Now viewing: quality control

Ken Roney
By Ken Roney
Tuesday, September 10, 2013 - 11:34

In part two of this series we discussed Process Control and, specifically, what “control” means. The control chart is very helpful tool in identifying unwanted variation in a production process. The goal of statistical control is to identify process variation and to determine which variation is beyond our control, that is, variation that is inherent to the particular process we are employing, and which variation is “special” or assignable. This type of variation is outside of the “normal” variation we typically see in a process. It can be identified using statistical tools and reduced/eliminated from the normal process.

First, let’s discuss variation. In a perfect world there would be no variation. Whenever we turned on a machine it would turn out the exact same part every time we used it. As you are well aware, we don’t live in this perfect world. There are many inputs to a given production process that must all work together to produce parts that will satisfy customer requirements. Machines can fail, internal parts wear out, temperatures change, molding tools wear over time, materials can subtly change from lot to lot, etc. By measuring key characteristics of the finished part we can evaluate the process that made it. Using control charts we can identify “normal” variation from special cause variation (a mold wear issue or machine settings that are not set properly) and take steps to eliminate these special causes and return the process to “normal”.

Two charts regularly used at Elite Plastics are Individual Charts and Xbar-R charts. Both charts feature a Center Line which is the mean (average) for the particular data set being studied and upper and lower Control Limits, which are statistically calculated from the same data set. Measurement data for key characteristics are collected and entered at regular intervals over time.

This example shows the center line and upper and lower control limits of a contr

This example shows the center line and upper and lower control limits of a control chart.

Note that specification limits are not used for these charts. Control charts show process variation. It is up to the company to adjust the process to match the specifications required by the customer.

The control limits, calculated statistically, represent plus/minus three standard deviations from the average (mean). These lines represent the threshold at which the process output is considered to be statistically “unlikely”. In other words, the control limits represent the division of the “natural variation” of a process from the “unlikely” variation that is occurring due to special or assignable causes that can be eliminated from the process. Data points that appear outside of these control limits or unusual data runs (data increasing or decreasing over 6 or more successive measurements) should be investigated so that the source of the assignable variation can be determined and eliminated. An advantage of identifying and dealing with this variation early is that steps can be taken to prevent problems before any out-of-specification parts are produced. This maximizes machine and material efficiency which, in turn, lowers production costs.

This concludes our three part series on Statistical Process Control at Elite Plastics. I hope that this has been helpful in describing our efforts to maximize our production efficiency by building quality into our production processes. 



By Ken Roney
Friday, August 9, 2013 - 15:30

As customer demands grow ever more complex and tolerances tighten, it is incumbent on us, as injection molders, to continually innovate and improve our processes to meet these growing demands. The tools of statistical process control are very helpful in understanding our process capabilities, identifying unwanted variation, and “fine-tuning” our processes. Overall, it enables us to efficiently and consistently meet our customer’s increasing demands for quality, cost and delivery.

Statistical techniques have been around for decades, having been most primarily developed by Walter Shewhart at the Bell Labs in the 1920’s. Since then many companies worldwide have implemented a wide variety of statistical tools to help reduce costs by working more efficiently; by reducing variation in their production processes. This in turn results in reduced scrap or re-works. These tools include things as simple as graphing to more complex tools/analysis including Pareto Analysis, Histograms, Capability Analysis, Fishbone Charts and Control Charts. Statistical software is available to make the “data crunching” quick, easy, and easily interpreted and used.

At Elite Plastics we use our statistical software package to provide us a quick view of how our injection molding processes are preforming by analyzing data from internal key product characteristics and/or customer driven product features. This data can be viewed as a line graph (individual charting), or by stratifying the data into a histogram (including process capability indexes – Cp, Cpk) or control chart (typically x-bar/R). These tools allow us to quickly identify our process condition and give solid data back to our process engineers in the event they need to investigate a production process issue.

The Value of SPC for Elite Plastics

It’s critical, when dealing with a large array of customer parts, to understand our machines capabilities and production processes. We work with a wide variety of part sizes, shapes, and complexities every day. Being able to make quick decisions based on statistical data, developed both during initial qualification as well as during in-process production, gives us flexibility in production and allows us to develop sound process controls. In turn, reducing scrap and re-work costs while maximizing our production activity.  In an upcoming blog I’ll discuss two of our most commonly used tools – Capability Indexes (Cpk) and Control Charts.

Chris Passanante
By Chris Passanante
Tuesday, September 4, 2012 - 16:26


In the last post we announced our new machining capabilities at Elite Plastics. This week I want to touch on the benefits of plastic machining.

  • Cost
    • Depending on order quantity, utilizing machining over a tool can result in significant savings. For lower volume parts, tooling isn’t necessary – eliminating the cost of tooling, and reducing the NRE (non-recurring engineering).
  • Prototyping
    • Need a quick prototype of a new part? Machining allows for fast turnaround of prototypes, while also making prototyping of plastic parts affordable, by cutting out the tool cost.
  • Quality control
    • Having our own machining capabilities means increased control over our quality and production. By not having to sub contract machining, everything can be done in house. This means less handling and paperwork and ensures the GM Nameplate quality and level of detail.
  • Fine Trimming/De-gating
    • Many parts require a smooth finish. Machining can be used to remove the gate from an injection molded part, making the surface smooth and gate free.
  • Taping holes
    • Machining allows the ability for holes to be taped rather than using an unscrewing tool. The result is a huge cost savings, depending on quantity, and allows better control of threads and holes.
  • Tooling or Machining
    • There are many factors to consider when deciding whether or not to use a tool or machining. Well, machining may eliminate the tooling cost it increases the price per part, so depending on volume size tooling may be more affordable. But if you need to change an existing part, secondary machining may make more sense.

To learn more about the benefits of plastic machining at what capability is the best fit for your part, please contact Elite Plastics.