What Makes an Injection Mold Cost More

Most people expect the price of a mold to follow the size of the part. A bigger part means a bigger block of metal and a bigger number. Size matters, but it is rarely what surprises anyone. What drives a mold's cost is how the mold has to be built to fill the part and then let it go.

A mold is two blocks of metal that close, fill with plastic and open again. Anything about the part that stops it pulling straight out, that a cutter can't reach, or that the plastic won't fill cleanly adds work to the mold. That is why two parts that look almost the same can need very different molds, and why one small feature can change how the mold is built and what it costs. Four features cause most of it.

Undercuts

An undercut is anything that hooks back on itself: a snap, a side hole, a groove around a boss. The part can't pull straight out, so the mold needs a side action or a lifter, a moving piece that slides clear before the part is ejected. That is more to design, more to cut and more to fit, and it is a moving part that has to keep working for the life of the tool.

Often the design can be changed to avoid the undercut. A hole turned to face the direction the mold opens, or a snap formed through a slot in the wall below it, can take the side action out.

Deep, narrow ribs

Ribs make a part stiff without making it thick. In the mold, a rib is a slot cut into the metal. A cutter can only reach so deep before it is too long and thin to cut cleanly, so a deep, narrow rib is often burned in with sinker EDM instead. That means machining an electrode in the rib's shape first, then eroding it into the mold. It works well, and it adds a step. what sinker EDM is for explains where it fits.

Texture and draft

Every wall that runs in the direction the mold opens needs a slight taper, called draft, or the part drags as it comes out. A textured surface needs more, because the texture grips the mold. A common rule of thumb is at least one extra degree of draft for every thousandth of an inch (0.025 mm) of texture depth. A part designed with straight walls and then given a heavy texture can need its walls changed, and that changes the mold.

Wall thickness

Thick sections cool slower than thin ones and pull inward as they shrink, leaving sink marks on the surface. Walls that change thickness sharply can warp. Thick sections also lengthen every cycle on the press, because the part has to cool before it can come out. The fix is usually to core out the thick areas, which changes the shape of the mold.

Why there is no price calculator on our site

Instant quoting works well for many simple machined parts. For a mold, a form can't ask the questions that change the price. Could this undercut go? Does that face need texture? Is this wall thick because it has to be? I'd rather lose an enquiry than hand someone a number I expect to move, because the first number becomes the expectation and the real quote then reads like a broken promise.

So our quotes start with a conversation, by phone, Zoom or in person. The first conversation is free, and we keep it short: long enough to understand what the part has to do and to work out what we still need to quote it. Then we gather that information and build the quote from the shop's historical data and our internal systems. The written quote prices setup separately from each part, so a smaller order is priced on its own terms. There is no minimum order.

What to send for a first conversation

You don't need all of this. Anything helps.

• A 3D file if you have one (a STEP file is the most widely readable), or a sketch, a photo or a sample
• What the part does, and what it touches or fits into
• Which faces a customer will see
• The material you have in mind, and why
• How many you need now, and how many you might need later

Some of what a mold has to allow for never shows in the CAD model. One strut-tower part had to come out round. Plastic pulls and shrinks as it cools, so the cavity was cut deliberately oval, and the finished part came out round. The model showed a round part. The mold had to be built for an oval one.

If your part started as a 3D print, read why a printed part is not ready to mold first.

If you only do one thing before asking for a mold price, mark the faces customers will see. That decides where the gate, the ejector pins and any texture can go.

Related services: Aluminum tooling, molding and machined parts Shop capabilities