Stern Rubber Company has three locations located in Minnesota; two plants are in Staples and the third is located in Aitkin. The Aitkin plant, or plant number three, is dedicated to producing rubber to metal bonded gate valves for two of our customers: American Valve and Hydrant in Beaumont, Texas, and Waterous Company in South St. Paul, Minnesota; these valves are used in the water industry meeting NSF 61 requirements.
The process for manufacturing these products is by bonding rubber to cast iron valves by injection molding; we clean and prep the cast valves, apply bonding agent material to the cast valves and encapsulate the casting with EPDM rubber. EPDM rubber is an approved rubber used in the water industry since it has good resistance to ozone, oxygen, and both inorganic and organic chemicals. With its temperature range running between -70⁰F and up to +300⁰F, it finds itself resistant to seawater and fresh water. EPDM is not reactive to chlorinated water, which makes this synthetic rubber an excellent application for the water industry.
The injection molding press at Stern Rubber Company plant number three takes the EPDM rubber into the press by strip as a thermal set elastomer (TSE), into the barrel where there is a screw that warms the rubber to temperature which is still below the operating temperature of the mold. The EPDM rubber is then injected through channels in the tool, passing into the cavities of the mold and encapsulating the valve castings. Vulcanization occurs when temperature, pressure and time creates a crosslinking of the chemical bonds in the rubber. Rubber injection molding is a faster process compared to compression or transfer molding, since the rubber is preheated before it is injected into the mold’s cavities, which reduces the required cure time. In addition to preheating the rubber, we also preheat the castings before they placed into the mold, which also reduces the time to vulcanize the rubber.
A metal mandrel is used to position the casting in the correct position inside the mold and is removed by a hydraulic press after the injection press completes vulcanization. The next step in the process is the parts are inspected, excess rubber flash is removed and the rubber sprews (feeds) are removed. Our experienced operators apply a black epoxy resin to the area of the valves which had stand-off pins, before adding plastic ears which allow the gate valve to slide easily inside the valve body. Once through our quality inspections the gate valve and packed on a pallet and are ready to ship to Beaumont or South St. Paul.
We have been Supplier of the Year for the past eleven years creating high quality gate valves, gaskets and rubber products for American Valve and Hydrant with deliveries that are always on-time. In 2014, Stern Rubber had 100% on time delivery and 100% quality.