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|Prototyping For Inventors|
This article is re-printed courtesy of Jack Lander
Copyright © Jack Lander, www.inventor-mentor.com
Prototypes usually go through several stages of development, starting with a relatively crude “concept” model and hopefully finishing with a model that “looks like, works like” the eventual product. To aim for a prototype that approximates the final product, you need to select prototyping processes that imitate the volume-production materials and processes that will be used when the product is in full production. Much of this selection is common sense. Mass-produced products and product components today are made primarily in three ways: You can injection mold them out of plastic, stamp them from steel, or die cast them from a zinc alloy.
Prototypers are often specialists in certain processes, and you may need more than one source in order to complete your prototype. Also, there is usually more than one way to imitate any volume-production process, and you can often reduce your costs dramatically by knowing a bit about the options. For example, if the main component of your final product will be an injection-molded plastic part, you can imitate this in your prototype in several ways: by machining the component from solid plastic; by fabricating it by gluing together several pieces of plastic; by casting it from a two-part resin in a silicone rubber mold; by using the “additive manufacturing” processes of SL (stereolithography), SLS (selective laser sintering); or by what is called 3-D printing, a process that jets wax or plastic, and builds a three-dimensional part. Each has its place, its cost structure, and its characteristic time from start to completion. The process that is perfect for a corporation may be way too expensive for the average inventor, who is willing to wait a week or two for his or her parts in exchange for a low price.
This appendix covers the main options for imitating volume-produced parts. Some of these processes you can accomplish yourself. Others require enormously expensive machines and are only practical through job shops. In any event, it will pay to have a working knowledge of these processes to obtain the best prototype possible for the money. At the end is a “Resources” section listing the phone numbers and addresses of the suppliers and vendors that are mentioned.
A concept model typically shows only what a product will eventually look like. It helps investors, potential partners, and the people you contact for market research to get a better idea of your product’s appearance. So-called “appearance models” are concept models that simulate the look of the final product, and they are used for initial brochures, publicity releases, and trade shows when the final product is not ready. It also helps the inventor get a sense of the proportion and style of the final product.
Using Balsa Wood
Description: This makes for an “odds-and-ends” prototype that is a do-it-yourself project. It uses tools and material found in the home workshop or readily available from hardware stores, craft supply shops, building supply “supermarkets,” and auto supply stores. Balsa wood is perfect for products that have curves.
Products used for: Any.
Cost range: Free to $25 for most.
Resources: Local hobby shops and craft suppliers have useful publications that will guide you in a wide range of tabletop processes for cutting and working with balsa wood.
Making your prototype: However crude, making a prototype out of balsa wood and other found materials is often useful as a concept model. Because such a prototype is fast to make and inexpensive, you can remake it several times until it looks and feels right.
Using Hobby Shop Products
Description: You can make concept models yourself using the foam, plastic, wood, glue, and metal parts found at most hobby shops.
Products used for: Any.
Cost range: Free to $25 for most.
Resources: Local hobby shops and craft suppliers have both materials and how-to books.
Making your prototype: One of the easiest materials to work with in making large items is foam board. This sign-making and self-supporting board consists of front and back surfaces of white cardboard laminated to a core of Styrofoam. It is approximately a quarter-inch thick and can be cut using a hobby knife or a jigsaw. Foam can also be easily curved. Start by making closely spaced parallel lines using a ballpoint pen or a dull table knife on the inside of the part you want curved. Then curve the piece, and it will hold its new shape. You can join pieces of foam with a glue gun or one of the special glues (sold at craft supply stores) made for Styrofoam.
For thick sections, plain Styrofoam is sold at craft shops in a wide variety of shapes. If you intend to photograph the finished item, balsa wood, also available in thick pieces as well as thin, can be sanded and painted. Balsa wood’s main advantage over pine or poplar is that it is very easy to carve. However, it is generally too soft for functional prototypes, which are best made from maple if strength is needed. Maple has a fine grain, and it can be sanded and painted for a near-perfect surface.
