First-time-correct injection molded parts, Part 1

Reducing scrap is the low-hanging fruit in strengthening your bottom line, so make the parts right the first time.

How many companies have slowly gone out of business because of poor margins? How many companies are in business but holding on by single-digit margins? The difference between being in business and being out of business can be surprisingly small. In these lean times, a lot us are looking at ways to drop money to our bottom lines. Reducing our scrap—or the overall cost of quality, if our accounting is advanced—is an easy way to make up at least some of the difference.

There is a fairly big difference between scrap percentage and the cost of quality. This subject has been covered in different ways through the years. Philip Crosby used “the price of nonconformance” and Joseph Juran used “the cost of poor quality,” just to name a couple. The term “cost of quality” refers to the costs associated with providing poor-quality product or service. Your scrap percentage is typically the amount of bad parts out of the total amount produced. Your cost of quality, however, is the amount of money and time you spend achieving your yield. You can think of it in terms of value-added vs. nonvalue-added activities. To illustrate this: If you have inspectors running around ensuring your scrap rate is zero, your cost of quality can be extremely high.

Once we get through that line of thinking, we quickly gravitate to prevention methods vs. detection methods. But in order to be effective, the prevention needs to be as close to the source as possible, particularly from a time perspective. “First time correct” is often used as the goal. But how do we get there? The old saw “you can’t manage what you don’t measure” holds true here. Many new technologies and tools have come out through the years that have allowed us increased insight into the molding process. A lot of these are like the “check engine” light on your car’s dashboard—they might not tell you exactly what is wrong, but at least a clear signal is being sent that something is wrong. Many variables are easy to measure; we just need to take the time to do it.

On the opposite page is a fishbone (or Ishikawa) diagram for a number of variables that contribute to the outcome of a molded part over a short time period. It breaks down the causes of an effect into four areas: machine, method, material, and man. The list, of course, would be much longer if we considered longer-term affects. If you are able to measure or control these, your pathway to lowering your cost of quality should be much clearer. It will also help to assign someone to be accountable for these so your fate is not left to chance. With a little patience, some careful studies, and a team effort, any molder can dramatically improve its level of control over its process and watch the scrap decrease.

Not everything in the diagram will apply to all shops, and it is quite likely there are elements missing, but it can be used to set up a framework to organize your project and then break it down into bite-sized pieces.

Author Mike Miller is director of engineering at contract molder New Berlin Plastics (New Berlin, WI).

Part 2 addresses each of the four main variables in arriving at correct parts the first time: machine, method, material, and man.