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March 9, 2004

5 Min Read
Tooling Corner: Using the science you have

Editor?s note: Bill Tobin is a plastics processing consultant and president of WJT Associates.

Most mold designers use 3-D simulations. Most customers of mold designers want filling and cooling simulations (?Sim? or ?Sims?). Sadly, most of the people who look at these Sims haven?t a clue what they are looking at. The Sims are usually nothing more than pretty pictures with rainbow colors that are supposed to show something.

However, these finite element analysis (FEA) programs give some interesting data: filling pressures and cooling times. If we go back to the basics, it is possible to optimize a cycle and save money using this data.

The basic principles in molding are:

  • Fill with the fill.

  • Pack with the pack.

  • Cool with the cool.

  • Eject smartly.

  • Close the mold and go back to the beginning.

    The basic (generic) excuses for ignoring the facts are these:

    1) We always did it that way (so why change?), or,

    2) It?s convenient (it?s worked in the past, so why not now?).

    Let?s stay away from the generic excuses and play with the cards we?ve been dealt: You?ve run the Sim on the part, and you know you can fill it, and you know it will cool with minimal distortion. You?ve made this Sim work, assuming some gate sizes and locations. But how are you going to get the plastic to this gate at this pressure?

    Go back and independently do a Sim on the gate and runner system. Round runners accommodate almost all gate configurations. Trapezoid, half-round, and all the other generic runner geometries tend only to support subgates. Using your experience (generic excuse #1), size the runner system and the gate and see if you can actually deliver the required pressure to the gate through the machine. Relying on experience usually makes the runner system too big.

    Now look at the runner as it cools. Will it cool before the cavity is filled? If the runner is too small, it will. Remember, an increase in diameter is a square function of cross-sectional area. A small increase in diameter is a big increase in the ability to fill.

    Now look at the gate. The rule of thumb is this: Use between 50% and 100% of the nominal wall thickness as the depth of the gate, with as much as possible as the width. This is your window to fill and pack with. The cross-sectional area shows how easily you can fill. The thinnest cross section gives you the time it takes to freeze off.

    It is always interesting to see folks make round subgates. Because they are round, the diameter determines the fill and cooling times.

    But what would happen if the gate size was a ?cat?s-eye?? A cat?s-eye gate intersects the edge of a blunt subgate. The gate cross section is an oval with sharp ends. Its surface area is large compared to its cross-sectional area. A cat?s-eye gate fills faster and shuts off faster than a conventional subgate.

    Calculate the time you need to fill and see if the gate will still be open when you get to the packing portion of the cycle. If the gate freezes prematurely, you?ll either be cold injecting?forcing solid material through the gate?or you?ll continually get short shots and sinks in your part.Finding the optimum runner size is usually an iterative process. To keep the guessing to a minimum number of Sims, use the Newtonian method: Use your best guess for a runner diameter as your first trial. Use something much smaller as your second. Your next try should be somewhere between the two. Look at this set of numbers and times compared to your first two trials.

    This midpoint trial should show a dramatic improvement over either the larger or smaller trial. Throw the useless trial out. You can now re-center somewhere between your mid-point and whichever other set of data worked best, and so on. Usually, with one or two more guesstimates, you?ll zero in on the optimum sizing.

    Now that you think you?ve optimized everything, do a sanity check. Can you still fill with the fill, and pack with the pack? If not, go back and begin again. If you can, continue to the next paragraph.

    To cool with the cool, look at the part and the runner. The cooling is determined by the thickest cross section of molten plastic. Use your Sim again. The time it takes the thickest cross section (usually the runner) to get below the heat distortion temperature determines the cooling time.

    Assume your ideal runner size is .213 inch diameter. It?s not likely you?ll ask the toolmaker to hop down to Cutters R Us and pick up a few 5/47-inch radius ball-end mill cutters. Unfortunately, the mold builder usually gets the most common close cutter and uses that (see generic excuse #2). This is not good. CNC machining matches full round runners easily. Molds cost as much as a nice car or an excellent house. Why not use custom cutters? Making a slightly larger runner will cost you in cycle time and material every time the mold opens. Using custom-built cutters costs you only once.

    While most FEA programs have some significant predictive defects in them, you can use the information you have to make some good guesses. Remember, if you make the runner or gates too small, it?s easy to make them larger. If they are too large, most molders write off this long cycle time to one of the excuses above to avoid the expense of welding and re-machining.Molders don?t sell parts. They sell machine time, materials, and expertise. Expertise determines your profit.

    Contact informationWJT Assoc., Louisville, COWilliam J. ?Bill? Tobin(303) 604-9592[email protected]

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