2026-10-23 · JCH Design & Manufacturing

Injection molding terminology: the words in your quote and what they commit you to

Cavity, family mold, mold class, hot runner, draft, gate, T1, knit line, regrind: the injection molding terms that show up in quotes and sample reports, in the order you meet them, and what to ask about each.

Filed under Engineering and services

Short answer: Injection molding terminology is the shop vocabulary that fills a mold quote, a DFM report, a sample report and a purchase order: cavity, family mold, mold class, hot runner, side action, draft, gate, T1, short shot, knit line, regrind and the rest. Most of these words are not just descriptions. Each one fixes a cost, a lead time or a quality limit. A buyer who reads "1+1 family mold, P20, cold runner, Class 103" should know what was just agreed to. This note goes through the terms in the order you meet them, and what to ask when you see each one.

Injection molding terminology matters because the factory writes its commitments in it. A quote that says "4 cavities, hot runner, H13" has made promises about piece price, tool cost and tool life that a quote saying "1 cavity, cold runner, P20" has not, even if the drawing is the same. For a plain A-Z list of definitions, see our injection molding and sourcing glossary. This note is about reading the documents.

Terms in the mold quote

Cavity, multi-cavity and family mold. The cavity is the hollow that forms the part. A multi-cavity mold is, in one molder's definition, "A mold with multiple cavities of the same part that is typically used to reduce piece-part pricing for higher volume parts." A family mold is different: "A mold capable of producing two or more different parts." Family molds save tooling money when you need, say, a lid and a base in equal numbers. They cause trouble when the parts are very different in size or volume, because both cavities must run together, and one part can be hard to fill well without overpacking the other. Ask how the factory will balance the fill, and whether one cavity can be shut off if you only need one part.

A closed injection mold base on a tool-room bench, with tall guide pins on top, brass cooling-water fittings along the side and blue springs in the ejector housing, and a second mold on a stand behind it — Mold bases in a tool room. The tall pillars are guide pins, the brass fittings on the side connect the cooling-water lines, and the blue springs return the ejector plate.

Mold class and tool life. Fictiv, a manufacturing platform, defines the term this way: "Injection molding tool life is the number of production cycles that an injection mold can complete before it needs to be replaced or repaired." In the US, molds are often described by the old SPI classes, 101 to 105. Of the top class, Fictiv writes: "SPI Class 101 molds are the most expensive and are built to withstand over 1 million production cycles and produce plastic parts with tight tolerances and complex geometries." Class 105 is a prototype tool. If a quote does not state the class or an expected shot count, ask for one in writing. "Lifetime mold" with no number is not a specification.

Steel. The steel name in a quote is a proxy for tool life. Fictiv gives typical ranges: "For example, aluminum tooling usually supports 10,000 shots or fewer." "Because P20 is significantly harder than most aluminum grades, injection molds made of this tool steel can support 50,000 to 100,000 shots." "Hardened steels for injection molds include H13, which has an expected tooling life between 500,000 and 1 million shots, and S136 stainless steel, which has a tooling life of 1 million shots or more." Those are typical figures, not guarantees; wear also depends on the resin (glass-filled grades are abrasive), on maintenance and on how hard the tool is run. Match the steel to your real annual volume, not to the cheapest line on the quote. Our note on what an injection mold really costs shows how steel and cavity count move the price.

Hot runner or cold runner. The runner carries plastic from the sprue to the gates. In a cold runner mold it solidifies every cycle and is cut off as scrap or reground. A hot runner keeps the plastic molten inside the mold, which removes most of that scrap and can shorten the cycle, but, as Wevolver puts it, "However, the cost of a mold with a hot runner system far exceeds one with a cold runner system." It is a trade of tool cost for piece price, so it usually pays only at higher volumes.

Side actions, slides and lifters. A side action is "A sliding cam arrangement in the mold that allows for the molding of parts with undercuts." An undercut is any feature that would stop the part pulling straight out of the mold: a side hole, a snap hook, an inward lip. Every slide or lifter adds cost and something else to maintain. If the quote lists more side actions than you expected, ask which feature drives each one; a small design change can sometimes remove it.

Texture. A matte or grained finish is etched or blasted into the steel, usually specified by a Mold-Tech or VDI number. It needs extra draft (more below), and it is normally applied after the first samples are approved, so changing it later is tooling work, not a setting.

Terms in the DFM report

A design for manufacturability (DFM) report is the factory's review of your model before steel is cut. Our DFM note covers the process; these are the words you will see in it.

An injection mold half on a bench, a black molded part still seated in the steel, with bronze-bushed guide holes at the corners and lifting rings on the side — One half of an injection mold, with a molded part still seated in the steel. The bronze-bushed holes at the corners take the guide pins that line the two halves up.

Draft. Protolabs defines it: "Draft is a taper applied to the faces of the part that prevent them from being parallel to the motion of the mold opening." Without draft, parts drag and scratch on ejection, or stick. On how much, Protolabs says "a general rule of thumb is 1 degree of draft per 1 inch of cavity depth, but that can change with the aforementioned factors." Texture raises the requirement: "On finishes, Protolabs requires a minimum of 3 degrees of draft for a light bead-blast finish (PM-T1) and 5 degrees of draft for a medium bead-blast finish (PM-T2)." When a DFM report asks for more draft, it is usually protecting your cosmetic surfaces.

