Glossary of Plastic Injection Moulding

Glossary Beginner 10 minutes to read 📅 Updated on 14 May 2026

Glossary of plastic injection moulding

This glossary brings together 65 technical terms essential for plastic injection moulding and plastics engineering: process, materials, defects, tooling, standards. Each definition is Short and operational, designed for design offices, product engineers and industrial project managers.

Chaque terme a une ancre directe (par exemple #retassureusable from any page of the site. Updated by the Hybster Design Office.

A

ABS (Acrylonitrile Butadiene Styrene): Amorphous thermoplastic combining rigidity, impact resistance and good paintability. Widely used in automotive, electronics, and domestic appliances. Sensitive to UV without additives.
Anisotropy: A property of a material whose mechanical characteristics vary depending on the direction; in 3D printing, it's referred to as anisotropy, whereas in plastic injection moulding, it's more commonly termed material orientation in fibre-rich areas.

B

Ejection battery: Subset of the mould consisting of ejectors (pins, blades, plates), their ejector retainer plate and their ejector plate. Mechanically pushes the part out of the cavity after cooling, under the action of the press's ejection cylinder.
Smudge: Excess material formed at the junction of mould parts (parting line, sliders, ejectors) when injection pressure exceeds clamping force or the mould is poorly machined. Visible at the edge of the part, requires trimming.
Two-material (injection moulding): Process for injecting two different thermoplastics into the same part, in a single cycle or by transfer. Allows for the combination of properties (rigid + flexible, opaque + transparent, structure + aesthetics).
Valve gate nozzle: Nozzle hot canal equipped with a mechanical or hydraulic needle valve that opens and closes the injection point. Eliminates the visible gate mark on the part, allows for sequential multi-cavity injection and guarantees a clean start without bavure. Indispensable for premium-look parts and balanced multi-impression moulds.

C

Rising slope: Inclined tooling piece fixed to the fixed part of the mould which forces lateral movement of a slide during opening/closing. Also locks the slide in the closed position during injection to resist pressure. Also called a «wedge» or «locking heel».
Hot channel: A distribution system that maintains the melt between the press nozzle and the mould cavity. Reduces material waste and improves quality, but tooling costs are higher than for cold runners.
Cooling channel: Classic system where material solidifies in the channels between cycles, forming a carrot to be recycled or disposed of. Cheaper mould but greater material waste.
Submarine gate: Sloped feed channel opening beneath the joint plan in an area not visible from the room. At the’ejection, the carrot is automatically separated from the workpiece by shearing, thus avoiding any visible marks and any manual retouching operation.
Carcass (mould): Mechanical mould frame structure, excluding cavities. Includes bases, cavity plates, guide pillars, cylinders, ejection battery. Often purchased as standard (DME, HASCO, Meusburger, Strack) to save time and tooling costs.
Carrot (or jet): Solidified plastic block in the main feed channel, between the press nozzle and the impressions. Recovered with each cycle with the parts on the mould cold channels, it must be recycled or thrown away. The hot channels Remove this material loss.
Guide pillar: Treated steel cylindrical element that ensures precise alignment of the two mould halves during closure. Works in conjunction with a bronze or treated steel guide bush. Misalignment causes smears, visible markings and premature wear of the mould.
Conformal cooling: System of Cooling whose channels faithfully follow the geometry of the imprint, manufactured by direct metal laser sintering (DMLS). Reduces the cycle time from 20 to 40 %, eliminates the Net curtain and improves surface finish. Used on critical moulds (large flat surfaces, transparent parts, areas difficult to cool conventionally).
Injection cycle: Complete sequence: mould closing, plasticisation, injection, holding pressure, cooling, opening, ejection. Its total duration (cycle time) directly conditions the part cost. See cycle time.

D

Shed angle: The angle given to the vertical walls of a part to facilitate its ejection without sticking. Typically 0.5° to 2° depending on the material, the surface finish of the mould and the depth.
Design for Manufacturability: Design optimisation process for manufacturability: drafts, uniform thicknesses, ribs, sprue positioning. Reduces defects, part cost, and cycle time from the design phase onwards.

E

Ejector: Ejection element which pushes the part out of the mould at the end of the injection cycle. Common forms: cylindrical (pin), flat, blade, sleeve (threads), compressed air. The positioning and number of ejectors are calculated via simulation to prevent deformation during mould release.
Tubular ejector: Tube-shaped ejector variant that slides around a central core. Allows for the ejection of hollow cylindrical parts (plugs, sleeves, tips) without marking their underside or risking misalignment. Often used in conjunction with a central ejector for tubular parts.
Ejection:: Final phase of the cycle where the part is extracted from the mould, usually by an ejection system (pins, plates, strippers). The system must avoid visible marks and deformations.
Footprint: Cavity machined in the mould that gives the part its shape. A single-cavity mould produces one part per cycle, while a multi-cavity mould produces several (2, 4, 8, 16...).
Venting: System of small channels or interstices integrated into the joint plan which allow trapped air in the mould to escape during filling. Insufficient venting leads to filling defects, burns (Dieseling), weld lines marked or smears. Typical depth: 0.01 to 0.03 mm depending on the material.

