Injection Molding Glossary
Use this glossary to understand common injection molding, tooling, material, and quality terms before uploading a CAD file or reviewing a manufacturing quote.
Overview
Injection molding terminology can affect design choices, quote assumptions, tooling complexity, and quality expectations. This page explains key terms in practical buyer-facing language.
Each definition is written to help engineering and sourcing teams understand what the term means, why it matters, and when it may show up during DFM review, tool design, sampling, or production.
For project-specific decisions, NetProto can review your CAD model, material direction, tolerance needs, and production goals before quoting or launch.
Glossary Terms
Use this expanded table as a quick reference when reviewing part design, tooling feedback, material choices, or quote details.
| Term | What it means | Why it matters |
|---|---|---|
| DFM | Design for manufacturability review that checks whether a part can be molded efficiently and reliably. | Helps catch geometry, tooling, material, and tolerance risks before mold build. |
| Tooling | The mold and related hardware used to form the plastic part. | Tool design affects lead time, upfront cost, maintenance, cycle time, and part quality. |
| Cavity | The shaped space inside the mold that forms the outside geometry of the part. | Cavity count and construction influence production rate, tool complexity, and unit cost. |
| Core | The mold feature that forms internal geometry, holes, recesses, or the inside of a part. | Core design affects ejection, cooling, wall thickness, and tooling complexity. |
| Draft angle | A slight taper added to vertical walls so the molded part can release from the tool. | Insufficient draft can cause sticking, drag marks, cosmetic defects, or ejection damage. |
| Wall thickness | The thickness of the molded plastic walls throughout the part. | Consistent walls improve filling and cooling while reducing sink, voids, and warpage. |
| Rib | A thin reinforcing feature used to add stiffness without making the part too thick. | Good rib design can strengthen a part while avoiding sink marks and heavy mass. |
| Boss | A raised post or cylindrical feature often used for screws, inserts, alignment, or assembly. | Boss design affects strength, sink risk, cracking, and assembly performance. |
| Gate | The entry point where molten plastic flows into the mold cavity. | Gate location can affect cosmetic marks, filling behavior, weld lines, and trimming. |
| Parting line | The line where two mold halves meet. | Parting line placement affects appearance, flash risk, shutoffs, and tool construction. |
| Undercut | A feature that prevents simple straight-pull mold release. | Undercuts may require slides, lifters, inserts, or part redesign, increasing cost and lead time. |
| Sink mark | A surface depression caused by uneven cooling or thick plastic sections. | Common around ribs, bosses, and thick walls if geometry is not balanced. |
| Warpage | Part distortion that happens as plastic cools and shrinks unevenly. | Can affect assembly fit, flatness, appearance, and dimensional reliability. |
| Shrinkage | The dimensional change that occurs as plastic cools after molding. | Material shrinkage must be considered in tool design and tolerance planning. |
| Weld line | A line where two flow fronts meet inside the mold. | Can affect appearance and mechanical strength depending on location and material. |
| First article inspection | Initial inspection of molded samples against drawing or project requirements. | Confirms whether tooling and process assumptions produce acceptable parts. |
| A-side | The cosmetic or cavity side of the mold that typically forms the visible outside surface of a part. | A-side decisions affect appearance, texture, gate visibility, and where cosmetic standards should be protected. |
| B-side | The core side of the mold, often containing ejector pins, side actions, and features that form the inside of the part. | B-side planning affects ejection marks, internal geometry, undercuts, and tooling complexity. |
| Barrel | The heated section of the injection molding machine where resin pellets are melted before injection. | Barrel temperature and material preparation influence melt quality, flow, and final part consistency. |
| Bridge tool | An interim mold used to produce parts before a long-run production tool is ready. | Bridge tooling can support validation, pilot builds, or early demand while production tooling is finalized. |
| Bumpoff | A flexible undercut feature that can release by bending or stretching during ejection. | Bumpoffs can reduce tooling complexity, but they require material, geometry, and durability review. |
| CAD | Computer-aided design data used to define the 3D model, drawings, and geometry for manufacturing review. | Clean CAD files help quoting, DFM review, tool design, and dimensional planning move faster. |
