Printer Friendly

What it takes to mold to Cpk 2.

Molding as few as 3.4 defective parts per million is a realistic goal--provided that it is approached deliberately and with the full cooperation of your customer and toolmaker.

Improvement is possible for anything that can be measured. Cpk is a measurement of process capability that can tell molders the expected level of defects for a given part. The field for improving Cpk in molding is wide open.

Several years ago, when a handful of molders began using this measure, a Cpk of 1 was considered acceptable. And in some circles, for some parts, it still is. A Cpk of 1 means statistically that 2700 defective parts will be produced for every million parts molded.

Today, a Cpk of 1.5 is considered to be excellent quality, but it still results in 1350 expected defective parts per million. Depending upon the application of the product, this may be an acceptable quality standard. But most applications, such as in the medical field, require a higher minimum standard of Cpk 2, which results in only 3.4 defects per million. If Cpk is this high and statistical process control (SPC) is used to monitor and correct any drifts in processing conditions, then manufacturing yields essentially zero defects.

Is Cpk 2 attainable? Yes. Is it easy? Far from it. If Cpk 2 were easy, everyone would be attaining it and it would not be a worthy standard.


Why be so fanatical over a few rejects? Why not just inspect them out of the process? First of all, 100% inspection is very expensive and it is also fallible. In addition, when you manufacture to levels of Cpk 2 or higher, you can look forward to some or all of the following good things:

* Lowest total cost because parts are more functional throughout the entire manufacturing and delivery channel.

* More consistency of product from part to part and from lot to lot.

* Added value to your customer because, over time, he can eliminate incoming inspection and order smaller quantities or more frequent shipments according to his needs.

* Reduced rejects and labor involved in assembly because mating parts fit better.

* Improved end-user satisfaction.

* Reduced product failures and associated warranty costs.

* Reduced scrap.

* Increased sales, profits, and market share.


Cpk is the process capability index, a statistical ratio that compares the molding-process capability to the product-tolerance band. Cpk is not a difficult concept to understand. After about an hour or so of tutoring, most technically oriented shop-floor people will master it. The following paragraph contains a technical definition, but don't feel like you have to master the math behind Cpk to understand its importance.

To calculate Cpk for a specific part feature, you are concerned with three numbers: the process average, the standard deviation (the average amount of variation in the feature from one part to the next), and the tolerance limits. Cpk is calculated by obtaining the process average for a given part characteristic, then subtracting from that number the closest tolerance limit upper or lower specification limit). Negative signs are ignored. The result is then divided by three times the standard deviation (3 sigma).

Cpk = the lesser of:

(USL - Mean) / 3 Sigma


(Mean - LSL) / 3 Sigma

A Cpk of 2 means that six standard deviations will fit between the process average and the closest specification limit. Only 0.00034% (3.4 parts per million) of a normal distribution falls outside of [+ or -]6 sigma.

Remember, this statistical process must be applied to every critical feature of a part. If a part has four critical features, and we are molding it to Cpk 2, then the Cpk for all four of those features must be 2 or the objective has not been attained.

To improve Cpk a molder can do any or all of these things:

* Move the process average closer to the center of the spec limits. This will maximize the distance between the process average and the nearest spec limit.

* Reduce the standard deviation. The relationship of sigma to Cpk is inverse. A smaller deviation yields a higher Cpk.

* Widen the spec limits. This should be the last option, to be considered only if the product tolerance limits are unnecessarily tight to begin with.


Although Cpk 2 is an enormously worthwhile objective, it is difficult to achieve and there are many steps along the way. Furthermore, once participants get to the end of the chain of steps, most find it necessary to double back and begin all over again. It is never ending. Here, in the broadest of terms, are the steps in the process:

* Molder/customer commitment. The molder cannot achieve Cpk 2 by himself. Cpk 2 is not something that can be tacked on once tooling arrives at the shop. The tool must be designed for this level of quality. Therefore, design for manufacturability must be a consideration in the earliest possible stages of product development. The customer must be willing to bring in the molder as a partner in the product-design stages. This calls for a high level of trust and communication and requires some up-front planning and co-engineering. This investment ultimately results in shorter product development times, higher quality, and lower costs.

* Design for manufacturability. The molder needs to know as much as possible about the part--what it does, how it functions, which features are considered critical, and why. This knowledge enables the molder to recommend changes in the part design that will result in a simpler and less costly mold design.

Omission of this step can result in a complex mold design with multiple timing mechanisms and side actions. Complex molds are more costly to build and maintain and generally produce lower levels of quality over time. By understanding as much as possible about how the part must function, the molder can help the customer by challenging assumptions that push the project in the direction of a more complex mold.

* Design the tool for Cpk 2. The urgent need to get a product to market as quickly as possible means that various stages of product development must happen simultaneously. Tool design may begin with input from initial design specifications. Market research and prototype testing frequently result in changed product requirements and specifications. The tool designer needs to stay on top of these changes and incorporate them into his work as quickly as possible.

A critical aspect of mold design is flow simulation and analysis. Flow analysis indicates where molded-in stresses in the part will be. Controlling these can have a huge impact on part quality. By experimenting in software--not in hard steel--with different numbers of gates, gate types, and gate locations, the designer can optimize these factors for maximum part durability and dimensional stability.

During the design stage, special attention is paid to critical dimensions and how closely these may be held, given the shrinkage factor for the material specified. If that material has a high shrinkage factor and there is a critical dimension across a relatively long section, then it may not be possible to maintain a tolerance of [+ or -]0.001 in. at anything better than a Cpk of 1. At this point it may be necessary to question either the material of choice or the need for such a tight specification.

