In the plastics industry, material choice is crucial to the performance of a company's products. While some properties of a material may give a product a certain edge, that same material may possess any number of drawbacks. The several grades of nylon, which offer a multitude of advantages such as a good operating temperature range, have been "taking the heat" and, at the same time, "putting the heat" on their competitors.

Nylon, or polyamides, was the first of the modern engineering thermoplastics and has flourished over the past 55 years mostly because of its cost/performance ratio and attractive blend of properties. Two general types of nylon exist: those prepared from a diamine and a diacid (nylon 66) and those prepared from an amino acid or amino acid derivative (nylon 6). The material's broad processing range allows fabrication by almost all of the common thermoplastic processes, including injection molding, extrusion (tubing, pipe and profile, sheet, monofilament, blown and cast film, extrusion coating, and wire and cable jacketing), blow molding, rotational molding and thermoforming.

Nylon's Advantages/Disadvantages
When considering nylon's properties, the majority of industry experts found difficulty in selecting one standout benefit. "What makes nylon unique is its combination of properties -- high compressive strength, good friction coefficient and low cost," said Marc Licharz, president of Licharz Engineering Plastics Inc. in Peekskill, NY.

Kendra Kenney, marketing manager for Ensinger Inc. in Washington, PA, further complimented the material's many benefits. "Nylon can withstand higher temperatures and strength than acetal or UHMWPE, is tougher than PET, has a lower cost than PTFE/acetal formulations, bearing grade PEEK or ULTEM or PPS, and has the ability to be cast in large parts and near net shapes to reduce machining costs and material waste," she said.

"I think nylon's best advantage is its versatility," added Jody Walker, the advanced engineering plastics product manager for TMX AIN Plastics Inc. in Detroit, MI. "Nylon has outstanding mechanical properties which allow it to be used in structural as well as bearing and wear applications. Nylon makes design engineers' jobs very easy. The engineer doesn't have to consider a lot of different engineering plastics when designing parts for low to moderate temperature bearing and wear or structural applications."

A Guide to Machining Nylon The machining of nylon is most beneficial in short runs, when the cost outweighs the building of the mold. Some machining may still be necessary on nylon that has been cast or molded. The use of sharp steel or carbide tools that will produce a continuous chip is advantageous. Although the machining of nylon is relatively easy, the removal of the chip is one of the biggest challenges faced by those who machine nylon. If the correct tool is used, the chip will come off in a continuous long "string." It is important to direct the chip away from the revolving chuck so that it does not become entangled in it. One solution is to have air blowing through a spindle that then blows the chip out through the back of the machine. Another solution is to set up a manifold with air pressure in a tube that vacuums the chip away from the machine. The following are suggested guidelines for the machining of nylon. For the turning of cast nylon 6, use a clearance angle of 15 to 20 degrees, a positive rake angle of 0 to 5 degrees, a side angle of 45 to 60 degrees, a surface speed of 800 to 1,600 feet per minute, and a feed of 4 to 20 mils per revolution. For the milling of cast nylon 6, a clearance angle of 10 to 20 degrees, a rake angle of 5 to 15 degrees and a feed rate of 10 inches to 30 inches per minute at 500 to 1,500 rpm are recommended. When drilling large diameter holes (1-inch diameter and larger) in nylon, a slow spiral (low helix) drill ground to a point angle of 118 degrees with a lip clearance of 9 to 15 degrees is best. The lip rake should be ground off and web thinned. Then drill a small (max. 1Ú2-inch diameter) hole at a speed of 600 to 1,000 rpm using a positive feed of approximately 0.005 inch to 0.015 inch per revolution. Avoid hand feeding the drill because "grab" can occur and stress cracks may develop. A secondary drilling at a speed of 400 to 500 rpm at approximately 0.008 inch to 0.020 inch per revolution is required to expand the hole to 1-inch diameter. Then progressively drill a slightly larger hole using the same technique as described until you arrive at the desired diameter. Sawing is generally used to cut off thick-walled parts with a relatively thin blade. Rip and combination blades with a 0-degree tooth rake and a 3- to 10-degree tooth set are best. Hollow ground circular saw blades without set give smooth cuts to 3Ú4 inch. Blades with set can sometimes reduce frictional heat. Tungsten carbide-type blades wear well and produce a good finish. For the sawing of cast nylon, a clearance angle of 20 to 30 degrees, a positive rake angle of 2 to 5 degrees, with a linear blade speed of 1,600 feet per minute and a pitch of 115 to 310 mils is recommended. The guidelines for the machining of nylon 66 run very similar to those of cast nylon, but with the following exceptions: For the clearance angle of the milling operation, an angle of 10 to 20 degrees and a clearance angle of 5 to 15 degrees for drilling and boring are recommended. With that machining operation, a rake angle of 10 to 20 degrees and a point angle of 90 degrees are suggested. Sidebar information courtesy of Kendra Kenney, marketing manager for Ensinger Inc., in Washington, PA, and Jody Walker, advanced engineering plastics product manager for TMX AIN Plastics Inc. in Detroit, MI.

