Solid Ground Curing
Solid Ground Curing is a liquid based Rapid Prototyping technique developed in 1991 by Cubital Ltd. Cubital was found in 1987 as an internal R&D unit at Seitex Corporation Ltd. SGC is based on the principle of the photo polymerisation. Whole layer is polymerized at a time unlike “point by point” in Stereolithography.
Solid Ground Curing, also known as the Solider Process, is a process that was invented and developed by Cubital Inc. of Israel. The SGC process uses photosensitive resin hardened in process uses photosensitive resin hardened in layers as with the Stereolithography (SLA) process. However, in contrast to SLA, the SGC process is considered a high-throughput
production process. The high throughput is achieved by hardening each layer of photosensitive resin at once. Many parts can be created at once because of the large work space and the fact that a milling step maintains vertical accuracy. While the major advantage of the method is it, offered good accuracy and a very high fabrication rate. (8 times faster than other existing RP Processes)
The principle behind SCG process is Photo Polymerization and Mask Generation. Process is same as Stereolithography, only change being the whole layer is cured at a time, making the process faster.
Mask Generation: The mask is created from the CAD data input and “printed” on a transparent substrate (the mask plate) by an nonimpact ionographic printing process, a process similar to the Xerography process used in photocopiers and laser printers. The image is formed by depositing black powder, a toner which adheres to the substrate electrostatically.
This is used to mask the uniform illumination of the UV lamp. After exposure, the electrostatic toner is removed from the substrate for reuse and the pattern for the next layer is similarly “printed” on the substrate.
Photo Polymerization: Parts are built, layer by layer, from a liquid photopolymer resin that solidifies when exposed to UV light.
Polymerization is the process of linking small molecules (known as monomers) into chain-like larger molecules (known as polymers). When the chain-like polymers are linked further to one another, a cross-linked
polymer is said to be formed. Photo polymerization is polymerization initiated by a photochemical process whereby the starting point is usually the induction of energy from the radiation source.
A catalyst is required for polymerization to take place at a reasonable rate. This catalyst is usually a free radical which may be generated either thermally or photochemically. The source of a photochemically generated radical is a photoinitiator, which reacts with an actinic photon to produce the radicals that catalyze the polymerization process.
The free-radical photopolymerization process is schematically presented in Figure below. Photoinitiator molecules, Pi, which are mixed with the monomers, M, are exposed to a UV source of actinic photons, with energy of h . The photoinitiators absorb some of the photons and are in anν excited state. Some of these are converted into reactive initiator molecules, P•, after undergoing several complex chemical energy transformation steps. These molecules then react with a monomer molecule to form a polymerization initiating molecule, PM•. This is the chain initiation step. Once activated, additional monomer molecules go on to react in the chain propagation step, forming longer molecules, PMMM• until a chain inhibition process terminates the polymerization reaction.
Fig. Schematic for a simplified free-radical photo polymerization
The longer the reaction is sustained, the higher will be the molecular weight of the resulting polymer. Also, if the monomer molecules have three or more reactive chemical groups, the resulting polymer will be cross-linked, and this will generate an insoluble continuous network of molecules.
During polymerization, it is important that the polymers are sufficiently cross-linked so that the polymerized molecules do not redissolve back into the liquid monomers. The photo polymerized molecules must also possess sufficient strength to remain structurally sound while the cured resin is subjected to various forces during recoating.
Fig. Solid Ground Curing process (Courtesy Cubital Ltd.)
The process starts with the creation of three dimensional composition of the parts to be made in the next run and the definition of layer thickness. The Solider computer then slices the whole composition together into layers and generates a precise raster image of each layer.
This image is sent to the mask plotting unit in the machine and a high resolution, precise optical mask is generated by means of electrostatic charges and black toner powder.
The optical mask is then positioned precisely above the workpiece which already been spread with a thin layer of Liquid photopolymer and under a high power UV lamp (2000W) ready to be exposed.
A shutter is opened for about 3 seconds and the resin is exposed and cured by the light passing through the mask. The pattern of
the cross section of the model at this layer is fully cured due to the high power lamp and the length of exposure time. Unexposed areas on the layer surface remain in liquid state.
The mask, which is actually a plate of glass, moves back to the mask plotting unit and is being discharged and erased, ready for the next cross section to be drawn.
The work piece now passes under an aerodynamic sucker that pumps away all the residual liquid from the surface, leaving behind only the cured pattern.
A thin layer of melted wax is then spread over the surface, filling all the voids and cavities left after the removal of the residual liquid. A cold plate is lowered on to the surface of the layer, cooling down the wax and solidifying it. Now we have fully solid layer that is made partly from plastic and partly from wax.
The workpiece now passes under a milling disk that trims off the layer's surface down to the desired thickness, creating a flat, smooth surface ready for the next layer. A new layer of liquid photopolymer is then spread and the whole process starts once again.
After the last layer is done, we have a block of wax, within which the model or models are embedded. The wax is melted away in a microwave oven, or by using a hot air gun or even using plain warm water and the finished model is ready for use.
The work station or the equipment manufactured by the cubital Ltd. Is as shown in the figure below.
Fig. General layout of a SCG Machine
Models and Specifications
Cubital’s products include the Solider 4600 and Solider 5600. The Solider 4600 is Cubital’s entry level three-dimensional model making system based on Solid Ground Curing. The Solider 5600, Cubital’s sophisticated high-end system, provides a wider range and options for the varied modeling demands of Solid Ground Curing. Table below summarizes the specifications of the two machines.
Parallel processing. It has a high speed throughput that is about eight times faster than its competitors. Its production costs can be 25% to 50% lower.
Fig. Parallel Processing of several parts at a time. Self-supporting. The solid wax supports the part in all dimensions
and therefore a support structure is not required. Fault tolerance. It has good fault tolerances. Removable trays allow
job changing during a run and layers are erasable. Unique part properties. The part that the Solider system produces is
reliable, accurate, sturdy, machinable, and can be mechanically finished.
Minimum shrinkage effect. This is due to the full curing of every layer.
High structural strength and stability. This is due to the curing process that minimizes the development of internal stresses in the structure. As a result, they are much less brittle.
Requires large physical space. The size of the system is much larger than other systems with a similar build volume size.
Wax gets stuck in corners and crevices. It is difficult to remove wax from parts with intricate geometry. Thus, some wax may be left behind.
Waste material produced. The milling process creates shavings, which have to be cleaned from the machine.
Noisy. The Solider system generates a high level of noise as compared to other systems.
Comparison with other RP Techniques
The table below shows comparison of SCG with other RP Techniques with respect to different parameters.
Selection of the SCG Process
The flow chart below shows when to select SCG process for rapid prototyping and when not to.
General applications. Conceptual design presentation, design proofing, engineering testing, integration and fitting, functional analysis, exhibitions and pre-production sales, market research, and inter-professional communication.
Tooling and casting applications. Investment casting, sand casting, and rapid, tool-free manufacturing of plastic parts. Mold and tooling. Silicon rubber tooling, epoxy tooling, spray metal tooling, acrylic tooling, and plaster mold casting.
Medical imaging. Diagnostic, surgical, operation and reconstruction planning and custom prosthesis design.