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The Troubleshooter, Part 36: Balancing family molds
December 30, 1999
9 Min Read
This article continues our series of troubleshooting reports from one of the leading on-the-spot problem solvers in the molding industry. Bob Hatch is manager of technical service and customer support for Prime Alliance, the Des Moines-based resin distributor. Before his present assignment, Bob managed a molding operation for 25 years.
When I ran my own molding shop, the molds that we hated the most were family molds. I say we because I was still learning and I generally adopted the likes and dislikes of those around me. They hated family molds so therefore did I.
I have changed my mind the last several years about family molds because I am gradually starting to understand them. A good friend of mine, Jerry Jenson, is responsible for my change of heart when it comes to family molds. Jerry convinced me that all I needed to do was attack family molds one cavity at a time, starting with the biggest part, and working my way down to the smallest.
So when a box arrived in my office with a runner and a couple of parts that were from a family mold, obviously molded out of acrylic, I jumped at the chance to use Jerry’s suggestions to solve the customer’s problems. First, I looked at the runner and could see that the sprue was too small to feed the runner system properly. Second, the runner was unbalanced and I knew this would complicate things. Third, I could see shear splay on the side of the largest of the three parts that were attached to the runner. Fourth, I could see long lands on every gate. Last of all, I could see that all three gates were the same size for each part, which is crazy since the volume requirement for each part was different.
So I started with the sprue and made a note to open the sprue bushing O diameter up to .312 inch from the current .155 inch. This would allow the nozzle orifice to be opened up from the current .130 inch to .290 inch. This change alone will pretty much quadruple the flow of material through the nozzle into the sprue using the same amount of fill and pack time. Plus we could gain a little more control over the fill speeds and times with the larger size nozzle orifice, sprue, and runners.
Balancing the Runners
The main runner is currently .250 inch and that should be just about right for acrylic. The subrunners are all .250 inch except for the one closest to the sprue, and it is .220 inch. This was probably done in an attempt to balance the pressure at each gate. Actually, for unbalanced runners, the subrunners should get larger as you move away from the sprue, with the subrunner closest to the sprue the smallest in diameter, and those farthest away from the sprue just a step down from the main runner diameter. This is just an old way of balancing unbalanced runners.
I always hate it when the runner plate is already cut and then they ask me what it should have been. I prefer to get in on the tool print early and help the customer size the sprue and runners correctly from the beginning, but that rarely happens. The sprue O diameter should have been .312 inch, the main runner .250 inch and the subrunners closest to the sprue could have been .150 inch. Then the next subrunner out from the sprue would have been .175 inch, the next one out would be .200 inch, and with just a little tweaking from the injection speed the mold would have probably filled out fairly evenly.
What can we do here when the main runner is .250 inch, the same as most of the subrunners? Well, I will suggest he recut the main runner to .275 inch, which can still be fed by the new .312-inch sprue O diameter without losing much pressure. Very little extra regrind will be generated by doing this and I’ll bet it will help the pressure balance quite a bit. This way the customer will have a step down for each of the subrunners, which we need. It won’t be perfect but if he wanted perfect he would have used a balanced runner system in the first place.
The gates are a different story. Gate depth is equal to 75 percent of the thick section of the part for acrylic, which would be 75 percent of a .200-inch ring around the part for the big part. So the gate should be .150 inch deep instead of the current .065-inch depth. What a difference right there! But to stay steel safe and gain the respect of toolmakers everywhere, I will recommend going to .100 inch deep from the current .065 inch.
The width of the gate on a bigger part like this would be about two times the depth or .200 inch in this case. The land should be one-half the depth, not to exceed .030 inch for any material. This gives us a gate .100 inch deep and .200 inch wide with a .030-inch land for the big part. The depth and land will be the same for the other parts. The only difference will be in the width, since the width of the gate determines the volume of material that will get through the gate during the fill and pack times.
This is where Jerry’s ideas about family molds could be used. Forget what I just told you, and try this. Cut the runners right up to within .030 inch of each of the cavities. Take a hand grinder and cut the gate in for the biggest part. Shoot a few shots and grind a little more until you get the biggest part to fill and pack using fairly normal heats, speeds, and mold temperatures. I like to run acrylic at 425F, with the mold at 90F, injection pressure at 800 psi, hold pressure at half that, and injection speed at medium or medium fast.
Now start with the next biggest part, cut the gate in, and adjust until you get it to fill. Repeat until you have all the parts filling and packing with the same heats, speeds, and pressures needed for the first part. You can run production this way, or take the mold out and have the toolmakers clean everything without changing the hand-ground dimensions. It sounds pretty crude, but it works for Jerry, and I have heard him tell many stories about how this technique saved the day for him.
Venting is pretty important for acrylic parts, so I suggested this customer look at the areas to vent. First, vent the runner at the sprue puller and at the end of each runner by cutting in vents that are .003 inch deep, as wide as the runner being vented. Go out from the parting line .060 inch with the .003-inch depth, and drop into a .040-inch-deep channel to atmosphere. I know .003 inch seems deep, but I like to feel the feather flash at the ends of the runner so we know we are getting rid of the air.
Part vents, on the other hand, are only .001 inch deep and I recommend either perimeter venting or at least one vent per parting line inch. Perimeter vents can be used on every kind of part, but they seem to work really well on round parts. Remember that the depth of the vent is what causes flash, not the width. This is why we can use perimeter vents and not get flash at the parting lines. Perimeter vents are .001 inch deep with a .060-inch land that drops into a .040-inch-deep channel to a race track that is vented to atmosphere.
Individual vents are cut in at one vent per parting line inch and are still .001 inch deep but .200 inch wide, with a .040-inch land with a .040-inch-deep channel to atmosphere. All the vent lips (runner, perimeter, or parting line) need to be draw polished to an A1 finish to make them self-cleaning.
So our suggestions for this customer are to increase the sprue bushing, the nozzle orifice, and the main runner diameters. Leave the subrunners like they are and vent the runner to get rid of any excess air. The gates on all parts will be sized according to their individual wall thicknesses. The width of each gate will depend on how much plastic is needed to fill and pack each part.
Small parts will have a gate with a width equal to the depth, while larger parts will have gates two or three times wider than they are deep; the land is always one-half the gate depth, never to exceed .030 inch. With these three parts, I would use a gate the same width as it is deep for the small parts; on the larger part I would make the gate twice as wide as it is deep.
I faxed a note to the customer relating these changes and waited a few days to let the changes soak in. Then I called to get his reaction. He didn’t seem to be upset with the recommendations when I spoke with him. He said he gave the changes to the toolroom and they were working away, doing their thing. He promised to call when the changes were finished and when they had a chance to get the mold in and run some samples.
Sometimes it seems to take so long to make changes in the toolroom, especially when I know these changes could be made in a couple of days. I guess the toolmakers just don’t like making promises they may not be able to keep. Of course I don’t always know how busy they are, either.
Anyway, I heard from them a few days later and the parts were running a lot better and so far they have their fingers crossed hoping to keep the parts running as sets and without shear splay, short shots, or cracking at the gates. I have a better feeling about how well these parts are going to run from here on out than they do, but I like to have a positive attitude about things such as these. Someone told me a long time ago that the optimist and the pessimist end up at the same points in life, but the optimist has a better trip. I suspect most toolmakers are pessimists and most molders are optimists. It kind of fits, from my vantage point.
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