Greg Fraser of Nylatech Inc. in Everson, WA, also praised nylon for its use in the engineering industry and its replacement of metal in many applications. "Relatively high tensile (11,000 to 14,000 psi), compressive (15,000 psi at 10 percent deflection) and load bearing strengths, combined with a low coefficient of friction (0.36 to 0.10) allow for a long-wearing mechanical part which requires lower maintenance than a similar metal part."

While most complimented nylon's diversity, some had favorites as far as advantages. "The advantage of nylon over other materials is its great resiliency," said Scott Dukesherer, engineering plastic specialist for Cadillac Plastics in Troy, MI. "Nylon will deform under load but return to its original shape 75 times easier than other materials."

"Nylon's most important property is its inherent wear resistance," said Mike Oliveto, marketing manager of DSM Engineering Plastic Products Inc. in Reading, PA. "On a cost-per-pound or volume basis, nylon's wear resistance is the greatest among engineering thermoplastics."

According to Oliveto, the addition of internal lubricants can greatly enhance nylon's bearing properties, expanding its load/velocity capability, increasing its bearing capability by a factor of five and reducing the wear rate by as much as 85 percent. "Nylon modified with internal lubricants provides the greatest bearing capabilities of the general-purpose plastic bearing materials, surpassing UHMW, acetal and PET. Solid lubricant modified cast nylons have limiting PV values exceeding 15,000 unlubricated and 30,000 with periodic lubrication and the lowest wear rates. Molybdenum disulfide is also added to extruded and cast nylons to improve the bearing properties over unmodified grades, although the benefits are not as pronounced as the newer solid lubricants," he said.

Though it garners much acclaim, nylon does have its drawbacks. "The greatest limitation of nylon is the dimensional stability of machined and molded components," said Oliveto. "Despite its low coefficient of linear thermal expansion, nylon's elastic modulus is lower than that of other engineering plastics."

Walker also cited nylon's poor coefficient of linear thermal expansion and water absorption properties. "These properties affect dimensional stability and create design concerns when tight tolerances are necessary in an application," he said.

Moisture absorption ranks as the largest dilemma faced by manufacturers and users.

Dealing with Moisture Absorption
"When cast nylon absorbs moisture, various mechanical properties are affected," said Licharz. "Parts which require higher notch impact strength or resilience will perform better if they have absorbed moisture, since these two properties increase as moisture content rises. Tensile strength, compressive strength, hardness and friction coefficient will all decrease as moisture content rises."

According to Licharz, depending on the moisture content, dimensional changes may also occur. "The expansion of cast nylon due to moisture absorption is approximately 0.15 percent to 0.20 percent per 1 percent absorbed moisture. The application conditions of the part must be known in detail to properly design the part," he said.

The rate of moisture absorption depends on temperature, crystallinity, humidity and part thickness. Under atmospheric conditions, nylon absorbs moisture very slowly. In normal ambient conditions (70F, 50 percent RH) cast nylon absorbs 2.5 percent moisture to a depth of 0.040 inch to 0.080 inch over the course of about 40 days, Licharz said. "That means that by the time the material arrives in your shop, it will usually not absorb any more ambient moisture. This initial absorption occurs fairly quickly, but additional absorption slows progressively with the depth of penetration," he explained.

A number of steps can be taken to minimize the effects of moisture absorption, Licharz said. One simple step is to switch from regular cast nylon to OILAMID, an oil-filled nylon. OILAMID has a slightly lower saturation point and a better friction coefficient.

Another suggestion is to secure long wear strips with one fixed hole and slot all other holes. This allows linear expansion of the strip as moisture content increases, Licharz said. In severe cases, he warned that conditioning the material may be required.

