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There is a common method to troubleshoot all molding defects. Before you begin, go through all the places to look and give yourself the reason (or lack of one) that would cause this defect. If you’d made good parts before, the only reasonable cause for any defect is “something changed.”Here’s an example of this thinking.

Bill Tobin

August 12, 2009

10 Min Read
Troubleshooting flash

There is a common method to troubleshoot all molding defects. Before you begin, go through all the places to look and give yourself the reason (or lack of one) that would cause this defect. If you’d made good parts before, the only reasonable cause for any defect is “something changed.”

Here’s an example of this thinking.

I was recently discussing flash with a guy. He whipped out his calculator, did some serious finger thumping, and then announced flash was not possible with the conditions at hand. Then I showed him the part, complete with flash.

All defects stem from a cause. The fickle fairies of fate don’t just come down and curse the mold. Flash is probably the simplest and most common defect to understand. Molded parts are actually created by the concept of flash: Molten material got into an area, cooled, and became solid before ejection. Simply put, if there’s a hole and the liquid plastic can get in it, it will.

Let’s go through the various areas of molding to see if we can highlight some of the causes of flash.

Environment

This is the plant’s cleanliness, temperature, operating procedures, or something else. The only cause of flash that can be considered in this section is some kind of mechanical blockage. I’ve actually seen screwdrivers left in the mold when it closed. The next shot had flash. The next day the employee head count had one fewer person.

Operating procedures are always a cause of every defect. You must show the plastic an acceptable set of conditions, the same way for every shot, if you want consistently good parts. These must be universal conditions, understood and agreed upon by every technician. Look at a molding machine—megawatt heaters, a couple inches of steel from the outside of the barrel to the screw, an excessive amount of pressure to drive the screw forward, and material shooting into a mold at such enormous speeds and pressures that unless we wrap the cavities in at least a couple inches of steel, they will crack. Now with that said, why should every shift have to change operating conditions because the plant cooled down at night and warmed up during the day?

Catch a clue: The plastic is too dumb and it’s too dark in the machine and mold for it to know day from night. However, there are a number of factors that could require the next shift to change the setup:
• If your process conditions are set too close to the edge, even a few degrees difference in the plant temperature can cause changes. During the day it’s hot. Process techs bring in 36-inch fans to keep the operator cool. Unfortunately, the fans also cool down the barrel and overheat the material. At night the fans are shut off, and the high settings must be lowered. And so the battle goes.
• A burned-out heater may be compensated for by overheating adjacent heaters.
• Every tech hooks up waterlines differently.
• The plant cooling water (the infamous and ever-variable tower water that is sensitive to how machines are hooked into the system as well as to the differential from the plant temperature to the evaporative ability of the tower) changes by a few degrees from day to night.

Equipment

Machine
1. Pressure is resistance to force. If the force on the plastic is greater than the force holding the platens together, the mold is blown open. This can occur through leaky valves, or if there’s no ability to maintain pressure heads, and so on. This requires a maintenance solution.
2. Using the principle in #1, if the platens aren’t square in relation to each other, one side of the mold will close up very firmly while the other will be loose. This can cause a drop in clamp pressure. This also requires a maintenance solution.
3. Overtightening bolts in the mold clamps will begin to pull the threads out of the platen. This will leave high spots on the platen. While the actual platens might be square to each other, the high spots will not allow the mold to close properly. This requires a maintenance solution: Check each time a mold is hung by scraping the platen with a straight edge. Any high spots encountered should be stoned flat.
4. While this can be either a machine or mold cause, look at the temperatures of each half. If they differ more than 10 deg F from each other, consider the hot half is usually the cavity holding the guide pins and the cold half is usually the ejector side holding the ejector bushings. Cool down the bushings and heat up the pins just before full lineup. It is possible to keep the mold from closing when the pins jam into the holes without full engagement.
5. Can the machine tonnage generate 3.5 tons per square inch of projected area for the mold face? If not, use a larger machine; a smaller one can’t generate enough clamp pressure.
6. The machine’s shot capacity should be between 20% and 80% of the required shot. Lower than 20%, it’s like shooting rabbits with an elephant gun—too much power and no control. Use a machine with a smaller barrel. More than 80% and you won’t get a consistent melt. That’s like shooting a Kodiak bear with a .22 caliber—it only makes it mad.

