Testy Plastic Tubes

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A little venting, a little runner sizing, and badda bing! Perfect parts.

This month I received a runner system feeding two identical parts. The parts were clear, which means a polystyrene, acrylic, polycarbonate SAN, clear ABS, or some other amorphous material was being used.

Testy plastics tubes For once, a note was enclosed with a phone number to call and ask for Tom. So I called and found out that Tom, who happens to be an acquaintance from many years ago, was having trouble with this runner system causing a small percentage of short shots in this tube-looking product.

A day later I started to review Tom’s parts and, sure enough, it was very noticeable that he was having issues with sprue and runner sizing. But was that all that needed to be changed? I looked the parts over carefully with a 5X magnifier and could see a sharp corner where the ring gate attached to the parts, but this was OK since this design detail helped to initiate the degating process of separating the parts from the ring gates.

I checked the end-of-fill areas for evidence of lack of venting and found surface burning where the air was being compressed by the material being packed here. Trapped and compressed air shows up in three stages: First, we see dullness on the part’s surface; second, it turns to a grainy look; and third, it burns the part surface and produces a black residue. This part only showed evidence of stage one.

Sizing for the whole package
So we had two areas to fix: the undersized sprue and the inadequate venting. The sprue O-diameter was .160 inch and the root diameter, or where it attaches to the runner, was .260 inch. This was not big enough for the length of flow this material had to travel to fill and pack the parts. This could have been the source of the whole problem and I could have stopped there, but you know me; I wanted to get it all sized correctly so we could forget about this mold ever being a problem.

I moved on to the main runner feeding the gate. With a ring gate of .140 inch, we needed a main runner diameter or depth equal to this dimension or as much as 50% bigger to allow the material passing through this section to travel without restrictions.

This made the requirement for main runner sizing at least .140 inch deep and possibly as deep as .210 inch, instead of my measurement of .130 inch. I preferred .210 inch because an additional benefit could be lower barrel heats and a faster cycle time for old Tom.

Next, I looked at the sprue O-diameter. With our newer, larger .210-inch main runner, the sprue O-diameter needed to be .312 inch.

I had difficulty estimating the nozzle orifice size as it left vestige on the end of the sprue. With the existing sprue O-diameter at .160 inch, the best measurement I could make put the nozzle orifice diameter at either .050 or .060 inch. I suspected it was .061 inch since this is a common pilot hole size drilled in a new nozzle. But I could tell this nozzle orifice hadn’t been drilled out to what it should have been to meet the sprue, which was .250-.290 inch.

This was all that was needed to optimize this sprue and runner system. Can you imagine how much better this mold would run with these restrictions to flow and the bad pressure losses eliminated?

Venting
Venting this part was a little more difficult than most thanks to its long length and the amount of air in the cavity and core area. About all you can do in a case like this is to use vacuum venting or cut a relief area where the cores come together. Now, you could just run the vent to atmosphere, but I prefer to cut the relief area into the core, and then vent across from where the air is being trapped with the same vent dimensions we would use for normal parting line venting.

In this case, the vent needed to be .001 inch deep and .200 inch wide, with a land of .040 inch. Then from this .001-inch-deep vent, drop into a .040-inch-deep channel to where the relief area has been cut into the core. The vent lip should be draw polished to make it self cleaning. That was all it would take to get rid of the slight amount of dullness on the end of the part.

I gave Tom a call, got his credit card number to cover the cost of my review, and filled him in on the details of my proposed changes. He was glad to get the suggestions, and then asked me where to get more information on how I arrived at these new numbers. I told him they were in many of the 100 case studies in my book (On the Road with Bob Hatch).

Another point I made to Tom is that I did not know the material being used, but if I had I would have used the free section of the database at www.ides.com to look up the current material and check the melt flow to see if changing material would have additional benefits for this particular mold. All in all, it was a good optimizing job for an old friend and I collected some consulting money in the process.