Volumes of production
The key feature of contour crafting is that it is an automated building method, however buildings can, of course, only be erected one at a time.
Unit price vs. capital investment
Allowing for the fact that multiple houses can be built using a single machine, Dr. Khoshnevis estimates the cost of building an average-sized American house at between a fifth and a quarter of the current cost of building a house by conventional means.
Speed
Construction using this process can build a 2,000 square-foot house, including electricity and plumbing, in less than 24 hours.
Surface
The use of the various types of trowel produces a good concrete surface, one that requires no preparation before painting. A painting system may even be incorporated within the contour crafting process itself.
Types/complexity of shape
The shape is limited only by the CAD drawing and the normal physical forces that apply to buildings, though even shapes such as arches can be extruded through the nozzle.
Scale
Dr. Khoshnevis suggests that this method can be used for anything from a small house to a high-rise structure.
Tolerances
The nozzle assembly that can move in six axes allows for very high tolerances on a large scale.
Relevant materials
Cement, with additives such as fiber, sand, and gravel.
Typical products
This is a process that is offering the building industry a new way to construct permanent houses, buildings, and complexes, as well as temporary emergency shelters.
Similar methods
On this scale, the process is unique. The CAD-based system makes it similar to many smaller scale rapid-prototyping processes (see, for example, stereolithography [SLA] p.246).
Sustainability issues
With its high-speed “printing” system, contour crafting could dramatically reduce construction times and therefore energy consumption in comparison to traditional building techniques. There is no wasted or excess material as the concrete is built up accurately in layers to the exact contours of the structure.
Further information
www.contourcrafting.org
www.freeformconstruction.co.uk
Stereolithography (SLA)
Product |
Black Honey bowl |
Designer |
Arik Levy |
Materials |
epoxy |
Manufacturer |
Materialise |
Country |
The Netherlands |
Date |
2005 |
This beautiful, open-cell structure is an excellent example of the highly intricate and complex forms that can be built up using this process.
Stereolithography (SLA) is one of the best known methods of rapid prototyping. Driven by a CAD file, components are produced by a laser, which scans a bath of photosensitive resin, building the components layer by layer. The ultraviolet laser beam is focused onto the surface of the liquid, tracing the cross-section of the part and turning successive thin layers of the liquid into solid. The solid part remains below the surface of the resin throughout the process, because it is seated on a bed that is lowered gradually, allowing the component to be built up in layers.
All rapid prototyping technologies give a geometrical freedom that no other processes do. SLA is typical in that it allows for the testing of components before entering into mass-production. Your choice of process is dependent on the geometry of the part, the surface quality required, or the material that you want to use. Selective laser sintering (SLS) (see p.252), for example, cannot match SLA for quality.
SLA is an accurate process, although not the most accurate, and it can be applied to a range of materials, although not to as many as vacuum casting. (This is a method of producing small batches of identical components that are generally used for prototyping or modelmaking. It involves producing an original master that is cast into a silicone mold. The mold is subsequently filled with plastic resins. A vacuum is applied and the resulting parts are very accurate, with fine detail and thin wall sections.)
1 This image, of designer Patrick Jouin’s CI chair, shows the finished product being raised from the liquid polymer. During actual production the only part visible is the very top edge of the chair as it is formed by the laser.
2 The finished chair is seen this time with a white block that acts as an internal support for the seat during the forming process, without which the chair would collapse.
3 The completed chair before removal of the support block.
4 The finished chair in all its translucent, ghostlike glory.
– Unlimited geometric freedom.
– Good surface finish.
– No intermediate steps between the CAD model and finished object.
– High unit costs.
– Only photosensitive resins can be used.
– Inaccuracy in two directions.
– Often needs support structures.
– Not as rapid as many other prototyping processes.
Volumes of production
Due to the time it takes to build up a product, SLA is strictly limited to low-volume production.
Unit price vs. capital investment
No tooling, and, even with a fairly high unit price, it is still the most cost-effective way of making prototypes.
Speed
Dependent on a number of factors, including the volume of the part, the material used, and the fineness of the step that is set by the operator. Another factor is the orientation of the component: If, for example, a beverage can is made lying down, the process is quicker, although less accurate, than when it is made standing up, which requires more passes with the laser.
Surface
The “stepping effect” as a result of the layering can be controlled by the thickness of the step. Also, shallow gradients will produce lines similar to contour lines on maps. Steep gradients and vertical walls will have smoother surfaces, but in both cases the part may need sand blasting.
Types/complexity of shape
Anything that can be drawn on a computer.
Scale
Standard machines can allow for a 20 by 20 by 24-inch building area. For anything bigger than this the components must be made in several sections and joined together. However, some companies make their own machines, producing components several feet long.
Tolerances
Height is the least accurate dimension, because of the increased number of passes that the laser has to make, but tolerance is generally ±0.1 percent plus 1/25 inch.
Relevant materials
Ceramic, plastic, or rubber can be used. More commonly, engineering polymers such as acrylonitrile butadiene styrene (ABS), polypropylene, and acrylic mimics are used.
Typical products
The word “typical” has no application here, since you can make anything that you want.
Similar methods
Vacuum casting (see above), selective laser sintering (SLS) (p.252), and inkjet technology (p.240).
Sustainability issues
Stereolithography requires a UV laser to cure the resin, and this is very energy intensive as cycle times can be quite slow depending on the complexity of the part. The additional support structures required for the majority of moldings can increase material consumption and waste. However, the uncured liquid resin is washed off the finished part and can be recycled back into the process to help minimize material use. As with all rapid production methods, tooling is eliminated and in the future local production will eliminate transportation costs.
Further information
www.crdm.co.uk
www.materialise.com
www.freedomofcreation.com
Electroforming for Micro-Molds
Product |
micro-mold |
Manufacturer |
Mimotec |
Country |
Switzerland |
A close-up image of the finished part (top) shows the scale achievable, as does the mold (beneath) that has a pinion cavity of only 1/425 inch and a micro-inscription on the side. The plate (as the presence of the needle demonstrates) is only 1/5 by 3/8 inch, and 1/20 inch thick.
Swiss company Mimotec has developed the process of electroforming (see p.164) to the extent that it can be used to make micro-molds. Before describing the Mimotec process itself, however, I need to make it clear that micro-molding is not the same as “miniature” injection molding. Micro-molding is closer to the seriously minuscule nano-end of the scale, rather than just small-scale molding, with parts being produced that can weigh as little as a few thousandths of an ounce with details that measure only a few microns thick.
Although the principle behind micro-molding is reasonably conventional, the methods used to produce the molds are rather fascinating. Micro-molds can be made by a number of different methods, including a micro-milling technique (where material is cut away). Mimotec, however, has harnessed the fine detailing achievable with electroforming to produce the most minute of molds.
The Mimotec process starts with an unpolymerized layer of photo resist deposited on a glass plate. This is then exposed to ultraviolet light through a mask of the final shape, which causes the exposed resist to polymerize, leaving the nonexposed area to be washed away. The remaining part is coated with gold followed by a further layer of resist. The part is built up in this way to produce a more complex part, which acts as the molding block and incorporates holes through which plastic for the component can be injected. This process is just one of many new methods of forming nano-scale components, and it is an excellent demonstration of the ever-advancing research that is going on in this field of production engineering.
– Capable of extreme precision.
– Low setup costs for electroforming make it good for prototyping.
– Making micro-molds in this way is a fairly slow process.
– Restrictions in current technology mean that only nickel and phosphonickel alloys can be used for the micro-molds.