Defects are part of plastics manufacturing. When a defect causes a halt in production, the manufacturer has to dig deep to find the source of the problem, following a method, asking the right questions and using whatever intellectual tools he or she has to attack the problem at its root.

The Fishbone Diagram: Breaking Tunnel Vision During Problem Solving

Manufacturing defects can usually be traced to an issue in one of six categories: people, processes, equipment, materials, environment, or management. The fishbone diagram is a wonderful mental exercise that nudges people out of their natural tendency to develop tunnel vision during problem solving.

The fishbone diagram starts with an arrow pointing to the problem. From that central “spine,” secondary spines point away from the problem toward potential culprits, such as human error, bad materials, faulty machines or problems with the process. Hypothetical causes are indicated by branches off those spines. Causes of the the cause are called secondary causes and are indicated by more sub-lines as shown on the example diagram.
This exercise is effective at getting the problem solvers to broaden their thinking by enabling them to visualize the possibilities, run thought experiments with different variables and help isolate the likely source of the defect.

The Five “Whys”: Ask Why Until There Are No More Questions

When the problem-solving team begins to identify the cause of a defect, it can dig deeper with the Five Why’s technique. A person might start with a simple question such as: “Why did this happen?” When that question is answered, in order to dig deeper, the next question about where/when/how/why can be asked, and so on.

For example:

  • “There is a spot on a mirror. Why did this happen?”
  • “We don’t know, but we observe that the spot is white, irregularly shaped, and is located on the back side of the mirror.”
  • “Can you feel it by touch from the back or the front?”
  • “Just the back side. You can’t feel it on the front”
  • “Why is there a bump on the back side? If you look at the raw material prior to forming, are there bumps in the film masking?”
  • “No, the raw material is smooth”
  • “Why is there a bump being created in our process?”
  • “We think the bump is metallic. When viewed under a microscope the bump looks like molten metal that has resolidified”
  • “How did molten metal get on the back of the mirror”
  • “Perhaps the filament was defective and ejected liquid metal during the evaporation phase”
  • “Does the filament need to be changed?”
  • Yes, it is brittle and failed during the evaporation cycle, ejecting liquid metal prior to all the metal being evaporated.
  • Did you change the filament and retest?
  • Yes, the process is working perfectly now.
  • How can you prevent this from happening again?

If enough “why” questions are asked (usually at least five), like a curious child, eventually the team will run out of “whys.” When they run out of “why” questions, they’re probably at the root of the problem.

Manufacturing defects are like dandelions. If you deal with them only at the visible surface, they are almost certain to return. Finding and eliminating problems at the root is the only way to ensure long-term continuous improvement.