, PBT, PEEK); higher shrinkage, opaque.
Short shot	Defect: incomplete filling — the melt front stops before the cavity is full.
Shot size	Maximum melt volume a machine can inject per cycle; total shot should use ~20-80% of it.
Shrinkage	See mould shrinkage.
Side action (slider)	Cam-driven moving steel forming an external undercut; adds from ~€1,000-5,000+ per direction.
Silver streaks	See splay.
Single-cavity mould	Tool with one impression; cheapest tooling, highest unit cost.
Sink mark	Defect: surface depression from local shrinkage, typically opposite ribs, bosses or thick zones.
Snap-fit	Moulded-in deflecting hook fastening two parts without hardware.
SPI classes 101-105	Tool-life grading: from >1M cycles in hardened steel (101) to <500 prototype shots (105).
SPI finish	Polish grading from A1 mirror through B/C semi-matt to D blasted textures.
Splay	Defect: silvery surface streaks, usually moisture flashing to steam — a drying problem.
Sprue	The main feed channel from machine nozzle into the mould; also the direct gate for single cavities.
Stripper plate	Ejection plate pushing the part off the core evenly; enables zero-draft faces.
Stress whitening	White marks at stressed or bent areas, from micro-cracking of the polymer.

T

Term	Definition
T1 / T2 / T3 samples	Sequential trial shots from a new or modified tool, measured and corrected toward approval.
Tab gate	Cold-runner gate feeding through a sacrificial tab, reducing shear stress and jetting.
Thermoplastic	Polymer that melts and re-melts (ABS, PP, PA); the standard injection-moulding material class.
Thermoset	Polymer that cures irreversibly in the mould (epoxy, phenolic, BMC).
Three-plate mould	Tool with a floating runner plate enabling pin gates and automatic runner separation.
Tolerance	Permitted dimensional deviation; ISO 20457 classes apply to moulded parts.
Tool / tooling	The mould and its mechanisms; 'tooling cost' is the one-time build price.
Tunnel (submarine) gate	Angled cold-runner gate shearing off automatically at ejection; standard for automated small parts.
Two-plate mould	The simplest tool construction: cavity half plus core half, opening at one parting line.

U

Term	Definition
UL 94	Flammability rating for plastics: HB, V-2, V-1, V-0, 5VA from least to most stringent.
Undercut	Feature preventing straight ejection (side holes, hooks); needs bump-off, lifters, sliders or redesign.
Unit price	The recurring per-part price: material + machine time ÷ cavities + overhead; excludes tooling.

V

Term	Definition
Valve gate	Hot-runner gate with a mechanical pin sealing each cycle; cleanest witness, per-drop control.
VDI 3400	European EDM texture standard: VDI 12 (Ra ~0.4 µm, finest) to VDI 45 (Ra ~18 µm, coarsest).
Vent	Shallow channel letting air escape the cavity; blocked vents cause burns and short shots.
Vestige	The witness mark a gate leaves on the part.
Void	Internal cavity in thick sections from shrinkage without packing compensation.
V/P transfer	The switch from velocity-controlled filling to pressure-controlled packing.

W

Term	Definition
Warpage	Defect: distortion from differential shrinkage — bow, twist or cup.
Weld (knit) line	Seam where two melt fronts meet; strength depends on temperature, angle, venting and material.
Witness line	Visible mark left by the parting line, gates or ejectors; placed on non-cosmetic faces by design.