Making Plastic Balls; Solid or Hollow

[Geof]

In the thread on a home made injection molding machine I asked this question; "...what plastic is used for the balls in track balls? I want a quick/cheap and precise way to make rigid plastic balls 2.750" diameter that are accurate to +/-0.001 for size and concentricity. Quick/cheap means less than $20.00 including material and labor costs."

plastibob provided one answer:

The trackball that your referring to is some type of thermoset material, like a cellulosic, polyester or melamine.......Like anything the more you get the lower the cost, I'm not sure what kind of quantity your looking at.

My answer and comment was; "Initially a hundred or so, but I don't really expect to find anyone to supply the precision I am asking for at $20 each for this quantity. Which is why I am interested in finding out what is the material with the idea of injection molding blanks that could be machined to size and precision."

mactec54 answered:

Do you want it to be a solid or just a thick wall I did mold some ball's at one time around 1.750 dia after molding they were tumbled & polished they came out to be with in .0001 no machining needed a lot of hour's in the tumbler to size them

So I figured I would start my own thread to give more explanation for what I am getting at, pick brains and get feed back on some maybe nutty ideas.



Do I want them solid or thick wall?

Yes, and this is not really a facetious answer. I am working, with others, on developing a trackball intended for use by people with a physical disability such as limited motion, poor muscle control leading to spasmodic motion, muscular dystrophy which results in loss of strength. Our concept is to have both solid and hollow balls and also incorporate other ideas to make the balls easy to move when stroked gently but resistant to sudden fast movement. And of course everything has to be done as in-expensively as possible because financing is limited.

Incidentally how do you mold a hollow ball? Rotational molding?

I had this idea of making molds that were hollow metal hemispheres that screwed together to form a hollow ball. These would be filled with a charge of some plastic (I don't know what), screwed together and then put in a heated tumbler or some device that rolls them around randomly so the charge gets distributed evenly over the inner wall of the mold. After a while they would be cooled down, which would cause the plastic to contract so it would easily fall out when the hemispheres were screwed apart.

Making the hemispheres precisely enough so that when they were screwed together the inside formed a spherical cavity precise to my +/-0.001" tolerance should not be too difficult. However, there is the problem of compensating for thermal expansion of the mold and then an even greater thermal contraction of the hollow plastic ball formed in the mold, but the plastic balls should have good concentricity; maybe we can figure out how to accept a +/-0.01" in diameter provided the ball is concentric.

Comments? Suggestions?

[mactec54]

Hi Geof

To make the balls hollow is quite easy you make the normal injection mold & then when the shot is pushed into the mold a stream of gas is injected at the right time (it is called gas assist) which will give you a very nice part they could also be made from styrene & hard coated

[Geof]

Originally Posted by mactec54 .......To make the balls hollow is quite easy....

After reading your link I think your "is quite easy" exaggerates a little bit ; it looks like quite precise control is required over timing and temperature.

I would be surprised if this technique could produce hollow balls guaranteed to be balanced; i.e. with the cavity truly spherical and centered in the ball. This is necessary for a trackball otherwise the ball will tend to drift to put the center of gravity at the lowest point as soon as the user lets go. Also I rather doubt anyone would take me seriously if I tried to order 100 another 4 or 5 zeroes would be needed to attract attention.

[plastibob]

I don't think gas assist is viable for this application. That is a large dia and you wouldn't have a consistent wall thickness. I have used gas assist for thick handle type applications and I have always fought cosmetic issues on parts like that, thinner parts where you trying to save money it's a good option, (an example would be a coat hanger). I have made golf balls out of ionomer that have been blown with nitroge, however they were only 1.68 dia. One option may be to mold two halves and ultrasonically weld them with a shear joint, then finish them.

[mactec54]

Hi Geof

The other way is you have a machine shop it would/is not to hard to make the hollow balls that you need in 2 parts

[plastibob]

Originally Posted by Geof Incidentally how do you mold a hollow ball? Rotational molding?

yes. You also could blowmold a thin walled part, but forget about concentricity, plus you have vestige where the parison is squeezed off.

[ImanCarrot]

I'd contact manufacturers of pool balls or snooker balls.Perhaps your design could be modified to use something like that? Would certainly be less than $20 per ball I'd immagine. I think they make them using two parallell plates which rotate against each other- the only shape this can make is a perfect sphere.

Worthwhile looking into?

[Geof]

Originally Posted by mactec54 Hi Geof The other way is you have a machine shop it would/is not to hard to make the hollow balls that you need in 2 parts

That one I have sorted out, two halves screwed together and then finish turned. This is why I figured I could make spherical molds that screwed together to rotary mold hollow balls as I suggested above.

The problem with making plastic balls this way is that I cannot find a source for different colored plastic round stock, only black or white Delrin. It is also expensive on material because a lot of it gets machined away.

I had considered a variation on making two halves like plastibob suggests but threading and screwing them together rather than ultrasonic welding. I have a fully equipped CNC shop but do not have ultrasonic welders.

The idea of getting something off-the-shelf as ImanCarrot suggests has been tossed around but finding something the size we prefer seems to be impossible.

I am considering the rotating plate lapping method for down the road when this project is a success. For the short term I have a ball finishing machine that polishes the ball between two rotating cups; this is a standard method for making spheres out of rocks.

[mactec54]

Hi Geof

Rotational molding won't work for you as you will not get a consistent wall thickness,

Screwing together will put it out of balance as well

So you would have to have a material that you can glue together ABS/Acrilc Etc

Acrilc you could cast it into a mold glue the 2 halve's to gether & have any colour you want

Or you could do what I said to start with Injection mold with gas assist there are some good molders out there that could do this as I said EASY Geof not EXAGGERATING eather I have done this & the order was for 400,000 Ball's which we delivered
Most would/will charge $25,000 to $40,000 for a mold for this part so this is not for your part

So for a hundred or even 2 hundred balls they would be way past the $20 each

So make yourself a mold & cast them in Acrilic & have some fun doing it I mean fun Geof
because this is fun stuff to do

[plastibob]

There is no way your going to get an even wall with gas-assist. You could look into compression molding a solid which is how the track ball is made now. Casting is a good idea other that the weight may be too much.

[mactec54]

Hi plastibob

I could guarantee the part to have a wall thickness of no greater variation than .005 or better in wall thickness if done as I have said there are a lot of things you have to do to your mold to make this hapen

The parts that you have been trying to do like your coat hanger if you are not getting even wall thickness then your mold needs work also how you are controlling the gas flow plays a big part of getting this to work

Blowmolding you say has vestige not if you inject on the inside of the closed mold you only have a small hole in the finished part which you can plug after molding if you need to, This all comes in the design of product & mold as you will see in the photo no vestige is possible

[plastibob]

Generally speaking if your using gas-assist your not concerned with what the inside looks like. I have about 20 different gas-assist parts made over the years built by many shops and non of them had a remotely uniform wall through out the part. BTW I used the coat hanger as an example. My parts were high end consumer products and auto motive applications. In all fairness I have never needed the inside held, what was your part used for? As far as blow molding is concerned your either using a parison which you have to clamp off or a preform that has blow rod - which leaves the small hole your referring to, but I think that does Geof no good. I'm curious what are the types of things to do to the gas-assist mold?