Hot Melt Adhesive Principles (1 of 2)

I’m about to have an exam on adhesives as part of my undergraduate studies and so I thought it would help my own revision if I took some time to talked about the underlying theory of adhesives. Specifically, hot melt adhesives and how they can be related to a reprap and its printer bed.

When most people think of adhesives they think crazy glue, super glue, wood glue, epoxy resin and so on. In reality any substance that is applied to a surface, known as the substrate, and then adheres to it could be considered an adhesive. Some other common examples include paints, sticky tape, sticky labels, blutack, and yes, hot extruded plastic onto a print bed. The plastic acts like a glue that sticks to the bed. In order to design the best print bed surface a good understanding of the underlying concepts involved is a must.

The physical strength of the bond made between extruded plastic and the print bed is determined by two deceptively simple factors:

• The type and strength of the bond formed between an adhesive, in this case the extruded plastic, and the substrate, which in this case is the printer bed, at the molecular level.
• The total bonded surface area at the molecular level.

This is assuming that your only printing indoors at room temperature, in a dry environment, that the print bed is clean from containments (oil, dust etc.)  and that both the plastic and print bed surface are much stronger than the bond formed between them. To understand the last part think about why you would not print onto a tissue paper printer bed..

The type and strength of the bond is determined by which plastic you use and the bed surface. The more polar a material, the stronger its bond will be. Unfortunately HDPE is extremely non-polar, making it hard for anything to bond to it, or it to bond to anything else. ABS is slightly more polar, and when coupled with its lower melting temperature may explain why its a popular choice for printing. For the most part, changing the type of plastic is not really an option, so I will mainly focus on different materials for the printer bed.

As an interesting side note, in the milk bottle manufacturing industry they are required to print the expiry date on the sides of each bottle. However due to the HDPE bottle being so non polar the ink wont stick and can be rubbed off with your figures. To overcome this one method is to oxidise the surface by running it over a hot flame for just a split second prior to printing the date on. This oxidised surface is dramatically more polar and thus, the ink will stick.

So how do you know what materials are more polar than others? How polar a material is depends on all sorts of factors which requires a lot of chemistry to explain. Thankfully, there is an easy way to get a good estimation at home. If a material is very non polar, such as a freshly waxed car roof, water will bead up (maintain the smallest surface area) on the surface and refuse to spread. So if you sprinkle some water on the surface of a milk bottle (HDPE) or a teflon frying pan (PTFE) it will bead up and run off.

A water droplet beading up on a non polar leaf

On the other side of things, if a surface is very polar water will either want to spread out and make the largest surface area in contact with the material. This is because the attraction of the water to the material (adhesion)  is stronger than the attraction of the water to its self (cohesion). Some examples of polar materials are exposed (unpolished) wood and paper (cellulose).

However this isnt the whole story. The physical shape (roughness) of the surface also has a dramatic impact on how well water will spread and in turn, how well hot plastic will spread. The better the spreading, the more likely a stronger bond between your extruded plastic and printer bed.

This post is already far to long so I will talk about how surface features increase bond strength and some suggested ideas for printer bed materials at some point in the future.