Bondo, which is a plastic filler used in automobile body repair work, can be added to wood, plastic, foam board, and so on to produce compound curves and fillets, as well as to fill joints. A compound curve is a secondary or more complicated curve than you can make with just a piece of foam or wood. For example, if you have a curved computer monitor with a small half moon on the top, that half moon is a compound curve that you could produce with Bondo. A fillet is a small curve that joins two flat parts. Apply Bondo in layers no more that an eighth-inch thick to minimize cracking and shrinking. Bondo begins to harden in a matter of minutes, so be cautious about mixing too much or adding too much hardener, which affects its cure time. Bondo can be sawed, sanded, and painted for outstanding effects.
Sculpy is a ceramic imitation that can be “fired” in your kitchen oven. It can be molded into almost any shape, like modeling clay. It is available from craft stores.
“LOOKS LIKE, WORKS LIKE” PROTOTYPES
Appearance and concept models are not functional. In fact, they will often fall apart if handled too roughly. “Looks like, works like” prototype parts are not only functional but they look like the final product. The inventor uses them for market research with customers and for functional tests to see if the product will really work. They typically are not as strong as the final product, but they will last long enough for some product testing.
Using Stock Plastics
Description: This covers cutting and gluing together stock plastic shapes—such as sheets, tubes, and round and rectangular bars—using standard consumer hand and power tools.
Products used for: Many parts that will be plastic injection molded in production can be assembled from stock plastics shapes.
Cost range: $10 to $100.
Resources: Try national plastics suppliers such as AIN, Cadillac Plastics, and McMaster-Carr Supply Co. Smaller suppliers as well as plastic fabrication shops may be found locally.
Making your prototype: Stock plastics are readily available in a wide range of sizes: stock sheets of various thicknesses, bars and tubing of various diameters, and rectangular stock of various lengths and widths. These plastics can be divided into two main groups: commercial plastics and engineering plastics. The engineering plastics, such as nylon, Delrin, and polycarbonate, are so-called because they are not commonly used in high-volume consumer items due to their cost, and they have combined properties of strength, impact resistance, and resistance to chemicals that the less-expensive commercial plastics lack.
For prototyping, the main consideration, rather than cost, may be whether a plastic can be glued and successfully painted. Many common plastics are impossible to glue using off-the-shelf adhesives, and those that cannot be glued do not have acceptable paint adhesion either. The common plastics that are difficult to glue are nylon, Delrin (acetal), polyethylene, and polypropylene. Common plastics that are easily glued include acrylic, high-impact polystyrene, ABS, PVC (vinyl), and polycarbonate. Acrylic and polycarbonate are transparent; the others are opaque. Acrylic is brittle, but polycarbonate can be distorted or dropped without cracking. Acrylic is available in a scratch-resistant type. Polycarbonate, used for bulletproof windows, is more susceptible to scratching.
PVC and ABS are easy to machine using ordinary home workshop tools, including a table saw and router. If in doubt about which plastic to use for your prototype, start with PVC for opaque work and polycarbonate for transparent work. Glues for PVC and ABS are found at building suppliers and good hardware stores, or they may be ordered from plastics suppliers.
Using Stock Metals
Description: Applications include cutting a profile shape in sheet metal or bar stock, and optionally bending and/or joining to other parts by welding or screwing.
Products used for: Any part that imitates a punch press stamping or sheet metal fabrication.
Cost range: $100 to $500 for several pieces.
Resources: Job shops listed in the Yellow Pages under “Sheet Metal Fabricators,” “EDM” (electrical discharge machining), “Laser Cutting,” “Water Jet Cutting,” and “Chemical Milling.”
