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Will it blend? Recycling PLA plastic using liquid nitrogen and a blender

  • Mar 23 / 2014
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3D printing, Experiments

Will it blend? Recycling PLA plastic using liquid nitrogen and a blender

Recycling plastic for use in plastic extruders such as the Filastruder isn’t a new idea, and attempts have been made by a number of groups over the past few years to make recycling as accessible as possible. Presented here is a proposed method for grinding down old printed plastic into a size suitable for use in open-source filament extruders such as the Lyman Filament Extruder or the Filastruder. This following process was developed by UBC Engineering Physics student Scott Lawson.

Grinded PLA with red granules for size comparison

Grinded PLA with red granules for size comparison. Pieces too large can be filtered out and blended again

By cooling a polymer below its brittle transition temperature prior to blending, we solve a number of problems encountered by previous recycling efforts such as the RecycleBot. Once cooled, a common household blender can effectively grind plastic into sizes compatible with filament extruders.

Importance of recycling

Most plastic extruders on the market can convert small plastic granules into plastic filament, but it would be nice if there was an easy way to make the granules. Ideally, we could grind down old prints or perhaps other sources of plastic like water bottles to make 3D printing filament. At ~$30-40 per kilogram, commercially available plastic filament is expensive compared to the raw price of bulk plastic.

Proposed Method

Using liquid nitrogen, we cool PLA plastic below its brittle transition temperature to reduce the energy absorbed before failure. Unlike room temperature polymers, cold polymers can be easily chopped and blended into sizes compatible with filament extruders (see the Theory section below for details). The target temperature of approximately -20C (-4F) to -40C (-40F) is nothing compared to the blisteringly cold temperatures of liquid nitrogen (-196C / -320 F). A more commonly available substitute for liquid nitrogen is dry ice, which has a temperature of -78C (-109F).

Liquid nitrogen is used to cool PLA plastic below its brittle transition temperature prior to blending

Liquid nitrogen is used to cool PLA plastic below its brittle transition temperature prior to blending

The blender used is a Magic Bullet donated by Carly, one of UBC Rapid’s team members. As far as blenders go, the 250W Magic Bullet is not especially powerful and is weaker than many common household blenders. This really shows how much easier it is to grind cold plastic. The first step is to cool the plastic by submerging it in liquid nitrogen. Once cold, the liquid nitrogen is strained out and the plastic is placed in the blender.

The plastic being recycled was obtained from old rafts removed from previous prints

The plastic being recycled was obtained from old rafts removed from previous prints

Grinding the plastic takes about five minutes until a fine consistency is achieved. It’s a good idea to periodically filter out the smaller pieces and re-blend the larger pieces until everything is the right size for filament extruders.

Blending takes about 3-5 minutes

Blending takes about 3-5 minutes and can be accomplished by most household blenders

The final product is shown against commercially available granules for size comparison. Note that while many of the green pieces are even smaller than the red granules, there are some larger pieces that would need to be filtered out and blended again prior to use in a filament extruder. For this trial, everything was grinded in a single run without stopping to filter out the smaller pieces.IMG_20140323_162647

IMG_20140323_162724

Many of the pieces are smaller than the red granules. Larger pieces can be filtered out and blended again

 

Safety

While grinding is easy to achieve with household equipment and liquid nitrogen / dry ice, this procedure is dangerous for a number of reasons and should be carried out with extreme caution and only with the proper training and safety equipment. At cryogenic temperatures, both metals and polymers are below their brittle transition point. The blades of the blender become very brittle and could shatter, cause serious injury or death.

Blenders are not designed to be operated with dry ice or liquid nitrogen, and you should never pour liquid nitrogen or put dry ice directly into the blender. This will almost certainly break your blender and can cause glass to shatter from the sudden temperature change. Place cold plastic into the blender but not liquid nitrogen or dry ice. We do not endorse attempting any kind of plastic grinding using a blender.

Never place liquid nitrogen in a sealed container. It will explode. Liquid nitrogen is constantly boiling off and if placed in a sealed container, pressure will build until it explodes. Always follow proper cryogenic safety procedures.

But where can I get liquid nitrogen or dry ice?

Depending on where you live, it may not be hard at all to obtain liquid nitrogen or dry ice. In fact, even some Walmart locations sell dry ice. Liquid nitrogen has to be stored in a vacuum insulated Dewar flask, or a vacuum insulated thermos. Almost all universities and cities will have suppliers for liquid nitrogen and dry ice.

Since dry ice and liquid nitrogen can be a hassle to obtain, it may be wise to save up plastic and grind a larger amount once in a while.

 Theory

Grinding down polymers into small granules is more difficult than it sounds. The difficulty  arises because polymers can absorb large amounts of energy before failure, due to plastic deformation. A typical polymer stress-strain curve is shown below.

A typical stress strain curve for a polymer
A typical stress strain curve for a polymer

Past the yield point, the long horizontal region on the stress-strain curve represents plastic deformation. During plastic deformation, the polymer will absorb large quantities of energy and it will become stronger as the polymer chains align. Grinding plastic with a common household blender can be difficult because the blades will become very hot and the plastic will absorb large amounts of energy.

However, by lowering the temperature of polymers below their brittle transition temperature, we can dramatically lower the amount of energy absorbed before failure. The graph shown below demonstrates the difference between brittle and ductile failure of metals, and a similar result is observed with polymers.

A brittle metal will absorb less energy than a ductile metal before failure
A brittle metal will absorb less energy than a ductile metal before failure because there is no plastic deformation

For ABS, the brittle transition occurs between -20C (-4F) and -40C (-40F). Below this temperature, grinding the plastic into granules becomes much easier. Each polymer has its own brittle transition temperature, although typical polymers will be brittle at -40C (-40F).