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How to streamline plastic injection mold qualification and process validation

Injection molding
Repeating mold qualification and process validation in the final production environment, which is typically required in the medical and automotive industries, is costly and time consuming. There are steps you can take to mitigate the process.

A moldmaker said recently that it sometimes takes him up to eight months to validate a large, high-cavity medical mold. Maybe this surprises you, but it doesn’t surprise many OEMs in the medical and automotive industries that have extensive mold qualification and process validation requirements.

Qualifying a mold and validating the molding process in the medical device (FDA requirements) and automotive (Pre-Production Approval Process or PPAP) industries typically requires that the validation process be performed in a specific molding machine that will also be used in production. Usually that means that the moldmaker first qualifies the mold either in-house (if the moldmaker has that capability) or uses an outside molder.

After the mold is shipped to the OEM or molder that will run production, mold qualification and process validation must be repeated in the press that will be used after the mold is installed in the production molding facility. Because proper validation that conforms to FDA or PPAP standards requires that the mold be validated in the press in which it will run, some OEMs have a protocol that calls for shipping the molding press to the moldmaker (if he or she is capable of performing mold qualification and process validation) or to the molder where the mold will run in production. That ensures that mold qualification and process validation will be done in the same machine that runs production. 

However, medical part validation protocols in many cases require re-validation of the molding process and re-qualification of the mold even if the press is moved slightly from its original placement on the production floor. That means the entire qualification and validation process needs to be repeated, a time-consuming and costly event.Today’s mold manufacturers are seeing greater demand for performing mold qualification and process validation in their facilities, and many have responded to that demand by adding appropriate capabilities and services. Some have created a specific area for molding machine installation that allows them to perform these critical processes—often adding assembly or other operations to develop an entire automated molding cell—prior to shipping the mold.

For example, StackTeck Systems Ltd. (Brampton, ON, Canada), a high-tech mold manufacturer specializing in the category of three- and four-level stack molds, responded to customer demand for complete turnkey molding systems that are qualified, validated and ready for production. “Nowadays, more and more of our customers are requesting complete injection molding systems that are ready to go when they hit the molding floor, and without being restricted to one molder,” explained Jordan Robertson, General Manager of Business Development.

To meet those demands, StackTeck created a test facility where molds, molding machines and automation can be integrated, qualified and process validated. The company recently expanded the size of its test facility and now has a total of 13 bays. “We can completely integrate machine, mold, product handling through automation and other downstream equipment,” Robertson said. “Imagine choosing the best injection molding equipment on the market to suit your needs and having it arrive at your plant fully debugged, ready to start production.”

These qualification and validation procedures are not a cheap proposition, however. Industrial Molds Group (Rockford, IL) tells of an extensive mold qualification and validation process for an automotive customer for which they quoted the entire program as a “turnkey job,” said Vice President Tim Peterson. “It wasn’t just the [mold] tryout that was the biggest expense—it was part inspection and process validation. In just the measurement portion of the validation process, we were $10,000 into it. It often costs $15,000 to validate a tool. In fact, if it costs the customer $20,000 to validate a tool, he’s getting a bargain.”

OEMs choose high-cavitation molds to reduce their unit manufacturing costs, yet the validation process can consume time and money if all of the mold’s cavities are being validated, not just 10 to 20%, as is the case for some high-cavitation molds (64 cavities or more). For medical device OEMs, validating only a certain percentage of the mold’s cavities is risky. With the FDA making moves to bolster its medical device rules, OEMs and their mold or molded parts suppliers cannot take risks.

For example, a 196-cavity medical mold with 10 critical dimensions in each cavity means there are 1,960 dimensions; 1,960 dimensions times one complete shot every hour for 24 hours equals 47,040 dimensions to measure in a 24-hour period times three (a 72-hour run), which equals 1,238,960 dimensions to check to validate this mold. At approximately $1 per dimension, you now have over a million dollars in the mold validation process, which is probably more than the cost of the mold itself. “The higher the cavitation of the mold, the more the validation costs,” says Steve Tuszynski, developer of the Algoryx system for mold qualification.

Multiply that by several times if a mold has to be moved into a different press or to another vendor, and the costs become unsustainable for OEMs operating in strictly regulated industries.

However, there are times when it may be necessary to move a mold to another press or even to another molding facility. In that case, many OEMs require that the validation process be repeated. It takes days or even weeks to revalidate the part and molding process, not to mention the additional time needed to collate the validation report and get final approval. In today’s global manufacturing environment, the ability to maintain process validation across global locations and in various molding presses has become extremely critical.