Making your prototype: Sheet metal fabricators are of two distinct types: those that make heating and air-conditioning ductwork and so on, and those that do precision work, which is nearly always what the inventor is seeking. Precision sheet metal fabricators cut shapes in sheet metal first by shearing from a large sheet, and then by stamping using a variety of steel in thicknesses from about .022 to about .100 inch. Less popular thicknesses are also available, and materials can be special ordered. Bending is done on a press brake. Drawing of metal, such as a bowl shape, is not a conventional process in most sheet metal shops, and must be prototyped by spinning. (Spinning vendors are found in the Thomas Register of American Manufacturers.)
Small parts that will be stamped when in volume production can be profiled using wire EDM, laser cutting, or water jet cutting. A wire EDM machine works something like a band saw, except the “blade” is a wire. The process is electrical, and is essentially the opposite of welding. The metal being cut is deposited on the wire. The machine is computer driven, and, except for threading the wire through internal holes or other closed cutouts within the periphery of the part, it is automatic.
Laser cutting is accomplished by using a computer-driven intense laser beam that cuts the profile and any internal cutouts automatically. Parts may be cut in stock up to about a quarter of an inch. For thicker stock, abrasive water jet is typically more economical.
Abrasive water jet cutting is done by a computer-driven machine that jets a needle-thin stream of water containing a very hard grit at pressures up to 60,000 psi. This process can cut metal up to about eight inches thick. Precision is lost as the thickness increases. Plastics, including foam, can be cut by water jets without the abrasive grit. EDM, laser, and water jet cutting require a computer program that is developed from your mechanical drawing. CAD drawings can be inexpensively amended for this purpose.
Chemical milling (or chemical machining) is a photographic process that is generally used for relatively thin parts. A drawing is made of your part and then reproduced several times on a photographic negative by the “step and repeat process.” A sheet of metal up to 24 inches long and up to about a 16th of an inch thick is coated with a photographic emulsion and exposed through the negative. It is then developed to expose the unwanted metal, which is etched away by a chemical spray. Blanks produced by any of the above processes can be bent to form parts that look convincingly as though they were produced by a punch press.
The cost of each of these processes varies according to the characteristics of your part, the kind of machinery a specific vendor uses, and how receptive the vendor is to working with a small job presented by an inventor. The only sure way to obtain the lowest cost is to submit a good mechanical drawing and request pricing from several vendors, perhaps two of each type of process for best results.
Many vendors of these processes also provide welding, usually TIG (tungsten inert gas) or MIG (metal inert gas) welding, either of which is much neater in appearance than “stick” welding. Parts that are around a 16th of an inch or thinner can be spot welded. Aluminum spot welding requires special machines that are not commonly found in sheet metal shops, but you can find them by calling shops listed under “Welding” in the Yellow Pages.
Plastics And Metals
Description: This involves removing material with cutting tools such as a drill press, lathe, and milling machine. Machining is purchased as a service from a prototyper or small machine shop.
Products used for: Any that are made from plastic or metal; usually for prototype parts that will be injection molded or die cast when in volume production.
Cost range: $50 to $1,000.
Resources: Prototypers and small machine shops found in the Yellow Pages, or in Inventor’s Digest classified ads, and by referrals from fellow inventors met through networking.
Making your prototype: A lathe is something like a drill press turned on its side, except that the work piece (the part being machined) is held in the chuck, rather than the cutting tool. Lathes create cylindrical shapes.
A milling machine—more precisely, a vertical milling machine—again resembles an upright drill press. The cutting tool, usually an end mill, resembles a drill, except it has a flat or round tip, and its edges are razor sharp. The work piece is held in a vise, or clamped directly to the mill table, and is moved against the revolving cutting tool. Movement of the mill table is in any of three axes: X, Y, and Z, which are left to right, front to back, and up and down respectively. Manually operated milling machines produce rectangular shapes, although CNC (computer numerically controlled) milling machines can produce nearly any profile that can be drawn using a computer.
