Introduction
A material viable for 3D Printing must be a thermoplastic material: It means that it becomes soft and moldable when heated and returns to a solid state when cooled. This process can be repeated many times and has to work well with:
- extrusion into Plastic Filament (for the production of the raw material), *second extrusion during the 3D Printing process,
- final application usage.
The two dominant plastics for 3d printing today are ABS and PLA.
General Material Properties
ABS - ABS as a polymer can take many forms and can be engineered to have many properties. In general, it is a strong plastic with mild flexibility (compared to PLA). Natural ABS before colorants have been added is a soft milky beige. The flexibility of ABS makes creating interlocking pieces or pin connected pieces easier to work with (LEGO bricks are made of ABS). It is easily sanded and machined. Notably, ABS is soluble in Acetone allowing one to weld parts together with a drop or two, or smooth and create high gloss by brushing or dipping full pieces in Acetone. Compared to PLA, ABS is much easier to recycle.
Its strength, flexibility, machinability, and higher temperature resistance make it often a preferred plastic by engineers and those with mechanical uses in mind.
PLA - Created from processing any number of plant products including corn, potatoes or sugar-beets, PLA is considered a more 'earth friendly' plastic compared to petroleum based ABS. Used primarily in food packaging and containers, PLA can be composted at commercial compost facilities. It won't bio-degrade in your backyard or home compost pile however. It is naturally transparent and can be colored to various degrees of translucency and opacity. Also strong, and more rigid than ABS, it is occasionally more difficult to work with in complicated interlocking assemblies and pin-joints. Printed objects will generally have a glossier look and feel than ABS. With a little more work, PLA can also be sanded and machined. The lower melting temperature of PLA makes it unsuitable for many applications as even parts spending the day in a hot car can droop and deform.
Storage
Both ABS and PLA are sealed off from the atmosphere to prevent the absorption of moisture from the air. Long term exposure to a humid environment can have detrimental effects, both to the printing process and to the quality of finished parts.
ABS - Moisture laden ABS will tend to bubble and spurt from the tip of the nozzle when printing; reducing the visual quality of the part, part accuracy, strength and introducing the risk of a stripping or clogging in the nozzle. ABS can be easily dried using a source of hot (preferably dry) air such as a food dehydrator.
PLA - PLA responds a little bit differently to moisture: in addition to bubbles or spurting at the nozzle, you may see discoloration and a reduction in 3D printed part properties as PLA can react with water at high temperatures and start de-polymerization. While PLA can also be dried simply, it is important to note that this can alter the crystallinity ratio in the PLA and will possibly lead to changes in extrusion characteristic (temperature..). For many 3D Printers, this is is not a big concern.
Printing process
ABS - Printing ABS gives often a notable smell of hot plastic. Ensuring proper ventilation in small rooms, that the ABS used is pure and free of contaminants and heated to the proper temperature in a reliable extruder can help a lot in reducing the smell.
PLA - PLA on the other hand, being derived from sugar gives off a smell similar to a semi-sweet cooking oil. While it certainly won't bring back fond memories of home-cooked meals, it is considered by many an improvement over hot plastic especially when you print at home.
Details printing accuracy
Both ABS and PLA are capable of creating dimensionally accurate parts. However, there are some differences of behaviour that must be taken in account when choose the material to print with.
ABS - The greatest hurdle for accurate parts in ABS can be a curling upwards of the surface in direct contact with the 3D Printer's print bed. Elements that can help for eliminating this issue are:
- heating the print surface (heated bed)
- ensuring it is smooth, flat and clean.
Additionally, some find various solutions can be useful when applied beforehand to the print surface. For example, a mixture of ABS/Acetone, or a shot of hairspray.
For fine features on parts involving sharp corners, such as gears, there will often be a slight rounding of the corner. A fan to provide a small amount of active cooling around the nozzle can improve corners but too much cooling can reduce adhesion between layers, eventually leading to a more fragile finished part.
PLA - Compared to ABS, PLA demonstrates much less part warping. For this reason it is possible to successfully print without a heated bed and use more commonly available print surface. Totally removing the heated bed can still allow the plastic to curl up slightly on large parts. Reduction of the printing speed can help in these cases.
PLA when heated, becomes much more liquid. The increased flow leads to stronger binding between layers, improving the strength of the printed part. If actively cooled with a fan, much sharper details can be seen on printed corners without the risk of cracking or warp.
In Summary
ABS - Its strength, flexibility, machinability, and higher temperature resistance make it often a preferred plastic for engineers, and professional applications. The hot plastic smell deter some as does the plastics petroleum based origin. The additional requirement of a heated print bed means there are some printers simply incapable of printing ABS with any reliability.
PLA - The wide range of available colors and translucencies and glossy feel often attract those who print for display or small household uses. Many appreciate the plant based origins and prefer the semi-sweet smell over ABS. When properly cooled, PLA seems to have higher maximum printing speeds, lower layer heights, and sharper printed corners. Combining this with low warping on parts make it a popular plastic for home printers, hobbyists, and schools.