Materials and Processes Used in Prototyping and Hardware Development: Part Two
By Anthony Giordano
Here at Tomorrow Lab, we utilize many materials and processes to design our client’s products. Some of these are off-the-shelf components, while others are specially developed by our manufacturing partners. Our method of learn-make-test-break-repeat is fundamental to our development process. The more we learn about how the materials we work with are handled, the more we understand how to make the best possible products.
The goal of this series is to explain a set of common materials and processes we use in our business as we prototype and fabricate products for our clients. Anyone interested in hardware product development should enjoy the encyclopedia approach of these articles. Since there are several topics to cover, this guide will be split into three parts; Part One: Electronics, Part Two: Housing — Low-volume Methods, and Part Three: Housing — High-Volume Methods, and distributed over the next few weeks. This is Part Two.
Part Two: Housing — Low-volume Methods
1. 3D Printing
Otherwise known as Industrial Robots - 3D printing is a subset of additive manufacturing, which differs from casting and other material processes. As such, the processes of adding material layer by layer is a common theme with 3D printers. However they differ in their choice of material and “melting” technology. At Tomorrow Lab we use 3D printers including the well-known MakerBot and the Zortrax (our favorite) to create rapid prototypes, or parts for prototypes, for our clients. Within a matter of hours, a proof of concept can be tested, and if necessary, conveniently scrapped and readjusted. Our lab typically uses FDM technology, but the three common 3D printing technologies are SLA, SLS and FDM printing.
1A. SLS (Selective Laser Sintering) and SLA (Stereolithography Apparatus)
Both the MakerBot and the Zortrax printers have a very similar setup, and utilize a laser to harden a curable material. The Z axis is controlled by the platform that the object rests on. As each successive layer is instantly hardened by the laser, the platform is lowered very slightly and the roller assembly pushes another even layer of material to be hardened on top. The SLS printers sinter powdered nylon 11 and 12 powered material, and the SLA printer contains a vat of UV curable liquid photopolymer. Because the materials used in each process differ, they each have unique chemical properties. The nylon material used in the SLS printers, have heat and chemical resistant characteristics, while the SLA Printers typically have a better surface finish, but generally cannot compete with the SLS heat and moisture deflection.
1B. FDM (Fused Deposit Modeling)
FDM printing has become one of the most accessible methods for 3D printing, and the MakerBot utilizes this technology. This machine involves a nozzle that extrudes a thermoplastic wound around a spool. Some describe it as a “robotically-controlled fancy glue gun”. The bottom layer is laid, then either the platform is lowered or the nozzle is raised, and another layer is added when the adjacent layer has cooled. FDM is great for modeling prototypes, and for form or fit testing. Sometimes, the 3D print is directly constructed in the material that will be used for final production. The failure rate can be a little high with this type of manufacturing, so you have to watch your print carefully. If you run out of material mid-print, you can pause the print and connect a new spool, but be careful to use the same kind of plastic.
Reference: Sd3d.com
2. Casting
There are many castable materials, including silicone, but plastic and metal remain the most commonly used in manufacturing because they are either already in liquid form, or can be liquified and then hardened. When it comes to metal and plastic, there are different advantages and disadvantages when choosing between the two. Plastics obviously do not have the same strength-to-size ratio as metals, but they are usually cheaper to manufacture and transport. Metals have greater resistance to mechanical stress and high temperatures. Metal designs are usually perceived to be of higher quality. And of course, weigh more, so can add cost to your shipping methods, especially from overseas.
2B. Metal Casting:
There are many ways to cast metal, but the three most common methods are sand, die, and investment casting. Sand is used when the complexity of the design is relatively low, and when surface smoothness is not a high priority or can later be machined and buffed. A casting mold is made using sand, a bonding agent, and water. The resulting putty is shaped over hard materials like wood or urethane, followed by a cavity when the sand has hardened. Once the materials used to shape it are removed, the cast is ready for molten metal.
Die casting, on the other hand, is used to make complex smooth shapes, which usually require little machining afterwards. However, because the dies are very expensive to make, designers and factories reserve this option for high volume production. Die casts are made using a two-part steel mold that is pressed together to create a cavity. Molten metal is than fed into the mold cavity under high pressure. Depending on the casting metal, the rate and speed of the feed may vary. Aluminum, magnesium, and copper take longer because they cannot be used in the faster, hot die chamber. Since cold chamber materials have a relatively high melting point, the cold chamber has a more rudimentary design, to avoid the metal solidifying as it is fed through the system.