* Design review/tool building. Prior to building the tool, the design is carefully analyzed to ensure its compliance with standards essential for fulfillment of the Cpk 2 objective. The designer must confirm that the process capabilities in the tool manufacturing process itself are sufficient to produce a mold capable of Cpk 2. For example, if a critical feature of the part is to be held within 0.001 in., then the tool builder must be capable of cutting all mold cavities to 0.0001 in.; otherwise, cavity-to-cavity variation in the tool will consume too much of the "design tolerance" for the part. The builder must also have measuring tools capable of verifying these dimensions. Ideally, if a mold dimension is manufactured to 0.0001 in., then the measuring tool must be capable of measuring to within 0.00001 in. for complete confidence in the result.

* Qualifying the mold. Once the mold has been built, it is time to see how well it works. Qualifying the mold involves setting the tool up in a press, establishing a comfortable process (with no extreme parameters), and molding some parts. Once the process has stabilized, samples are taken and critical dimensions are measured.

Based on these initial results, process and quality engineers make decisions to modify the tool and/or process. When these modifications are completed, another trial is performed and samples are re-evaluated. Again, based on the results, additional decisions are made to modify the tool or process. This process continues until all Cpk 2 process requirements have been met.

The process of mold qualification typically takes one to two months, sometimes longer. Balancing the output of multicavity molds is often a problem. Sometimes bringing one feature into the center of its tolerance band causes another to move out. Often "DOE" (design of experiments) is used to optimize the molding process to attain maximum Cpk values. This is a cost-effective method of identifying the key variables that affect critical part characteristics. Control limits on the key variables narrow or reduce variation. Loosened limits on the noncritical variables reduce product cost.

People often assume that higher quality means higher cost. This is not true. Higher cost occurs with indiscriminate tightening of product tolerances. A well-engineered product design and molding process is less expensive because emphasis and controls are placed on only those tolerances that are truly critical. Keeping this in mind results in more effective product tolerances, better fits, more manageable process controls, and less costly raw materials. These all yield high quality at lowest cost.


What happens when Cpk 2 has been accomplished? Is this an end of the partnering between molder and customer? Frequently the work continues. Sometimes there is good reason to go on to Cpk 3 or 4 or even Cpk 6 for some critical dimensions because other benefits may be obtainable throughout the product manufacturing and delivery system by pushing on to even higher quality levels.

At Nypro the quality commitment is "Cpk 2 or we won't take the job." It is essential that both molder and customer commit to this goal in a partnership effort. As a result of such partnerships, the average Cpk at Nypro for all critical dimensions on new production molds during the past three years is 3.5.

Beyond Cpk 2: A New Vision of Disposable Lens Quality

Vistakon, a division of Johnson & Johnson, has the highest yields in the industry in its contact-lens manufacturing--far greater than the industry average. One of the factors behind this achievement is a partnership formed with Nypro/Clinton in Clinton, Mass., to achieve a superior process capability for the disposable plastic mold used in the lens-manufacturing process. All critical dimensions for this component are at Cpk 2 or above and the most critical dimension is manufactured to Cpk 6 or better.

Nypro and Vistakon began their Cpk 2 partnership in 1986. During the following years, teams of design, process, and quality engineers have met every six weeks to drive continuous improvement in the manufacture of the plastic molds. Three years and about 500 "action items" (continuous-improvement projects) later, Cpk 2 was in effect for all critical dimensions and Cpk 3 was obtained for the most critical dimension--a radius measured in microns.

In the beginning, some of us at Nypro wondered if a Cpk of 2 was possible for this tight dimension. There were so many issues to be resolved, such as measurement methods, material stability, and process-control capabilities. But in 1989 we were at a Cpk of 3 and debating whether or not to push for more.

Nypro was already molding the most consistent product available and shipping it directly to stock. But the Vistakon engineers felt that an even higher Cpk could help improve yields in their own manufacturing process, and more importantly, higher Cpk for this particular dimension might improve consumer satisfaction with the product.

Since 1989, the Vistakon Cpk 2 team has met regularly at intervals of six to 10 weeks. During that time, Cpk for the critical radius has improved to a minimum of 6 and, for some products in the line, to a high of 9. For perspective, Cpk 6 equals 2 x [10.sup.-60] defects per trillion parts. The process variation in molding is less than 10% of the customer's allowed tolerance.

The only cost of this effort to Vistakon has been the time spent by its engineering staff and travel expenses when meetings were held at Nypro. The payback has been millions of dollars of improved yields and proces utilization at Vistakon and improved comfort for users of the contact lenses. Nypro has now assigned a full-time process engineer to Vistakon's headquarters to study ways in which refinement of the plastic molds can further contribute to product quality and profitability.
COPYRIGHT 1994 Gardner Publications, Inc.
No portion of this article can be reproduced without the express written permission from the copyright holder.
Copyright 1994, Gale Group. All rights reserved. Gale Group is a Thomson Corporation Company.

Article Details
Printer friendly Cite/link Email Feedback
Title Annotation:includes related article; measurement of process capability in injection molding
Author:Descoteaux, Alan
Publication:Plastics Technology
Article Type:Cover Story
Date:Nov 1, 1994
Previous Article:Sophisticated heavy-gauge forming starred in SPE parts competition.
Next Article:ISO 9000 for injection molders.

Related Articles
A new way to evaluate injection machine performance.
Gas-injection opportunities expand.
Automotive molder sees untapped potential for multicomponent molding.
What it takes to be a 'world-class' injection molder.
Rapid prototyping moves toward rapid tooling.
Powder injection molding breaks new ground.
Portable analyzers find what ails your process.
Thin-wall molding.
Microcellular foam promises big savings in injection molding.
Micromold Inc. Is Big on Tiny Parts.

Terms of use | Privacy policy | Copyright © 2021 Farlex, Inc. | Feedback | For webmasters