F

Clamping force: Clamping force, expressed in tonnes, that an injection moulding machine exerts to keep the mould closed against injection pressure. This is sized according to the projected area of the part and the material pressure.

G

Injection threshold: Melt entry point into the mould cavity. Common types: pin gate, edge gate, fan gate, tunnel gate (automatic degating).
Frost: Surface defect resulting in a granular, frosted or crystalline appearance on the part. Main causes: residual moisture in the material (PA, PC, ABS particularly sensitive), mould temperature too low, gas release or thermal decomposition. Solutions: pre-drying, mould temperature adjustment, degassing control.
Grain: Matte, granular or structured surface texture applied deliberately to the mould's surface to give the part a non-glossy appearance. Achieved by chemical etching, sandblasting or laser texturing. Specified according to VDI 3400 or Mold-Tech standards. Helps to mask micro-defects and improve the feel.

H

H13 (1.2344 steel): Chromium-molybdenum-vanadium tool steel, with high thermal strength. Used for dies subjected to intense thermal cycles.hot channels, loaded materials e.g. PA-GF fibre reinforced) or very high-volume moulds (several million cycles). Typical quenching: 50-55 HRC.
HDT (Heat Deflection Temperature): Temperature under load at which a plastic specimen deflects by a given amount. Key indicator for material selection in a thermal environment. Expressed in °C under 1.8 MPa or 0.45 MPa.
Hesitation: Short filling where the material flow slows down or stops locally in a thin area, with the plastic favouring thicker areas. Creates unexpected weld lines, incomplete parts or variations in appearance. Predicted by rheological simulation (Moldflow, Moldex3D) and corrected by uniform thickness or repositioning of the injection gate.

I

Plastic injection moulding: Process of manufacturing thermoplastics by melting and then injecting under pressure into a mould. Standard industrial process for the production of medium and large series. See the complete guide.
Sequential injection: Process with programmed opening of injection nozzles (usually gated, valve gating type) according to a time sequence. Allows the injection of large parts (bumpers, dashboards, bonnets) by eliminating or displacing weld lines and avoiding areas of over-pressure. Reserved for parts with high aesthetic or structural requirements.
Isotropy: Property of a material whose mechanical characteristics are the same in all directions. This is the case for unfilled injected thermoplastics, unlike additive manufacturing.

L

Weld line: A visible trace where two fronts of material meet during mould filling. Zone of lower mechanical strength. Controlled by gate position, rheological simulation and process settings.

M

MFI (Melt Flow Index): Melt flow index of a molten thermoplastic, expressed in g/10 min under a standardised load (ISO 1133). The higher the MFI, the more easily the material flows (typical for thin or long parts).
Moldflow: Market-leading software for Rheological simulation plastic injection (edited by Autodesk). Predicted fill, pressure, temperature, internal stresses, weld lines, withdrawal and Net curtain Before mould manufacturing. Tool DFM essential advancement for complex moulds and large series.
Mould tools: Tooling in steel or aluminium containing the impression(s), the injection system (sprue, runners, gates), cooling and ejection. A major project investment (€5,000 to €80,000+).
Tandem mould: Two-tier mould with compartments activated alternately by a hydraulic system: while one tier is injecting, the other is ejecting. This allows production to be almost doubled per cycle while maintaining a reasonable clamping force. Ideal for relatively flat, medium-sized parts produced in large series.

N

Rib: A stiffening element integrated into the part to increase its rigidity without adding weight or thickness. Designed with a draft angle and a thickness typically 50–70% of the main thickness to prevent sink marks.
Moving core: Mechanical component of the mould that moves (translates or rotates) to allow the demoulding of parts with internal undercuts or complex geometries (internal threads, through-holes). Activated by cams, hydraulic cylinder or electric motor. To be distinguished from Drawer, which forms external undercuts.

