Hot melt adhesive principles continued (2 of 2)

Reading back over my original post on adhesive principles I think I have may made it overly complicated. So consider this the key points from the first post.

• Having a good understanding of the principles behind how hot extruded plastic adheres to a surface is important as it allows you to make an educated decision over what print bed materials to experiment with.
• Hot extruded plastic such as ABS, PLA or HDPE, are poor adhesives as they will only form weak molecular bonds with most materials.
• Due to the weak bonding, the major determining factor in bond strength is the total area that the two surfaces, plastic and printer bed, contact at the molecular level.
• The total area that the two surfaces contact at the molecular level is primarily determined by the surface roughness, it temperature, and the type of bond it wants to form with the plastic.

Fist comes the theory, then I will relate it all to a printer bed surface.

What do I mean when I say ‘bonded surface area at the molecular level’?

When you think of two objects that are in contact with each other most people have a tendency to only imagine them touching at a level they can relate to. So when you imagine a 1cm square sugar pushed flush up against another 1cm square  cube you intuitively consider them to have a total area in contact that’s just 1cm squared.

Some sugar cubes!

But if you looked closer with a microscope you would soon see that only the outer most peaks that stick out from the surface in fact touch the other surface. So the total surface area in contact between these two rough surfaces could much less, say only a few millimetres squared. *

SEM Enlargement of Sugar Molecules (1)

On the other side of things, if you were to take a single drop of liquid and place it on the surface of the sugar cube, it may well have a surface area in contact with that cube that measure in tens of centimetres squared. That’s assuming that the liquid could perfectly wet the surface, that is, if it could fill every crevice and hole.

However a liquid will never perfectly wet a surface due to its surface tension. It will fall onto the surface, and then after a period of time will, reach an equilibrium where its at its making maximum contact with the surface.

So why is this all important and how does this all relate to some extruded plastic onto a printer bed?

When you extrude your hot plastic onto a printer bed it interacts in a few ways. These are:

• Due to weak molecular bonding between the hot plastic and printer bed material the hot plastic will want to spread out over the bed material and form a large interracial area (area in contact). Gravity plays little part in this spreading which is why you can print upside down with a Mendel.
• Due to the strong molecular bonding of the polymer chains, the fluid has a high surface tension and so does not want to spread over the surface, forming a weak bond and thus requiring people to use wasteful rafts when printing.

With enough time, an equilibrium is reached for that particular plastic and printer bed. At that point you have a curtain area squared that it is in contact at the molecular level that can be related to the size of the part your printing.

For each little bit of plastic that touches the printer bed at the molecular level you have a weak bond formed. The larger this surface area at he molecular level the more weak bonds you have. The more weak bonds you have, the more force you have too apply to break them all. Thus, the more force it takes to remove the object from the printer bed.

In short, the more molecules of extruded plastic that touch (‘wet’) the printer bed, the more weak bonds that are formed and ultimately, the more stuck your printed part is.

So how can you increase the molecular wetting of the extruded plastic? There are number of ways, some of which are currently being used. They include increasing the bed temperature, which intern decreases the surface tension of the plastic**. A heated bed also has the added benefit of allowing more time for the extruded plastic to reach that equilibrium before it falls below its Tg.

Finding just the right ‘roughness’ for the surface is also important as is trying different polar materials. However, this has again became much, much longer than I intended it to be and so this discussion must wait for another day.

* In reality no two atoms ever ‘touch’ in a physical sense and only their atmoic forces of repulsion and attraction interact.

**When you increase the temperature of a liquid you also decrease its surface tension. This is why you was your clothes and dishes with hot water, as its more able to spread into every pore.