As previously noted, one answer to these challenges has been to ship the molding machine to the moldmaker’s facility. This allows the mold and molding process to be qualified/validated together. The entire molding cell then gets shipped to the molder’s facility for production, where the mold and molding machine remain wedded for the life of the part.

The first step is to qualify the mold’s ability to make a part to print: Are all the dimensions to specification? Does the part meet any aesthetic or surface requirements?

After a mold is functionally operational and capable of making the part for which it was designed and built, the validation process begins, typically with the five components of the molding process—mold, press, tolerances, metrology and geometrical dimensioning and tolerancing (GD&T).

Validation then becomes a long, often complex process of synchronizing these variables, along with the press variables (temperature, pressure, pack and hold time) and material variables. At some point, conforming parts are produced when all these variables magically mesh at exactly the right time and place. Then, no one touches the press controls! (It’s like the old joke: What will the factory of the future look like? It will have a man and a dog. The man is there to feed the dog, and the dog is there to bite the man if he touches the machine.)

According to Matt Therrien, Northeast Regional Manager, at RJG Inc. (Traverse City, MI), a company specializing in scientific molding processes and procedures, validation—typically defined by a part dimension report—has to include the molding process itself that produced the parts. This will include the record/documentation of how it was developed—proof that it is capable of producing parts repeatably and consistently to dimension within the established control limits. Having the full roadmap from development to production allows a plastic part process to be portable (dimensions are a result of the defined part process). Everything is selected for a reason and should have background data to support it. That represents true “part process” development.

Fully documented dimensional and part process-related results satisfy the criteria that is typically outlined in any master validation plan. The proven results are transferable to any machine that can be verified to be capable of consistently delivering the repeatable plastic variables and conditions. The performance of the machine has to be documented/confirmed to be within the typical recommended target limits (i.e., acceptable shot capacity, consistent melt preparation and that it can hold consistent part tonnage, deliver a sufficient volumetric injection rate and is not pressure limited), Therrien explained. These results can be plugged into any standard validation and qualification report to satisfy the regulatory documentation requirements.

The idea is that the data from initial mold/part qualification and process validation can be used to easily and quickly reproduce the process in any machine (defined as capable), which then allows for the ability to transfer molds and increases flexibility in capacity management. While scientific molding principles help, there are many considerations when performing mold qualification and process validation procedures that can help in tooling transfers.

Keys to transferring tooling

The real key to transferring tooling from one press to another is looking at the molding process from the plastic’s point of view, not the machine’s. “Don’t just pay attention to the machine settings or set-up card—look at the actual values of the temperatures, pressures and times; understand the difference between the machines even if they are similar in tonnage and shot capacity,” emphasizes Vishu Shah, President of Consultek Consulting Group (Diamond Bar, CA), a group of scientists and engineering consultants to plastics processors globally. For example, to replicate the process from one machine to the other it is imperative that the melt temperature is accurately recorded and duplicated. Simply transferring barrel temperature settings will not work since many factors can affect the melt temperature. Half of the molders I know do not even have a pyrometer to measure the melt temperature. If they do, it is stuck back in a corner office somewhere. The melt temperature is not even recorded on most of the setup cards. Understanding the basics is important. One needs to learn to walk before attempting to run.”

Shah told PlasticsToday that it is “not completely true” that using scientific molding principles will eliminate the need for revalidation of molds transferred to alternate presses. It helps to follow scientific molding principles if you are paying attention to the plastic variables, said Shah.

Only four elements need serious attention when transferring molds to different locations:

Plastic pressure. Duplicating hydraulic pressure is not enough, since identical machines may have slightly different intensification ratios, altering machine pressure. On one machine you’ll set the same hydraulic pressure value as it was on the other machine, but the plastic pressure values will be different. Plastic pressure is what needs to be recorded and duplicated. Pay attention to what matters, the plastic pressure at the front of the machine, not the hydraulic pressure at the back of the machine.

Mold and melt temperatures. Scientific molding principles dictate that you pay attention to measuring the melt temperature of the material, not the temperature settings on the barrel. When you transfer the mold to another machine with a different controller and barrel size, you don’t know what you’ll get. If you match the melt temperature of the material, you’ll get the same melt quality regardless of the controller setting. Mold temperatures follow a similar logic: Measure the actual mold temperature—steel temperature, not the settings on the mold temperature controller—on both mold halves and duplicate that.

