Thermoplastics for AM: Semi-Crystalline vs Amorphous
A Brief Overview of the Thermoplastics Pyramid
Sometimes, the use of simple diagrams is the best way to get into a complex subject like semi-crystalline vs amorphous plastics. What better way to start than by introducing thermoplastics through the following pyramid:
The diagram above is a widespread tool used to sort out the numerous plastic polymers available today and improve critical decision-making processes in the industry.
There are dozens of versions of this pyramid on the web, each one with varying complexity and detail. Most of these pyramids, however, classify thermoplastics in the same ways:
- Vertically, by performance: Standard or Commodity, Engineering, High-performance, and sometimes Ultra-high performance
- Horizontally, by crystallinity: Semi-crystalline vs Amorphous, sometimes adding Elastomers
Materials placed at the top of the pyramid offer the best overall performances (mechanical properties, thermal stability, chemical resistance, etc.), but– as a rule of thumb– are also the most expensive and require the highest processing temperatures.
More on the performance classification:
Thermoplastics for AM: Breakdown by Performance
The crystalline dimension enables us to truly understand the nature of thermoplastics and how to make the best out of them in practical applications. But what’s the difference between an amorphous material and a crystalline material? The answer lies in molecular interactions.
The Science Behind Materials: Amorphous Vs Crystalline
Starting with the basics, we can say that the main difference between liquid and solid matter is that while the first can flow, the latter maintains a rigid structure. That solid structure can be organized in two, main ways: in a crystalline manner or in a random, amorphous manner.
Many solids are formed by crystal lattices, which are repeating, densely packed, geometrical patterns. Some examples include salts, gemstones, and most metals, which naturally form grains and prisms. They are crystalline materials.
Here is some spectacular footage of crystals forming:
However, not every solid material tends to form crystals. Solids can also form random structures which do not maintain any specific order, and this is what we call amorphous materials.
The most representative example is, without a doubt, glass, which is commonly and erroneously referred to as crystalline.
The image below illustrates simplified molecular structures of crystal and glass:
At a macro level, crystalline structures grant materials with distinctive properties. Crystalline materials tend to be stronger and stiffer than their amorphous counterparts. They also offer superior chemical resistance and tend to be opaque.
On the other hand, amorphous materials are more flexible and are ideal for impact resistance and optical applications.
Most polymers (i.e. materials with very large molecules) are amorphous, but some can partially form crystals and are therefore considered semi-crystalline.
With their spaghetti-like composition, they are unlikely to form lattice structures. Despite this, a number of thermoplastics are able to form semi-crystalline structures.
Indeed, some polymers have a tendency to partially fold into compact structures called lamellae. A semi-crystalline composition is an amalgamation of both crystalline and amorphous domains throughout the material.
Ultimately, by controlling how polymers crystalize, we can add crystalline attributes to our parts to make them stronger or more resistant to different elements.
Transition Temperatures of Thermoplastics
Unlike thermosets, thermoplastics won’t change their chemical composition as temperatures vary; they only change how their particles flow and rearrange. In simple words, a thermoplastic-made box could be melted into another shape, while a thermoset plastic-like silicone is permanently “set” into its final shape.
Given the reversibility of thermoplastics, what happens with polymer structures as they melt? What happens as they cool down?
To understand this, we must define three transition temperatures:
- Glass transition temperature (Tg): The point at which amorphous domains start to flow.
- Melting temperature (Tm): The point at which crystalline structures break.
- Crystallization temperature (Tc): The point at which crystalline structures rearrange.
As we cross over Tg, amorphous bonds break, and thermoplastics begin to flow. Now we can talk about viscosity– the resistance to flow. As temperatures increase, viscosity decreases and plastics progressively deform until they melt.
Practical example (see video below): A chewing gum, below its Tg, will be hard and easily snap in two. However, if you leave a pack of gum in the car on a hot, sunlit day (or even in your jean pockets), you will find it in a rubbery, sticky state at the end of the day.
The video below explains this in further detail, using ramen noodles and chewing gum as practical examples:
Since amorphous materials don’t have a defined melting point (Tm), Tg is all we need to understand an amorphous material’s behavior. In contrast, crystalline domains do have a transition temperature at which crystal bonds break, the melting temperature (Tm).
To translate this into an FDM printing scenario:
- amorphous thermoplastics can flow through the nozzle after it reaches Tg,
- while semi-crystalline materials require the nozzle to be over Tm.
Once the thermoplastic has completely melted, how do its molecules rearrange their structures as they cool down? Just between Tm and Tg, we get the crystallization temperature (Tc), the point at which crystals begin to form.
However, things aren’t that simple in practice. If the cooling happens too fast, crystals won’t have enough time to rearrange, resulting in a structure with a higher degree of amorphous domains.
Additionally, as crystals form, polymer chains become tightly packed, causing higher shrinkage rates and thus a higher tendency for warping and delamination issues.
Processing semi-crystalline thermoplastics implies many complexities and dedicated high-temperature 3D printers. Manufacturing thermoplastic parts becomes a more significant challenge now that we know about crystallization properties.
What implications does it have for FDM printing? How can we tune process parameters for optimal results? Undoubtedly, heating and cooling rates are essential.