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In the previous article on this topic, I explained the importance of prototyping to validate your design during product design and development. I reviewed the many plastic prototyping options available to us, including the three most popular rapid prototyping techniques. Part 1 discussed the first half of the 12 considerations that should be evaluated before choosing the best process. The second part will review six other considerations for the same prototyping technology to help you determine which method best suits your needs.

The following is a list of currently available plastic prototyping methods:

Here are the remaining six considerations for choosing the best prototyping method:

I will review each of these considerations in this article.

All previously listed procedures can be specified for prototype parts to verify overall appearance, interference check, ergonomics, and overall concept. However, part design verification based on specific material properties is much more difficult to complete. Prototype injection molding is the only prototyping option that can produce parts that are nearly identical to the production parts. The reason I describe it as almost the same is because tool design, quality, structure and materials, and processing conditions may introduce some differences between prototype injection molded parts and production parts.

The two minor prototyping options are CNC machining and FDM. CNC machined parts cut from a single flat plate of a specific plastic material will provide you with parts that are closely related to the production parts. It should be noted that thick-section slabs are either extruded or injection molded. It is rarely possible to achieve an exact match with the production material. On the contrary, a general match with a specific material is very likely. Examples include materials such as GP polystyrene, polyethylene, polycarbonate, acetal, and ABS. It is important to realize that machined slabs are limited to operations that can be performed on CNC machines and the availability of stock material sizes. In addition, processed parts should be annealed to reduce processing stress. People should also know that the molecular structure of extrusion or injection molded slabs is different from that of injection molded parts. This is especially true for glass reinforced materials.

Unlike CNC machining, FDM prototypes can be made of almost any thermoplastic material. The micro extruder allows you to extrude most thermoplastics into filaments for FDM printers. Except for tolerances, the printed part is geometrically identical to your 3D CAD file. The disadvantage of FDM is the anisotropic behavior of the printed parts. The material properties in the XY plane may be significantly different from the material properties in the Z axis. The degree of difference depends on the material, and most importantly, on the printer. Some printers claim that the Z-axis material strength is 90% of the XY plane. However, most printers are far from reaching this level of correlation. One of the most significant characteristic differences is the bending strength of the material. A simple solution is to print features such as snap locks as separate parts so that the build texture runs in the bending direction. A solvent or adhesive can be used later to bond the snap button to the main part.

The hand-made prototype consists of several smaller drills glued together and is limited to commercial stock materials such as CNC machined parts. Due to process reasons, hand-made prototypes can be more like injection molded parts; however, the finished prototypes are much more refined than CNC machined parts, because the main parts are usually glued by solvents or adhesives to several smaller parts Made together. Therefore, hand-made prototypes cannot be reliably tested for drop or impact.

Cast polyurethane, SLS and SLA are all limited to a small part of resin: cast polyurethane is limited to polyurethane, SLS is limited to nylon, and SLA is limited to epoxy or acrylic resin.

If the product is subjected to harsh chemicals, creep tests or impact tests, it is recommended that you test actual material samples under expected usage conditions. Testing prototypes may be too expensive or impractical. 

Sometimes you need to create multiple prototypes. When you perform destructive testing or distribute multiple prototypes to users for feedback or distribute to multiple sites for evaluation and testing, multiple prototypes are required. Choosing the ideal prototyping option is not as simple as you think. You must consider cost, investment, delivery time, quality, and what you are evaluating. The following table compares various prototyping options based on these considerations. It has been prepared as a guide. There is overlap between options, and your experience may vary, depending on the vendor you’re working with.

Rating: 1 = low, slow; 5 = high, fast

Product designers and engineers must maintain a balance between creativity, innovation and confidence, that is, the designs they propose will perform as expected after the project is completed. Prototyping and testing throughout the development process enables developers to effectively verify their ideas before the product design cycle is completed. This approach reduces the risk of investing hundreds or thousands of man-hours in a design that may eventually require a complete redesign. The designer must always analyze the trade-off between the cost and time of prototyping and the validity of the test results.

Some real-world testing and evaluation examples are listed below. Hope these examples can help you understand the process that is best for a particular situation.

