Tip of the Day 5: 3-Way Sorting

Tip” #1 describes fail-safe sorting. Fail-safe implies the use of the “Good” output to sort parts. That case is fail-safe, 2-way sorting.

If you use both the “Good” and the “Reject” outputs on the eDART then you can do both fail-safe and 3-way sorting. The sorting equipment is told to put parts into the “Good” bin only when the “Good” signal is on. Parts go into the “Reject” bin only when the “Reject” signal is on. If neither signal comes on then the parts are considered “Suspect”: i.e. we are not sure whether they are good or not.

Notice that this is still fail-safe. If the equipment fails, the power is off, the job is not started etc. then no parts are shipped to the customer. They will all fall into the “Suspect” bin. Of course the same mechanical considerations apply to 3-way: You must ensure that the parts actually go where the signals say they should go and do not get dropped or hang up somewhere.

On the eDART any time a level goes out of limits on a “Warning” line (with no rejects at the same time) then those parts go into the “Suspect” bin.

What are some benefits of 3-way sorting?

  • High Cost of Rejects

    You may have an expensive insert or expensive material and you do not want to set your alarms so tight that you throw out (“Reject”) some good parts. The alarms can be set for just those things that are known to be bad parts such as no pressure at the end of the cavity (shorts). Parts that may need some inspection can become suspects if “Warnings” are set a tighter level.

    The parts in the “Suspect” bin can then be manually sorted and, if good, can then be sold. But the sorting load is dramatically reduced to just the suspect parts instead of all of them.

  • Psychological Advantage

    Some people believe that if they find a good part in the reject bin (or one that “looks” good) then the sorting equipment (eDART) is not working. Having a third bin that contains parts about which the eDART could not decide shows that there is actually a gray area in the business of sorting plastic parts. The eDART can gain immediate integrity this way.

  • Alarm Optimization:

    If you find a lot of good parts in the suspect bin then you can incrementally widen the warning band. If you find a lot of bad parts in the suspect bin then you can incrementally tighten the alarm band. Thus fewer and fewer parts fall in the suspect bin without the risk of sending bad parts to the customer.