Research Papers

Variable Geometry Casting of Concrete Elements Using Pin-Type Tooling

[+] Author and Article Information
Troels H. Pedersen

Department of Management Engineering, Technical University of Denmark, Produktionstorvet Building 424, DK-2800 Lyngby, Denmarktroelshp@gmail.com

Torben A. Lenau

Department of Management Engineering, Technical University of Denmark, Produktionstorvet Building 424, DK-2800 Lyngby, Denmarklenau@man.dtu.dk

J. Manuf. Sci. Eng 132(6), 061015 (Dec 20, 2010) (10 pages) doi:10.1115/1.4003122 History: Received January 23, 2010; Revised November 13, 2010; Published December 20, 2010; Online December 20, 2010

Today, free-form parts are made in small production volume with time consuming methods and a significant amount of material waste. These methods include computer numerical control (CNC)-machining and the layered rapid prototyping techniques stereolithography apparatus (SLA), selective layer sintering (SLS), fused deposition modeling (FDM), 3D print, etc., which are well suited for smaller parts that are rich in detail. Variable geometry molds (VGMs) offer a different approach to small production volume. A die or mold can change shape between the castings, and parts with a different geometry can be made in the same mold. 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. The present VGM process is based on the so-called reconfigurable pin-type tooling (RPT) principle. The geometric possibilities have been examined using a proof-of-concept RPT test mold. Sixty closely packed adjustable pin-elements with hemispherical tops and a square section of 43.3×43.3mm2 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 toward the pins using a vacuum. The shape of the cast elements and the mold surface have been measured and compared. The RPT test mold 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. VGMs using the RPT principle can be used for making scale models of a range of free-form cast concrete façade elements. It is almost possible 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.

Copyright © 2010 by American Society of Mechanical Engineers
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Figure 1

Pin art, patented by Fleming (2)

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Figure 2

Three examples of principles considered for the VGM: a building block principle, expandable chains, and the RPT. As seen, the RPT principle (right) cannot mold or demold pieces with a negative draft (shown with black arrows, left and middle).

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Figure 3

A schematic overview of the process used for casting concrete elements in a RPT-based VGM

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Figure 4

Smoothening done with the interpolator. (a) The dimpled surface caused by the discrete design. (b) The interpolator absorbs the dimples and provides a smoother mold surface.

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Figure 6

Lead-screw actuation arrangements: (a) the principle used in the RPI test tool and (b) a principle more suitable for automation

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Figure 7

A concrete casting. Melamine covered plates were carved into shape to match the mold surface and enclose the casting on the sides.

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Figure 8

A section of the surface covering four pins and showing where measurements of dimples were made: (a) the distance between the lowest point and a line connecting two diagonally placed pin tops and (b) the distance from the lowest point and a line connecting the tops of two adjacent pins

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Figure 9

Dimple depth for different interpolator thicknesses

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Figure 10

Photos from the experiments with height differences. The circle marks the extended areas. The printed lines on the membrane show the larger extension for (a) the positive height difference compared with (b) the negative height difference.

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Figure 11

The hatched area marks where the vacuum affects the membrane: (a) a large area for the positive detail and (b) a smaller area for the negative detail

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Figure 12

(a) Vertical surfaces are possible, thanks to the extra membrane material borrowed from surrounding areas, seen from the top. (b) Same experiment seen from the side shows the curvature at the base of the mold.

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Figure 13

A facade model made from six RPT cast elements. The element shown in Fig. 1 is marked with an S.

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Figure 14

Depressions were mainly seen in the concave sections of the elements, matching the mold’s convex areas marked B. Convex sections in the elements marked A did not show any dimples: (a) a cast element and (b) the underlying mechanism.

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Figure 15

The raised pins tilt due to clearances between the pins

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Figure 16

(a) The membrane is stretched and drawn down between the pins and the clamping frame; (b) the free-hanging membrane can be supported, allowing more membrane material to be borrowed by the central part of the tool. P indicates the present position of the membrane and D indicates the desired position.



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