Mold making: how an injection mold is built, from steel to approved first article

Between the deposit and the first sample parts, a toolmaker turns your drawing into two blocks of steel. Knowing what happens in those weeks tells you what to ask, what to approve, and where a mold build loses time.

Filed under Molds and tooling

Short answer: Mold making is the work of turning a part design into a production injection mold: a review of the design for moldability, a mold design, steel bought to suit the shot count, CNC machining and EDM to cut the cavity and core, heat treatment where the steel needs it, fitting and polishing, then trial shots. A mold is not finished when the steel is cut. It is finished when parts from it measure right against your drawing.

Mold making is the step most product companies never see. The drawing goes out, a deposit is paid, and some weeks later there are sample parts. What happens in between decides the part's cost, its cosmetic quality and how long the tool will last, so it is worth knowing what the toolmaker is actually doing and what to ask at each stage. This note walks through how an injection mold is built, what it is made of, and where buyers lose time and money. What a mold costs is covered separately in what an injection mold really costs.

What a mold is made of

Strip away the vocabulary and a mold is two blocks of steel that close together and leave a part-shaped space between them. The Wikipedia injection molding entry describes the two primary components as the injection side (A plate) and the ejector side (B plate). Plastic enters through a sprue or gate, runs along "channels that are machined into the faces of the A and B plates," and the entry notes that these channels "allow plastic to run along them, so they are referred to as runners." From the runner it passes through one or more gates into the cavity.

The rest of the tool is there to get the part out and keep the cycle short:

• Ejector pins push the finished part, or the runner, out of the mold, usually from the ejector half. Where they push leaves a mark, so their position is a cosmetic decision, not just a mechanical one.
• Cooling is usually water passed through holes drilled in the mold plates and connected to form a continuous path. Cooling layout is a large part of cycle time.
• Draft is the slight angle on walls parallel to the opening direction, which eases the part out of the tool.
• Inserts. Cavities and cores are often divided into pieces, called inserts, which makes maintenance and venting easier and lets a worn or changed area be replaced without remaking the whole tool.

Mold making, step by step

The step most often rushed is the first one. Every change made on paper in stage 1 is cheap. The same change made after stage 4 means welding, re-cutting or a new insert.

Steel is chosen by shot count

The mold material follows how many parts the tool must make. The Wikipedia entry lists the usual choices as hardened steel, pre-hardened steel, aluminum and beryllium-copper alloy, and is plain about the trade-off: steel molds "cost more to construct, but their longer lifespan offsets the higher initial cost over a higher number of parts made before wearing out."

• Pre-hardened steel is supplied at roughly 38 to 45 on the Rockwell C scale, according to the same entry. It is less wear-resistant and is used "for lower volume requirements or larger components." It is machined in its supplied hardness, so there is no heat treatment step to schedule.
• Hardened steel is "heat treated after machining" and typically runs 50 to 60 HRC. The entry calls it "by far superior in terms of wear resistance and lifespan."
• Aluminum tools can cost substantially less and, when designed and machined with modern equipment, can be economical for tens or even hundreds of thousands of parts.
• Beryllium copper goes into areas of the mold that need fast heat removal.

The Wikipedia tool steel entry explains the grade letters you will see on a quote: "P-type tool steel is short for plastic mold steels," designed for zinc die casting and plastic injection molding dies, while H13 is a hot-work steel. It also notes that injection molds require tool steels "for their resistance to abrasion," which is what lets a tool run hundreds of thousands of cycles.

To make tool life a contract term rather than a hope, buyers in the US often use the SPI mold classes. A summary published by Plastikon Industries lists Class 101 at one million cycles or more, Class 102 at less than a million, Class 103 at less than 500,000 ("the most common price range"), Class 104 at less than 100,000, and Class 105 at 500 or fewer, for prototypes only. Name the class on the purchase order and the steel argument mostly settles itself. The mold quote comparison note shows how steel and class assumptions explain quotes that are three times apart.

Why EDM is in every mold shop

A milling cutter is round, so it cannot cut a sharp inside corner or a deep, narrow rib. Mold makers use electrical discharge machining for those features. The Wikipedia EDM entry defines it as a process "whereby a desired shape is obtained by using electrical discharges (sparks)," and notes that heat treated tool steels "that are very difficult to machine using conventional machining can be precisely machined by EDM." It adds that the process "is most widely used by the mold-making, tool, and die industries."

