What is HDPE
HDPE (High-Density Polyethylene) is a thermoplastic polymer with strong intermolecular forces and stiffness. HDPE is made from petroleum, and furthermore, from the polymerization of ethylene.
HDPE gains its high strength to density ratio property thanks to its lack of branches in the molecular structure. This allows for the molecular groups to close-pack which provides the strength and in certain instances, “crystallization,” depending on the processing.
The HDPE Polymer has great features that you can take advantage of for your 3D print. It is light, flexible, easy to dye and mold, has good insulating property, and also has a non-absorbent property that can be used for everyday materials.
Lastly, HDPE is water-insoluble, but can be dissolved with limonene. This is a feature that will be discussed later.
HDPE may sound foreign, but it’s more commonly found in your everyday life. They are in pipes, folding chairs, and in recyclable goods and common packaging such as bottle caps, etc. In fact, they are 30-35% of total world production of plastic materials.
Recyclability
As mentioned briefly, HDPE is a recyclable material known as a “number 2 recyclable plastic.” You will generally find the recycling symbol with a number “2” in the middle on your milk jugs, cleaning agents, laundry detergents, and even shampoo bottles.
In general, HDPE is used and reused to make items that need to outlast exposure of sunlight, extreme heat or freezing. Some examples include plastic lumber, and items at the park such as plastic waste bins, outdoor tables, park benches, and even bed liners for trucks.
What does this mean for 3D printing? This means that HDPE creates a new use for human-produced plastic waste. The finished or failed projects can be reused as new filament and overall be reused for other products in recycling.
The great news is, you can find the products that lay around in and around your house that are ready to be recycled as your own filament! Just consider it a DIY approach to 3D printing.
HDPE as Filament
HDPE is not as common as filaments such as ABS (acrylonitrile butadiene styrene) and PLA. For example, ABS is commonly found in our childhood favorite, Legos. Even so, HDPE is comparable to ABS in certain properties.
- Lighter and stronger than ABS prints
- Can be used as printing supports
Thanks to its light and compact structure, HDPE is stronger and lighter than ABS. In addition, it can also be used for other 3D prints by using its solubility feature of HDPE in Limonene.
HDPE can be paired with polymers such as ABS or PLA (polylactic acid) that will not dissolve in limonene then following the print melted away in the solvent.
FDM – Compatible
Most importantly, finding a compatible printer for an HDPE filament is crucial. In general, you will need an HDPE printer such as a general FDM printer that can print HDPE and also ABS.
When reviewing specs, you will need to find a nozzle (extrusion point) that can print at high nozzle temperatures – 230 and 360 degrees Celsius. In addition to a heated nozzle, you will need a heated print bed.
To keep up with its shrinking and its lack of self-adhesion properties, a heated print bed helps to compensate for these properties or lack thereof.
What is FDM?
Fused Deposition Modeling was originated by Scott Crump from Stratasys and now a common desktop 3D Printing technology for plastic parts. It is a popular choice by hobbyists and readily available.
An FDM printer takes a spool of filament and extrudes the polymer through a nozzle. The heat of the printer nozzle causes the filament to melt and the print is built layer by layer.
The Advantages of HDPE filament is
- Affordable
- Readily available
- Comes in various forms
- Easy to use
The Disadvantages of HDPE filament is
- Best for prototypes only
- Limited resolution
- Poor adhesion strength
Why HDPE
So why and when would you want to use HDPE for your project? Consider the features and benefits when deciding to use HDPE. Listed below are 4 features and the benefit obtained.
- HDPE dissolves in Limonene — your object needs a support that can easily melt away
- HDPE is lightweight — your object needs to be light, like a floating toy.
- HDPE is non water absorbent — your object needs to be water tight.
- HDPE is chemical resistant — your object will be exposed to harsh elements.
HDPE is FDA-approved which means it’s food safe, at least in its original manufacturing. Some use HDPE to print food containers or lids. Some recommend printing coasters and koozies with recycled HDPE, so the polymer isn’t directly touching the food.
The polymer is also widely used in industry for its chemical resistant property. You can readily find charts online by looking up the chemical you want to check compatibility with and searching for it with HDPE.
Why Not HDPE
Unfortunately, many people criticize printing with HDPE for its tricky printing complexities. Fan forums are full of stories of failed prints. So why is it so tricky?
- High temperature nozzle and bed requirement
- Tendency to warp
- Doesn’t stick well
HDPE polymer requires a rather high temperature nozzle and bed requirement for printing which isn’t always available in generic FDM printers.
When printing, it also needs to stay within the 230-260°C range. Don’t let it go too high because then the polymer is converted and the resulting fumes can be hazardous.
Additionally, due to its natural properties, HDPE can warp when printed and cooled. You may need to try a sample print to see how much your products shrink and if there are angles of the print that print unevenly. This has been a struggle for many hobbyists.
Lastly, although HDPE is known for stickiness, it also needs help for self-adhesion. HDPE prints generally require a base layer or raft to prevent warping and improve sticking, although some have complained about the opposite effect during the print. You can see by the opposite experiences; it is tricky by nature.
HDPE at Home
Lastly, for a fun weekend project, try extruding your own home-made HDPE filament from your milk jugs! Since HDPE is easily found in household goods, this will be a project worth trying out.
There are various ways to make your own filament and the best way to think about it is from your grade school projects where you had to use hot glue guns. There are some examples of videos that require basic extruding methods to spool some of your own HDPE filaments.
For example, in this video, you simply will need
- One empty carbon dioxide cartridge,
- Soldering iron
- Insulating material
- Piston for the cartridge
- Clamps
You can use the soldering iron as your extruding tip for your chips of milk jugs and plastic. Other methods of similar concepts (but with upgrades) can be found in this second video.
This idea of using recycled HDPE isn’t as foreign as one may think. Joshua Pearce, from Michigan Technological University, also had this idea of using recycled milk jugs and natural polymers for 3D printing filament.
Pearce provided this unit, RecycleBot, available as an open-source for everyone to have access on Thingiverse. RecycleBot also uses a similar method of melting shredded milk jugs into plastic HDPE filament.
Other designs are linked within the summary of RecycleBot on Thingiverse where you can try different designs and approaches for a spooler. Of course, if you wanted to buy your own spooler, there are options of buying easy spoolers such as Felfil’s filament extruder and other styles similar to it such as Makerbot’s.
Printing with HDPE
Printing with HDPE is possible! Although it isn’t commonly found, you can extrude filaments from everyday household items and help the polymer’s journey in the recycling process. Although it is a tricky print, it is also rewarding and offers great features. It’s worth a shot to find HDPE filament and try it as a build or at least as a support material for your next project.
