Fig 1: Light Cocoon, a car made from 3D printed parts. "Edag light cocoon" flickr photo by www.wbayer.com - www.facebook.com/wbayercom https://flickr.com/photos/wbayercom/16592649939 shared under a Creative Commons (BY) license
This is the Light Cocoon, a car that only weighs about 19 grams per square meter1. A 2010 Cadillac DTS, by comparison, weighs 151,000 grams per square meter2. The Light Cocoon was 3D printed by the German design studio EDAG using a metal fusion process called Selective Laser Melting (SLM). This and other 3D printing technologies are creating a library of products that are pushing the world ever closer to a manufacturing revolution, that some say is already here. GE is 3D printing Helicopter engines3. Laboratory scientists are printing human ears and kidneys from human cell materials. Soon we might be eating 3D printed pizzas and sandwiches wearing 3D printed t-shirts and bikinis. Additive manufacturing is creating workshops in everyone’s homes, turning houses to factories, bringing all shapes and all designs to everyone’s fingertips.
3D printing plastics
A typical 3D printer draws a line on the print surface by melting plastic through a heated nozzle. Stacked lines create a 2D surface. 2D Surfaces stacked on top of each other create a 3D shape with structure and volume. Advanced 3D printing technologies deposit powdered metal and then melt it using high power lasers.
While the number of available materials and techniques are ever increasing, currently two types of plastics dominate this technology: Polylactic Acid (PLA) and Acrylonitrile Butadiene Styrene (ABS)4. PLA is the "default" recommended material for many desktop 3D printers being useful in a broad range of printing applications, has low warp and is odorless. ABS is used for making durable parts that need to withstand higher temperatures.
In comparison to PLA filament, ABS plastic is less "brittle" and more "ductile." It can also be post-processed with acetone to provide a glossy finish. Besides these two common materials, there are Glow-in-the-dark plastics, NylonX with chopped carbon fibers, Polyethylene Terephthalate (PET) etc.
Heating the print bed - the importance of the glass transition temperature
In addition to the extruder that is maintained at a high temperature to melt the plastic, the print bed is also often heated for many materials.
When a hot layer is deposited on a colder bottom layer, thermal strains are generated in the structure that can result in warping of the surfaces. This is especially dangerous for the very first layer if molten plastic is made to come in contact with a cold print bed. Not only is the base of the structure now warped, but the poor adhesion can cause the layers to shift resulting in an even more deformed structure. The best way to avoid this to make sure that the temperature gradients across layers and between the base and the bed are controlled and sudden temperature changes are prevented.
For both PLA and ABS the extruder temperature is around 200 - 230°C. However, the print bed temperature for PLA is 40°C while it is 90°C for ABS4. The difference is due to the higher glass transition temperature of the two materials - PLA has a glass transition of 50°C while it is 100°C for ABS. Glass transition temperature is the temperature, below which the physical properties of plastics change to those of a glassy or crystalline state5. Ideally, the difference between the temperature of the print bed and the glass transition of the polymer must be less than 40°C, ensuring that the material does not change state during printing.
Fig 2: 3D printed structure with and without warping6
Typically this has been achieved by heating the print bed and maintaining it at a high temperature. Initially, PCB type heat beds were used that supplied heat to an aluminum top. These have now largely given way to silicone rubber heat pads that are attached to an aluminum plate which is in turn covered by a glass plate.
Aluminum is inexpensive, conducts heat rapidly and is suited for the introduction of an induction sensor to regulate the temperature. The surface on top of the heat bed is the critical layer for printing: it has to provide strong adhesion while printing ensuring that the material does not shift, and it has to be easy to remove after printing.
Several materials besides glass such as Kapton tapes, PET tapes and even artists’ masking tape with hairspray have been used before to provide a firm adhesion surface. However, glass is today the preferred material due to its transparency and ease of use.
Introducing NEXTREMA® glass-ceramics
SCHOTT AG’s NEXTREMA® series of glass-ceramics are considered a suitable material available in the market today for 3D printer heat bed applications.
To be an efficient print surface, the glass must have a non-porous surface, be a good conductor of heat (be transparent in the infrared zone), have a high service temperature and be resistant to thermal shocks. NEXTREMA® is not just a glass, it is a glass-ceramic. The ceramic infusions suppress thermal expansion and provide high heat conductivity which together ensure that these materials can withstand huge changes in temperature. This is shown clearly in the thermal expansion dependence of the NEXTREMA® line of glass-ceramics: the overall thermal expansion is below 1% for up to 700°C.
NEXTREMA® can withstand temperatures of up to 950°C which are perfectly suited to the requirement of common PLA and ABS type 3D printers. Besides the high service temperature and the thermal shock resistance, they are transparent in the IR region making them excellent conductors of heat. Further, they remain gas impermeable and chemically inert even at high temperatures ensuring that their surface properties do not change with use cycles.
The video below demonstrates the high thermal shock resistance of three NEXTREMA® line of glass-ceramics: transparent, opaque and translucent. A sheet of each of these NEXTREMA® glasses are heated in an oven to approximately 800°C. They are then taken from the oven and submerged into cold water. Traditional soda-lime glass heated up to merely 300°C shatters when exposed to such an extreme variation in heat. The NEXTREMA® glasses however come out unscathed with no trace of their violent past visible on their surfaces.
In addition to heat beds for 3D printers they are used in transparent windows for outdoor grills, protection for IR heaters, machine components, electronic, outdoor patios and more. More information about the specifics of these materials can be found here and here.
Video courtesy of SCHOTT © SCHOTT AG