Rapid Prototyping for Lighting Applications: Untangling the Material Selection for Plastic Parts
As new materials are incorporated into lighting and existing materials used in new ways, prototyping is more important than ever.
There was a time when anything to do with lighting had to be made out of glass or metal; glass because it was clear and metal because it could withstand heat. Early plastics were not sufficiently transparent or heat resistant to be of much use, but today all of that has changed.
Lighting technologies are more advanced than they were in the past, and polymer materials—in addition to their reasonable cost and ease of fabrication—offer a far wider range of characteristics that make them not just suitable for, but in many ways ideal for, lighting. As new materials are incorporated into lighting and existing materials used in new ways, prototyping is more important than ever. Metal, of course, is still widely used, and additive manufacturing (3D printing) methods like direct metal laser sintering (DMLS) and machining can be used to produce metal prototypes, but we’ll remain focused on prototyping with thermoplastics and thermosets.
In its various forms, plastic prototyping allows both the testing of well-conceived concepts and experimenting with new ones. Prototyping can answer a variety of questions about the suitability of the material and design and about the manufacturability of the part. But along with all of its benefits, a clear understanding of materials and processes is still needed.
Prototyping is used in many ways for lighting design. In the early phases of development, it can test form and fit. At this stage, materials need not be identical to production materials, so 3D printing processes like stereolithography are a suitable method of prototyping. In later stages of development, however, prototyping should ideally use materials and processes that are as identical as possible to those that will be used in production, making rapid injection molding the preferred method.
Benefits of Plastic
Plastic, in general, is half the weight of aluminum, corrosion and oxidation resistant, and less expensive to mold than metal is to die cast or machine; this allows fast, affordable production of large numbers of parts. In some cases, multiple metal parts that would require assembly can be combined into single plastic parts, further simplifying and reducing the cost of manufacturing. And when molded of suitable materials, plastic parts can incorporate features like living hinges and spring clips, simplifying both product design and operation.
Average light transmission of glass in lighting applications can range from 87 percent for standard clear glass to 98 percent for ultra-white glass. Light transmission for today’s clear injection-molded plastics is generally comparable, and includes frequently used materials like:
- Polycarbonate (PC) offers excellent impact resistance and has transmittance of up to 89 percent.
- Acrylic (PMMA) is less impact resistant than PC, but offers improved part quality in thicker applications along with light transmittance of up to 92 percent.
- Styron clear polystyrene resin offers high tensile strength and up to 90 percent light transmittance.
- Clear K-Resin is tough and flexible, and fills well in thin areas, which makes it an excellent choice for features such as living hinges. It offers light transmittance of up to 93 percent.
CNC machining is also a good resource for producing jigs, fixtures and non-lens parts such as housings or internal components for lighting applications. Clear parts can be achieved with the use of secondary finishing such as flame or vapor polishing as the surface of a clear material is typically hazy after machining.
Thermally Conductive Plastics
Today’s plastic resins can, in many cases, take the place of brass, copper or ceramic in lighting applications. While they cannot match the thermal conductivity of metals, specialized plastics today offer thermal conductivity up to 500 times that of conventional plastics, while offering all the advantages—lighter weight, part consolidation, corrosion resistance and ease of production—that plastics have over metal. These heat resistant materials can be used in LED luminaires, electronics, aerospace and automotive locations, housings and sensors.
Flexible liquid silicone rubber (LSR) has characteristics that can be particularly useful in lighting applications. Because it is a thermoset, it is not easily damaged by heat and can withstand temperatures up to 392° F (200° C). It is extremely stable, which makes it easy to clean and resistant to chemicals. It flows easily, allowing molding of extremely thin parts and of thick parts without risk of sink. It is light; flexible; and scratch, crack and UV resistant. It offers up to 400 percent flexibility and hardness ranging from 30 to 70 durometer. Its flexibility simplifies removal from the mold even if there are features that would be considered undercuts in a harder resin or if the part has negative draft. And it can allow up to 94 percent light transmission, making it suitable for primary or secondary lenses, light pipes, light guides and other components.
In addition to LSR’s excellent moldability, optical LSR has superior lighting-specific qualities.
- It is almost as transparent to visible and UV light as the best glass.
- It does not discolor with age.
- It is lightweight, and flexible with a durometer of about 70, reducing the likelihood of breakage due to blows or vibration.
- It is scratch and crack resistant and stable at temperatures up to 150° C.
- Its flexibility eliminates the need for separate seals and gaskets.
- And it can be easily colored before molding.
- It can be molded with a polished optical finish, and due to its low tendency to sink, it maintains its optical properties as it cools.
The additive manufacturing process of stereolithography (SL) can be a viable alternative for prototyping of optical components or manufacturing shapes that cannot be produced by molding. Because parts are built layer by layer in photopolymer resin, nearly any geometry can be reproduced. Parts can be hand-polished or sprayed with clearcoat to produce an optical finish. And while the process does not produce truly optical parts, it can use high-clarity materials and produce near distortion free parts.
Some available optical SL materials that mimic thermoplastics include certain grades of ABS that can be finished clear, making them ideal for lenses, transparent assemblies, flow testing components and high-humidity applications. Glass-filled polycarbonates are also offered, for example, in a clear, blue tint with high stiffness as well as transparent red with high resolution and temperature tolerance. Please note, however, that optical additive materials can degrade from UV exposure.
Physical prototypes can be particularly important in lighting applications because light distribution and heat dissipation are complex and difficult to model virtually. With literally hundreds of resin choices, both thermoplastic and thermosetting, material selection may also be a challenge. It may be necessary to produce multiple prototypes with different materials, although the same mold may be usable for multiple materials.
Component designs are typically developed using 3D CAD software, and those models, in turn, become input for prototyping processes such as stereolithography or rapid injection molding. These processes allow quick production of parts that can be assembled into complete product prototypes, testing of which can indicate the need for further development or readiness for production.
As an interim step, the rapid injection mold used to produce prototypes can be used to produce parts for market testing or as bridge tooling while hard tooling for high-volume production is being created. Overall, the use of appropriate prototyping methods throughout the development process can speed up the testing and adoption of new designs, the use of new materials, and the speedy introduction of lighting products to market.