A comparison: Amorphous vs crystalline polymers

What’s the difference between crystalline and amorphous polymers? They’re both high-temperature materials. The difference between the two lies in their molecular structure. Before you decide which to use, you need to understand the characteristics of each, as that will determine your injection moulding process. In this guide, we’ll cover:

What is an amorphous polymer?
What is crystalline polymer?
Examples of amorphous polymers
Examples of semi-crystalline polymers
Amorphous thermoplastics
Advantages of amorphous thermoplastics
Disadvantages of amorphous thermoplastics
Semi-crystalline thermoplastics
Advantages of semi-crystalline thermoplastics
Disadvantages of semi-crystalline thermoplastics
Injection moulding process for amorphous and semi-crystalline materials
Amorphous material
Semi-crystalline materials
The solidifying process
Semi crystalline vs amorphous polymers

What is an amorphous polymer?

First, let’s define amorphous, which means lacking a clear shape or form. When describing amorphous, the comparison used the most is that of cooked spaghetti. There’s no set order to spaghetti on a plate, which is exactly how an amorphous polymer behaves. 

Amorphous polymers have a randomly ordered molecular structure that lack a sharp melting point. The result is that amorphous materials soften gradually as the temperature increases. Amorphous polymers are thermoplastics, as they can be melted and immediately recast. 

Their lack of ordered structure allows them to bend and flex more easily than crystalline polymers, and they also tend to be more transparent. Due to their lack of order, amorphous polymers typically have lower mechanical strength and stiffness compared to crystalline polymers.

What is a crystalline polymer?

Now let’s define crystalline. The atoms, molecules and ions that make up the semi-crystalline polymer structure are arranged in an ordered way. But amorphous vs crystalline isn’t simply a matter of random vs. ordered molecules. Crystalline polymers have large amorphous areas, which is why we call them semi-crystalline polymers. 

Semi-crystallines also have sharp melt points. While amorphous materials soften gradually when the temperature rises, semi-crystalline plastics do not. Instead, they remain solid until a certain quantity of heat is absorbed. The materials then quickly change into a low viscosity liquid. This melting point is generally above that of the upper range of amorphous thermoplastics. Like amorphous solids, semi-crystalline materials are also thermoplastics. 

Examples of amorphous polymers

As the molecular chains of amorphous polymers are random, light is able to pass through them. These are therefore mostly translucent plastics. Amorphous polymer examples include:

• Polymethyl methacrylate (PMMA / Acrylic)
• Polystyrene (PS) 
• Polycarbonate (PC)
• Polysulfone (PSU)
• Polyvinyl chloride (PVC)
• Acrylonitrile butadiene styrene (ABS)
• Polyetherimide (PEI)
   

Examples of how some amorphous polymers are used include:

Examples of semi-crystalline polymers

Semi-crystallines with a fast crystallisation rate typically means the sizes of the crystals are larger than the wavelength of visible light. This in turn results in a large amount of light scattering. That is, the crystals in a semi crystalline structure block the light, giving this material an opaque appearance.

Common semi-crystalline plastics include:

• Polyethylene (PE)
• Polypropylene (PP)
• Polybutylene terephthalate (PBT)
• Polyethylene terephthalate (PET)
• Polyetheretherketone (PEEK)

Examples of how some semi-crystalline polymers are used include:

Amorphous thermoplastics

As with anything, there are advantages and disadvantages to using amorphous polymers. 

Advantages of amorphous thermoplastics

They’re easy to thermoform, for starters. Because these materials are isotropic in flow, they possess better dimensional stability than semi-crystalline plastics and are less likely to warp. Amorphous thermoplastics also offer superior impact strength and are best used for structural applications.

The materials bond well using adhesives. They also tend to offer excellent resistance to hot water and steam, good chemical resistance, and good stiffness and strength. PSU and PEI are especially good examples of amorphous thermoplastics offering these qualities.

Disadvantages of amorphous thermoplastics

The presence of hydrocarbons means they’re more sensitive to stress cracking. They also don’t perform well as bearings or wear components and have poor fatigue resistance. Amorphous thermoplastics tend to have lower chemical resistance and higher friction than semi-crystalline materials.

Semi-crystalline thermoplastics

A semi-crystalline thermoplastic also has its pros and cons. 

Advantages of semi-crystalline thermoplastics

Semi-crystalline polymers form tough plastics due to their strong intermolecular forces. They perform extremely well in applications involving wear, bearings, and structural loads. They also provide excellent chemical resistance, where amorphous materials do not.

You can expect very good stiffness and strength, good toughness, and a very low coefficient of friction.

Disadvantages of semi-crystalline thermoplastics

Semi-crystalline polymers’ sharp melting point makes them difficult to thermoform. These materials are anisotropic in flow, so they shrink more in the direction transverse to flow than they do along the direction of flow. This results in dimensional instability, compared to amorphous polymers. Also, the impact resistance of semi-crystalline materials is average at best compared to that of amorphous plastics.

They’re hydrophobic, chemically inert and possess low-surface energy, making them challenging to work with, despite these characteristics rating high for performance.

Injection moulding process for amorphous and semi-crystalline materials

As you’d expect, you’ll need to treat the moulding process differently for semi-crystalline and amorphous resins. Whichever you opt for, melt uniformity is key. Melt uniformity doesn’t merely mean melt temperature, but also melt consistency so that there are no swirls nor streaks, nor semi-melted pellets. You cannot have consistent dimensions and performance without melt uniformity.

Amorphous materials melt easier than semi-crystalline polymers, which can be difficult to melt uniformly. To aid the injection moulding process for either, follow these tips:

Amorphous materials

When moulding amorphous materials, overpacking should be a concern. Expect parts to stick in the mould. Also, parts can crack during ejection.

For optimum moulding conditions:

• Inject the material using high pressure
• Gradually decrease your pack pressure
• Reduce internal stress with high mould temperatures

Semi-crystalline materials

Underpacking should be a concern, which can cause sinks and voids and low part weight. Another major concern is incomplete crystallisation. This can result in warpage and shrinkage.

For optimum moulding conditions:

• Inject using moderate pressure
• Pack with consistent pressure
• Use high-mould temperatures to help crystallisation

The solidifying process 

Finally, here’s what you should know about the solidifying process:
 

	Hold Pressure	Flow through gate
Amorphous	Decrease over time	Stopped
Semi-crystalline	Maintain constant	Continues until end of crystallisation

Semi-crystalline vs amorphous polymers

At a glance, here’s how these polymers differ, including additional attributes.  

Semi-crystalline	Amphorous
Organised molecular structure	No ordered molecular pattern
Polymers are made out of atactic polymer chains.	Polymers are made out of syndiotactic and isotactic polymer chains
Weak attraction between polymer chains	Strong attraction between polymer chains
Low-density polymers	High-density polymers
High chemical resistance	Low chemical resistance
Translucent	Transparent

 

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