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The Making of the MV Wheel
By Randy Davis
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
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
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
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
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
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.
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.
There was a young Scottish lad named Angus who decided to try
life in Australia. He found an apartment in a small block and
After a week or two, his mother called from Aberdeen to see how
her son was doing in his new life.
"I'm fine," Angus said, "But there are some really strange people
living here in Australia. One woman cries all day long, another
lies on her floor moaning, and there is a guy next door to me who
bangs his head on the wall all the time."
"Well, ma wee laddie," says his mother, "I suggest you don't
associate with people like that."
"Oh," says Angus, "I don't, Ma'am, I don't. No, I just stay
inside ma apartment all day and night, playing ma bagpipes."
Today's special: Cars with tungsten canopies
Spiderman-mobile - Robert Stevens
Last year my son participated in his first pinewood derby and won the
grand championship for his pack. This year was his second pinewood
derby and his second grand championship for his pack. We took the
Predator design and turned it in to the Spiderman-mobile. We
implemented the rail-riding alignment technique using the Pro-Rail
Rider tool. Thank you for all that you do to help boys have success
with the pinewood derby. It has really helped build my son's
confidence and esteem.
Predator - Michael Tances
I raced this car in the adult class in 2010. The car won all the races
and had faster times than the pack winner. It is completely stock and
is exactly 5 ounces. I'm 64 years old and still like to win!
Race for the Cure - Kimberly Paul
This car took first place in the open division at our pack here in
Tulsa. The car ran really well thanks to the tungsten canopy purchased
through Maximum Velocity.
Share your car with our readers!
Do you have a car you would like to 'show off' to our readers?
If so, send us a photo of your car along with a brief description of
any special features. Also, please include your full name. If selected, we will include the photo and description in this newsletter.Please e-mail photos to:
Photos must be sent by e-mail in JPG format (minimum size of 640x480, maximum size of 1280 x 960). Please shoot photos from the front left of the car, similar to the orientation of this car:
Send only one photo per car, unless an additional photo is needed to adequately show a feature. Also, only one car per subscriber per year please. Thanks!
I have a specific question regarding the area where the wheel
touches the car body. I have heard that some people place small,
round stickers there before painting and then remove them after the
paint dries. Then they rub graphite into the wood area to reduce
friction. Do you recommend this technique? Or do you just paint the
car to a glossy finish, then rub graphite into the area (we plan to
clear-coat the car)?
A clear coat provides a slicker surface than bare wood. So just rub
graphite onto the wheel hub (not the car body). That way the body
will stay cleaner and you will have graphite only where it is really
needed. If you were not going to clear coat, then you might want to
mask off the area and rub graphite into the wood.
I have grooved axles, and wheels that have had their inner and
outer hubs coned. Since the outer hub is coned is there a need to
taper the backside of the axle head?
If you are using wheels that have a tapered outer hub, then, no,
you do not need to taper the axle head.
I have used tungsten cylinders inside our cars for the past few
years with great success. I was thinking of using the tungsten plates
on top of the car this year. Does placing the weight on top of the car
affect the speed or handling (versus being inside the car)?
Generally, lower weight provides better stability. So, if
possible, I recommend locating the plates directly on the bottom of
the car (or recessed into the bottom of the car).
Do You Have Questions that Need Answers?
Do you have a pinewood derby-related question? If so, send your question to: firstname.lastname@example.org. We answer all questions by e-mail, but not every question will appear in the Q&A section
of the newsletter.
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