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July 7, 2006
5 Min Read
Design and build a mold per part design XYZ to produce parts within tolerance and last for ABC cycles.? This is the standard wording on a purchase order, in which you and your company are betting your paycheck that this mold will generate product. Sometimes it even includes a reference to a set of specifications. But, it?s those pesky little bits we tend to forget to specify that can give the mold specifier and the molder a competitive edge.
The Little Bits
Mold life. Mold life is a constant source of contention because buyers think a mold will last indefinitely regardless how it is specified. Let?s assume you?ve specified the mold to last for 500,000 cycles. But within its production history the mold has lived at three different molders. How do you know how many cycles were run? The buyer will tell you the mold isn?t run into the ground but the molder is saying it?s dead. Who is correct?
Consider a second example. If you are making something proprietary, e.g. trendy eyeglass frames, you?d suffer a substantial loss if the molder ran thousands of extras and sold them off to a knockoff manufacturer. In this case you?d be obsessed about knowing exactly how many cycles have been run for production, scrap, and startups.
An inexpensive cycle counter that records up to seven digits can be buried in the mold under the ID plate. It is actuated by the ejector plate. When you want to know how many times the ejector plate has opened and closed, you remove the ID plate and read the number.
Deep-draw parts. If you calculated the amount of surface area of a deep part in the cavity, then the same calculation on the core, you?d find more square inches on the cavity side. Ejection has two suction breaks. The first is breaking the suction that allows the part to stick on the core side. The second is the part breaking free from the core. When considering cycle times, many molders will tell you if you open the mold too early it will stick to the cavity; if it doesn?t it will ?crinkle? during ejection. Because of this, some very expensive seconds are added to an ideal cycle time simply to get the part to eject.
The argument is always, ?losing time is always better than losing parts.? But what if you didn?t have to lose time? For a very small amount of money you can install air valves, sometimes called poppets, that range from 8-18 mm in diameter.
The poppets can be passive, being vented to atmosphere, or they can be actively pressurized. Using these air valves can easily allow between .5-1.5 seconds reduction in the overall cycle time, as long as the dimensions are still within the tolerance.
Closing up on a cam or ejector pins that aren?t fully returned. Return pins bend, hydraulic cylinders leak, springs break, spring-loaded ball bearing detents get jammed. This is only the short list of what causes the mold?s cams to be out of position when the mold closes.
Because of the natural resistance of a cam or rack-and-pinion, many technicians set the mold protection higher than normal so that the cam action doesn?t prevent the mold from closing.
Murphy?s Law clearly states the consequences of this thinking. Things that move wear out. Cams tend to fail simply because they move. The protection you can buy is a simple switch that will electrically not allow the mold to close until the cam is in its proper position.
You can also install switches under the ejector plate to stop ejector pins from either imbedding themselves into the cavity steel or being bent so they will break in a short when the ejector plate isn?t fully returned. The cost of these switches is cheap insurance to keep production going.
Better flow through your waterlines. In small molds there?s a tendency to put waterlines very close together. The setup tech is then forced to use large loops that on the ejector side of the mold have this nasty habit of rubbing on the doors of the machine until they ultimately leak. When the tech shortens the line, it gets kinked and its flow is restricted.
The problem is readily solved. Instead of buying straight male connectors, specify 45° or 90° connectors.
This is especially useful when cooling movable cores and space is at premium.
Purchase Productive Capacity
There are several companies that make their living selling gadgets that will improve cycle times, reduce setup times, and the like. While it is impossible to specify all of them in a set of tooling standards?if it gets beyond 30 pages nobody will read it anyway?it is incumbent on the engineer or specifying buyer to make sure that any component that aids production should be specified on the purchase order.
Remember, you never buy a mold. What you?re purchasing is productive capacity. The faster the mold runs, with the fewest unproductive episodes, the more profit the mold will generate.
Bill Tobin is a frequent contributor and consultant who also publishes an independent monthly e-newsletter with tips on injection molding. E-mail [email protected] to request to be put on the circulation list.
WJT Assoc., Louisville, CO
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