Wall thickness, sink and ribs. Even walls are the first rule of part design. Protolabs again: "This is one of the fundamental rules of plastic injection molding, and ignoring it can lead to sink, warp, and inaccurate or non-functional parts." Where thickness has to change, its guideline is: "Walls in any plastic-molded part should be no less than 40 to 60 percent that of adjacent walls, and all should fit within recommended thickness ranges for the selected material." Sink is the shallow dent that forms over a thick section, rib or boss. If the report flags a thick area, the usual fix is to core it out, not to accept the dent.

Parting line, A-side and B-side. The parting line is where the two mold halves meet, and it leaves a faint line on the part. The two halves have names: "Sometimes referred to as the stationary or cavity side of the mold, the A-side does not contain ejector pins and therefore it generally creates the visual side of the part." The B-side, or core side, holds the ejector pins, which leave small round marks. Tell the factory which surfaces are cosmetic, so the DFM can put the parting line and pin marks where nobody sees them.

Gate and vestige. "Gates are very small connecting points between a runner and a mold cavity", in Wevolver's words. Where the gate sits affects how the part fills, where knit lines form and where a mark is left: "There is typically a visible vestige when the gate is removed." Ask for the gate location in the DFM, and say how much vestige is acceptable on which face.

Terms in the sample (T1) report

T0, T1, T2. The first parts shot from a new mold are the T1 samples (some factories call an internal first trial T0). Fixes follow, and each new round is T2, T3 and so on, until the parts are approved. A T1 that needs no changes is rare; budget time for at least one correction loop.

Short shot and flash. A short shot is "The incomplete filling of a mold cavity which results in the production of an incomplete part." Flash is the opposite problem: a thin fin of plastic where the mold halves or inserts do not seal. The clamping force, or tonnage, is what holds the mold shut against the injection pressure. On a worn tool, these defects creep back. Fictiv's description of a neglected mold: "Months later, parts drift out of tolerance, or flash, sink marks, and short shots start appearing."

Knit lines, sink and warp. Knit lines (also called weld lines) are "Visual indications in a finished part that are formed by the intersection of two plastic flow fronts." They can be weak spots, not just cosmetic ones, which matters on a snap or a boss. Warp is distortion from uneven cooling or shrinkage. Our note on injection molding defects explains how to tell whether the tool or the process settings caused each one.

Shrinkage. Molding shrinkage is "The difference in dimensions, expressed in inches per inch, between a molded part and the mold cavity in which it was molded." Every resin shrinks at its own rate, and the mold is cut oversize to allow for it. This is why a resin change after the tool is built can put parts out of tolerance.

Terms on the purchase order

Shot and cycle time. "A shot of injection molded plastic fills the mold cavity, sprue, runners, and gates, not just the mold cavity." The cycle time is the time for one full shot, from mold close to ejection. Cavity count times shots per hour is your output, and it is what the piece price is built on.

Resin grade and regrind. Name the grade by supplier and product code, not just "ABS." Regrind is ground-up runners and rejects fed back into the hopper. State whether it is allowed and at what percentage; silence usually means some is used.

First article, golden sample and NRE. First article inspection measures the first production parts against every dimension on the drawing; see first article inspection. A golden sample is the signed reference part each lot is compared to, and it is only as good as the controls behind it. NRE is the one-time engineering and tooling cost; our NRE note breaks it down.

Tooling ownership and mold transfer. The quote should say who owns the mold once it is paid for. If you ever need to move it, read tooling custody and mold transfer first.

Five terms buyers misread

"Lifetime mold." Without a shot count or a mold class, it promises nothing. Ask for the number.
"Family mold" on parts with different volumes. You will make both parts every shot, whether you need them or not.
"T1 approved." That approves the samples, not production. First article and lot inspection still apply.
"Hot runner saves material." True, but the tool costs more. Check the payback at your real volume.
"Texture to be added." That is steel work after sampling. Agree the texture standard and the draft before the tool is cut.

Where JCH fits

JCH runs injection molding programs through vetted partner factories, and Jack's engineering team reads the quote, the DFM report and every sample report with the buyer, in plain terms, before money moves. If a term in a quote does not match the volume or the drawing, we say so before tooling starts. The injection molding page shows how a program runs from drawing to first shipment, and the mold step covers what happens while the tool is built.

Straight answers

What does cavity mean in injection molding?

The cavity is the hollow in the mold that forms the part. A one-cavity mold makes one part per cycle; a four-cavity mold makes four identical parts per cycle. A family mold has cavities for two or more different parts.

What is the difference between a hot runner and a cold runner?

In a cold runner mold, the plastic in the runner channels hardens each cycle and is cut off as scrap or reground. A hot runner keeps that plastic molten inside the mold, so there is little runner scrap, but the mold costs considerably more to build. Hot runners usually pay off only at higher volumes.

What is draft in injection molding?

Draft is a slight taper on the faces of a part that run in the direction the mold opens, so the part releases without dragging. A common rule of thumb is about 1 degree per inch of depth, with more needed for textured surfaces.

What is a T1 sample?

T1 samples are the first parts shot from a new mold, used to check fit, dimensions and appearance. Corrections follow and are re-sampled as T2, T3 and so on until the parts are approved. Approving T1 samples is not the same as approving production.

What do SPI mold classes mean?

SPI mold classes, from Class 101 to Class 105, describe how long a mold is built to last. Class 101 is built for over a million cycles, and Class 105 is a prototype tool for a few hundred shots. Ask for the class or an expected shot count in writing on any mold quote.