Variable Geometry Casting of Concrete Elements Using Pin-Type Tooling

Background: Today, free form parts are made in small production volume with time consuming methods and a significant amount of material waste. These methods include CNC machining and the layered rapid prototyping techniques SLA, SLS, FDM, 3D print etc., which are well suited for smaller parts that are rich in detail. Variable geometry moulds (VGM) offer a different approach to small production volume. A die or mould can change shape between the castings, and parts with different geometry can be made in the same mould. VGM is used in a multitude of applications such as sheet metal forming of parts for aircrafts, trains and cranial prostheses. The present project focuses on VGM for free form concrete facade elements, which in contrast to previous VGM projects uses a liquid raw material and involves the use of only a small amount of force. Method of Approach: The present VGM process is based on the so-called reconfigurable pin-type tooling principle (RPT). The geometric possibilities have been examined using a proof-of-concept RPT test mould. Sixty closely packed adjustable pin-elements with hemispherical tops and a square section of 43.3 x 43.3 mm create a dimpled surface that is evened out using an elastic interpolating layer. Castings with concrete and plaster are made on an elastic membrane that is sucked towards the pins using a vacuum. The shape of the cast elements and the mould surface have been measured and compared. Results: The RPT test mould can produce a large variety of free-form geometric shapes. It is possible to make straight vertical surfaces and even horizontal surfaces with dimples of only 0.3 mm. Part details can be made down to the size of a pin with hole depths up to 65 mm and protrusions up to 19 mm. Repeatability is better than the measurement uncertainty. Conclusions: VGMs using the RPT principle can be used for making scale models of a range of free-form cast concrete façade elements. It is possible almost to remove the imprints from the pins by using the right interpolators, but the dimples could also be a visually attractive characteristic of the process that could be valued by architects. Large hole depths and smaller protrusions are possible.