There are two kinds you will hear about. Sinker EDM uses a shaped graphite or copper electrode to burn its negative into the steel, which is how ribs, logos and sharp-cornered pockets are made. Wire EDM cuts through the steel with a thin wire, for inserts and through-shapes. Because EDM can work after heat treatment, the entry notes, "dimensional accuracy will not be affected by heat treat distortion." For a buyer the practical point is schedule: electrodes have to be designed and made before burning starts, so a part with many sharp details takes longer.

Cavities and family molds

A mold can have one cavity or many. More cavities cost more to build and less per part, and the right number follows annual volume. The Wikipedia entry notes that some very high-volume molds, like those for bottle caps, can have over 128 cavities. A family mold makes several different parts in one tool. It saves tooling money, but the parts must fill evenly, and if one part needs a change the whole tool goes back to the toolroom. The cost note covers how cavity count and runner type move the price.

T1 is a trial, not an approval

The first shots off a new tool are called T1. They show whether the tool fills, ejects and cools as designed. They are not proof that the tool is right. Approve the tool on a measured first article, with critical dimensions recorded against the drawing, not on a part that looks good in a photo. The golden sample note explains why a single good-looking part is a weak standard, and precision injection molding covers what tolerances to put on the drawing before the tool is cut. Adjustments are easier to make by removing steel than by adding it, which is why many toolmakers leave critical dimensions "steel safe" for the first trial and cut to size from the measurements.

What to ask the mold maker before steel is cut

1. Which steel, which hardness, and which SPI class is this tool built to?
2. How many cavities, what runner (hot or cold), and where are the gates?
3. Which features need slides, lifters or EDM, and are they priced line by line?
4. How many trial rounds and how many sample parts are included in the price?
5. What does the dimensional report at approval include?
6. Who owns the tool, where will it be kept, and how will shot counts and maintenance be recorded?

The last question matters as much as the steel. A mold paid for by you still sits in someone else's building. The tooling and IP page sets out the ownership wording to get in writing.

How Jack runs a mold build

Jack starts with two answers: the annual volume and the material. Those decide the steel, the cavity count and the runner before anyone draws a gate. He specifies the tool with the toolmaker in Mandarin, so the drawing is not translated twice. Typically several weeks to first trial shots, depending on part complexity; Jack confirms timing in your quote. The tool is approved on measurements, it carries a nameplate, and its custody record shows where it is, how many shots it has run and when it was last maintained. The mold step of the process shows how it fits between the prototype and production, and the injection molding page covers the molding itself. If you have a part that needs a tool, send the drawing and Jack will tell you what the mold should be built to.

Straight answers

What is mold making?

Mold making is building the steel or aluminum tool that an injection molding machine fills with plastic to make a part. It covers the design review, mold design, machining, EDM, heat treatment where needed, fitting and polishing, and trial shots until the parts measure right.

How long does it take to make an injection mold?

It depends on the size and complexity of the part and the number of cavities and mechanisms. Typically several weeks to first trial shots, depending on part complexity; Jack confirms timing in your quote. Plan the launch from the approved first article, not from the day steel is cut.

What steel are injection molds made from?

Pre-hardened plastic mold steels (P-type, such as P20) for low and mid volumes, hardened steels such as H13 for high volumes and abrasive resins, and aluminum for prototypes and bridge runs. The right choice follows the number of parts the tool must make.

What is the difference between T1 and first article approval?

T1 is the first trial of a new tool: it shows whether the mold fills, ejects and cools. First article approval comes after any corrections, when parts are measured against the drawing and the critical dimensions are recorded and accepted.

What is an SPI mold class?

A way of stating how long a mold must last. Class 101 is built for a million cycles or more, Class 103 for under 500,000, Class 104 for under 100,000 and Class 105 for prototypes of 500 or fewer. Naming the class on the order settles what steel and construction the tool needs.

How this note was made: researched and drafted with AI assistance, checked against the sources listed below and edited at JCH Design & Manufacturing.