| Cam / side action | A moving mold component used to form or release features that cannot be handled by a straight-pull mold. | Side actions can solve undercuts but usually add tool cost, lead time, maintenance, and design constraints. |
| Clamp force | The force used to keep the mold closed while molten plastic is injected into the cavity. | Insufficient clamp force can cause flash, while oversized press requirements may affect production economics. |
| Core pin | A fixed mold element used to form holes, slots, or recessed features in the molded part. | Core pins affect hole quality, cooling, tool robustness, and whether tall thin features are practical. |
| Cycle time | The time required to complete one molding cycle, including close, inject, cool, open, and eject. | Cycle time is a major driver of unit cost and can be affected by wall thickness, material, and cooling. |
| Direction of pull | The direction the mold opens and the part releases from the tooling surfaces. | Pull direction determines draft needs, undercut risk, parting line placement, and whether side actions are needed. |
| Drying plastics | The pre-processing step used to remove moisture from resins that absorb water before molding. | Poor drying can cause cosmetic defects, weak properties, bubbles, splay, or inconsistent molding behavior. |
| Edge gate | A gate placed along the parting line, usually on the outside edge of the part. | Edge gates are common and practical, but gate vestige and flow direction should be considered early. |
| EDM | Electrical discharge machining, a moldmaking process used to create precise features that are difficult to mill. | EDM can support sharp details, deep ribs, and tight tooling features, but may add toolmaking time. |
| Ejection | The step where the cooled molded part is pushed or released out of the mold. | Ejection planning affects pin marks, part distortion, release reliability, and where draft is required. |
| Ejector pins | Pins in the mold that push the part out after it cools enough to release. | Pin location can leave marks and should be coordinated with functional and cosmetic surfaces. |
| Family mold | A mold that produces multiple different part numbers in the same molding cycle. | Family molds can reduce tooling count but require careful balancing of part size, fill, material, and demand. |
| Fillet | A rounded transition between surfaces, often used where ribs, bosses, or walls meet. | Fillets improve flow and reduce stress concentration, but overly large radii can create thick sections. |
| Flash | Thin unwanted plastic that escapes at mold shutoffs, parting lines, or other small gaps. | Flash can require trimming, indicate tooling or processing issues, and affect fit or appearance. |
| Flow marks | Visible patterns created as molten plastic flows and cools inside the mold. | Flow marks may affect cosmetic acceptance and can be influenced by gate location, material, and processing. |
| Glass-filled resin | Plastic resin reinforced with glass fibers to improve stiffness and strength. | Glass-filled materials can improve performance but may increase brittleness, wear tooling, and affect warpage. |
| Gusset | A triangular reinforcing rib used to support walls, bosses, or structural features. | Gussets add strength without thick mass when designed with proper thickness, draft, and radii. |
| Hot tip gate | A gate style that injects material directly into the part face without a cold runner. | Hot tip gates can reduce runner waste, but gate location and cosmetic vestige must be reviewed. |
| Insert | A mold component or part feature added separately, either permanently in the tool or during molding. | Inserts can enable complex geometry or embedded hardware, but they affect tool setup and cycle time. |
| Jetting | A flow defect caused when resin enters too quickly and snakes through the cavity before bonding smoothly. | Jetting can create weak or visible flow patterns and may require gate or process changes. |
| Knit line / weld line | A line where separate plastic flow fronts meet and rejoin during filling. | Knit lines can affect cosmetics and strength, especially near holes, ribs, bosses, or multiple gates. |
| Living hinge | A thin flexible plastic section that connects two molded sections and allows repeated bending. | Living hinges require suitable resin, flow direction, thickness control, and gate planning. |
| LSR | Liquid silicone rubber, a thermoset elastomer processed through liquid injection molding. | LSR can support flexible parts and seals, but tooling, material handling, and flash control differ from thermoplastics. |
| Metal safe / steel safe | A tool design approach that leaves extra metal so a feature can be enlarged by machining later. | Steel-safe planning helps support revisions after sampling without fully rebuilding the mold. |
