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June 4, 2003

7 Min Read
Tooling Corner: Qualifying tools for automotive molding

toocor.jpgEditor?s note: Umberto F. Catignani is president of Orbital Plastics Consulting Inc. This article is adapted from a dialogue on the website of our sister publication, Injection Molding Magazine.

Automotive parts leave molding plants by the truckload every hour of the day, but not until the manufacturing process has been approved. A mold purchaser for an auto industry Tier One/Tier Two supplier says that his company requires that all of its tooling sources provide an Automotive Industry Action Group (AIAG) Level 3 Production Part Approval Process (PPAP) with each mold. They generally run these molds in-house and use an RJG (Traverse City, MI) processing philosophy (evaluating melt rheology) to establish that the mold/machine/material combination is capable of repeatability. The questions posed by the mold purchaser can be summarized as follows, with responses from Mr. Catignani provided.

  • Is a first-article inspection report and proof of repeatable processing enough to qualify the mold?
    In order to conform to the AIAG guidelines, a first-article inspection and proof of repeatability in and of themselves are not sufficient to qualify the mold. This also depends on what you deem proof of repeatable processing and how you conduct your first-article inspection.

    Prints are typically developed by internal product or component engineers with input from the end customer. If your first-article inspection meets print (is ?clean?), then you may proceed with the qualification process. If not, the component engineer must determine if the nonconforming dimension(s) will affect the overall fit and performance of your part.

    The component engineer may modify the Upper and Lower Specification Levels (USL and LSL) to bring the dimension within tolerance or insist on the current USL and LSL.

    Tooling engineers should have enough foresight to build a majority of the dimensions steel safe. If so, your tooling vendor can remove core and/or cavity steel, adding plastic to your part and bringing the off-specification dimensions to print upon re-sampling. It is not always possible to build every dimension steel safe. In such situations, you must rely on your tooling and component engineers for design expertise.

    Note that resampling should not mean reprocessing. Some toolbuilders and automotive part suppliers are guilty of this and modify the injection molding machine process by one or more parameters. If you reprocess, your first-article inspection is no longer representative of the original process and must be conducted again. Bringing several out-of-specification dimensions into specification by changing machine parameters inevitably produces new out-of-spec dimensions that were previously within the USL and LSL.

    Some tooling vendors and engineers in the industry know this to be true but proceed with reprocessing and claim that all dimensions meet print. What you really have in this situation is a semicapable tool that conforms to certain aspects of the part print, depending on the chosen process.

    The manufacturing engineers, who are ultimately responsible for molding good parts, will never be able to mold a part that meets every dimension on the tool-build print since this was never accomplished with the original inspection process. In effect, their department is heavily burdened by shortcuts used during the toolbuild process.

    Bringing several out-of-spec dimensions into specification by changing machine parameters inevitably produces new out-of-spec dimensions that were previously within the USL and LSL.

  • Is a first-article inspection report and proof of repeatable processing enough to qualify the mold?
    One must define repeatability from a quality point of view. A repeatable process implies stability and predictability, but is it capable? A stable and capable process is consistent, predictable, and has variation that falls within the Upper Control Limits (UCL) and Lower Control Limits (LCL). A normal curve applied to the variation must have 99.99966 percent of its units fall within ±6 standard deviations of the average.

  • Do I really need to have the tooling vendors perform a capability study?
    Absolutely, this is proof to you and your end customer that the process is stable, predictable, and capable before the mold is released to your production facility. To demonstrate process capability, the tool vendor may initially conduct a mini- or short-term capability study by running 20 to 25 consecutive shots and picking one to three statistical process control (SPC) dimensions such as length, width, and height.

    For example, if your part is nested during assembly, we suggest using the overall length and width. If mating to another part, choose a dimension that represents the expected consistency of the fitted part feature. Your quality and manufacturing department should discuss these dimensions with your end customer.

    If the part print calls out Key Product Characteristic (KPC) dimensions, use these vs. the SPC dimensions for the short-term capability study. After running statistical analysis, identify the worst performing cavity or cavities and use them for the capability study.

    The capability study consists of 300 shots, with every third shot numbered and saved for analysis. Using consecutive shots introduces the element of time into your capability study. It may provide insight into process and machine fluctuations that occurred.

  • Do my tooling suppliers need to provide a Level 3 PPAP?
    No, however, this is required if you intend to purchase parts from the tooling supplier for use in production, and these parts are subsequently released to your customer. For example, a situation may develop in which the tool is delayed significantly, and your plant is unable to retrieve, set up, debug, and run a capability study for an internal Level 3 PPAP before the production start date.

  • Is there a quicker, less costly way to prove that the mold functions and produces parts correctly?
    Yes, nevertheless, your tooling vendor must perform first-piece inspection and a capability study to qualify the mold. Your customer may require mechanical testing. There are ways to reduce this cost (see Figure 1, below).

    Total tool cost: $80,000
    Mold cost: $60,000
    PPAP cost: $20,000
    Capability study, 32-cavity mold
    32 cavities, three dimensions/cavity, 300 sample parts yields 28,800 data points
    Figure 1

    Sample parts can decrease to 100 from 300. Dimensions measured are either KPC or SPC, which will be greater than one. For illustration purposes, assume three dimensions. After the short-term capability study, pick several poorly performing cavities. Assume the quality department is comfortable with four of the worst performing cavities. Therefore, four cavities, three dimensions per part, and 100 shots yields 1200 data points.

    We must also include data points from the short-term capability study. Assume three dimensions, 20 samples, and 32 cavities yielding 1920 data points. Thus, total data points measured are 3120.

    Compared to the original setup, that is over an 89 percent reduction in the number of data points. If we factor in setup costs of $2000 and assume a linear relationship between PPAP costs and data points, cost is decreased by approximately $16,000?reducing the total tool-build cost by 20 percent (see Figure 2, below).

    New total tool cost: $64,000
    Mold cost: $60,000
    New PPAP cost: $4000
    Capability study, 32-cavity mold
    Four cavities, three dimensions/cavity, 100 sample parts yield 1200 data points
    Short-term capability study, 32-cavity mold
    32 cavities, three dimensions/cavity, 20 sample parts yield 1920 data points
    Figure 2

  • Does the auto industry really need this done twice?
    The auto industry requires that the supplier of production parts must have documentation supporting PPAP status. If your tooling vendor is to supply production parts on a short-term basis, then they must be at full PPAP status with other supporting documentation like process failure error mode analysis. Other instances requiring additional PPAP include:

  •  If the mold is moved outside the four walls of your production facility, even if the make and model are identical in an adjacent plant.

  •  If the mold is moved inside the four walls of your facility, even if the make and model are identical.

    However, in the latter example, you may be able to avoid the additional PPAP if your plant uses a universal process card that focuses on actual processing parameters. Such parameters include actual plastic material temperatures, fill time with associated fill rates, actual melt pressures that factor in machine amplification ratios, and so on.

  • Do toolmakers have a better idea on how to qualify that a mold makes parts to print?
    Someone in the tooling industry may offer an alternative procedure or practice. Nonetheless, if you desire to sell parts to the automotive industry, which subscribes to AIAG standards, then you must obey the rules.

    Incidentally, the Food & Drug Administration, which regulates the use of plastic components in medical applications, is looking to adopt many principles outlined in AIAG manuals for initial and long-term process validation studies. They view the procedure as straightforward, proven, and already adopted by many injection molders.

    Contact information
    Orbital Plastics Consulting Inc.
    Atlanta, GA; Umberto F. Catignani
    (404) 849-6714; [email protected]

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