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The injection molded part defect known as Flash is that thin layer of plastic that flows outside of the cavity where the two halves of the injection mold meet. If the flash has to be manually trimmed off by an operator or some other employee, it becomes a labor-intensive, high cost problem. Plastic parts with flash are usually put in the scrap grinder or just thrown away, depending on the plastic material and company guidelines. In my last blog post, I explained that the plastic molecules only know the four conditions: plastic pressure, plastic temperature, plastic flow rate and the plastic cooling rate. The plastic material knows nothing about injection pressures, injection timers, barrel temperatures, or any of the other machine controls found on a modern injection molding machine.
Back to the flash problem – If the flash part defect is allowed to continue cycle after cycle, it will eventually depress the metal on the mold, seriously damaging the mold and causing downtime and expensive mold repair. You want to avoid this expense at all costs. It can kill your profit on a molding job. So it is very important to eliminate flash as soon as you start to see it occurring.
So what would the plastic molecules have to say about a flash problem? The molecules would tell you that pressure forced them to flow into the parting line of the injection mold. What are the causes and solutions to a flash problem based on the four plastic variables cited above?
Plastic pressure in the cavity– There is a very good chance that the pressure of the plastic in the cavity is exceeding the clamp force of the injection molding machine. So, one obvious answer is to increase the clamp force. If the clamp force is already at its maximum, the plastic molecules would say “Hey, you have to decrease the plastic pressure in the cavity.
Plastic temperature – On molds for thin-walled plastic parts, which are the type of mold most likely to have flash, an increase the plastic temperature will allow the molder to use lower injection pressures to fill the mold. So the molecules would say “Make us hotter”
Flow rate – Fill rate will have little effect on flash because it doesn’t affect cavity pressure unless a high fill rate continues after the mold is full. That would cause a spike in cavity pressure, easily causing flash. So your molecules wouldn’t have much to say about their flow rate.
Cooling rate – Could cooling rate (mold temperature) affect flash problem? A cooler injection mold will freeze and seal off plastic that is trying to squeeze into the parting line. It is unlikely that a cooler mold will eliminate flash, but it will reduce it. So your molecules would say “You can try cooling us down, but do the other stuff first”.
The Paulson method of teaching problem solving and injection molding cycle time improvement is to first analyze the cause of a problem from the plastics point of view, then determine what machine controls will change the plastic conditions and therefore correct the plastic behavior and solve the plastic part defect.