P

P20 (1.2311 / 1.2738 steel): Pre-treated chrome-molybdenum steel delivered directly at 28-32 HRC, ready for machining. Most common steel for mussel shells and medium series footprints (50,000 to 500,000 cycles). Excellent compromise between machinability, durability, and cost for the majority of industrial moulds.
Polyamide: Family of high-performance engineering thermoplastics (PA6, PA66, PA12) with excellent mechanical properties. Good resistance to fatigue, wear, and oil. Hygroscopic: requires drying before injection.
PC (Polycarbonate): Transparent amorphous thermoplastic with very high impact resistance. Used in optics, electronics, and automotive. Sensitive to solvents and UV without specific treatment.
Polyethylene: Semi-crystalline thermoplastic available in two main variants: HDPE (rigid) and LDPE (flexible). Good chemical resistance, low cost. Widely used in packaging, containers, non-technical parts.
PEEK (Polyetheretherketone): High-performance thermoplastic used in aeronautics, medical, and advanced industries. Excellent mechanical, thermal (HDT > 150 °C), and chemical properties. Very expensive.
Joint plan: The parting surface of the two halves of the mould (cavity side and core side). Its design directly influences the aesthetics of the part, ejection, the presence of flash, and access to undercuts.
Stripping plate: Alternative ejection system to spot ejectors: an entire plate slides around the core to push the part out of the mould cavity. Particularly suitable for thin parts (cups, thin housings, covers) where ejectors would mark the plastic. Also called a stripper plate.
Plasticizing:: The stage of the cycle where the solid polymer (pellets) is gradually melted by heating and shearing in the press screw. An essential prerequisite for injection.
PMMA (Polymethyl methacrylate): Transparent thermoplastic known as «acrylic glass», with excellent optical clarity and good UV resistance. Used in lighting, lenses, signage. More brittle than PC, more scratchable.
SPI polishing: Classification SPI (Society of the Plastics Industry) standardising mould surface finishes in 12 levels. SPI A1 (3 μm diamond mirror polish) for optical parts. SPI A2/A3 (high gloss) for premium parts. SPI B1/B2/B3 (Mechanical policy) standard. SPI C/D (sablé) for matt aspects.
Fibre de verre (Glass Fibre): High-precision thermoplastic semi-crystalline technical material with low friction and good mechanical strength. Ideal for moving parts, gears, and technical supports.
PP (Polypropylene): Versatile, economical and chemically inert semi-crystalline thermoplastic. Often glass-fibre reinforced (PP+GF) for technical applications. Used everywhere: automotive, packaging, furniture.

R

Cooling:: The longest phase of the cycle (often accounting for 60–70% of the total time), during which the part solidifies in contact with the temperature-controlled mould. Optimised through the design of the cooling circuits.
Reassure: Cavity or sink mark on the surface of an injected part, caused by uneven shrinkage of the polymer during cooling. Avoided by uniform thickness, holding pressure, and proper rib positioning.
Withdrawal: Volumetric reduction of an injected part after cooling, due to polymer contraction. Varies depending on the material (PE, PP have high shrinkage) and direction (parallel or perpendicular to flow). Compensated for by mould design.

S

Rheological simulation: Digital modelling of the melt polymer flow in the mould prior to manufacturing. Validates the gate positions, runner sizing, and balance of impressions, the placement of the weld lines and the effectiveness of the Cooling. Common software: Moldflow, Sigmasoft, Moldex3D.
Stack mould: Multi-level mould with stacked cavities, opened simultaneously in the same press. Multiplies the number of parts per cycle (x2, x3, or even x4) without proportionally increasing the required closing force (only the projection of one level counts). Reserved for very large runs of flat or thin parts (cups, lids, packaging, trays).
Stavax (M333 ESR steel): High-purity stainless steel (M333 ESR Uddeholm, equivalent 1.2316) particularly suited for cavities requiring SPI polishing A1/A2 or a VDI texture Corrosion resistance and extremely fine grain make it the material of choice for visible parts, transparent applications, and medical applications.
Overmoulding: Process consisting of injecting a plastic material around an insert (metal, electronic, other plastic) already positioned in the mould. Used for the integration of connectors, sealing, and multi-material functions.
Insert overmoulding: Specific variant of overmoulding where a metal insert (threaded nut, pin, electrical contact, stiffener) is placed manually or robotically into the mould before injection. The plastic encases it and mechanically secures it to the part. Very common for integrating screw fixings into plastic housings or contacts into connectors.

T

Cycle time: Total duration of a part's production cycle (T = Ti + Tc + Tm + Te). Key economic indicator: it determines the pace and therefore the unit cost. See the guide.
Thermoplastic: Polymer that can be melted and solidified multiple times without significant loss of properties. Main category in plastic injection (unlike thermosets, which are irreversible).
Drawer (mechanism): Movable mould mechanism for creating undercuts or side holes that cannot be directly demoulded. Increases mould complexity and cost.
Tolerance (ISO 20457): Acceptable variation range of a dimension of an injection-moulded part. ISO 20457 standard structures tolerances in plastics moulding via TG grids, according to material, process and target quality level.

V

VDI 3400 (texture): German standard for mattifying textures applied to electrical discharge machining (EDM) impressions. Common levels: VDI 12 (very fine, almost satiny) to VDI 45 (marked texture). VDI 30 to 36 Typical for matte consumer parts (automotive interior, household appliances). Hides surface defects and improves perceived quality.
Sheer curtain: Deformation of an injected part after ejection due to differential shrinkage (uneven thicknesses, asymmetric cooling, material orientation). Avoidable by DFM design and balanced cooling.

Hybster Team

Hybster Engineering Team

Design Office – Plastics Design & Engineering

The Hybster Design Office brings together the company's plastic engineering, mechanical, and industrialisation engineers. The team supports projects from the ideation phase through to series validation, incorporating Design For Manufacturing (DFM), rheological simulation, material selection, and mould design. It serves the automotive, electronics, electrical, EV charging, and industrial sectors.

Injection moulded part design DFM Simulation Moldflow Subject choice Moulds for conception ISO 20457 Tolerancing