Page 33 - Injection Molding Advanced Troubleshooting Guide
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14 1 Troubleshooting Methodology
Why? Lack of drying time.
Why? Hopper was not loaded in time.
Why? Machine did not have an auto loader and had to rely on a material handler
to load the dryer.
Why? A decision was made to not spend the money to purchase the auto loader.
Using the 5 Why process helps drive down to the ultimate root cause by continuing
to dig for the root cause. There will be times when the root cause is determined
before 5 Whys have been asked and in some cases, it may even be required for
additional questions to be asked.
1.4.7.2 Fishbone Diagram
The fishbone or Ishikawa diagram was developed by Kaoru Ishikawa in the 1960s.
The diagram is frequently called a fishbone because of the fish-like shape that the
diagram is drawn as. A fishbone typically looks at man, method, machine, mea-
surement, and materials as categories of potential root causes. Each of the five
categories is placed on the top line of the fishbone diagram and then additional
bones (lines) are added below it to detail potential root causes.
As the 4M troubleshooting methodology was developed it became clear that the
key categories molders need to worry about are molding process, mold, machine,
and material. As mentioned previously the additional M of the man/woman can
impact any of the 4M’s.
1.4.7.3 Scrap Recording Sheets
One of the keys to effective troubleshooting is to have good data. There are many
types of sheets that can be put together to capture the required scrap data. At the
basic level, having a Pareto chart of the various defects is critical.
Another useful tool may be a tally sheet that documents scrap by the hour through-
out the day. This can help to determine if a defect if it occurs at specific times or
more during specific shifts. Sometimes these tally sheets may show that at the
start of each shift the scrap increases until the operator gets up to pace. See Figure
1.6 for an example of a scrap tally sheet; notice there is a box for each hour of the
shift for specific defects.
An additional scrap tracking sheet that can be useful is a printed-out example of
the part. The operator than simply makes a mark where the defect occurred on the
part. Using this type of tally sheet will help establish if a defect is in a consistent
location or scattered throughout the part. In some cases, leaving a part at the
machine to mark the defect locations can also work.
