注塑模具FLASH.doc
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1、FLASH Flash can be defined as excess plastic material forced out of the cavity. This occurs at any point where two mold surfaces meet. MACHINE EXCESSIVE RESIDENCE TIME IN BARREL Explanation: Ideally, a shot size should equal 50% of the capacity of the barrel. That results in processing one cycle whi
2、le preparing for the next cycle. However, this is a general statement because, depending on the material, the ratio can be as small as 20% for non-heat-sensitive materials such as polypropylene and up to 80% for heat-sensitive material such as PVC. As the ratio drops, the time of residence by the ma
3、terial in the barrel increases, and the material gets hotter. It will flow much easier at this point and enter areas where it could not at its normal viscosity. This results in a flashing condition in those areas. Solution: Strive for a 50% shot-to-barrel ratio. This is ideal but can go as low as 20
4、% if the material is not too heat sensitive (like polypropylene) and up to 80% if the material is extremely heat sensitive (like PVC). It is not a good idea to empty the barrel every shot because more time will be required to bring the next mass of material up to proper heat and degradation may occu
5、r. EXCESSIVE INJECTION PRESSURE Explanation: If too much injection pressure is used, the clamp unit may not be able to hold the mold closed, especially on a machine with a hydraulic clamp. The mold will blow open and plastic material will leak out around the perimeter of the parting line. This leaka
6、ge is called flash. It can also occur between any mating steel surfaces of the mold such as around ejector pins and slides. Major mold damage may occur as a result of flashing. Solution: Reducing the injection pressure reduces the tendency for the material to flash. In addition, make sure there is a
7、 properly raised shutoff land around the perimeter of the cavity. This will focus the clamp force and allow less clamp tonnage to be used. Without the shutoff land on the mold, the machine may not be able to generate enough clamp force to keep the mold closed under normal injection pressure. HIGH BA
8、RREL TEMPERATURES Explanation: High barrel temperatures have the same effect as long residence time. The plastic material becomes more fluid than it should and can enter small openings and crevices that it could not normally enter at the right viscosity. Solution: Reduce the barrel temperature to th
9、at recommended by the material supplier. And, remember to keep the profile set so the material is heated from the rear towards the front of the barrel. EXCESSIVE CYCLE TIMES Explanation: If the total cycle time is too long, there is a good possibility that the material is overheating in the barrel.
10、This will cause the material to become too fluid and creates the potential for flashing in areas where it would not normally do so. Solution: Reduce the cycle time. Normally, this can come from the cooling portion of the cycle, but make sure the other functions are not excessive. For instance, injec
11、tion hold time only needs to be long enough for the gate to freeze. After that, the hold pressure has no effect on the material in the cavity. So, the hold time is an area that should be considered for time reduction. Other functions should also be analyzed. INADEQUATE CLAMP FORCE SETTING Explanatio
12、n: In both hydraulic and mechanical clamp machines, the clamp unit position must be set at the beginning of a molding run and readjusted as the run progresses, due to thermal expansion of the mold and machine. If this readjustment requirement is ignored, the clamp unit position may shift to the poin
13、t of not fully closing the mold against the incoming injection pressure, and flashing will occur at the parting line. Solution: Size the mold to run in the proper machine. This is done by calculating the molding surface area (area of the part to be molded) and multiplying it by a factor of from 2 to
14、 6. The higher number is used for stiff material (like polycarbonate) and the lower number for easy-flowing materials. That will give the number of tons needed to keep the mold closed, assuming there is a proper shutoff land on the mold. You must calculate the total area so include all cavities and
15、the runner system. MOLD IMPROPER PARTING LINE SEAL Explanation: The parting line(s) of a mold must be machined to very close tolerances and parallelism to seal properly when the mold is clamped shut. If the parting line is not parallel, or is otherwise improperly machined or designed, molten plastic
16、 material will be forced out of the areas that are not closed tightly, and flash will form. Solution: Check for proper parting line seal. Make sure there is a shutoff land around the perimeter of the part. There should also be pads around the leader pins at the same height as the shutoff land to ens
17、ure parallelism when the mold is clamped. Use a dial indicator to check the flatness (or parallelism) of the parting line surfaces. They should be within 0.002 (or less) over the entire parting line surface. IMPROPER VENTING Explanation: If vents are machined too deeply for a specific material the m
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