Lathes and milling machines are mainly used to machine metals, but they are also used for machining plastics. The best machining plastic is Delrin, though PVC, ABS, and nylon machine well. Polycarbonate, while easy to machine, does not produce a beautiful finish like the other plastics. And acrylic is touchy. When drilling any plastic, there is always the danger of the drill grabbing the plastic—which spoils the work and can injure the machinist—but acrylic takes first prize in this respect. Brass and copper are even more dangerous to drill. Never, never drill acrylic, brass, or copper without clamping it securely to the drill press table. Drilling any of these with a hand-held power drill is also dangerous, even when the work piece (the part being machined) is held in a vise. The drill may be grabbed from your hand as the drill bit breaks through the far side. Experienced machinists can regrind drills to take the spiral edge off and thereby prevent the grabbing.
Prototypers usually have their own preferred suppliers, but small machine shops may work nearly entirely in metals and may not have convenient sources for plastics. You will usually be able to save money by hunting down the plastic stock yourself, in which case you are entitled to receive the unused portion back from the prototyper.
Casting Plastics At Room Temperature
Description: A two-component resin, similar to epoxy, is mixed and poured into a silicone rubber mold. The reusable mold is made from two liquid silicone components that are mixed and poured over a master pattern. The process can be learned for home shop production. A good source of information for this tactic is the Model Makers Handbook (Alfred Knopf) by Albert Jackson and David Day. Another source is Castolite Inc., a manufacturer of many prototype molding compounds; write to them at PO Box 391, Woodstock, IL 60098.
Products used for: This works for any parts that are made from plastic, but usually parts that will be injection molded when in volume production and are smaller than a grapefruit.
Cost range: $50 to $150 if you do it yourself.
Resources: Polytek Development Corporation’s catalog. Vendors that cast parts are those that provide SL and SLS services.
Making your prototype: This process starts with a model of the part you intend to cast. This model can be made from metal, plastic, wood, or even Ivory soap if the part is not delicate. Molds are made in two steps. The model is suspended by a string half an inch above the bottom of a small container, such as a plastic refrigerator container, and a two-part mix of silicone rubber (which becomes the mold) is poured halfway up the model. This is allowed to cure overnight, and it becomes the first half of the mold. Then, a mold release (Vaseline is excellent) is applied to the exposed top rubber surface to prevent the next pour from adhering. (Silicone won’t stick to anything except itself.) The model is then turned over and suspended over a container into a two-part mix of silicone rubber that just touches the first part of the mold. When the second pour is cured, the mold is removed from the container; the model is removed from the mold by splitting the mold where the two parts meet. Then channels are cut, or holes drilled, to provide an inlet for the plastic and an outlet for air.
A two-part plastic resin, usually polyurethane, is mixed and poured in the mold; this is cured for several hours. The cast part is removed, the inlet and outlet channel runners are trimmed off, and you have a part that closely resembles one that came from an expensive permanent injection mold. Polyurethane is available at hobby shops in a wide range of hardnesses, from soft as rubber to hard as acrylic.
Casting Metal Parts At Low Temperatures
Description: This covers the melting of alloys at less than 600˚F and casting them in silicone rubber molds. This can be done at home.
Products used for: Parts that will be die cast when volume produced.
Cost range: $50 to $100.
Resources: Polytek Development Corp., McMaster-Carr Supply Co., and building supply and plumbing supply stores.
Making your prototype: Cerro alloys are available in melting temperatures as low as 158°F. These alloys start at about $14 per pound. The lowest cost alloy is Cerroshield, which melts at just under the boiling point of water. McMaster-Carr’s catalog number for Cerroshield is 8921K23. Plumbers solder, which does not contain (poisonous) lead, melts between 500˚ and 600˚F and is less expensive than Cerroshield. Either of these alloys produces a part that is not as strong as the eventual die casting but is generally satisfactory for a prototype. Castings below 600˚F can be made in the same silicone rubber molds as is used for casting plastics (see above). Dust the mold with talcum powder to produce the best surface finish.
