Optimized ball check valve for fluid filling system
26 June 2018
A check valve permits a fluid to flow in one direction only. This application was selected for the Swiss AM Guide 18, as it demonstrates how an iterative development process and the consideration of trade-offs result in a successful AM serial part.
A so-called check valve or non-return valve is a component that allows a fluid or gas to flow only in one direction of the component. If the fluid tries to flow in the opposite direction, the valve blocks it. As a standard component, such a one-way valve is employed for many applications in fluid technology, pneumatic systems and process engineering. The simplest form of check valve is a ball check valve, which consists of a housing with two openings, a ball, and a spring.
In the closed state of a ball check valve, the pre-tensioned spring presses the ball against the constriction meaning the narrow part of the housing. In case fluid tries to flow from the right opening to the left opening, the ball blocks a possible flow. In the open state of the valve, fluid flows from left to right. Due to the dynamic pressure of the fluid, the ball is pushed to the right and a gap is formed at the narrow section, allowing the fluid to pass through the check valve.
3D Druck Tech AG had the task to develop a ball check valve for a small series production of a filling system in which a chemical solution is transferred from a larger container to smaller devices. In the filling system, a non-return valve allows air to flow from the outside into the container during the bottling of the solution, thereby preventing a compression of the container. The valve only permits fluid to flow into the container and the chemical solution itself cannot escape through the valve.
In the development of the check valve, 3D Druck Tech AG had to meet several require-ments. To ensure the tightness of the valve in the closed state, a high surface quality is needed at the constriction where the ball contacts the housing. As the valve is exposed to a chemical solution, the durability of its components must be ensured. The specified available design space corresponds to a relatively small size of 9 X 9 X 25 mm. As the check valve is employed as a final component, a cost-efficient and reproducible manufacturing concept is required.
Advantage of using 3D printing
Additive manufacturing was chosen by 3D Druck Tech AG because it allows the fabrication of small-sized components with complex, thin-walled geometries in an economically viable, small series production. Laser melting was therefore selected to manufacture a chemically resistant check valve made of stainless steel.
First Design Iteration
For the first design iteration, the basic idea was to build an integral check valve consisting of three parts – a rubber ball, a spring and an additively manufactured valve housing. The valve housing corresponds to a rotationally symmetric body with a conical connection for tubes of different diameters. If the housing is fabricated in longitudinal direction, no additional support structures are needed. Laser melting makes it possible to realize a wall thickness of 0.4 mm. During post-processing, the housing is removed from the build plate and sandblasted. The check valve is assembled by inserting the spring and pushing the rubber ball through the larger opening. The advantage of this first design iteration is a reduced number of parts and thus lowered assembly effort. However, as the constriction, meaning the contact surface between rubber ball and valve housing, is not further post-treated, it is not optimally sealed against fluid flow.As it can be seen in a microscope image, the surface is rough and contains unmolten powder particles. Further post-treatment of this functional surface is possible but difficult for conventional machining due to insufficient accessibility, and expensive in case of more advanced post-processing technologies such as electropolishing.
The second Design Iteration
The second and final design iteration achieves an improved sealing of the check valve with a concept consisting of five separate parts – a metal ball made of stainless steel, a spring, a sealing ring, a valve housing and a valve plug. The valve housing and plug are additively manufactured. The upper edge of the plug is turned for a high surface quality and seals the check valve together with the metal ball. During assembly, the spring andthe ball are first inserted into the valve housing. The plug is then pressed into the housing plug together with the sealing ring. Although the number of individual parts is higher in comparison to the first design iteration, the tightness of the valve is nowensured. Only a low pre-tensioning of the spring is necessary and the post-processing Networkof the constriction as a functional surface is simplified. The assembly of the valve consists of standard components in addition to the two additively manufactured parts. Compared to the first design iteration, the valve housing has a more conformal and space-saving shape with a diameter of only 7 mm and a length of 22.5 mm.
Other developments of the check valve
The two design iterations represent only an excerpt of the overall development process in which 3D Druck Tech AG has continuously developed and improved the original design concept for the check valve. The application example illustrates well that the devel-opment of additively manufactured components is an iterative process, in which the interplay between design, manufacturing and post-processing needs to be considered as a whole to achieve optimized results.
In this application, it is the combination of additive manufacturing, standard compo-nents and conventional post-processing which enables the fabrication of the check valve for the specified requirements. Due to the space-saving design, approximately 400 valve housings can be fabricated in a single batch job for a small series production. The final assembly of the valve is carried out using standard components such as metal balls, springs and sealing rings. In this way, a cost-efficient production of check valves is possible which meet the necessary requirements such as chemical resistance and fulfill the specified functionality with an optimized sealing