Tolerances and Quality of Injection Molded Nylon Screws
Micro Plastics' molded nylon components are accurately molded of Type 6/6 Nylon. They are tough, resistant to corrosion, light weight, and good electrical insulators. Nylon components conform to irregular surfaces, provide vibration dampening, and offer an inexpensive alternative to more expensive metal parts. Molded threads approximate UNC/UNF 2A and 2B. Metric approximate 6g/6H.
Tolerances are generally held to +/- .005 (0,13) on dimensions up to .625 (15,875), +/- .008 (0,20) on dimensions from .626 (15,90) to 1 inch (25,400), +/- .010 (0,25) on dimensions from 1.001 inch (25,425) to 1 1/2 inch (38,100), +/- .015 (0,38) on dimensions larger than 1 1/2 inch (38,100), and +/- .015 on all fractional dimensions unless noted in part section (millimeters in parenthesis.) However, it is often more sensible to try samples or small order quantities before changing from one product to another. Plastic molding has varying degrees of quality. It often involves small amounts of flash, knockout pin marks, gates, sinks, and other slight imperfections. Screws and bolts are gated on the threaded end. It is possible for the gate to leave an extra projection of up to .025".
Nylon screws have been available for many years commercially. They are either machined on a screw machine or injection molded. The machined parts are made from extruded nylon rod stock and can be held to relatively close tolerances during the machining process. Nylon, however, has the unfortunate property of absorbing moisture from its environment which causes significant changes in its dimensions. So even though parts may have been accurately machined, a moist atmosphere can cause fit problems. Conversely, parts that have been moisture conditioned "lose" moisture causing dimensional problems. If the overall length of a screw grows, it obviously affects the threads per inch thus causing a tight fit. Changes in the moisture content can also have an affect on the outside diameter, pitch diameter, and the root diameter.
Injection molded nylon screws also suffer from moisture problems, but in addition, have inherent tooling problems. All molded nylon screws come from mold cavities parted along the length of the screw. These cavities have two halves that must be matched exactly to produce a perfect part. The cavity halves are doweled together with leader pins when the mold closes to provide the match required. Other locking devices are also employed to hold the halves of the mold in alignment. In the course of molding production quantities, these leader pins and locks suffer some wear. The mold base holding the cavities wears. The molding machine wears causing slight press platen misalignment. Small amounts of dirt on the mold can cause slight mismatch. From the time the tooling begins to run in production, the entire process begins to work against perfect alignment. Mold temperature changes from room temperature to production temperature causes the mismatch condition to worsen. At some point in time, the various factors affecting mismatch reach a condition that causes the molded screw to be unsatisfactory to the user. Then it is necessary to rework the mold to bring it up to its original condition. Since nylon is somewhat flexible, generally molded nylon screws are not held to as quite close tolerances as metal screws. A slight mismatch and minor flash at the parting line is not as serious as it would be in a metal part. End users who submit nylon screws to go-no-go gauges without trying the fastener in its actual functional application are generally causing unnecessary difficulties for themselves and their supplier. The fact is that because of the many areas of potential process variables, nylon screws made by injection molding are simply not measurable by the same methods as metal screws. They just are not capable of being manufactured to as close tolerances as metal screws.
The development of the tooling for nylon screws, plus the molding conditions, is a carefully kept secret. Tooling such as this is an evolutionary process. The quality of parts being manufactured today with today's tooling is obviously superior to those made in the beginning. Slight improvements have gradually been made over the years. Since we have production for nearly 3,000 sizes, if we make tooling improvements it is not practical to go back and remake 3,000 molds each time we make an improvement. We do, however, rework and replace as we go along any unsatisfactory tooling. This is a continual process with new sizes being added constantly and at the same time remaking tooling whenever the circumstances require it.
Occasionally, a few unacceptable parts may escape our quality control procedures. If you receive unsuitable parts, let us know immediately. We will do whatever we can to correct them.
All nylon screws are molded in an natural color resin. If you require black color, or some other identifying color (not a color match) these parts are made by dyeing the nylon screw with "RIT" dye in a water solution held at about 200 degrees F. which adds color but also moisture to the nylon. As stated previously, nylon screws change considerably with the loss or gain of moisture. Nylon screws, as they come from the molding machine contain about .5% moisture. After dyeing in a "RIT" dye, the moisture content increases to about 3-4%. Low moisture nylon is more rigid than moisture conditioned material. Impact strength and toughness increases with moisture addition. Flexibility also increases with higher moisture content which causes difficulties for the user on occasion. If you need a solid molded color, a closer than normal tolerance, or a special grade of nylon or other plastic material, we can furnish whatever variation or specialty using special molds and set-up. This, of course, adds considerably to the cost. We are here to serve you, so tell us what you need.