For several years, Maximum Velocity has been offering a bulk car kit similar to kits offered by BSA, PineCar, et al. When we first introduced the kits, we used PineCar-brand wheels. But in the spring of 2010 we decided that the time had come to have our own “MV” wheels. Thus, we entered the world of plastic injection molding.
We decided that if we were going to have a wheel made, it had to be very high quality. So we chose the best characteristics from the various wheels on the market:
- Internal, three gate injection: Improves wheel accuracy (no mold mark on the tread surface),
- Hard plastic (actually styrene): Less flexibility to improve performance,
- Solid sidewall: Results in a stronger and more accurate wheel,
- Light weight: Lighter wheels are faster,
- Clean transition from tread to sidewall: No tread marks on the edge of the wheel.
The end result is shown in Figure 1.
Figure 1 – MV Wheel Design
The next step was to find a molding company that would work with us to get a high quality wheel. We were fortunate to find a company (MoldWorx1) in our area that was interested in the project. Then came the education; I learned that injection molding consisted of four phases:
- Part Design Phase
- Mold Design Phase
- Mold Creation Phase
- Injection Molding Phase
Part Design Phase
The design phase consists of creating a 3D model of the part to be molded. During this phase, the design of the part is usually adjusted to allow for manufacturability. The end result is a 3D model of the part, agreed upon by both the customer and the molding company.
Figure 2 – Blueprint of 3D Model
Mold Design Phase
Next, the mold itself must be designed. This is where the injection method and the runner system must be decided upon. “Runners” are the paths for the molten plastic to get into the mold cavities (and the resulting waste product – see Figure 3). If you have ever built a plastic model, then you will recognize the waste plastic connecting the various parts as the runners.
Figure 3 – Runners on MV Car Parts
The runner system shown in Figure 3 would be used on wheels that have the mold injection point on the tread of the wheel (like old BSA wheels and PineCar wheels). To have the injection point on the interior of the wheel requires a more sophisticated runner system (see Figure 4). In this mold, when the wheels are released they are automatically freed from the runners. The only sign of the runners are the three small dimples on the interior of the wheel. Although this method generates more waste plastic, the three gate system results in the most round wheel (least runout) available from injection molding.
Figure 4 – Runners on MV Wheels
The final mold design is in two parts (A and B), and is shown in Figures 5 and 6.
Figure 5 – Side “A” of Mold
Figure 6 – Side “B” of Mold
Mold Creation Phase
Next, the mold must be manufactured. This is the most costly part of injection molding as the mold must be precision machined so that it functions properly and produces high quality parts. Most of the mold is fashioned on a computer driven (CNC) milling machine. However, much of the detail in the mold cavities is too fine for the CNC. This fine detail is fashioned using custom made graphite electrodes. When high voltage is applied, the electrodes etch the desired detail into the mold cavity. The electrodes self-destruct so many of them are required. For the eight-cavity, MV wheel mold, eighty electrodes were used.
Injection Molding Phase
Finally, it is time to make the parts on the injection molding machine. The machine has several sections, shown in Figures 7 through 9.
Figure 7 – Plastic Pellets in Hopper
The hopper holds the styrene pellets. The white pellets are the base plastic, while the black pellets are added to adjust the color.
Figure 8 – Heating Section
The pellets are fed into the heating section, where they reach the proper temperature for injection into the mold.
Figure 9 – Molding Section
The mold halves are pressed together, injected with plastic, cooled with water, and then pulled apart. The parts are then ejected and fall into a bin below. The runners are grasped by a robotic arm (partially seen at the top of the photo), lifted out, and dropped into a grinder. A portion of the ground up runner is fed back into the pellet hopper. The rest is discarded.
The cycle time of the mold machine affects both the quality and cost of the part. The major step that affects the cycle time of plastic injection molding is cooling the part area – the better the cooling system, the quicker the material will “set”. This also happens to be one of the most expensive areas of a mold. Typically, more dollars spent on the cooling system results in a faster cycle time and a lower part cost.2
When the cycle is slow, the parts cool down while still in the mold so that they are fully hardened when they are ejected. This achieves the highest quality part, but the longer cycle time means that the machine must be run for a longer period of time to finish the production run (higher cost). On the other hand, if the cycle time is faster, the cost is lower but the parts are still malleable when ejected. This can cause slightly warped parts.3 So, quality and cost must be balanced to achieve the best cycle time for the part being molded. For the MV wheels, the cycle time was set such that the parts were adequately cooled before ejection from the mold.
The final result of this project was the MV wheel shown in Figure 10. These wheels have a lower variance than most pinewood derby wheels.
Figure 10 – MV Wheels
The MV Wheel specifications are:
- Wheel Diameter – 1.192 inches
- Wheel Tread Width – 0.319 inch
- Wheel Bore Diameter – 0.095 inch
- Wheel Weight – 2.4 grams
1Visit the MoldWorx web site at: www.moldworx.com
2As an example, MoldWorx told me that they saw a 128 cavity mold run at a 5-6 second cycle time (very fast). But the customer likely spent $750,000 for that mold.
3This is a major reason why there is so much variance in most pinewood derby wheels, even within a given mold number. If the cycle time is a little too fast (which is very common, as lower cost is the main objective for most companies), the wheels will still be soft when ejected, leading to a slight warpage.
From Pinewood Derby Times Volume 11, Issue 8
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