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HOW A SHAFT SEAL IS MADE
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Manufacturing quality seals is a multi-step process.
by Rick Hudson
Having examined, in our previous two issues, both the anatomy and function of a shaft seal, I’d like to use this final installment of our shaft seal series to outline how a typical seal is made. Multiple manufacturing processes, all with their own technical challenges, must coalesce in order to produce a high quality shaft seal. The diagram at right shows how it all comes together. Numbers in the text refer to specific steps along the way.
PREPARATION Rubber recipe ingredients are carefully selected and precisely weighed (1). Thanks to sophisticated computer controls, the process cannot move forward unless the right amounts of the correct ingredients are chosen. Once assembled, the ingredients are fed into an internal mixer that uses rotors to heat and mix the compound (2). The rubber then goes onto a mill that further kneads it to ensure uniform dispersion of the ingredients (3). The resulting rubber sheets are dipped in stearate to reduce stickiness (4), then air cooled to prevent premature curing, or scorch (5). Samples are taken for batch testing (6). If approved for production, the rubber is fed through an extruder to produce pre-forms (small pieces of rubber compound) for use in molding (7).
Most shaft seals have metallic cases that have been press-stamped from strip steel (8). After stamping, the mild steel cases are placed in a rotating basket and submersed in a series of tanks designed to both clean the cases and apply a coat of zinc phosphate (9). This coat helps prevent rust, and it roughens the case’s surface, making the metal more amenable to being bonded to rubber during molding. Adhesive is generally sprayed on the case and then oven- or air-dried as a last step before molding (10).
But what about the mold itself? Mold making begins with a design generated by an engineer based on the desired final dimensions of the seal (11). Typically made of hardened tool steel, the mold is produced using computer numerical control (CNC) equipment (12). Once cut, the mold is hand-polished to a blemish-free finish, then cleaned prior to use or storage (13).
MOLDING Molding is where rubber is shaped, cured, and bonded to a case via heat and pressure (14). There are three main molding methods: compression, transfer, and injection molding. They differ in how the rubber enters the mold, but all three methods can be used to make shaft seals. Our illustration shows compression molding. Each cavity of the mold is loaded with a metal case and a rubber pre-form, then the mold is closed for simultaneous shaping and curing of the rubber, as well as bonding of the rubber to the case.
FINISHING After the molded seals are taken from the cavities, any excess rubber (flash) is removed. The sealing lips may be knife-trimmed in an automated process (15). Automation may also be used to insert garter springs, which were produced previously in a separate preparatory stage (16), into the sealing lips. Spring insertion may be done by hand for low volume, unusual seal designs or for very large seals (17). The seal lips are greased if the application requires lip lubrication. Though each step of the manufacturing process is carefully controlled and monitored to ensure product quality, a final audit inspection is conducted on a sample of the finished parts taken from a randomly selected box (18). All parts are shipped when this final inspection has been successfully completed (19). I hope you’ve found this series on shaft seals helpful. For more information, visit our web site at www.rlhudson.com and download a copy of our Shaft Seal Design Guide. Just click on “Knowledge Base” in the menu bar. And please do not hesitate to call us at 1-800-722-6766 if we can help you design or select the right shaft seal for your application.