Cooling. Heat transfer between the mold and melt must be kept the same between two machines. How much water is going through the mold and how much water is coming out of it? Measure the flow rate of the water to make certain that flow is always turbulent and identical. You must also pay attention to the water temperature and water pressure.

Times. Fill times, pack and hold times, and cooling times are the times that are the process settings and must be matched. Pack and hold and cooling times are settings based on a study, which can be simply duplicated, but fill times are actual values based on a set injection speed—a flow rate is achieved, and this flow rate must be duplicated.

“If you pay attention to these four elements, you can duplicate the process from one machine to another,” said Shah. Scientific molding helps you develop a robust process and duplicate these process parameters. To reach the stage of a developed robust process, however, one must conduct design of experiments (DOE). This is often not done for a couple of reasons: People think it involves difficult math and statistical techniques and takes time. Both preconceptions are false.” (Suhas Kulkarni of FimmTech has written several articles and given several talks explaining how simple the technique is for injection molding and how to reduce the number of runs and measurements of the parts.)

“You have to do the scientific molding studies, but you still have to do the DOE,” Shah explains. “Many people don’t understand how to conduct the DOE properly. If you perform a DOE on a 16-cavity mold, you may have over 500 dimensions to check, for example, and that takes a very long time. To save time, you have to conduct the DOE properly and know what variable makes the most difference. Scientific molding helps, yes, but not completely, unless you couple it with the proper way of doing a DOE. There are only a few things you need to pay attention to with DOE, based on the morphology of the material and the part design. There are no general guidelines on the effect of the process parameters on the part dimension. In some cases, cooling time may increase the dimension; in other cases it may not. On the same part, the melt temp may make a difference in the length but not on the width. Usually, it is the pack and hold pressure that has the most effect on dimensions.”

Scientific molding principles do help, but proper DOE saves time. Things vary, so it’s important to establish a process window, and that is what DOE does, Shah emphasizes.

“Many molders think that decoupled molding—separating injection from pack and hold—is scientific molding. It’s not,” says Shah. “Scientific molding is going back to the science of molding—chemistry, rheology, math, physics and so on. A lot of it is just common sense,” Shah adds, noting that machine controllers “have gotten far ahead of an average technician’s ability to comprehend—10 or more different pressure settings and velocity profiles, a variety of programmable sequences and more. One manufacturer even offers a two-day training course just to learn how to operate controls.”

Temperature, pressure, time and speed should be the basics, Shah emphasizes, then you peel back the onion. “These controls are all useful gadgets, but until you understand the science of molding, you can’t comprehend all these complex controls,” he notes. “The industry has gotten way too advanced in the simple things. I know people who’ve worked in a molding plant for 15 years but don’t know how to measure the melt temperature and do not know the difference between hydraulic and plastic pressure.”

Planning for a mold transfer

Jared Sunday, a technical sales manager with Raumedic, a custom plastics processor of injection molded and extruded medical components headquartered in Helmbrechts, Germany, provided some good advice on mold transfers in a recent issue of sister publication MD+DI. Before transferring molds to a new vendor, you need a “clear roadmap” for the process, and Raumedic has developed a successful approach.

Define asset ownership. This includes not only the mold(s), but all spare components, tool drawings, inspection fixtures and gages, end-of-arm tooling, manifold controllers, assembly fixtures, printing fixtures, welding fixtures and any functional test fixtures. You should also identify any current tooling condition issues and processes needed for manufacturing.

Prepare last shot samples from the mold. Send three to 10 complete shots from the mold in its current condition to the intended supplier. Use these parts for a complete evaluation of the visual and dimensional aspects of the current product. This pre-work will highlight any potential pitfalls of a revalidation effort.

Validation strategy. A strategy should be put into place that satisfies all quality requirements. The supplier should write, control and execute to the approved protocol; the customer’s role would be to approve overall process flow, review potential deviations, and approve submitted packages. This minimizes resources required and places accountability at the site of manufacturing. Validation protocols should be approved ahead of time by quality, engineering, procurement, and manufacturing team members. A change to protocol after the tool transfer risks extending sampling or metrology timing.

Sunday commented in MD+DI that customers often ask for the same parts they were getting before. “If that is truly the requirement, then a reduced nominal process point capability study or a revalidation based on equivalency may be the preferred path forward,” said Sunday. “Rely on an experienced supplier to interpret quality aspects and recommend a strategy that will meet the requirements.”

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