Finally, the most complex option for highly detailed objects: investment casting. There are many steps involved in the process starting with a gelatin mold created around a solid object. The solid object is removed, and wax, such as microcrystalline, is painted on the walls to the desired thickness of the final product. After the gelatin is removed, a ceramic clay is painted on and baked. The clay exterior is later heated to keep it from breaking from shock when it is introduced to molten metal. Once the mold and metal cool, it is placed in a vibrating machine that slowly breaks off the clay shell, leaving a smooth, intricately shaped metal. This final product usually only requires a little machining relative to other methods.
References: Premier Die Casting Company
3. Plastic Casting
There are a wide variety of plastics, and each have their material properties. The plastic casting process begins with resins. Natural resins, are viscous liquids excreted from plants. Over the years, synthetic forms have been created, and are the main building-block of plastics. Resins are made up of masses of polymers, which are made up of monomers — carbon-based molecules capable of forming chain-like structures. The polymers exhibit characteristics different than the individual subunit monomers. The monomers are typically found in crude oil. When petrochemical plants receive a refined oil from a refinery that contains mostly monomers, a hardening agent or catalyst can transform these into polymers.
Some common types of polymers include: Acrylic, polycarbonate, or polyurethane — which may be more recognizable by their commercial names Acrylan, Lexan, and foam rubber, respectively. The polymer, either homogeneously constructed by a single type of monomer, or from a variety of monomers, can be arranged in two categories: thermosets and thermoplastics. Because thermoplastic resins consist of long molecules without side chains, they may be repeatedly melted and solidified. Because thermoset plastics have a cross-linked molecular structure, they cannot be remolded into a new object. Thermoplastics are easily recyclable because they don’t degrade when they are melted, which is why they make up 80 percent of all plastics produced.
4. Thermoforming
As previously mentioned, thermoplastic resins can be repeatedly melted, a property used in thermoforming. Thermoforming — in its most basic form — is the heating of a thermoplastic to a malleable state, and forming it into the required shape before cooling. Casted or machined aluminum typically creates the mold cavity. The various processes available in thermoforming differ in how the plastic is applied to the the mold. The most basic method is mechanically forming it, which is done by making a plug that is more or less a positive of the mold and press it into the mold cavity. Other methods include air pressure typically utilizing a vacuum element to suck the plastic into the mold. For this to work, little holes must be drilled into the mold cavity. In addition to the vacuuming, some machines also have air pressure on the back side of the sheet to force the plastic into the grooves. This creates sharper more detailed edges and increases the range of plastic thickness that can be thermoformed.
References: Thermoforming, Bargainorganite, Nobel Prize Chemistry, Earth Resources, Casting
5. Wire EDM
A Wire EDM (Electrical discharge machine) is one of the most robust cutting tools available. Not only can Wire EDM cut through some of nature’s hardest materials (such as titanium, tungsten and diamond), the cuts are so smooth they typically do not require any surface finish or polishing. Since the Wire EDM process involves electricity, more conductive materials tend to work better.
Wire EDM or Electrical Discharge Machining, is very similar to a CNC Milling in that it can cut very intricate shapes due to its +/-.0001 high accuracy potential. However, the electrically charged brass wire that the EDM machine uses never actually touches the surface that it is cutting. This brass (sometimes copper) wire is a taut thin wire, usually .10mm — .30m, which can be imagined as a miniaturized conductive floss stick .The cuts come from the inherent high temperatures that arise from the high voltage 1000 A arks between the brass wire and object cut. A CNC servo system controls the distance between the cathode brass wire and anode cutting material. This process happens while it is inundated in a dielectric fluid, which keeps the large sparks at bay and helps remove the debris as well. To create highly accurate designs, manufacturers use kerosine as their dielectric fluid choice.
6. In-house Circuit Board Options
In 2014 as part of in internal project, we repurposed a MakerBot Thing-o-Matic into a circuit engraver to attempt to make quick PCBs in house. It worked (mostly) for a fun experiment, but it left a lot to be desired in terms of PCB quality. In an effort to upgrade, we purchased an Other Mill in 2015 to make same-day PCBs for our more rapid 4–8 week Phase Zero project scopes. It works pretty well, and cuts boards quickly, though because we generally use off the shelf components while prototyping in the early stages, we haven’t had a chance to utilize its full potential.
Stay tuned for Part Three: Housing High-volume Methods which will cover CNC Mill/Routing, Extrusion, Rotomolding, and Injection Molding!
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