Fast Injection Molds in China: Tooling in 10 Days

Your first shots could be in your hands in 10 days, not 45. Ask almost any shop how long injection tooling takes and you will hear the same story. It is a story worth knowing, because it is mostly true for the way most shops work. It is just not the only way to do it.

The standard story

A typical injection mold takes about 30 days to build. Then the first parts come off it, and you find out what needs to change. Mold revisions usually add about 15 days on top. During that time the team is dialling in the tolerances on the parts: a boss sits a little off, a snap fit is too tight, a screw hole does not line up.

Behind this sits a habit called steel-safe. When a toolmaker cuts a mold, the cavity is made from steel, and steel only goes in one direction. You can cut steel away, which adds plastic to the part. You cannot put steel back, which would take plastic away, without re-making the cavity. So the toolmaker starts conservatively: holes a touch big, bosses and ribs undersized, walls a touch thin. A hole is formed by a steel pin, so a fat pin gives a big hole. Then, after the first shots, they trim the steel until the part hits its numbers.

That is a sensible way to protect the tool. It is also exactly why the process has a built-in revision loop, and why you hear "30 days, plus another 15 for revisions" from just about every shop you ask.

What if I told you there are 10-day injection molds?

Your first reaction is probably the same as ours was: no way. That is not possible. They must be fake, or terrible.

There is some truth and some fiction in that reaction.

The fiction is that a fast mold is a toy or a trick. It is a real mold, often aluminium or a softer tool steel, that makes real injection-molded parts in your real resin. The plastic part itself is the real thing.

The truth is that fast molds are built differently. They are simpler, they are built for a short run, and they depend on you giving the toolmaker everything up front. We will get to the catch below. But a real mold in about 10 days is something we see regularly.

How it works: a cluster, not a factory

A fast mold does not come out of one very clever factory. It comes out of a tightly interconnected cluster of small specialists: steel suppliers, CNC shops, EDM shops, mold-assembly shops and injection shops. They sit close together, they know each other, and they hand work to each other the same day.

The steel is cut to block and delivered next door. The CNC shop roughs the cavity. The EDM shop burns in the fine detail. The assembly shop fits the mold base and checks the fit. The injection shop is already waiting with a machine to run the first shots. Nobody waits on a freight booking, a quote cycle or a purchase order approval, because everyone has done this with everyone else many times before.

That is how it is fast, and it is also how it is cheap. Each stage is done by people who do only that stage, with machines that are already paid for. There is very little overhead between steps.

This is a secret supply chain in the outskirts of Zhejiang. It is not secret because anyone hides it. It is secret because you cannot find it by searching. It is invisible from the outside and known mostly to those of us who have spent years on the ground hunting for it.

When a fast mold is the right answer

A fast mold is a good fit when you need a housing or a toy in about 2 weeks and you do not want to go the 3D printing route. Maybe the part has to be in the real material. Maybe you need a pilot run, a trade show or a first sale. Maybe you want identical parts in front of customers, and a printed part will not behave the same way.

If you know what you are doing, you can get 5,000 parts in 14 days. That is a first production batch, not a prototype.

The catch: you get one shot

The downside is simple, and it follows directly from the speed. You get one shot. The usual revision loop does not exist here, because there is no time for it. So the work moves to you, up front:

• Supply every mating part. Send the toolmaker every part your housing has to fit around, not just drawings of them. Test fitment happens on the real parts.
• Supply every screw. The exact screws, inserts and fasteners you plan to use. A boss that works with one screw may not work with another.
• Specify the surface finish up front. Texture, polish, gloss: decide before the steel is cut, because the finish is part of the mold, not an afterthought.
• Do not change a requirement mid-way. Not a dimension, not a color, not a logo. A change once steel is cut means delays, and in this process the delays hurt, because the whole schedule is built on nothing changing.

None of this is difficult. It just asks for the kind of discipline most teams only develop after a few expensive molds. Do it carefully once and the process rewards you.

