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A key stage in the product design and development process is to verify your design. Design verification can be achieved through simulation or prototyping. Prototyping is usually the preferred method for design verification of smaller plastic parts because of its cost-effectiveness, accuracy, and convenience.
Not long ago, designers and engineers had only two prototyping options-machined/hand-made prototypes or prototype injection molded parts. Today, more options are available, the most popular of which is 3D rapid prototyping. However, each prototyping method has its advantages and limitations. This article will review the pros and cons of various plastic prototyping options to help you determine which method best suits your needs.
The following is a list of currently available plastic prototyping methods:
The best option should be selected according to your requirements. The recommended list of important considerations includes:
Not long ago, there were few options for creating plastic prototypes. About 25 years ago, all plastic prototypes were machined, hand-made, cast, or, to a very limited extent, injection molded using temporary tools. There is no other choice.
Making plastic prototypes is extremely expensive, time-consuming and very inaccurate. Only a very limited set of shops offer prototype injection molding, which is very expensive and has a long lead time. This option is rarely used. Therefore, most plastic prototypes are manufactured by hand or in some cases CNC machined.
We must also remember that since 3D CAD is still in its infancy, the design is mainly detailed in traditional 2D. In that era, 3D CAD was mainly wireframe or very primitive solid modeling. The engineering "class production" prototype is hand-made based on manual interpretation of 2D drawings. Due to the interpretation or error of the model manufacturer, as well as the limitations of the tool, this usually results in a significant deviation from the printed product. The designer patiently waited for weeks or even months for the prototype, which has always been a bottleneck in the design process. This is a heartbreaking experience, because people never know what the design will become. In addition, people always question the accuracy of the model, and never know whether the model manufacturer has fabricated a design that will disguise the error until the part is formed.
Today, 3D CAD modeling, computer simulation, and many prototyping options have exponentially accelerated the design process. Projects that used to take a year to develop can now be completed in a month or two, sometimes even in a few weeks! It is now possible to accurately reproduce all the subtle details of the 3D CAD model, including drafts, radii, embossed logos and many other key features, without any human interpretation or errors. Of course, this paradigm change is the result of 3D CAD, 3D printing and CNC machining. However, if any of these prototyping options are misused, the result may lead to wrong conclusions. Designers should choose the prototyping method that best suits their requirements. I will review these prototyping options based on the 12 considerations mentioned earlier.
Before deciding on the method of prototyping, you should ask yourself: What is the purpose of the prototype? There are many different reasons for the use of prototypes. For example, you may need a realistic-looking non-functional model to show to marketing or sales staff. The model must have the correct surface finish, color, graphics and materials to accurately represent the final production design so that the audience can evaluate the appearance. In other cases, your prototype will be subjected to harsh chemical or impact testing. You may need a prototype to evaluate human factors, such as the quick release of the force of a plastic cap or bottle cap. You may want to evaluate the ease of assembly and maintainability. In the end, you may just want to quickly evaluate a concept at the lowest cost. All of these situations will lead you to choose different prototyping options.
The appearance model does not necessarily require internal features or individual parts. If the part is small-less than 10 inches. Cube-it may be very suitable for FDM printing in ABS. However, if the part is as big as a refrigerator, it may be very suitable for hand-made using REN shapes or wood. If a refrigerator-sized product has a hinged door and a finished interior, it may be most suitable for a combination of methods including FDM, vacuum forming, and wood. As a designer, it is important for you to understand how the prototype is manufactured, because you may need to provide the model maker with CAD files for each part of the entire model. These parts will be very different from the parts actually produced.
The model should be distinguished from the prototype according to its sole purpose to convey the appearance and beauty of the design concept and the function of the product. Although the model may lack internal details, the complexity and skills required to develop a high-quality aesthetic model can be quite challenging. The planning and skill levels of designers and model makers are equally demanding. Designers need to design parts specifically for non-functional models and a set of production parts. For example, the non-functional product model shown below requires the following components:
3D printed parts are most suitable for finely constructed FDM or SLA. Resilient decoration can be printed in elastomer using SLA technology, because FDM will produce a surface that is too rough. Instead, medical devices can be manufactured using a combination of CNC-processed REN shapes, FDM, and hand-made sheets.
The lead time and turnaround time to manufacture one or more prototypes are usually key considerations in the development process. Sometimes the fastest way to make a prototype is to pick up a tool, cut a piece of plastic into a shape that represents the part, and test it. Although this is not the most accurate way to make a prototype, it is the cheapest, fastest, and most convenient method. Compared with printing, the manufacturing speed of medium to large parts is usually much faster, especially when they are designed as large planes. The second fastest method is internal 3D FDM printing or CNC machining, depending on your equipment. Today, most companies and design companies have in-house 3D printers that can print mid- to high-quality parts on demand. Independent prototyping facilities usually convert 3D CAD files into parts based on the following general turnaround time:
It should be noted that the turnaround time listed in the table is a realistic general time range. Depending on the workload, the complexity of the project, and the number of parts, some suppliers may be able to respond faster or slower.
