A few moldmakers have adopted a new practice for bushing retention using snap rings, which helps create a more robust, more reliable and more easily serviceable mold.

It replaces the conventional retention methods: a small cap screw that overlaps the bushing lip at one point. In the new method, retention forces are distributed uniformly around the ring rather than concentrating on just one point of the bushing lip.

The idea started in the Midwest, where three or four moldmakers have made it standard practice and half a dozen others are in varying stages of converting. Timesavings per mold set average four to five hours for simple molds containing eight to 10 bushings. On bigger, more complex mold sets with 40 to 50 bushings, the timesavings mount up proportionately.
    

   Bushing Retention
 Bushing retention locks pin-guide bushings in place in the mold block. The bushings are made of hardened metal, as are guide pins, to resist wear as they rub against each other during mold open/close cycles. If either member did wear, the mating mold halves could get out of alignment and produce bad parts. Likewise, if the bushings shift while the mold is in operation, bad parts could be produced and the molds and machine could be damaged. A safety hazard also could be created by a damaged mold part or machine part breaking loose like a piece of shrapnel.
    

  Snap Ring Process
 In the new method, a snap ring holds the bushing in place, and the groove to accommodate that snap ring is machined with the same tool as the bushing hole itself, and on the same centerline. Cutting the groove saves mold machining and assembly time while making the mold stronger (see Bushing Retention:

  Cap Screw Versus Snap Ring Sidebar). After opening the bushing hole, moldmakers only need to put a different tip—a slotting tool—onto the toolshank already in place, add an orbiting step to the machining program, and they will have the snap ring slot. Total machining time: four to eight minutes, or about one-fourth the time of creating the screw hole. There are no slow machining operations such as drilling and tapping, and design time is not needed to locate the retaining screw.
 

The new practice hinges on retention by snap rings rather than socket-head screws. The process centers on:
 1) Machining a snap-ring groove in the counterbore that accommodates the bushing’s lip. Using a tool with interchangeable tips, the grooving step is done on the same centerline as the bushing bore itself, with the toolshank still in place in the spindle.
 2) Inserting the bushing and snap in the retaining ring.
  The snap rings go in and out in seconds following a simple process:
 1) Compress the snap ring with conventional snap ring pliers.
 2)  Place it in the groove.
 3)  Let go.

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