Designing With Nylon (Part 3)

By Glenn Beall

Related links: Features of Nylon (Polyamide)

Nylon with reinforcement differs from straight resin in simple ways, such as volume per pound, and in more complex ways, such as performance properties and gate positioning.

Reinforcing a material to improve its performance is at least as old as adding straw to mud to produce adobe building blocks. These bricks were more durable than those made of mud alone. Injection molders reinforce plastic material for the same reason. It was the reinforcing of thermoplastics that allowed the replacement of metal in heavy load-bearing applications.

The modern reinforced plastics industry as we know it today started in 1935 when the Owens Corning Fiberglass Co. learned how to economically produce continuous glass filament. The first applications relied on thermosetting material as the matrix to bind together and shape the glass fibers.

Injection moldable, reinforced thermoplastics were introduced in the early 1950s by the Fiberfil Corp. (now DSM Engineering Plastics). The first application was a military land mine housing. The transportation industry, which became the largest market for reinforced plastics, started with a Lincoln Continental instrument panel.

Large quantities of reinforced thermoplastics are now being injection molded, but these articles have never discussed how to design with those materials. Nylon is the largest-volume reinforced thermoplastic material. It, therefore, seems appropriate to discuss reinforced thermoplastics in this third, and last, part of the nylon series.

Some definitions
Today people erroneously use the words fillers and reinforcements interchangeably. Reinforcements improve the physical properties of a plastic material. Fillers, on the other hand, give the plastic material bulk. On a volume basis, fillers reduce a material’s cost and must, therefore, be lower in cost than the plastic material. A filler may or may not improve a material’s physical properties.

A reinforcement is any fibrous material that is stronger than the plastic material. All kinds of natural and synthetic fibers have been used. Glass fiber accounts for approximately 95% of the market. The use of carbon and wood fibers is small, but increasing.

Reinforcing fibers have an aspect ratio. Their length is greater than their width. Fillers have little or no aspect ratio. The typical glass fiber is .0004 inch in diameter. The minimum fiber length that will provide significant reinforcing is .008 inch. This length produces an aspect ratio of 20:1.

Fibers increase a material’s strength by distributing a load over many molecules. The longer the fiber, the more molecules there are to absorb the load. The typical fiber length used in injection molding is limited to .125-.187 inch, as that is the length of the pellets. In 1985 a British company learned how to coat longer glass fibers. This technology allowed the production of .500-inch-long pellets and fibers. These longer fibers provide more reinforcement than shorter fibers.

In a glass-fiber-reinforced thermoplastic, the fibers provide greater strength, heat resistance, and reduced mold shrinkage. The plastic material contributes chemical and electrical properties, color, surface finish, and processing characteristics. Common glass fibers used in injection molding have a tensile strength of around 500,000 psi. Adding 30% by weight of these glass fibers to nylon 6/6 results in the impressive improvement in properties shown in the table below. There are many different nylon 6/6 plastics and this table lists average values.

Glass is dense. Adding it to nylon 6/6 increases its specific gravity and reduces the volume of a pound of that mixture. This reduction and the increase in cost per pound must be considered in specifying a glass-fiber-reinforced material. The large-volume market price for these materials is included in the table.

Reinforced materials are different
In these times of thin-walling, design engineers are frequently disappointed in the lack of stiffness in the resulting part. A common reaction is to add glass fibers and try it again. Glass fibers increase a part’s stiffness, but they also change a lot of the part’s other characteristics.

For example, a nylon 6/6 part is isotropic; it will for all practical purposes have uniform physical properties in all directions. Adding glass fibers will make the same part anisotropic, i.e., with nonuniform physical properties. The primary reason for this lack of uniformity is that the fibers orient in the direction of melt flow.

In one set of experiments on a .050-inch-thick end-gated part, 90% of the fibers were oriented in the direction of flow. A similar part with a .250-inch thickness had only 5% of the fibers oriented in the direction of flow. The thinner the part, the more anisotropic the properties will be.

A part with a high degree of fiber orientation has a high tensile strength and stiffness in the direction of orientation and parallel to the length of the fibers. The tensile strength and stiffness transverse to the length of the fibers is much weaker.

Mold shrinkage with nonreinforced crystalline materials, such as nylon 6/6, is slightly greater in the direction of flow than transverse to the flow direction. Fiber-reinforced parts are the exact opposite. Mold shrinkage is approximately 50% greater in the direction transverse to flow. These differing shrinkages are of little concern with nonreinforced parts. The large differences in mold shrinkage of fiber-reinforced parts result in warpage, which is a constant source of problems for molders.

Conventional wisdom teaches that gating a part in the center produces the shortest flow path, while allowing the melt to reach all extremities of the cavity at the same time. This is ideal for nonreinforced materials. There is no universal agreement, but many molders believe that fiber-reinforced parts should be gated at one end. The logic for this gate location is that you will at least know in which direction the fibers are oriented.

Part design tips
With a few exceptions, the design criterion is the same for reinforced or nonreinforced nylon parts. A good minimum wall thickness for a fiber-reinforced part is .125 inch. Thinner walls will have more problems resulting from fiber orientation.

• Glass-fiber-reinforced nylon parts are stiffer and more abrasive. Cavities, and especially cores, must be polished and free of undercuts with a minimum draft of 1° per side.
• Highly polished, blemish-free surfaces are difficult to guarantee. The best results are achieved with a warm cavity that produces a resin-rich surface.
• Sharp corners are to be avoided, as they impede melt flow and break fibers.
• Holes can be molded, but the resulting weldlines may have only 60% of the material’s tensile strength. When the two flow fronts that form weldlines meet, the glass fibers from one flow front do not cross the weldline to mingle with the second flow front. While using fiber-reinforced materials, every effort must be made to avoid locating weldlines in heavily loaded, high-appearance areas. The reduced concentration of glass fibers in the area of the weldlines may also result in a nonuniform surface appearance.
  

June, 2006 - Reprinted with permission from Injection Molding Magazine. Copyright © Canon Communications LLC.