Although injection molding is the most popular plastic molding process, there are many other processes that can be used to make plastic products. Most 3D rapid prototyping is optimized for injection molded parts, because mold investment and part complexity are the most extreme for this process. Other processes such as blow molding, vacuum molding, rotational molding and extrusion molding are also suitable for processing plastic parts. Each process has its own unique design criteria, material selection and part geometry type.
For example, blow-molded bottles are usually made of polyethylene (PE), polyethylene terephthalate (PET), or polypropylene (PP). It is currently not possible to use any rapid prototyping method to make a prototype of a PE blow-molded bottle on a 1 mm wall. However, blow molded PE bottles can be prototyped by creating SLS or FDM blow molding molds and molding some samples. There are multiple vendors that provide such services. The same is true for other plastic molding processes, where the mold is either 3D printed or CNC processed, and a small number of parts are molded with actual materials. Making prototypes of plastic parts based on actual materials and molding processes can provide you with the most accurate representation of the final production parts. The injection molded prototype parts in the actual material are almost the same as the production parts. They can be tested and evaluated based on the properties of the specified resin, the stress in the mold, and the tolerances of the parts. Large-scale structural foam molding parts are most suitable for CNC manufacturing or FDM, and the part density is 70% to 80%. Since structural foam molded parts are usually large, you may need to divide them into smaller parts, then print and assemble them into one larger part.
Plastic parts usually need to meet very strict tolerances. Tolerances are especially important for features such as snap fits, press fits, sliding parts or optical components. Controlling the tolerances and surface finish in the prototype depends to a large extent on the material, equipment, prototyping process, part size and build speed. FDM usually produces parts with tolerances ranging from /- 0.008 to /- 0.02 inches. SLA tolerances are much stricter than FDM parts, resulting in part tolerances ranging from /- 0.004 to /- 0.007 inches. SLS parts are just like those SLA. CNC parts can be machined to /- 0.0005 inch reamed holes and /- 0.002 inch non-cumulative holes. This is the most accurate prototyping option.
Due to the large number of process-related variables, the tolerances of cast polyurethane or epoxy resins are very inaccurate. Tolerance stacking starts to accumulate from pattern, silicone mold, resin shrinkage, and finally a secondary processing operation. The tolerances for cast polyurethane can range from +/- 0.01 to as much as +/- 0.06 inches, depending on all the variables previously cited.
One of the major challenges of any prototyping method is to accurately reproduce the fine design details. Retaining part features highly depends on the same parameters mentioned earlier, as listed below:
Below is a brief review of the resolution and accurate reproduction of design features for each prototype option.
The presentation of the details of the handmade prototype obviously depends on the craftsman. A skilled craftsman can surpass the resolution of most rapid prototyping machines; however, most hand-made prototypes are rough and very basic. The reason is obviously based on efficiency, cost and skill level. Therefore, this option is at the bottom of the list and is used to accurately replicate design features.
CNC machining is at the bottom of the list and is used to reproduce the fine design details of injection molded parts. There are two main reasons for this ranking. First, most of the features of injection molded parts cannot be processed from a single piece of plastic. CNC machines are limited by tool diameter and reachable depth. For example, because there is not a long enough tool with a diameter of 0.09 inches, it is impossible to machine a pair of two-inch deep ribs 0.09 inches apart from a piece of plastic. Therefore, the CNC prototype is divided into many smaller parts and glued together to form the desired final part, or the injection molded part must be redesigned for the CNC machining process.
Casting polyurethane is often specified as a cost-effective method for replicating 5 to 100 parts. The reproduction of the part features is as good as the original pattern used to create the silicone rubber mold. It should be reminded that the quality of cast polyurethane parts is highly dependent on the suppliers, employees and QC procedures they adopt. The reproduction of fine details largely depends on the casting technology, mold quality, and all secondary operations inherent in the process. Although castings can reproduce details as finely as fingerprints, the high amount of labor inherent in this process often results in very inconsistent quality.
The temporary injection mold will provide you with a prototype almost identical to the final production part. It should be noted that the reproduction of design details depends on mold quality and processing conditions. This option is also the most expensive and time-consuming turnaround time.
Based on process, part geometry, tools and materials, vacuum formed parts and composite parts are significantly different from injection molded parts. Prototypes for either of these two processes are usually large parts—greater than 2 x 2 feet—essentially a surface with mounting features glued to the back after molding. The parts formed in these processes usually do not have fine details.
The second part of this two-part series has now been published and covers the remaining six notes.
If you have any questions, please feel free to call 516-482-2181 or contact me via email, [email protected].
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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