Mold
1. Components: check the part and the runner for an artificial vent—e.g., a paperclip, duct tape, metal shim
2. Look at the front and back sides of the mold before clamping it in. Are there any loose bolt heads stopping the mold from hanging square?
3. Dumb checkup: Use a torque wrench and tighten all the bolts on both halves of the mold to the same amount. Different torque can cause the mold to bend.
4. If this bending has been a continuing problem, look at the ejector system. Are there enough pillar supports so that the ejector plates aren’t bending under the pressure of injection?
5. Are the ejector-half holding plates thick enough to not bend? Do they have enough bolts holding them together?
6. Have you used Prussian Blue? (If you don’t know what this is, ask a moldmaker.) Put it on one half of the mold; then, under full clamp pressure, check to see if every cavity is shutting off. If not, machine away material beyond the shutoff to increase the effective tonnage as well as rework it.
7. Is the mold shutting off square? If not, why not? Fix it.
8. With a flashlight look into the leader pin holes. Remove all the crushed and impacted pellets/parts/crud that lives in the bottom of the hole. If one or more holes is filled with something, the mold can’t close regardless of pressure.
9. Plastic always fills a cavity. Did somebody close up on something? Or did they smoosh the mold on a part and bend or damage something? Operators would rather blame it on a freak occurrence. Regardless of who the freak is, fix the mold.

Process variables

Heat
1. Check the mold heat as described above.
2. Material heat has little to do with flash.
3. Mold heat has little to do with flash, except in the extreme case of heating the side with the guide pins and cooling the side with the bushings. Since these have a precision fit when almost closed if the pin is heated (expanded) and the hold is cooled (contracted), if it hasn’t seized up already, the mold will probably not fully close.

Pressure
1. Pressure only blows the mold open after it is filled. You can blow open the mold if several cavities are filling early, if you try to put too much material under high pressure, or if packing pressure is too high. Fill pressure and fill time are related.
2. Fill pressure is really only used to get the material moving. Once moving, its viscosity drops to where the machine usually can’t keep up with it. However, you can overdo it.

Time
1. Fill time should be the least amount until 95-98% of the mold is filled. The last 2-5% of the time will be consumed from going from fill to pack. If you slam fast-moving, low-viscosity melt into a full mold, it is relative easy to flash it.
2. Packing time should only be maintained until the gate freezes. It should have little to do with flashing. Cooling never flashes anything.

Speed
1. The speed of fill is what allows viscosity to drop. Fast fills coupled with adequate venting and appropriate melt temperatures fill molds and do not flash them. Switching to packing mode slows the speed, thereby increasing the viscosity, thereby reducing the possibility of flash. It’s like having a pickup truck full of bricks going 60 miles per hour and coming up to a stop sign. The whole trick is to know when to put on the brakes.
2. Heat is a minor component of inherent viscosity; therefore, the speed of screw recovery (also backpressure) should be considered. In an ideal process, the screw stops turning and is fully decompressed only a short time before the mold opens.

Position
1. This is a combination of both machine and process maintenance. The screw must return to the same spot every time. Thinking differently, the cushion must be maintained. Cushions are designed to pressurize the plastic in the cavities to offset the shrinkage of cooling material. However, trying to put 10 quarts of anything into a gallon bucket is overdoing it. Cushions too large don’t transmit pressure to the cavity; too small and there’s not enough material to transmit pressure. Find the balance. The amount of cushion will be determined by the return position of the screw. Since most processes are “switch on position,” if the screw picked up too much material, the mold would have already been filled before it switched to packing mode. Maintain the machine, monitor the process.
2. The full-open position should be the minimum space allowable to eject the shot. However, when a hefty setup tech gets in between the platens to dig out a stuck part, he usually doesn’t reset the platens. Position is proportional to speed and time. More distance changes the cycle time that changes the temperature, which changes the viscosity that could be a cause of flash.

Troubleshooting method
In short, here’s the process:
1. Look at the variables
• Environment
• Machine
• Mold
• Heat
• Pressure
• Time
• Speed
• Position
2. Find the cause
3. Remove the cause either by changing the machine, mold, or process

You’ll usually solve the problem. Remember that the first three causes require hard (permanent) solutions (a tooling or machine correction), which is a one-time expense. The last five causes usually use soft (process) solutions. This means every time you run the mold, you have to remember and implement this little tweak. Soft solutions are the most expensive and scrap-prone.

Do two setups and call me in the morning.

Consultant Bill Tobin ([email protected]) is a regular contributor to IMM. You can sign up for his e-newsletter at www.wjtassociates.com.

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