Introduction
The purpose of this article is to provide an outline on techniques that can be used to prototype an invention and make a small volume production run. This article will not cover the techniques in detail. There are multiple resources available online for each of the techniques mentioned in the article. Rather this article will show how to combine the techniques to produce a prototype and multiple copies. By using these techniques or variations, an inventor or entrepreneur would be able to make a prototype and multiple copies. In addition, by using the techniques I outlined in a previous article I wrote on low costs packaging and inventor or entrepreneur would be able to make a fairly complete product that could be test marketed or presented to a potential licensee.
Limitations
First some warnings about the techniques outlined below. They are rather labor intensive. Not hard physical work per se, but it does require learning and developing new skills. Further, the number of copies that can be made using silicone molds varies and is dependent on the geometry of the components being molded. Typically the lifetime of a silicone mold varies from as few as 25 copies to several hundred. Another factor is how much care is taken in extracting the components. The rougher the mold is treated the shorter its life.
In addition, these techniques are not practical for making intricate prototypes such as complex mechanical devices. These techniques are more suited to making prototypes composed of a few parts (1 to 3) with either a single component or where multiple components are glued together.
Finally care must be taken in selection of suitable casting material. Be sure to use materials that are safe for the prototypes application. Do not use toxic materials especially in any prototype to be used in the handling of food or by children.
Making the Prototype Pattern
The pattern is the initial prototype and will be used to make the molds to create copies. The techniques that can be used to make an initial prototype include but are not limited to those outlined below. Use any method that will yield a usable pattern. Please do not feel limited to this list of techniques. Feel free to be as creative in coming up with a method to make the pattern as you were in coming up with the idea.
Sculpting
One way to make a prototype pattern is to sculpt the shape in wax, clay or other suitable sculpting material. The problem with wax or clay is that they are either brittle (wax) or deformable (clay). Some clay can be fired, heated, so that they are no longer deformable. However brittleness is still a problem. There some sculpting materials such as Magic-Sculpt (available at Tap Plastics) that remains pliable for a couple of hours and then cures to rock hardness. In order to produce the desired shape sculpting tools are usually required. Sources for these tools are easily found on the Web.
Carving
Another technique very similar to sculpting is carving. A pattern could be whittled out of wood for example. However if the material is porous, the surface will have to be sealed using a suitable surface treatment such as a sealer or primer. To make the carving job easier an electrically powered rotary tool can be used. As with sculpting, sources for the tools and techniques associated with carving are easily found on the Web.
Rapid Prototyping
Generally to use a rapid prototyping technique a Computer Aided Design (CAD) three dimensional model is required. This model is exported to a suitable file format such as a stereolithographic file. The file is then read by a computer automated machine that makes the part. The processes are either additive or subtractive. In an additive process material is added by solidifying or fusing material. A computer controlled laser may solidify a liquid by a photochemical process or adherence of solid material by sintering or fusing. The part's position is also computer controlled. These processes are called by a variety of names such as SLA (stereolithography), SLS (selective laser sintering) and FDM (fused deposition modeling). More information can be found at the following site http://en.wikipedia.org/wiki/Stereolithography.
An example of a subtractive process is computer numerical controlled machine tooling (CNC). In this case the computer controlled machine tool removes material to form the parts.
There are numerous companies offering prototyping services. In fact, many SLA companies have Web sites where the CAD model can be uploaded, a price quoted, parts ordered and purchased via a credit card payment. The process is almost completely automated. However, there is a check done on the uploaded file to confirm the intentions of the customer. Sometimes there are problems with the file. On the other hand, the CNC process has a much lower level of automation. A program has to be written for the part model and stock material has to be loaded into the machine. The program may be generated by software but as before a check is required to confirm intentions and fidelity of the file.
Silicone Molds
After a prototype pattern is finished a silicone mold can be made using room temperature vulcanization (RTV) process. Most of the RTV materials are non toxic and are easy to use. Further silicone molds can take temperatures up to around 500 degrees Fahrenheit and can be use to cast low temperature casting metals such as pewter. Since there are numerous sources on the Web that provide instruction on the technique it will not be covered here. Materials for RTV molding are available on the Web and through Tap Plastics. However the following pointers are offered. First use gloves not just to keep the materials off your hands but also to keep finger prints off the prototype pattern. The molding material is able to produce extremely fine surface detail. If a finger print were on the pattern it could pick it up and it would appear on all your copies. Second, the material is very fluid so make sure your mold frames are well sealed. In addition, pay attention to the parting lines of your mold - where the mold comes apart to remove the casting. Make sure there are no areas where the silicone rubber can flow to trap the part. For example, the handle of a cup would require that the mold would part on a plane through the center of the cup and parallel to the handle. There would be handle impressions on both sides of the parted mold showing a profile of the cup and handle.
Depending on the prototype pattern it may be possible to make a one piece mold that is completely open on one side. This type of mold is usually used for low temperature casting metals. For a two piece mold be sure and use mold release on the first part before casting the second. If this step is neglected the first and second halves will fuse together and tarp the prototype pattern. Also for the two part mold make sure not to forget to place the spews and a fill port. Spews are vents that allow trapped gases to escape and the mold is filled until the resin flows out of them. When the casting is set and the mold is parted the spews and the fill port will leave a column that has to be cut off. Therefore be sure to place them where it is easy to touch up the surface. The spews need to go on the highest portion of the pattern so that air is not trapped in the mold. Spews and the inlet can be made by sticking rods vertically on to the prototype pattern and hold them in place with a supporting structure connected to the mold frame. In an adhesive is used be sure that it will not interfere with the curing of the silicone molding material.
Use mold release liberally on the relevant surfaces of the prototype pattern and all surfaces of the spews and fill port. This is important to make sure that the castings can be removed and to prolong the life of the mold.
To prevent the formation of bubbles on the surface of the prototype pattern, paint the surface being molded with the molding material. To keep bubbles from forming in the body of the mold, pour the silicone compound slowly and try to thin out the molding compound over the lip of the pouring container to burst bubbles already in the material. It is very difficult not to introduce bubbles while mixing. Usually a vacuum degassing chamber is used to remove the bubbles. However, with enough care the bubbles should not be a problem for most prototype patterns.
Finally for a complex three dimension casting consider a bottom up approach. That is, support the prototype pattern using the top of the molding frame and fill the frame up to its surface. The pattern needs to be securely held in place or it may float. As with the spews and fill spout make sure that whatever supporting structure is put in place does not irreparably mar the surface of the casting.
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