Making Plastic Balls; Solid or Hollow
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?
An open mind is a virtue...so long as all the common sense has not leaked out.