Thursday, September 15, 2016

Plastics

This post is a basic overview of plastics, some common types, and their properties. There's a particular emphasis on some of the plastics used in digital fabrication processes (3D printing and vacuum forming).

Etymology

The word plastic is derived from a Greek word (plastikos) meaning "capable of being shaped or molded". Plasticity is the general property of all materials that are able to irreversibly deform without breaking. This occurs to such a degree with plastics that their name is an emphasis on this ability.

Types

There are two basic types of plastics:

  • Themoplastic
  • Thermoset

The primary difference between these two is that thermoplastics can be remelted back into a liquid and then reshaped and reused. Thermoset plastics always remain in a permanent solid state. 

Thermoplastics Plastics

Thermoplastics pellets soften when heated and become more fluid as additional heat is applied. The curing process is completely reversible as no chemical bonding takes place. This characteristic allows thermoplastics to be remolded and recycled without adversely affecting the material’s physical properties.

Most materials commonly offer high strength, shrink-resistance and easy bendability. Depending on the resin, thermoplastics can serve low-stress applications such as plastic bags or high-stress mechanical parts.

Thermoplastics Pros

  • Highly recyclable
  • Aesthetically-superior finishes
  • High-impact resistance
  • Remolding/reshaping capabilities
  • Chemical resistant
  • More environmentally friendly manufacturing

Thermoplastics Cons

  • Generally more expensive than thermoset plastics
  • May melt if heated


Thermoset Plastics

Thermoset plastics contain polymers that cross-link together during the curing process to form an irreversible chemical bond. The cross-linking process eliminates the risk of the product remelting when heat is applied, making thermoset plastics ideal for high-heat applications such as electronics and appliances.

Thermoset plastics significantly improve the material’s mechanical properties, providing enhances chemical resistance, heat resistance and structural integrity. Thermoset plastics are often used for sealed products due to their resistance to deformation.

Thermoset Pros

  • More resistant to high temperatures than thermoplastics
  • Can be used for both thick to thin wall products
  • Higher levels of dimensional stability
  • Cost-effective


Thermoset Cons

  • Cannot be recycled
  • More difficult to surface finish
  • Cannot be remolded or reshaped


Common Plastic Types and Uses

A brief list of some commonly used plastics: 

Polyurethanes (PU) – Used in cushioning foams, thermal insulation foams, surface coatings.
Polycarbonate (PC) – Used in compact discs, eyeglasses, riot shields, security windows, traffic lights, lenses.
Polylactic acid (PLA) – A biodegradable, thermoplastic derived from lactic acid commonly used in 3D printing.
Acrylonitrile butadiene styrene (ABS) – Used in 3D printing, electronic equipment cases (computer monitors, printers, keyboards).
Polycarbonate/Acrylonitrile Butadiene Styrene (PC/ABS) – This is a blend of PC and ABS that creates a stronger plastic. Used in car interior and exterior parts, and mobile phone bodies.
Polyester (PES) – Used in fibers, textiles.
Polyethylene terephthalate (PET) – Used in carbonated drinks bottles, peanut butter jars, plastic film, microwavable packaging.
Polyethylene terephthlate Glycol-Modified (PETG) – Used in vacuum formed products.
Polyethylene (PE) – Used in a wide range of inexpensive uses including supermarket bags, plastic bottles.
High-density polyethylene (HDPE) – Used in detergent bottles, milk jugs, and molded plastic cases.
Low-density polyethylene (LDPE) – Used in outdoor furniture, siding, floor tiles, shower curtains, clamshell packaging.
Polyvinyl chloride (PVC) – Used in plumbing pipes, window frames, flooring.
Polypropylene (PP) – Used in bottle caps, drinking straws, yogurt containers, appliances, car fenders (bumpers), plastic pressure pipe systems.
Polystyrene (PS) – Used in packaging foam ("peanuts"), food containers, plastic tableware, disposable cups, plates, cutlery, CD cases.
High impact polystyrene (HIPS) - Used in refrigerator liners, food packaging, vending cups.
Polyamides (PA) (Nylons) – Used in fibers, toothbrush bristles, fishing line.


3D Printing Plastics

A comparison of the properties of some common 3D printing plastics: PLA versus ABS.

PLA - The wide range of available colors as well as translucent material is often desirable. PLA can also have a glossy feel to it. The plant based origins are appealing as is the semi-sweet smell as compared to ABS. When properly cooled, PLA generally has a 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.

ABS - Its strength, flexibility, machinability, and higher temperature resistance make it often a preferred plastic for engineers, and professional applications. ABS has a petroleum base which is less applealing to some. The hot plastic smell deters some from its use. The requirement of a heated print bed means there are some printers simply incapable of printing ABS with any reliability.


Recycling

The greatly expanding use of plastics in the early 20th century resulted in environmental concerns due to its slow decomposition rate after being discarded. Thermoplastics can be remelted and reused. Thermoset plastics can only be ground up and used as filler material (although the purity degrades with each reuse).



  1. PET (PETE), polyethylene terephthalate
  2. HDPE, high-density polyethylene
  3. PVC, polyvinyl chloride
  4. LDPE, low-density polyethylene,
  5. PP, polypropylene
  6. PS, polystyrene
  7. Other types

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