Easy to be hard: Medical Manufacturing

Milling plastics is just easy enough to be difficult.

Milling plastics might at first thought seem to be a snap, but the variety of available plastic materials and their differing physical characteristics can make the task more complicated than it first appears.

Many plastics offer ease of fabrication, low weight, strength and the ability to hold close tolerances, which make them excellent for use in for many structural, wear and bearing applications. Although injection molding is arguably the most common way to form plastics, in some cases milling plastic parts is more cost effective or is simply necessary (see sidebar on page below).

Milling can be employed to make prototypes or short production runs of plastic parts without the expense of creating a mold. Implementing engineering changes for a milled part requires only changing a CNC program rather than reworking a mold. And milling can achieve tight tolerances and contours not possible via molding, making it possible to complete some parts in a single operation rather than a two-step mold-and-secondary-machining process.

Plastic Basics

While milling may be the best way to manufacture a particular part, machining plastic workpieces presents several challenges. Chief among them is controlling and dissipating the heat machining produces because plastics generally have much lower melting points than metals. For example, the acetal plastic Delrin has a melting point of about 350° F, while the melting point of aluminum is 2,000° F. Plastics also lose heat more slowly than metals, and, when heated, plastics expand up to 10 times more than metal alloys, according to Quadrant Engineering Plastic Products, Reading, Pa.

The variety of plastic material compositions presents a range of machining characteristics that basically defy generalization. For example, the Hytrel thermoplastic elastomer from Dupont Engineering Polymers, Wilmington, Del., is available in more than 50 grades. The selection includes “soft grades” with a flexural modulus below about 240 MPa as well as hard grades with higher flexural modulus values. There is no sharp transition point; machining conditions will vary gradually from type to type, according to Dupont.

Courtesy of Quadrant EPP

A 3 “-dia. cutter, tooled with six inserts and running at 2,500-rpm and a 10-ipm feed rate, mills a sheet of Quadrant Ertalyte TX PET-P.

Application support from plastic material suppliers is a starting point for developing milling processes. Quadrant, for example, has outlined basic plastic material characteristic information in its Quadrant Materials Triangle, available on its Web site. Intended to help designers pick the right plastic for their products, the matrix groups thermoplastic materials by material and performance criteria, including hardness, strength, heat resistance and dimensional stability.

Higher-performance materials have higher heat deflection temperatures and lower coefficient of thermal expansion rates and therefore are less affected by heat generation during machining, according to Quadrant. Machining parameters, however, reflect the advanced materials’ additional strength. Typically, with general engineering plastics, the company recommends the use of more aggressive feed rates than those applied with advanced materials. Cutting speeds can vary widely, as well. For example, Quadrant recommends starting-point milling speeds of 200 to 500 m/min. for some polyvinylidenefluoride (PVDF) and nylon (PA) plastics with melting points under 350° F, but suggests speeds of just 25 to 75 m/min. for polybenzimidazole (PBI) materials, which can perform reliably at temperatures well over 400° F. Quadrant added that PBI materials may also require the use of CVD diamond-coated or PCD tools for optimal tool life.

Fill It Up

As is the case with advanced metal alloys, increasing the performance capabilities of a plastic generally increases machining difficulty. One example is plastic filled with glass or carbon fibers, which increase strength, insulation abilities and dimensional stability. Lance Nelson, president of tooling and workholding manufacturer 2L Inc., Hudson, Mass., said the filled plastics are “wicked abrasive and chew up cutting tools,” and slower cutting speeds may be required to conserve tool life. “Glass is almost as hard as carbide,” he said. “If you run a very high surface speed, you will generate heat and the carbide tools will break down.”

Ed Padgett, co-owner of Padgett Machine Inc., Tulsa, Okla., which focuses on defense products, said his shop cuts a variety of plastics, but stays away from filled materials. In many cases, machining filled plastics requires special biohazard handling techniques. “From the envelope in which these plastics are being machined, that air has to be evacuated because it has glass particles in it,” Padgett said. He noted that shops may machine the filled materials on one or two dedicated machines in an enclosed room with its own circulatory system.

The Right Tool

Top efficiency in any machining scenario requires tools that are engineered in response to the material’s specific characteristics. Jeff Davis, vice president of engineering at carbide tool maker Harvey Tool Co. LLC, Rowley, Mass., said specialty shops that process only plastics typically use machining processes and tools developed to maximize productivity in those materials. “Then, there are also plenty of shops that do plastics occasionally, and they are not looking to optimize—they are just looking to get through the job,” he said. That may involve applying a 2-flute, general-purpose HSS mill to cut plastic. “In many cases that works, but very often, the geometry, relief or the flute depth isn’t right.” That can produce poor surface finish, chipping, chatter, melting and even burning.

