on the right, two star assemblies that snap together, pin inserted between
Injection Molding—Star and Pin
This might be the most challenging assignment of the term.
Making an injection mold is a complicated process. First you must envision the part you are making, this week something like a Lego block, a part that can be assembled into a larger whole. This creates issues of fit and tolerance, including shrinkage during the molding process. Second you must be able to describe, in IDEAS (an extremely powerful program with an equally backward interface) a CAD model of your part, in reverse as it will be cut into an aluminum block. Third, you must learn to use the injection molder, a step I have not yet taken, since the molder was broken until the day before the final review.
Did I mention that you have to make a toolpath and use the CNC mill?
This entire process takes folks in industry quite some time and for good reason they charge thousands of dollars to produce such a mold. We are given one week.
This is the concept diagram for the parts that I want to make. The pin will fit into the star assembly. Each mold will produce one pin and one star, and since two stars assemble together with mortise and tenon fit, this mold will produce twice as many pins as stars.
on the right, two star assemblies that snap together,
pin inserted between
First, I made a CAD model in IDEAS. The simple pictures below can hardly betray the aggravation of learning to use software that will be replaced soon by something easier to use. Unfortunately importing an IGES (International Graphics Exchange Standard, or something like that) does not work, so forget about your favorite CAD modeler and prepare to spend hours in IDEAS trying to figure out how to snap to point.
[ open GL cad model image coming ]
Second, I made a toolpath in IDEAS, encountering many issues along the way. For example, if you want to cut a 1/16" pocket, IDEAS will not let you use a 0.0625" cutter to do this—it must be a smaller diameter—the largest that worked for me was .060". This will result in a slight error in your part as the tool center is not accurately described. But for 2.5 thousandths of an inch, I didn't blink an eye. OK, generate toolpath, I'll file off that 2.5 thou later...
[ toolpath image coming ]
Third, I went to the Compumill and spent hours learning how to use it. The tutorial helps but the instruction manual in the notebook, as well as John DeFrancesco's assistance, were invaluable. Also Juan Rivas and Jorge Martinez helped me a great deal. Thank you all. Below the final part is shown.
injection mold closed, black circle is around one alignment
pin
injection mold open
Note that I made an error cutting the sprues, the channels where the plastic will flow into the mold. This is because for the life of me I couldn't figure out how to get IDEAS to cut these surfaces—some of the simplest in my model. But I think that the mold will still work, though it will waste a little plastic each time. Also, I have not yet dealt with the air vent, that is required to allow the plastic into the mold channels—I will make that by just passing a sharp object across the mating faces.
I hope that eventually somebody will fix the injection molder. Still to be seen is whether the sprues function, whether the parts will twist off the little tree formed by the sprues (i.e. does the constriction at the end of the sprue work well?), and whether the star assemblies fit together.