The aesthetic model can be very simple or very complex. In all cases, the product function is not important unless the function is directly related to the appearance in some way. For example, functional doors, buttons, and handles can affect the appearance. This requires a certain level of detail to clearly communicate the design intent. Internal spaces may be exposed, requiring them to meet overall appearance standards. Therefore, all prototyping options are applicable except for prototype injection molding. Large and complex electronic devices with hundreds of internal components can be clearly represented by solid or hollow housings, which contain the fine details of all external components visible to the viewer. The cost of these prototypes is invested in the artistry and craftsmanship of the model maker-mixing paint, masking, painting, sanding, polishing, and meticulous resolution of details are heavy and costly.

When you are trying to optimize the consideration of human factors, prototypes are essential. Parameters such as weight, balance, finger clearance, comfort, and handle design all require extensive research and evaluation before the design is finalized. An ergonomic prototype does not need to be 100% functional. Their level of functionality depends on their impact on ergonomic considerations. For example, if the prototype is evaluated based on its acceptable comfort when held in hand, the handle profile must be precisely defined. If the prototype's trigger, button, or latch tactile feedback is being evaluated, the force distribution of these characteristics should be accurately represented. Other features, such as the position of the handle, the gap between the legs, the visibility of the display, etc., must be accurately represented in the prototype. In most cases, simplified non-functional models can be used for these studies. The model may not need finishing or painting. For most ergonomics research, a simple single-color original model may be sufficient; however, if color, surface texture, or material hardness is critical, a finished prototype may be required.

According to the type of ergonomic research, hand-made prototypes have the characteristics of high efficiency, low cost and speed. They can be made from a variety of materials, from clay, plaster or wood to plastic and metal. If a more accurate model is required, all other processes are applicable except for prototype injection molding. The determinants are reduced to cost, availability, and final requirements.

Part size is an important factor when deciding on the best prototyping options. The size of the parts can range from microscopic to piano-like. A small number of microscopic parts is very suitable for rapid prototyping. Several companies offer specialized 3D printers that are specifically designed to print parts as small as a few microns. Of course, the choice of materials for this process is limited. Although most injection molded plastic parts meet the standard 12 inches. Cube volume, larger parts can be printed as a single piece on a larger machine. The current volume of FDM machines exceeds 1000 x 1000 x 1000 mm. These very large machines are available on the market, but most 3D printing manufacturers do not have them. Instead, they print large parts in smaller parts and then glue them together to make a larger part. Since all processes can produce large parts larger than 2 x 2 feet, the geometry, complexity and tolerances of the parts must be considered when determining the process. Prototype injection molding is certainly not a cost-effective option. However, cast polyurethane, vacuum forming and composite materials are very suitable for medium to large parts. In fact, if the materials and processing parameters are similar, vacuum forming and composite prototypes can be the same as production parts.

When choosing a prototype option, cost is always an important consideration. Some processes require a large upfront investment, while other processes require no investment at all. Some processes are labor-intensive and incur high unit costs, while others are more efficient and cost-effective. The following table compares processes based on investment and unit cost.

The unit cost of all the prototyping processes listed above (except injection molding) may vary greatly due to external finishes. Compared with functional prototypes that require a lot of manual work to complete with level 1 paint, graphics, and other details, functional prototypes that require almost no work are much cheaper. The cost of FDM parts may double or triple, depending on the quality of finish required. Generally, FDM parts require more manual and finishing time than SLA parts, which have higher resolution. CNC machined parts are the most cost-effective for functional prototypes that must be executed under stress or other environmental conditions.

Prototypes usually require clear parts. With the exception of SLS, all prototyping processes discussed in this article can produce transparent parts. SLS is mainly limited to nylon and glass fiber or carbon fiber reinforced composite materials. However, if we increase the requirements for optical clarity, the options will become narrower.

FDM can use PET and acrylic materials to produce transparent parts, but the parts will not be optically transparent. Light will pass through, but due to layering, visibility will become blurred. Hand-made and CNC machined parts made of acrylic can be polished to optical clarity. Polyurethane can be cast into colored or transparent materials to form optically transparent parts. Generally, castings require some minimal polishing to obtain a clear finish. It should also be noted that if the processing is improper, the casting sometimes entrains air bubbles. Vacuum formed parts can be formed from transparent polycarbonate or acrylic sheets. Of course, if the mold is highly polished, injection molding can use almost any thermoplastic to produce optically transparent parts. The best rapid prototyping method for producing transparent parts is SLA. The prototyping of lenses or optical quality parts is much more complicated than the prototyping of transparent windows. Transparent windows do not need to be as perfect as optical quality windows, which must transmit light almost without distortion. Optical lenses not only require distortion-free geometric shapes, but the refractive index of the material must match the refractive index of the production part.