What adds time, and what does not

Not every feature costs the same. Slides and side horizontal features usually add little or no time. A good toolmaker handles them as part of a normal day.

Slides fit your own experience too: see the story below, where the shop added one and the part got better.

Be careful with anything else. Undercuts that need lifters, angled holes and other special features will increase the time. They are not impossible, but each one adds machining, fitting and risk. If you can redesign a feature to avoid a lifter, do it. It is often the cheapest day you will ever save.

Why fast molds wear

Fast molds are fast partly because the cavity is cut from softer material. For short runs that means aluminium, or a soft tool steel such as P20 (often bought pre-hardened, so it is machined and used without a separate heat treatment). Long-run molds use hardened steels such as H13 or S136, which take far longer to machine and cost more. Soft metal cuts quickly, but it wears sooner.

The wear shows in predictable places. Small cores and thin features go first. At the gate, where plastic enters at speed, the steel erodes. At the parting line, the edges peen over and stop sealing, which is where the flash comes from.

Don't design for the mold: the draft story

Our first China mold, I did everything the textbooks say. I added all the draft angles. I shaped the part so the mold needed no slide, and I was very proud of my work. I even hired a mold consultant in Canada to do the basic mold layout.

Then the mold shop rotated the part 90 degrees, added a slide and changed all the draft angles. Their explanation was simple: it produced a nicer part.

The irony is that the features that became nicer were the ones I had added to reduce mold complexity, not for looks.

Adding draft makes your model hard to change and does not reduce mold cost or time.

That is our rule for standard housings, toys and consumer electronics. Automotive is probably the exception. Unless you're an expert, you're likely doing it wrong.

So give the mold shop a clean model plus your requirements: which faces are cosmetic, which parts mate with it, and what finish you want. Let their mold engineers choose the draft, the parting line and the slides.

Three things on the part do still need your decision, because the shop cannot guess them.

• Gate and ejector-pin marks. Every molded part carries a small scar where the plastic entered (the gate) and round dimples where pins pushed it out. Say where they may land before the steel is cut, and keep them off the cosmetic face. If you do not choose, the toolmaker will, and it may be the face your customer looks at.
• Surface finish. Finish is named by standards: SPI grades (A to D, from mirror polish down to rough stone finish) for polish, MT numbers for texture, and VDI 3400 for spark-eroded finishes. Choose one and write it down. Texture adds time and cost, because it is an extra process on the steel.
• Material and shrinkage. Plastic shrinks as it cools, and each resin shrinks by a different amount. The mold is cut oversize to allow for it. Change the resin after the steel is cut and your fit changes with it. Choose the resin first, and tell the toolmaker.

Get feedback before steel is cut

Send the 3D file for design-for-manufacturing feedback first. A good toolmaker will flag thin walls, trapped undercuts and awkward gate positions in a day. A change on screen is free. A change in steel is the delay you cannot afford.

What to check on the first samples

The first shots are called T0 or T1, the first article. They are not a finished product. Check them for four things:

• Fit: do they go together with the real mating parts and the real screws?
• Shrinkage: are the key dimensions where you expected?
• Warpage: are long, flat or thin parts straight?
• Flash: is there excess plastic at the parting line or around moving parts?

Write down what you find before the next shot, not after.

These molds wear out

A fast mold is not built for ever. Somewhere around 20,000 pieces you may start to see more flash than you did at the beginning: the thin film of plastic that squeezes out where the two halves of the mold meet, as the steel wears and the fit loosens.

Why this is usually fine: most tooling runs one production batch and is then abandoned. The product changes, the design moves on, the order is not repeated, or the part is redesigned before the tool is worn in. It is rare that a tool runs batch after batch for years. If yours is one of those, you can build a harder tool for the long run, once you know the part is a winner.

The economics

A small fast mold costs about $2-3k. A regular small mold costs about $5-10k. Here is how the options sit side by side, using only the facts above:

You do the math.

Now you know about the secret supply chain too. Could this speed up your first production batch?

Working on something like this? Get in touch.