Non-Return Valves Sealing Surface Design
When asked by one of our customers to get our opinion of a recently published article in Plastics Technology Magazine by Mr. John Bozzelli, we answered:
I think that John is over-simplifying the science involved in getting a valve to shutoff and specifically shutoff in a short period of time without leaking through the full axial movement of the screw, especially considering that the viscosity of the polymer fluids that the average molder uses everyday can vary by a factor of ten with varying injection speeds. There is only one similarity between the engine valve which he describes and the sliding ring NRV used in the plastics industry and that is that it is a SEAT. In our industry there are many more variables to be concerned with such as;
- The viscosity of Thermo Plastics is 10,000 1,000,000 times more than that of water… let alone air.
- The pressure within the plastic fluid changes which is dependent upon the viscosity which is influenced by the heat that is being applied via the heater bands on a cyclic nature and shear heat from the screw which can change due to slippage between the polymer and the screw as well as the residence time.
- The pressure within the plastic fluid that is measured across the valve which is dependent upon the design itself and the factors mentioned above.
- The relative speed of the screw during injection forward.
- Fillers that are part of many of the polymers that we use today. The type of filler will determine if it will deter complete shutoff because they are solids and do not change shape unless they are exposed to compressive forces that are greater than they can resist.
I believe that the seat is important but if there is a poor hydraulic design no matter what type of seat is employed, complete and quick shutoff will not occur. I am a believer that the best designs are ones that have a minimal pressure drop across the shutoff mechanism which is the ring on a sliding ring design and a poppet or ball on one of those namesake designs. If there is a lower pressure differential across the shutoff mechanism then when the screw moves forward on a ring valve for example, the displacement of the fluid between the faces of the ring and rear seat will take place quicker and therefore less material will traverse upstream along the flight channel. In this case, the theoretical displacement of the polymer fluid and the actual will be closer. When the two faces touch, then and only then can pressure be built and complete shutoff occur, this is where the seat design comes in. I think that the best way to assure a shutoff between two components is to assure alignment between them. A screw and ring that sits within a barrel does not assure a rigid alignment because of tolerances and eventually greater differentials because of wear, therefore I believe and have tested that a flat seat is the best way to assure a seal. After all, the caps on the milk container that John mentions in his article are all flat surfaces.
Our “MDP” performance sliding ring valve (Patented US #7,527,493) has the lowest pressure drop in the industry because the front seat and rear seat see virtually identical surface areas as opposed to most valves that have flutes that retain the ring’s forward movement. We have had rave reviews from our customers regarding shutoff performance…and no bridging issues. In addition, we design an Automatic version of the “MDP” design and we sell a “Spring” loaded poppet valve as well. Overall, I am pleased with our offerings and can make a recommendation for your company and will guarantee performance. In fact if you record shutoff performance now I would love to get data from you after using one of our designs.
Michael F. Durina