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the little bit of extra, where did it come from?
Definition: The thin, surplus web of material which is forced into crevices between mating mold surfaces during a molding operation, and which remains attached to the molded article.
Flash, this defect which may be caused by numerous faults of the mold, process, and material and is one of those simple but not simple problems. What is meant is that one has to first determine when the flash is occurring. In a simple explanation flash is the result of putting too much material into the cavity. Yet it may also mean that the clamp did not hold, or the tool is damaged. In general as stated previously flash is that excess material hanging onto the part.
When did it occur? Did it occur on filling the part, or did it occur on packing out the part? To determine this, one can simply take pack and hold pressure off of the press setting. Please note that on some presses one may have to also take time off of the setting to prevent packing out. The results than have to be observed:
Is the part the same with flash? If flash is still there this than would mean that cavity is filling and packing (to a degree) on the fill stage of the setup conditions.
Is the part short? If so than has it flashed?
If no flash and short than one can than conclude that the part flashes on the pack phase of the process, while if short and flashing than one may need to look both at the clamp conditions, but also inspect the tool as to damage which leads to flash.
So what has been learned from above? That by eliminating pack and hold pressures/function one can than make a fair determination as to when flash is occurring. This than brings the fact that flash may occur during fill, or during pack, but also because there is mold damage.
If during fill we have flash than the following area may wish to be examined:
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What is the transfer method, time or position?
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What % full is the part at said transfer point?
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Is the transfer time consistent, meaning on 10 consecutive shots is the fill time/transfer point the same? Are the parts consistent?
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If a multicavity tool or family tool is it one particular cavity or multicavity issues.
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What is the mold condition and surface of the mold?
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What is melt temperature?
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What is clamp pressure?
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What are grade / viscosity of materials?
If during pack
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Check clamp force
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Is flash at vent area?
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Is it being overpacked? Check part thickness.
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Is there a velocity point / setting on pack which is too high?
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Is material degraded? Not dried properly, too hot, etc.
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Is the material too hot?
While the above list contains some common areas of concern they are not total or in any way in order of importance.
An example of a real flash issue occurred too many years ago, but the parts started to flash, and this was on a multicavity tool which used LDPE and ran great. In looking at the parts it was found that the flash occurred to one side of the tool more than the other but was seen in all parts. In observing the face of the mold there was no buildup, or anything holding open the tool. The processor decided to raise the clamp pressure which solved the problem for a bit of time. All along another issue was occurring and that was that the parts would occasionally get caught on one leader pin resulting in an alarm. After a bit of running and more parts caught the flash came back. Upon inspection it was found that the leader pin punched out the plastic which was building up in the leader pin bushing, thus holding open the tool. A solution to this issue was to mill out at the back of the bushing so that these plugs would fall out, and eventually an ejection system was devised to eliminate the catching problems which lead to the other flash problem.
So while it is important to look at when the flash occurs, it also very critical to observe those things which are simple and often overlooked, is there something just holding the mold open so it does not close totally to prevent flash but is allowed to clamp over? Is another issue occurring which over time can lead to a current issue.
Too many times processors live with little issues that over time multiply into larger concerns.
SLSILVEY
04122014
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