[Behind the scenes] – Injection molding

Every day, we come in contact with objects that are injection molded – one of the cornerstone technologies that define our modern world. Injection molding allows designers to create extremely complicated parts with relatively low part cost after a (usually expensive) initial investment in an injection mold.

Injection molds are often closely guarded trade secrets – Legos is said to bury worn-out molds in the concrete of its new buildings. However, since we are an open sourced project, working closely with our manufacturing parters, I am able to give you a behind-the-scenes look at how injection molds are built.

Here’s the facility at Western Tool and Mold - my manufacturing partner for all the plastic brackets and components. It is run by an American gentlemen by the name of Collin Wilkerson. Their company specializes in aerospace injection mold construction, as well as specialized engineering resins, such as the EMS Grivory material used in the OpenBeam brackets. I’ve worked with Collin and his team for over three years with two different companies and would trust them with my most demanding projects.

The injection molding process starts with CAD data, transmitted electronically via email. Often, a drawing package will accompany the 3D data. Younger engineers tend to neglect the importance of a good engineering print, but this is a formality that cannot be overlooked. For one, the drawing is part of a legal contract between the buyer and the supplier. It governs everything from material selection, color and texture information, to packaging material, inspection standards, and delivery terms. (Oh, you didn’t want all your parts to be stuffed into a box in newspaper? You should have put something in your drawing…) The drawing also functions as a tool to convey features not in the cad model – such as locations for the gate (where the plastic is injected into the mold), allowable positions for the ejector pins (to remove the part from the mold), and cosmetic requirements on the part (the bottom of a toaster has less of a cosmetic requirement than the front display housing on an iMac).

An injection mold can essentially be thought of as two big blocks of steel housing various mechanical components to allow cooling and heating of the mold, the “ejector pins” to remove the part from the mold after the mold opens, guide and alignment features to keep the two halves of the mold aligned properly with each other, as well as the actual steel cavity into which molten plastic is injected under high pressure to form the part. After the mold maker receives the CAD files, drawings, and other requirements, a tooling engineer takes over and starts the mold design process. Tooling design is a very specialized art, and the folks that work on these are truly master craftsmen in their own right. Part geometry, tolerance requirements, resin selected (and the shrink rates of the resin as it cools) are all considerations that the tooling engineer must take into account when designing the mold. Today, advanced simulation software can help simulate the flow of hot plastic resin under pressure into the mold to help predict and prevent issues such as part warpage, sink marks on part, and filling problems with a mold. The tooling engineer will then create computerized programs to drive the robotic machines involved in cutting the steel pieces into all the components of a mold.

The majority of the metal cutting takes place on a computer numerically controlled (CNC) milling machine. At Western Tools, this work is predominately done by Mori Seki machining centers.

These machines are so accurate, they are subjected to export restrictions by the Japanese government under International Traffic of Arms Regulations (ITAR). The reason being is that these machines have the accuracy required for machining the complex 3D curvatures required by modern nuclear weapons.

Before each program segment for the CNC machine is executed, a laser scanning probe verifies the tool geometry and compensates for wear and tear of the tool. After each program segment, the laser reinspects the tool for breakage and alerts the operator if a breakage occurs. A hot-spare for each tool exists in the tool changer magazine – so without human intervention, the machine can rerun that particular segment of program code. This allows for “lights out” operation and frees up a highly trained tool maker from “babysitting” a machine.

Not all parts can be cut by a CNC mill. sharp inside corners and awkward geometry may make material removal by an end mill difficult, if not impossible. In this situation, a different trick is used.

This is an EDM machine – and any mold shop will have a few of them. It uses a high voltage electric discharge to burn away metal. The nice thing about EDM, is that it doesn’t really care how hard the material is. Normally, your cutting tool has to be harder than the material you are cutting – and for hardened tool steel, only specialized ceramic materials called carbides are hard enough to cut them. But here, a relatively soft copper rod is slicing through a piece of tool steel with relative ease.

EDM machine comes in 2 shapes – sink EDM and wire EDM. In the example above – a sink EDM is using an electrode to make a straight cut through the steel. The cross section cut takes on the exact same shape as the electrode.

A wire EDM machine, on the other hand, allows for free-form cutting. In this machine the wire is threaded through automatically, and the top and bottom position of the wire can be individually driven. The wire itself acts as the cutter, or an impossibly thin band saw. (Imagine a hot-wire foam cutting machine – same principle, except this is cutting hardened steel. Keyways for slides, ejector pins and coolant lines can be cut this way. When the machine is done with one hole, it can automatically cut the wire, move to the next position and rethread the wire through the hole – again allowing for “lights out” manufacturing.

For the finishing touches, part numbers, ISO material identification codes and recycling information are engraved into the part as part of good engineering practice. Finally, the mold is textured by a chemical etching process. This does two things; it makes the part less glossy and plastic looking, and in the case of parts that are glued / labeled, it also increases the surface roughness of the plastic to allow for a better adhesion.

Simultaneously with all the cutting, polishing and assembly work, the operations team is hard at work, sourcing the plastic resins. Often, a color chip is provided to the molder and color mixing and matching occurs. Larger annual usage (one metric ton of resin and above) may have custom compounded color resin to ensure lot-to-lot consistency. Saudi Basic Industres (Sabic, formerly GE Plastics), for example, have patented formulation for Apple products. This is how every Macbook power supply brick and every Magic Mouse have the same glossy white look. For smaller players like myself, lot codes are important if we care about color matching, due to the tendency for color to vary from batch to batch.

This ought to give you an appreciation of how much work goes into the fabrication of an injection mold. Again, it is no small feat to finish a mold like this in five weeks, from CAD file upload to first shots. It takes an incredibly well tuned organization (and fantastic client-vendor relationships) to make this happen.

Here are closeup pictures of the OpenBeam Mold cavities:

You can see the ejector pin guides here, corresponding to the CAD model screenshot:

And finally, here are the first shots off the molds!

(Note the Tam Labs Part number (TL625-000100), revision level number (-002), material code (PA66+GF30) and ISO recycling code molded into the recess of the part as good engineering practice)