Casting Metal Parts At High Temperature
Description: This is a foundry process of creating metal parts from various molds. It requires outsourcing to a vendor and cannot be done at home.
Products used for: Parts that will be die cast when produced in volume, or parts that will be cast using the same process in volume production.
Cost range: From one to several thousand dollars.
Resources: Thomas Register of American Manufacturers or the Yellow Pages.
Making your prototype: Investment casting (also known as the “lost wax” process) uses a wax pattern of the part you wish to cast. This pattern is invested (coated) with a plaster, the wax is melted out, and metal is poured in. This ancient process has changed little in four or five thousand years. Creating the wax pattern can be done by the SL, SLS or MJM methods (see “Computer-driven Additive Processes” below for descriptions of these methods).
Plaster casting is a service offered by some of the die-casting vendors for customers who must prototype their parts. The model used to make the plaster mold is made by any of the same processes that are used for investment casting. Plaster molds, like investment molds, are destroyed in removing the parts.
Sand casters represent the “smokestack” era of manufacturing, but they are still predominantly used in the casting of large parts, especially cast iron. The master pattern consists of a model of the part that is split in half and mounted on a board. A fence is placed around the board; a sand, clay, and binder mix is poured over the pattern; the half-mold is inverted; and the pattern is removed. The opposing half is made the same way. The two halves are put together, molten metal is poured, and when cooled, it is removed as the casting. The pattern is often made from wood, but for smaller parts it can be made from plastic by the SLS process.
Computer-Driven Additive Processes
Description: A three-dimensional image is created using a computer that then drives a variety of equipment to produce prototype parts. The image is amended to create a machine program, which drive the X, Y, and Z axes of the machine.
Products used for: Any parts that will be made by injection molding when in volume production, or parts that become patterns for plastic or metal casting.
Cost range: $500 to several thousand dollars.
Resources: Thomas Register and Yellow Page listings under “Stereolithography,” “Selective Laser Sintering,” and “Rapid Prototyping.”
Making your prototype: Rather than remove material, as in machining processes, these processes add material or solidify it to produce a net shape without waste. The SL (stereolithography) process uses a liquid plastic that is hardened by an ultraviolet laser beam. A platform is positioned about two human hairs below the top of the liquid. The machine program, which has “sliced” the 3-D computer image into hundreds of thin digital slices, scans and hardens the plastic, thereby creating the bottom “slice” of the prototype. The platform drops down one slice (about .005 inches), and scans again, bonding the new slice to the first. This process is repeated slice by slice until the entire height is reached. The plastic used in SL is relatively fragile, and it is often honeycombed to reduce the amount of material used and the process time. The parts are seldom used directly in prototypes except to check fits and to visually evaluate the concept. The selective laser sintering (SLS) process is much the same as SL, except that a powdered plastic is used. The plastics used are stronger than that of SL and can be used directly in prototypes.
Multi-jet modeling (MJM) is a wax jetting process that builds a wax prototype layer by layer, as in SL and SLS. The wax model can be used as a pattern for silicone rubber molds, or as a sacrificial pattern for investment casting or plastic casting.
NEGOTIATING WITH PROTOTYPERS
The inventor should think carefully about exactly how many parts he or she will need, now and in the foreseeable future, before contacting any vendor. Much of the cost of making a prototype is in the time spent planning the method, setting up machines, and communicating with the inventor. This time is the same whether the prototyper makes a single piece or 10, and of course, the cost of this time is spread across whatever quantity is bought. Thus, the cost of 10 pieces may be only twice the cost of a single piece.
In any event, the inventor should always ask that the nonrecurring costs (programs, special tools, and so on) be spelled out as a separate item in the quote. And every RFQ (request for quotation) should ask for pricing in quantities of 1, 3, 10, and 25 (or whatever quantities you like) before the order is placed, while the vendor’s “pencil is still sharp.” Afterthought quotes solicited for repeat orders often come in high because the vendor holds the tooling and has the advantage.