What are Polylactic Acid's (PLA) properties and how do they affect your 3D printing experience? We aim to address this question in this article. We’ll go through every relevant mechanical and thermophysical property of PLA and let you know how it compares to other thermoplastics like Acrylonitrile Butadiene Styrene (ABS) and Polyethylene terephthalate glycol (PETG) and why it matters. Buckle up for an applied material science ride that you can’t find anywhere else!
- Printing temperature: The printing temperature of PLA must be above the point at which it starts to act like a liquid, because PLA can be amorphous, it doesn’t have a melting point like most other materials, but it does start to flow well at around 160 ⁰C. PETG begins flowing around 230 ⁰C and ABS is around 240 ⁰C. A higher printing temperature also means that the plastic decreases in temperature more to get to ambient temperature, so higher melting points result indirectly in more warping and curling (all else equal). What else matters for warping and curling? The coefficient of thermal expansion, which we’ll talk about next.
- Thermal Expansion: The coefficient of thermal expansion (CTE) describes how much a material grows or shrinks as its temperature change. A large CTE means that the material’s dimensions are more sensitive to a given change in temperature. PLA has a coefficient of thermal expansion of 68e-6/⁰C while ABS has a CTE of 90e-6/⁰C and PETG has a CTE of 68.4e-6/⁰C. You can see that ABS has a significantly higher CTE than PETG and PLA, and this is partly why ABS is so prone to warping and curling.
- Heat Deflection Temperature: The heat deflection temperature of a material is the temperature at which the plastic starts to soften and easily bend under a load. A high heat deflection temperature is what people commonly refer to as “heat resistance.” PLA has a heat deflection temperature of 55 ⁰C, while ABS is at 100 ⁰C, and PETG is at 71 ⁰C. You can see that ABS has the greatest temperature resistance, then PETG, and finally PLA has the least.
- Strength: Strength is a confusing subject for many, because it depends on the manufacturing process, it can refer to strength under several different types of loads, and is often confused with toughness (next subject). Typically, in plastics strength refers to the tensile yield strength of the bulk material (this ignores interlayer adhesive strength). To find out how to make the strongest 3D printed parts possible, check out this article. Tensile yield strength is the stress that the material can take before it is permanently deformed, so that when the load is removed, the plastic does not return to its original shape. The tensile strength of PLA, ABS, and PETG are respectively: 60, 40, and 50 MPa. You can see that PLA is actually the strongest, but we know that it doesn’t do well under repeated loading or impacts. This leads us the next subject: toughness!
- Toughness: Toughness is a lot like strength, but a little different. It is technically the integral of the stress and strain curve captured during strength testing, but understanding it doesn’t have to be that complicated. Rubber is tough, but weak. You can hit it with a hammer and it doesn’t break, but if you grab it and pull you can rip it in half easily. Ceramics (like kitchen plates) are not tough, but they are strong. If you hit them with a hammer they will shatter, but you can’t easily pull them apart. The tougher a material is, the greater its maximum elongation before breaking, and the stronger it is against impacts. PLA, ABS, and PETG have impact strengths of 16, 200, and 101 respectively. You can see that PLA is not very tough, ABS is very tough, and PETG is in the middle.
- Stiffness: Stiffness describes how much a material bends under a given load. Steel is stiff while rubber is not. This property is usually characterized by Young’s Modulus (also called Modulus of Elasticity). The values for the PLA, ABS, and PETG are as follows: 3.6, 2.0, and 2.1 GPa. You can see that PLA is by far the stiffest of the three materials. It is often the case that high strength and stiffness are paired with low toughness (just like ceramics), and the opposite is true as well (like rubber).
- Thermal conductivity: Thermal conductivity impacts the ease with which you can print the plastic. A higher thermal conductivity means that heat more quickly conducts from the surface of your filament into the center of the filament. A plastic with a higher thermal conductivity will be easier to extrude, and it will cool faster (if density and specific heat are equal) than materials with lower thermal conductivity. PLA, ABS, and PETG have the following thermal conductivities: 0.16, 0.1, and 0.2 W/(m*K).
- Latent heat of fusion: The latent heat of fusion describes how much energy is required to melt a substance. Think about ice water. What is the ice doing? It keeps the water at its melting point (32 ⁰F = 0 ⁰C) until all of the ice melts. As the ice melts, energy is stored in it. Likewise, it takes energy to melt PLA. In fact, it takes approximately 93.6 kJ per kg of PLA (depending somewhat on the crystallinity of your particular PLA) to melt PLA. This compares to 110 kJ/kg of ABS. In other words, the heater on your hotend has to work 15% harder just to melt the ABS, and it of course has to work even harder to keep the nozzle at the elevated printing temperatures for ABS. This is partly why PLA is easier to 3D print with than ABS.
You now understand most of the material properties that affect the quality of your 3D prints and the ease with which you can 3D print things. Hopefully this helps you understand the 3D printing process just that much better. If you have any questions, comments, suggests, or high-quality trolling, please leave that in the comments section. Thanks everyone!