According to Davis, a fine surface finish is crucial in most plastic parts. As a result, cutters designed to mill plastic have geometries engineered that cut cleanly and minimize heat generation, including sharp edges, acute relief on the back side of the cutter and flute depths much larger than those common in metalworking.

Courtesy of Harvey Tool

The extreme flute depth of Harvey Tools’ single-spiral “O” flute tool provides maximum room for a large plastic chip to flow cleanly from the cut and carry away heat with it.

As an example, Davis cited Harvey Tools’ plastic-focused single-spiral “0” flute tool, which “almost looks like a corkscrew.” Depending on the individual tool, the flute depth extends to the center or past the tool’s core, eliminating the core diameter typical of a metalcutting tool. “The flute depth is gigantic, so there is plenty of room for the chip to exit,” Davis said.

The configuration is weaker than a tool with a large core diameter, but a tool for milling plastic generally isn’t subjected to the power and torque experienced by a metalcutting tool. When machining plastic, “The cutter can be more sleek and dynamic. It doesn’t have to be beefed up with corner radii, edge prep and special coatings.”

Harvey also polishes the tool’s large flute valley “to give the chip very little to stick to. We don’t want to have any texture or hangups that might keep the chip from being thrown,” Davis said. “If you can cut [the workpiece] solidly and cleanly [to create a big chip] you get the finish you are looking for and any incidental heat ends up in the chip.”

The differences in application requirements and of the machining characteristics of different plastic materials may prompt the use of different tool designs, according to Davis. For example, a tool with two straight flutes would be more appropriate than a single-flute, high-helix tool for milling abrasive-filled plastics. The dual flutes share abrasive wear and minimize fraying of fiber strands by cutting the workpiece without the upward or downward pull that a helix/spiral produces. Or, in an application where surface burrs are undesirable, a “downcut” tool with a right-hand cut/left-hand spiral design would force the chip, and the burr, down through the bottom of a through-hole so that they are not on the part surface.

The cutting strategies employed in plastic milling also differ from those employed in metalworking. On a typical metal workpiece, it is normal to rough a contour and leave a few thousandths of an inch excess material for a finishing pass. “You cannot do that in plastics, because in the finish cut you are taking very little material, and most of the heat either stays in the tool or the part because the chip can only absorb so much,” Davis said. “You want to take as big a chip as possible with the most feed and the least speed.”

Courtesy of 2L

Vacuum systems can help firmly locate plastic and oddly shaped workpieces.

Courtesy of Quadrant EPP

Vic Maturi, machinist (left), and Jim Hebel, manager of technical services and application development, for Quadrant Engineering Plastic Products discuss milling of a plastic workpiece at the company’s North American headquarters in Reading, Pa.

Quadrant’s Hebel concurred with this strategy, saying DOC is critical, and lighter is not better. “Too light of a pass will cause the cutter to essentially rub the surface. For finish cuts, we recommend a minimum of 1⁄32 ” DOC,” he said.

Steven Lawver, prototyping machinist and CAD technician at experimental P/M lab Matsys Inc., Sterling, Va., uses a variety of machining technologies to process plastic materials for both professional and personal projects. Regarding a basic approach to milling plastic, he said, “Feed rates and speeds similar to those used for milling aluminum are a good starting point for most plastics. But if you have some scrap to work with, adjust the feed rates up from zero to find the point at which the bit will cause the material to climb, catch or chip on the bit,” and then back off. Some plastics are more forgiving than others. But the rule of thumb is that chip clearing and cooling are far more vital with plastics than would be otherwise.

Coolant Controversy

Suppliers and end users agree that it is crucial to keep the plastic milling operation as cool as possible, but they don’t always agree on how to do it. 2L’s Nelson acknowledged that there is divergence of opinion on the use of coolant. “Some people say never use coolant, while some say you have to use coolant. As far as the cutting tool end of it, you should always use coolant. That is the bottom line answer,” he said.

But those promoting dry machining of plastics using air-jet cooling point out that some plastics are hydroscopic: they absorb water. “Nylon is very hydroscopic, and some people don’t want to use coolants with nylon because it causes it to swell and change dimensions,” Nelson said.

While it is true that some materials absorb moisture, Quadrant has found that components are not on a machine long enough for this to be a factor, according to Jack Sharp, toolroom/machining manager. “The only time this may be an issue is if a part were to sit in a puddle of coolant overnight or during an off shift,” he said.

The company always recommends the use of a water soluble coolant to keep the part cool and reduce the effects of heat generation, noting that coolant also helps clear chips. Water-soluble products are specified because some plastics with lower chemical resistance can be discolored or cracked by the rust inhibitors in oil-based coolants.

While plastic milling generally is best performed with coolant, Nelson agreed there are specific applications, such as medical parts, that must be run dry to avoid the chance of contamination of a component that may later be implanted in a patient.