]The Wimoto’s enclosure, like the vast majority of consumer products, is made of plastic. That plastic is formed using a process called injection moulding.
Believe it or not, getting to the point of being able to make a plastic part is actually quite laborious and complex. Moulds often cost upwards of USD$10,000 and very complex, high capacity moulds can even cost as much as a house. It takes weeks of iterative work to make a single injection mould, but a mere seconds to form each individual part once you move to manufacturing. A typical mould has a lifetime of a million or so ‘shots’ (impressions) and sometimes multiple parts can be shot simultaneously in ‘multi-cavity’ moulds. Our moulds will have a lifetime of around 600,000 shots and are ‘multi-cavity’ – we get one normal enclosure and one modified enclosure with a hole in the side (think Wimoto Grow), every time the injection moulding machine fills the mould with molten plastic resin and then ejects it.
In the plastics industry, they tend to refer to the moulds they make as “tooling” rather than moulds. Different iterations of the tooling refinement are numbered from zero upwards, so that the first time you use your tooling and inspect the results it’s called “T0” (tooling zero) and then “T1”, “T2, “T3” etc. In general, even experienced tooling manufacturers have some minor imperfections on the first test of the tool for the “T0” proofs such as excess flashing (the bits of plastic that bleed out where the two halves of the mould meet), sinking or warping due to too much heat or thick walls, etc. Most companies expect that by T1 or T2 iterations, all these defects are gone….but sometimes it can take even longer to get things perfect!
If there are major issues with the tooling, which has various sections cut from a solid block of steel or Beryllium-copper, you order more tooling steel (very high quality, expensive steel), and literally start cutting all over again. Ouch!
Even a part that’s small and looks relatively simple can actually be very complicated to make a mould for. Any time there are details that are not on the top or bottom face, you need to employ side-actuators — hydraulics that move a piece of tooling in and out and create the impression or holes that you need. Add relatively high pressure and heat to the mix, and you can see why this is a very specialist job and you want to pick your partners wisely.
To be able to get the moulded part to release from the mould, you’ll actually find that the majority of parts have a slight angle on their sides called “draft”. Designers generally design their parts square, and then the tooling engineers analyze the part using a process called “Design for Manufacture” (DFM) to decide how much draft to apply to the sides (usually between zero and five degrees) and other recommendations to make sure the part forms correctly and will also get ejected from the mould.
To further complicate things, you require different kinds of machinery to cut the tooling steel. The first is a machine called a CNC mill (Computer Numerical Control). The name CNC is a leftover from years ago when these machines had big display pads covered in buttons that allowed a ‘programmer’ to set the machine’s tool path and make cuts (in three different axis) out of the material being machined. These days, the machines are hooked up to computer systems and literally take a 3D CAD file (such as the ones we design in Autodesk Inventor) and cut the shapes out of a block of steel.
CNC has some limitations:
– Because it uses a bit that looks like a drill bit, it’s virtually impossible to get “squared” corners with CNC. You can keep reducing the diameter of the bit to get closer and closer to square, but that approach follows the law of diminishing returns.
– Getting completely smooth faces and edges is hard. Even something that looks relatively smooth in steel looks very poor once reproduced in plastic. Consumers expect very good finishes in plastic.
– While CNCs are very, very accurate today, somethings even tighter tolerances are required. Imagine how tight the fit needs to be between two parts that need to be weather resistant.
– Small features are hard with CNC. There comes a point where the smaller CNC bits are too fragile to work with and they break a lot, especially with a material as tough as tooling steel.
So, CNC is really used to create “rough tooling”. But what happens next?
Enter the EDM (Electrical Discharge Machine). I’m pretty comfortable around most machines, but the EDM machines scare me a bit 😉 Imagine controlled lightening and you get a pretty good idea about how this machine removes tiny pieces of steel in a highly controlled manner. The EDM machine is used to smooth out surfaces by taking away continuous, tiny ‘spots’ of material and then moving the electrode head horizontally or vertically.
After traditional EDM is done, for our parts, we’ll also use a variant on EDM called “wire EDM”. This uses the same principle of operation as EDM, but is used to make very smooth and tight tolerance cuts in material like a very, very accurate bandsaw. If you think of the edges of our Wimoto enclosure, this is how we make the sides and edges nice and smooth. Here’s a video of the EDM process that explains things better than I can.
At this point, you should have a relatively good quality, detailed mould. There will still be some surface imperfections, albeit tiny. There’ll be some ‘slag’ left behind from the EDM process, for example. The moulds are polished with abrasive materials to clean the imperfections and provide a slippery surface so the plastic won’t stick.
It’s time for the test shots. The moulds will move from the machining floor of the factory to the production floor and be inserted in an appropriate machine. The machine is chosen based on the number of parts per hour that need to be manufactured and the amount of material at a desired pressure it can force into the mould.
The injection mould machine is loaded with a sample quantity of the chosen resin. Resins can be simple such as the ABS (many consumer electronics products) and Polyethylene (think milk and juice containers) we all know and love. Or they can be more esoteric. Our resin falls somewhere in the middle and is a mixture of ABS (relatively inexpensive and the most common plastic for parts), polycarbonate (for strength and precise details), and a UV additive to stop the plastic from degrading in sunlight (which contains ultraviolet light).
There’s a lot of science in injection moulding and engineers calculate the flow characteristics of the resin to make sure it’ll reach every nook and cranny of the mould, but sometimes nature has a way of interfering. So, multiple test shots at different temperatures, cooling times, and pressures are done to find the combination that produces the best, most consistent results.
One of these “T0” test shots is sent to the customer (us) for review. This will happen in the next couple of weeks for Wimoto.
Assuming there are no major “fit and form” issues, we will approve this T0 shot and the factory will again polish the moulds to get closer and closer to the final finish quality. More test shots will be performed, and some will get sent to us for approval.
Finally, once everyone is happy with the mould and how it performs, texture will be added to the mould so that the final part has a slightly texture and doesn’t feel slippery to the touch.
During all of the above, the mould is travelling back and forth between the workshop and production floors. Bear in mind that even a tiny mould can weigh 10kg+ (20lbs+) and large ones can weight hundreds of kilograms/pounds.
I hope this post helps explain a little more about the manufacturing of the enclosures and the journey we’ve been on. Below are some pictures of our ‘rough tooling’. You can see the CNC machine marks that need to be removed with the next steps quite clearly. The copper coloured pieces are called “inserts” and sit inside the master mould. The grey steel pieces are the cavity itself and you can see channels and gates for the molten plastic to flow through.
Feel free to ask questions or provide feedback. Hopefully you’re enjoying these posts and they’re making you feel more involved in the process and closer to the action! 🙂