"During conditioning, parts are rough-machined, then submerged in warm water for one to two weeks," Licharz noted. "During this time, the parts will be saturated to a depth of about 1Ú2 inch. They can then be machined to their final dimensions and will absorb very little additional moisture."

Replacement Material
The majority of industry experts agreed on the most common applications of nylon being in the engineering market or replacing traditional metals. "With current casting technology, a grade of nylon is available to provide excellent service in a wide variety of engineering applications," said Fraser. "Most commonly used are bearings, bushings, pulleys, gears and wear strips."

The material is not geared toward just one industry though, Walker said. "Nylon applications can be found in every industrial, manufacturing or processing market segment from food processing plants to waste water treatment facilities. Nylon's outstanding mechanical properties allow it to be used in applications ranging from wheels and rollers to even sheaves used on the booms of heavy lifting cranes and equipment," he said.

Licharz said he sees nylon's greatest demand in bottle filling and handling machinery, citing such items as star wheels, timing screws and guide liners.

"The durability of nylon has made it the material of choice for load bearing and wear components on heavy-duty construction equipment and process machinery, despite the introduction of newer general-purpose engineering thermoplastics such as UHMWPE, acetal and polyesters PET and PBT," said Oliveto. "Nylon slide pads for telescoping booms, cast and machined sheaves for mobile cranes and gears for noise reduction on process equipment continue to provide design engineers with cost-effective performance benefits not possible with traditional materials, including thermoset laminates and newer thermoplastics."

According to Mike Plague, sales representative for Curbell Plastics in Buffalo, NY, unique applications for nylon include heavy-load wear plates and locator pins in stamped body panels for the auto industry and base plates for textile cutting machines.

What a material does for a particular product is the main issue for companies when deciding which plastic to choose. Fraser summed up why one would choose nylon by listing a few simple requirements. "A good rule of thumb is any application where a significant weight savings (1Ú7 over bronze), high impact resistance, prolonged part life and lower replacement and maintenance costs are desired; cast nylon is probably an excellent alternative," he said.

According to Fran Alder, sales marketing manager of Cast Nylons Ltd. in Willoughby, OH, "Near net shape castings are ideal for parts with a finished weight over 3 pounds. Purchasing a casting near net size minimizes the milling time necessary to create an eccentric-shaped finished part."

Growing Trends for Nylon
Like all thermoplastics, nylon has its disadvantages, although engineers are constantly searching for ways to improve its properties. With newer technology and broader imaginations, though, what was once deemed impossible for plastics is now becoming a reality.

"More nylon parts will be cast to size and require less machining and material waste," said Larry Phippen, president of Adapt Plastics Inc. in Rockford, IL. "More fillers will be added to nylon for better physical properties and longer wear."

Dukesherer also sees progress on the horizon for nylon. "In the next decade I think you could see nylon improving in such properties as moisture absorption, temperature and a wider variety of castable shapes. The ability to hold tighter tolerances with nylon would open up a wider variety of applications."

"I expect to see an emphasis on nylon near net shape castings as a means of reducing the cost and drop or scrap associated with the machining of nylon parts," said Oliveto. "Machine shops are increasingly conscious of the drop being generated from machining operations. Also expect to see an increased use of cast nylons that will be available in larger sizes typically associated with extrusion processes. Large 4-inch by 10-inch cast sheet and 10-foot cast rod is now available in certain sizes, enabling the production of larger parts and improved yields versus 2-inch by 4-inch sheet and 4-foot rod historically supplied.

"The use of nylon gears and sprockets will be expanding through the introduction of nylon/steel composite materials. This product, consisting of nylon cast onto specially-treated steel cores, provides a mechanism for rigidly attaching a plastic gear to a shaft, enabling greater use of the quiet, lubrication-free operation of nylon gears and sprockets," Oliveto added.

While some foresee the betterment of current nylon, others, such as Licharz, anticipate a new crop of nylon grades to hit the market. "Nylon is a mature product. Generally, the existing grades of nylon differ only slightly from each other. Over the next decade, some manufacturers will continue to introduce new grades, but this is mainly for marketing purposes. Manufacturers which make many different grades will not be able to make large production runs of each grade and will either price themselves out of the market or cut back on the variety of grades and colors they offer. The end effect will be better pricing for the customer."

Plastics Machining & Fabricating
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