| Mold release spray | A spray applied to the mold to help difficult parts release during molding. | Frequent release spray can indicate ejection or draft problems and may not be ideal for production stability. |
| Multi-cavity mold | A mold with multiple cavities that produce more than one part per cycle. | Multi-cavity tools can lower unit cost at volume but require balanced filling, higher tooling investment, and demand planning. |
| Nozzle | The machine component that transfers molten resin from the barrel into the sprue or runner system. | Nozzle setup affects material delivery, leak prevention, and process consistency. |
| Overflow | An extra area where material can flow beyond the part to improve fill or edge quality, later removed. | Overflows may improve certain features but add trimming, material use, and secondary handling. |
| Packing | Additional pressure applied after filling to compensate for shrinkage and improve part density. | Packing can reduce sink and voids, but too much pressure can cause flash, stress, or sticking. |
| Pickout | A removable mold insert that stays with the part after ejection and must be manually reset. | Pickouts can solve difficult geometry but slow cycle time and add operator handling. |
| Post gate | A gate style that injects through a post-like feature, often leaving a vestige that must be trimmed. | Post gates can help feed certain geometries, but trimming and vestige location should be planned. |
| Press | The injection molding machine used to clamp the mold and inject molten plastic. | Press size, clamp force, shot capacity, and setup affect cost, availability, and production planning. |
| Runner | The channel that carries molten plastic from the sprue to the gate or cavities. | Runner design affects fill balance, material waste, cooling, and cycle efficiency. |
| Short shot | A molding defect where the cavity does not completely fill with plastic. | Short shots can point to flow restrictions, thin walls, poor venting, low pressure, or unsuitable material. |
| Shot | The amount of molten plastic injected during one molding cycle. | Shot size must match part and runner volume so the machine can process material consistently. |
| Shutoff | A mold surface where two tool faces seal against each other to form holes, clips, or openings. | Shutoffs require draft and careful design to avoid wear, flash, or fragile tooling features. |
| Sprue | The main channel where molten plastic first enters the mold from the machine nozzle. | Sprue and runner design influence material flow, waste, cooling, and gate performance. |
| Straight-pull mold | A mold that opens in one direction without side actions or special moving features. | Straight-pull designs are usually simpler, faster, and less expensive than tools with complex actions. |
| Texture | A surface finish applied to molded part faces for appearance, feel, or hiding minor imperfections. | Texture usually requires additional draft and should be defined before tool finish is finalized. |
| Tunnel gate | A gate cut through the mold body that can place the gate away from the exterior visible surface. | Tunnel gates can improve appearance but must be reviewed for geometry, trimming, and filling behavior. |
| Vent | A small opening that allows trapped air to escape as plastic fills the cavity. | Poor venting can cause burns, short shots, weak fill, or cosmetic defects. |
| Vestige | The small mark or leftover material at the gate after the runner or gate is removed. | Vestige location matters for appearance, fit, assembly, and trimming requirements. |
Term Groups
These categories help visitors understand where glossary terms usually appear during the quote-to-production process.
Terms that describe the molded part itself and how geometry affects moldability.
Terms that explain how the mold is built and why tooling decisions affect cost and lead time.
Terms that connect resin selection and processing behavior to finished part performance.
Terms that appear when samples, inspections, and repeat production are being reviewed.
Glossary FAQ
These answers help connect glossary terms to practical quoting and DFM conversations.
No. A basic understanding helps you interpret feedback, but NetProto can review your files and explain the terms that matter for your specific part.
Tooling, undercuts, tolerances, material selection, cavity count, and secondary operations often have the largest effect on cost and lead time.
DFM feedback often uses terms like draft, wall thickness, gate, sink, and parting line. Understanding them makes it easier to evaluate changes and approve a manufacturing plan.
Yes. For injection molding projects, NetProto can clarify tooling assumptions, material choices, tolerances, production quantities, and any design risks behind the quote.
Next Step
Upload your CAD file and NetProto can review the design, clarify key manufacturing terms, and help turn glossary knowledge into a practical quote path.