Prototypes similar to production usually require custom color matching. FDM, casting, CNC machining, vacuum forming and injection molding can produce parts with in-mold colors. However, it is impractical to prototype any part with custom molded colors. Ordering customized colored resins for FDM filaments, injection-molded pellets, or vacuum-formed sheets is very impractical, costly, and time-consuming. SLA and SLS parts are usually limited to the use of natural material colors-SLA is white, translucent beige or water transparent, SLS is white. Custom color-matched polyurethane will never be exactly the same as injection molded thermoplastic. Therefore, almost all prototypes are painted with precisely matched pigments to meet strict color standards. When applying paint to a prototype, you must carefully define the close-fitting features that must mask the paint. Paint thickness can sometimes accumulate more than five thousandths of an inch, causing interference. 

Prototypes are very helpful for testing and verifying your design before it is completed or released for production tools. Some of the more common tests and evaluations performed on the prototype are listed below:

The first three depend entirely on materials and manufacturing methods. As mentioned earlier, ideally, the same material should be used to determine material-based testing and evaluation. However, in most cases, this is neither practical nor cost-effective, and you must find alternative ways to perform these tests. A viable alternative is to isolate the attributes you are interested in testing and replace the material with suitable alternatives. An example might be to test the feeling of snapping. For example, if your production part is designated as polycarbonate, and you need an SLA prototype for fine detail, you can replace polycarbonate with an epoxy resin with the same modulus as polycarbonate.

Evaluating the ease of assembly, especially for compact electronic products with a large number of connectors, small screws and snap fits, is a key step in the design process. The ideal process for this type of assessment is the SLA, which can accurately and cost-effectively replicate very fine details. Larger assembly studies can be evaluated using FDM or even hand-made models, depending on the development stage. 

Evaluating product safety is critical for most products, which have become more and more complex with each generation. All the procedures discussed in this article can be specified for testing product safety. If safe use is directly related to material properties, the options are limited, as mentioned earlier. However, if you are interested in evaluating the ideal location of a panic button or protective cover, sometimes a handmade foam model is sufficient. The ideal process is highly dependent on the level of detail required to represent the feature being evaluated.

Hand-held products, including power tools, sporting goods, medical equipment, etc., are essential to maintain balance and comfort during use. The feel of a product can have a significant impact on its acceptance or rejection in the market. Therefore, as a designer or engineer, you must verify these parameters early in the design process. These parameters are a function of the mass distribution within the component and the material specified for the external structure. Non-slip overmolded elastomers usually provide a certain level of additional tactile comfort, which must be felt by the individual in order to be properly assessed. A simple handmade wood or cardboard model may be sufficient to assess balance in the early stages of design. However, it may be more appropriate to use a more refined, printed FDM model in the middle of the project, and then use a very refined, completed SLA model later in the project. The main factors that determine the best process are cost, personnel, skills, and equipment.

Hope these two articles provide you with enough information to help you determine which prototype options best meet your requirements. The boundaries between each of them may be a bit blurred, but there are pros and cons. If you have any comments or questions, please feel free to contact me at [email protected] and we can talk. Good luck for your next project.

Michael Paloian is the President of Integrated Design Systems Inc. (IDS) in Oyster Bay, New York. He holds a bachelor's degree in plastics engineering from the University of Massachusetts Lowell and a master's degree in industrial design from the Rhode Island School of Design. Paloian has in-depth knowledge of part design in a variety of processes and materials, including plastics, metals, and composites. Paloian holds more than 40 patents and was the chairperson of SPE RMD and PD3. He often speaks at SPE, SPI, ARM, MD&M and IDSA conferences. He has also written hundreds of design-related articles for many publications. He can be contacted by phone 516/482-2181 or by email, [email protected].

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