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The difference between amorphous and semi-crystalline plastics

clock 2 minutes | 23 Aug 2019

The difference between amorphous and semi-crystalline plastics

Amorphous and semi-crystalline plastics are both high-temperature polymers. 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 molding process. Let’s take a look.

Amorphous thermoplastics

These include mostly translucent plastics:
  • Polymethyl methacrylate (PMMA / Acrylic)
  • Polystyrene (PS)
  • Polycarbonate (PC)
  • Polysulfone (PSU)
  • Polyvinyl chloride (PVC)
  • Aacrylonitrile butadiene styrene (ABS)
  • Polyetherimide (PEI)

These 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.

The pros

This makes them easy to thermoform. Because thee 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.

The cons

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.

Non Threaded PCB Spacer
  • Polystyrene (PS)

Non Threaded PCB Spacer

LED Light Pipes
  • Polycarbonate (PC)

LED Light Pipes

Snap Rivets
  • Polysulfone (PSU)

Snap Rivets

Connector Adapter Dust Cap
  • Polyvinyl chloride (PVC)

Connector Adapter Dust Cap

Semi-crystalline thermoplastics

Common semi-crystalline materials are usually opaque and include:

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

Unlike amorphous thermoplastics, semi-crystallines have a highly ordered molecular structure with 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.

Pros

Semi-crystalline polymers form tough plastics because of 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.

Cons

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

Push-In Full Face Flange Protectors
Uniscrew Caps
  • Polypropylene (PP)

Uniscrew Caps

Transistor Insulator
  • Polybutylene terephthalate (PBT)

Transistor Insulator

Socket Head Cap Screw
  • Polyetheretherketone (PEEK)

Socket Head Cap Screw

The molding process

As you’d expect, you’ll need to treat the molding process differently for amorphous and semi-crystalline plastics. 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-crystallisation polymers, which can be difficult to melt uniformly. To aid the injection molding process for either, follow these tips:

Amorphous materials

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

For optimum molding conditions:

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

Semi-crystallised 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 molding conditions:

  • Inject using moderate pressure
  • Pack with consistent pressure
  • Use high-mold 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-crystallisation Maintain constant Continues until end of crystallisation

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