Applications and guidelines for nylon use.

Nylon, the oldest of the engineering thermoplastics, was introduced by DuPont at the 1939 World’s Fair in New York City. Since that time nylon has grown to become the largest-volume engineering material. The attributes that account for this material’s success were discussed in the February 2006 “By Design” article (imm net.com/articles/2006/February/2785).

This article is concerned with injection molded nylon applications and design details. Nylon covers a wide range of products, but its largest market is for fiber. Products include hosiery, undergarments, brush bristles, rope, heavy-duty industrial fabric, and, believe it or not, Americans annually consume 3 million miles of nylon dental floss.

Part design tips

Wall thickness. Nylon is an easy-flow material. This allows it to be molded in thinner sections than other materials with similar physical properties. For example, a 48-inch-long part can be molded with a .190-inch-thick wall. Walls as thin as .010 inch can be molded, but demonstrate an increase in shear and molecular orientation, which can result in nonuniform physical properties. A better minimum wall thickness is .040 inch, which can provide a flow length of around 10 inches.

Nylon is a high-mold-shrinkage-factor crystalline plastic. It is not ideal for molding parts with thicknesses greater than .250 inch. Thicker walls are susceptible to uncontrollable mold shrinkage, internal voids, and molded-in residual stress.

Variations in wall thickness should be limited to 10-15% of the part’s nominal wall thickness and be smoothly blended from thick to thin.

Corner radiuses. Nylon is a notch-sensitive material. Sharp inside corners prevent it from achieving its characteristic toughness. Maximum strength, impact resistance, and ease of flow are achieved with an inside corner radius equal to 75% of the part’s wall thickness. The minimum allowable inside corner radius is .020 inch.

Molding draft angles. Nylon is self-lubricating and frequently can be molded without draft angles. Many bearings and gears must be molded straight. However, nylon parts are easier to mold on shorter cycles if they are provided with 1?2-1° draft angles per side.

Projections. Many functional features, such as stiffening ribs, solid bosses, and snapfit latches, can be molded as projections off of a part’s nominal wall. Their thickness at the junction with the part should be limited to 50% of the part’s wall thickness. In cases where appearance and the absence of sink marks is critical, projections can be reduced to 40% of the part’s wall thickness.

Depressions and holes. The main problem associated with depressions is that they create weldlines. The core pins that form small holes are difficult to cool and are susceptible to bending. Weldlines can weaken a part while creating cosmetic problems. A properly molded nylon 66 part can have visible but commercially acceptable weldlines while retaining 95% or more of its original tensile strength.

Nylon’s ease of flow allows the use of low injection pressures. These lower pressures reduce the bending forces on the core pins that form small holes. All inside corners on holes should have standard radiuses. As the material shrinks, it grips the core pins. Providing molding draft on these core pins reduces ejection force, which allows shorter molding cycles.

Tolerances. As a general rule, the dimensional reproducibility of a plastic material is dependent on the polymer’s mold shrinkage factor. The higher the shrinkage, the broader the tolerances. Nylon is an exception. In spite of its high shrinkage factor, nylon is specified for precision parts such as gears, bearings, and aerosol valves. The tolerances that can be maintained are dependent on the part’s wall thickness. In one series of experiments, a .031-inch-thick nylon 66 part shrank an average of .010 in/in; a .125-inch part shrank .015 in/in; and a .250-inch part shrank .022 in/in. In other words, smaller tolerances can be held on thin parts than on thick-walled parts.

A “commercial” tolerance on a 1-inch-long, .125-inch-thick nylon 66 injection molded part is ±.0046 inch. A more costly “fine” tolerance on the same part is ±.0023 inch.

Nylon is the most often specified fiber-reinforced thermoplastic material. Considerations for designing with reinforced plastic will be the last part of this series.