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Computerized tolerance analysis helps molder meet quality goals.

A Cincinnati-based custom injection molder has developed a pretooling computerized capability analysis that predicts what quality results can be expected after a mold is up and running. Plastics Moldings Corp.'s "Predictor" program evaluates part dimensions and predicts areas in which shrinkage, tooling limitations, and process variations would prohibit achieving desired statistical quality levels during final production.


The value of the program, according to PMC sales manager Randy Smallwood, lies in its usefulness as an aid in pinpointing potential trouble spots early on, before a tool is built and when the part may exist only as a blueprint. The tooling analysis is based on data provided by material suppliers as well as PMC's own molding experience. The program is based on a materials database of thermoplastics and thermosets, including both commodity and specialty materials.


The predictor program was originally developed by PMC for in-house use to calculate the mold tolerance for a particular part. As the program evolved, it was expanded to take into consideration additional criteria, such as material shrinkage, process variations, and a customer's quality requirements. Eventually the program grew into a tool that PMC shares with certain customers to highlight potential problem areas in part design.

Taking these factors into consideration makes this program unique, according to Smallwood. Many available tolerance guides do not consider tolerance variations from cavity to cavity or the statistical quality requirements of the customer. By encompassing these factors, PMC says its program eliminates much of the guesswork and "gut feel" involved in having a mold built.

The program analyzes tolerance capability from two points of view. First, it calculates linear and anisotropic material shrinkage. Then, it takes into account process variations from shot to shot and from one material batch to another.

When PMC starts a new molding job or quotes a job for a customer, it inputs the part dimensions into the computer program, which then analyzes the capability to hold the desired tolerances. The materials database, which currently consists of about 20 resins, is built from information supplied by materials suppliers as well as PMC's own experience in molding specific parts, on which it has run capability studies. The database is created with a specific part in mind. It consists of four variables: high and low limits for material shrinkage and high and low limits for process capability. In making a prediction, a customer's Cpk (process capability index) requirements are also entered into the equation. The program will specify what tolerance can reasonably be held at the required Cpk value. If the tolerance can't be held to a specified Cpk, the program will calculate what Cpk can be met.

The program estimates tolerance capability for each dimension on a part and provides a summary report for all dimensions. The sample summary report provided here estimates the capability to hold desired tolerances on specific dimensions with or without "steel-safe" sizing. "With sizing" indicates acceptable mold tolerances based on process and material variations on dimensions that are steel-safe. "Without sizing" reflects total possible variations. The comments "capable" or "not capable" indicate whether the program predicts that the mold will be able to meet the customer's requirements.

Smallwood admits the system is not foolproof, and PMC does not guarantee its results. On the other hand, he says, it is a more scientific approach to part design that can eliminate a lot of costly engineering changes and downtime before a mold goes into production, especially when used as a conservative tool for predicting results. "Often customers will say dimensions aren't even close after a part is molded for the first time. It's a big problem," says Smallwood. "This is an additional tool to tell you on day one--not day 300--what you can realistically expect with your tolerances."
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Title Annotation:Technology News
Author:De Gaspari, John
Publication:Plastics Technology
Date:Apr 1, 1993
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