Introduction

The purpose of this article is to provide an overview of techniques that can be used to prototype an invention and make it into small-volume production. 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 could make a prototype and multiple copies. Also, by using the techniques I described in a previous article I wrote on low-cost packaging, the inventor or entrepreneur could make a fairly complete product that could be tested in the marketplace or presented to a potential licensee.

restrictions

First some warnings about the techniques described below. They are quite labor intensive. It is not hard physical work per se, but it does require learning and developing new skills. Also, the number of copies that can be made with silicone molds varies and depends on the geometry of the components being molded. In general, the useful life of a silicone mold varies from about 25 copies to several hundred. Another factor is how much care is taken in removing the components. The rougher the mold is treated, the shorter its life.

Furthermore, these techniques are not practical for making complicated prototypes, such as complex mechanical devices. These techniques are more suitable for making prototypes composed of a few parts (1 to 3) with a single component or in which several components are glued.

Finally, care must be taken in selecting the proper casting material. Be sure to use materials that are safe for prototyping. Do not use toxic materials, especially in any prototype that is going to be used in food handling or by children.

Make the prototype pattern

The pattern is the initial prototype and will be used to make the molds to create copies. Techniques that can be used to make an initial prototype include, but are not limited to, those described below. Use any method that produces a usable pattern. Please don’t feel limited to this list of techniques. Feel free to be as creative in coming up with a method of making the pattern as you were in coming up with the idea.

sculpt

One way to make a prototype pattern is to sculpt the shape out of wax, clay, or another suitable sculpting material. The problem with wax or clay is that they are brittle (wax) or deformable (clay). Some clays can be fired, heated, so that they are no longer deformable. However, brittleness remains a problem. There are some sculpting materials like Magic-Sculpt (available from Tap Plastics) that stay flexible for a couple of hours and then harden to rock hard. To produce the desired shape, sculpting tools are usually required. The sources of these tools are easily found on the Web.

carving

Another technique very similar to sculpture is carving. A pattern could be carved in wood, for example. However, if the material is porous, the surface will need to be sealed with a suitable surface treatment such as a sealer or primer. To make carving work easier, an electric rotary tool can be used. As with sculpture, sources for tools and techniques associated with carving are easily found on the Web.

rapid prototyping

Generally, to use a rapid prototyping technique, a three-dimensional computer-aided design (CAD) 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 additive or subtractive. In an additive process, material is added by solidifying or fusing material. A computer controlled laser can solidify a liquid through a photochemical process or the adhesion of solid material through sintering or melting. The part position is also computer controlled. These processes go by a variety of names, including 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 the computer numerically controlled (CNC) machine tool. In this case, the computer controlled machine tool removes material to form the parts.

There are numerous companies that offer prototyping services. In fact, many SLA companies have websites where you can upload the CAD model, quote a price, order parts, and purchase via credit card payment. The process is almost completely automated. However, a check is performed on the uploaded file to confirm the client’s intentions. Sometimes there are problems with the file. On the other hand, the CNC process has a much lower level of automation. A program must be written for the part model and stock material must be loaded into the machine. The program may be software generated but, as before, verification is required to confirm the intentions and fidelity of the file.

Silicone molds

Once a prototype pattern is complete, a silicone mold can be made using the room temperature vulcanization (RTV) process. Most RTV materials are non-toxic and easy to use. Other silicone molds can withstand temperatures up to around 500 degrees Fahrenheit and can be used to cast low-temperature molten metals like pewter. Since there are numerous sources on the web that provide instructions 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 tips are offered. First of all, wear gloves to not only keep materials off your hands, but also to prevent fingerprints from getting off the pattern of the prototype. The casting material is capable of producing extremely fine surface details. If there was a fingerprint on the pattern, you could detect it and it would appear on all your copies. Second, the material is very fluid, so make sure your mold frames are well sealed. Also, pay attention to the parting lines on your mold, where the mold parts to remove the casting. Make sure there are no areas where the silicone rubber could flow to catch the part. For example, a cup handle would require the mold to be split in a plane through the center of the cup and parallel to the handle. There would be handle impressions on both sides of the split mold showing an outline of the cup and handle.

Depending on the pattern of the prototype, 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 metal casting. For a two-piece mold, be sure to use release agent on the first part before molding the second. If this step is neglected, the first and second halves will merge and cover the prototype pattern. Also for the two-part mold, make sure you don’t forget to attach the nozzles and a fill port. Vomits are vents that allow trapped gases to escape and the mold fills until resin comes out of them. When the casting settles and the mold separates, the jets and fill port will leave a column that needs to be cut off. Therefore, be sure to place them where it is easy to touch up the surface. The jets should be placed in the highest part of the pattern so that air is not trapped in the mold. The jets and gate can be made by gluing rods vertically into the prototype pattern and holding them in place with a support structure attached to the mold frame. If an adhesive is used, make sure it does not interfere with the cure of the silicone casting material.

Use release agent liberally on relevant surfaces of the prototype pattern and on all jet and fill port surfaces. This is important to ensure castings can be removed and to prolong the life of the mold.

To prevent bubble formation on the surface of the prototype pattern, paint the molded surface with the molding material. To prevent bubbles from forming in the body of the mold, pour the silicone compound slowly and try to thin the molding compound over the rim of the pouring container to pop any bubbles already in the material. It is very difficult not to introduce bubbles during mixing. Typically, a vacuum degassing chamber is used to remove bubbles. However, with enough care, bubbles shouldn’t be a problem for most prototype patterns.

Finally, for a complex 3D 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 must be held securely in place or it may float. As with the vomitives and fill spout, make sure that any support structure that is placed does not irreparably mar the surface of the casting.