Molding is prevalent throughout the medical device industry. But to achieve consistency and repeatability in the fabrication of molded components and devices, some manufacturers employ a concept called scientific molding. Essentially, the advantage of scientific molding is that it takes the guesswork out of the molding process.

What Is Scientific Molding?
“Scientific molding consists of a methodical set of experiments to develop a process,” explains Brunson Parish, senior process engineer at Butler, WI–based MRPC, a custom manufacturer of silicone, medical rubber, and thermoplastic components and assemblies. “Optimizing those processes and determining preferential process parameters is achieved by relying on data. The alternative—reliance on guesswork, experience, or gut feelings—amounts to treating injection molding as a black art.”

Scientific molding was primarily developed for use in the thermoplastic injection-molding industry, adds Jeff Randall, MRPC’s vice president of engineering. It involves understanding the nature of the material to be molded and its preferred molding conditions. By understanding the material’s preferences, behavior, and response to process inputs, a manufacturer can optimize the molding process and produce the most consistent part possible.

The key properties of any thermoplastic molding process include the melt temperature; the plastic, or molding, pressure; the material’s cooling rate; and the material flow rate. By knowing these parameters, molders come to understand how significant they are independently of one another and how their interaction affects the molding process. Scientific molding starts with the understanding of controlling these variables.

For example, the flow characteristics of thermoplastic materials include their ability or their resistance to flow—in other words, their viscosity, Randall says. Resulting from internal sheer, the viscosity of thermoplastic materials changes with their flow rate. In general, as a material flows faster, it also flows easier. Understanding this relationship helps molders optimize the equipment so that all the processing properties are properly aligned to run within settings that produce the most consistent part.

“If you know that at a certain speed you need to fill the mold at a certain flow rate because the material becomes thinner the faster you go, you can optimize that speed,” Randall notes. “But if that speed is beyond the capabilities of the press to achieve repeatability on an ongoing basis, the molder is forced to make compromises and tradeoffs.”
In short, scientific molding is not a linear process technique; it accounts for the interaction of all the individual variables and tests, bringing them together to develop an optimized process.

Overcoming Molding Challenges Scientifically
As the medical device industry trends toward miniaturization, parts are shrinking and walls are getting thinner. But making smaller parts with thinner walls presents a challenge on the manufacturing side.

Producing smaller and thinner components is difficult because thermoplastic materials, by nature, don’t like to flow. They’re not like water, which flows easily when you push it harder, Parish says. “However, while they offer greater resistance than other materials, thermoplastics can be induced to flow better by modifying the manufacturing process. And that’s where scientific molding comes in.”

But materials can also exhibit many variations, Parish adds. “For example, the batch of material I run one month can exhibit behavioral differences when I run it the next month. I have a material that wants to vary and offers flow resistance, and I have a part design that is very restrictive. Thus, from a manufacturing standpoint, how do I balance these factors? That’s always a challenge.”

It is also possible to design a part, build the tooling, and begin the molding process, only to discover that the design is not manufacturable. By identifying and isolating many of the assumptions that go into the process of designing and manufacturing a part, scientific molding can help manufacturers avoid this stumbling block. It enables them to do a large amount of homework upfront on the development side so that they have more success when they proceed with manufacturing.

As parts have shrunk in size and their walls have become thinner, materials have also become more sophisticated. “For example, some materials try to replicate bone,” Randall comments. “In addition, there are many grades and classes of materials, some of which incorporate additives. All of these variations cause materials to behave differently during the manufacturing process.”

When a thermoplastic is heated in a barrel and the press and then injected into a cold mold to enable it to set up and resolidify, a balance must be struck, Randall states. “If I try to fill a very thin part over a fairly long distance, I have to fight the material as it cools, although I need it to flow out to the end of the part. In order to really understand the material and control the process to inject the material quickly without damaging or degrading it by handling it improperly, I have to have all of the molding factors working together.”

That’s where scientific molding comes in. It requires molders to observe the interactions among the different variables and adjust the process to achieve an optimally molded part.

Optimizing Defective Designs Using Scientific Molding
Among the benefits of scientific molding is that the concept can be employed to optimize poorly designed tooling and parts. For example, MRPC was approached by a customer that had worked together with another molder to design and develop an implantable knee component that exhibited too much variability and insufficient repeatability. Although MRPC had not worked on the initial design, inhibiting it from applying scientific molding principles to the development and design of the tooling and the part at the front end, it worked on the project at the back end to enhance or optimize the existing tooling and part.

“To do so, we applied scientific molding principles to analyze the behavior of the material used to make the part,” Parish remarks. “This material, polyether ether ketone (PEEK), is arguably the No. 1 material that flows like cement. Thus, it posed a series of challenges.”

In response to these challenges, the manufacturer prepared a series of scientific molding experiments to develop outputs based on the material’s viscosity versus flow rate. This process enabled the company to identify the ideal flow speed for the material.

The customer had reported that the previous molder’s part exhibited a high level of variability at the back end, probably a direct result of the fillability rate and the pressure at which it had been molded. To determine the correct fillability and pressure, MRPC conducted a pressure-loss study and a gate field analysis that enabled it to optimize when and for how long pressure should be applied. These steps also helped the company to understand how to densify, fill out, and achieve the material’s maximum properties once the correct pressure level had been determined.

“Throughout this process development phase, we identified a processing window with many dimensions, including temperature, speed, pressure, and time,” Randall comments. “As a result, we were able to identify a processing envelope in which we could mold the part consistently. Then, we performed a series of microstudies in which we processed several batches of parts at modified temperatures or speeds, each representing a different segment of the process envelope.” Although small dimensional differences could be observed between the parts produced under these varying conditions, they were consistent within their grouping.

While all of these processes were viable and able to produce stable parts, the customer subjected the parts to physical testing, determining that two batches outperformed the others. “From the standpoint of manufacturing robustness, we preferred one of these two batches,” Randall says. “Then, we conducted a smaller process development step based on where that batch was located within the overall process envelope. We didn’t just look at the dimensions or physical look of the part but also its critical functions, including its physical parameters and how it would be used in the field.” As a result of these considerations, the company was able to reduce part variation not only dimensionally but also from a performance standpoint. “Now, the customer has a part that is more consistent than that produced by the original molder and approximately twice as strong,” Randall adds.

Despite the proven advantages of scientific molding, probably fewer than 50% of manufacturers industrywide employ scientific principles and methodologies in their operations, Parish notes. “However, the trend is growing. On the medical device side, probably a smaller percentage of manufacturers use it, but the rate at which it’s growing is faster.”

The increasing prevalence of scientific molding in the medical device industry signifies the growing need to validate and provide data and evidence of process consistency—especially because this industry sector is directly tied to the life and health of people. “Thus, scientific molding will be adopted by the medical device sector much faster than it has been in other industries,” Parish predicts.

Bob Michaels is senior technical editor at UBM Canon.

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