Converting to hot trimming

Q: We have been changing over from cold trimming to hot trimming, and we've had some minor flaws at the parting line after cold trimming ...

... but when we hot trim we get repeating trim tears that seem to be located in the same areas of the forgings. We forge our parts in a platter with the forging nested in a manner that allows us to forge several parts at the same time. The trim tears are located at regions that correspond to the areas between impressions. We do not see trim tears on the outer regions of the forgings. Your suggestion?

A: The sketches you sent indicate that the flash developed in the blocker dies is significantly thicker than the flash in the finisher. Generally, that is not too bad, but when this occurs between impressions, the trapped flash metal has to have some place to go. Unless there is a gutter groove in the finisher die, this metal tends to flow through the adjacent forging causing a flow-through defect. This is probably what is causing the trim tears at that location. I suggest designing the flash thickness in the blocker to be thinner between impressions than in the finisher, especially if there is insufficient room for a gutter between the finisher impressions.

Concerning the lower frequency of trim cracks observed during cold trimming, I am not sure that they were not there but simply smeared over. To check this out, I suggest cutting a cross-section through the suspect regions to determine the depths of the flow-through defects. I am sure you would not like to have product failures due to such defects.

Flow-through defects are more often observed when forging structural rib-web parts, with surrounding ribs where the metal tends to flow out beneath the ribs unless the preforms and blockers are properly designed to encourage vertical flow. This usually means forging with surrounding flash that is thinner for the blocker impressions than for the finish impressions, but with a much larger radius at the flash land. Also, the webs are often forged slightly thinner in the centers (or at least no thicker) than the finishers to avoid this problem. To be sure that the metal is encouraged to climb into the upper dies, the finish die has a sharper radius at the parting line and essentially “digs into” the blocker flash, thereby choking the metal from rapid escape and preventing flow-thru defects.

For more than 40 years H. James Henning held key technical positions in the forging industry, including as director of technology for the Forging Industry Association, and as president of Henning Education Services, a Columbus, OH, firm specializing in customized education and training in forging technologies.

Guidelines and recommendations offered in this column are based on information believed to be reliable and are supplied in good faith but without guarantee. Operational conditions that exist in individual plants and facilities vary widely. Users of this information should adapt it, and always exercise independent discretion in establishing plant or facility operating practice.

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