Reprap Extruder’s: A few possible explanations.

Following on from my previous introduction to all things plastic, I thought I would try to help explain why home built plastic extruder act the way they do. The idea being that if you have a better understanding of the processes that are causing you problems, you can more likely design usable solutions. Please note that I am by no means an expert in this area, im just using what knowledge I have to give my best estimation of what may be taking place.

• Why does the plastic pop, smoke and out-gas if it gets too hot?

I covered this briefly in my last pot when discussing thermosetting plastics. When you heat a plastic up high enough, you not only start to break down the weak bonds between chains, but also the stronger bonds holding the chain together. When they start to fail you end up with some very reactive chemicals being exposed to the atmosphere. Some of these compounds will stay gaseous while others will react with oxygen to produce a dark sludge/charred material. The simplest and most obvious way to prevent this is to keep the temperature low.

Remember that the temperature you extrude at has a direct impact on the properties of the object your printing.This is especially true for ABS. If ABS is extruder in the upper temperature range the object it will be more resistant to high temperatures but have a lower strength. If its extruded in the lower temperature range it should be a stronger object.

• Why do some plastic filaments curl back on themselves when you extrude it into mid-air? Why not all plastics?

This is a more interesting, although not very important question. Not all polymers are created equal, in fact far from it. During the plastic manufacturing process (polymer syntheses or ‘polymerization’) there will always be different polymers of different lengths. Some will be very long chains, while others may only be a few units in length. These different polymer chain lengths have important implications for when they are extruded. Once melted in the heater chamber of a reprap device, each polymer will have the freedom to move around a little as its pushed out of the chamber. Due to friction with the chamber wall, the longer polymer chains are dragged along the chamber wall while the shorter ones migrate to the centre of the filament thats being extruded. This effectively forms a skin of long chain polymers at on the outside of the filament with a short chained core. Due to random distribution, at any one time there will always be slightly more long chain polymers on one side of the filament than the another.

After the filament of plastic is extruded it begins to cool and all polymer chains start to contact a little. The longer the chain, the more it contracts. So you now have a filament of plastic that has a uneven distribution of contracting long chain polymers on one side. This puts a stress on the filament causing it to curl upwards in which ever direction happens to have the most long chain polymers at that time. This problem should be more pronounced in plastics that have low levels of cross linking such as ABS and less so in HDPE. Take what I have just said with a gain of salt as this is just my take on the problem and not something I have inferred from experimental results…

Although this is interesting, it has little importance for a reprap extruder as its normally extruded so close to the bed/object that there is no noticeable effect. However it could become very important if you were trying to extrude long filaments onto a spool from plastic granules or shredded milk bottles.

• How does each extruded layer bond to the layer below it?

As previously discussed, all polymer chains are moving. The hotter the plastic, the more the chain will move. When you extrude a hot layer of plastic onto a cold layer of plastic below it, it will melt surface layer of the plastic below. As you now have an inter facial region of molten plastic you end up with the polymer chains diffusing into each other due to their random movements. Once cooled, the polymer chains that were at the surface are now entwined and bonded to form a seamless join.

On a side note, this is the same process used for solvent welding of plastics. When you apply a solvent it replaces the lose bonds between the polymers with its self. This frees the polymers to move and in effect lowers the plastics Tg until the solvent is removed, normally through evaporation. Solvent welding could be explored as a way to join different parts printed on a reprap device that are either too large or have complex geometry (overhangs) and as such can not be printed as a single piece.

• What exactly causes warping and why does a heated bed or chamber help?

As a polymer chain cools, it contracts. A very simple concept to imagine. So why then does a printed object want to peel up at the edges? Surly the whole object should contract equally, leading to an object that is a few percent smaller than what your printed when cooled to room temperature.. This would indeed be the case if an object had a random distribution of polymer chains in all directions and was laid down all at one time. However, when extruded in lines there is an uneven tension in the horizontal directions as it cools.

Possibly a lot more important than this is the temperature of each layer with respect the layer below it. Imagine a single line of plastic laid down on a printer bed. If it was left to cool it will want to contract and shorten. If it was not stuck to the printer bed it would just shorten in the same way that a rope shortens when a load is  removed. However as this line of plastic is stuck to the printer bed it can not move. Now imagine a second line of plastic laid down directly on the first, while the first layer is still hot, and then a third and forth and so on until there are 5 lines stacked on top of each other. Logically, you may imagine that if you left this to cool it would shorten like the unloaded rope analogy.

However in practice it seems that the lines pull up at the edges. I’m guessing this stems from the uneven way in which the object cools. The middle of the line of plastic cool slower than the out side edges and the whole object cools from the bottom up. Remember that where ever it cools there is the tension pulling inwards. So you now have an object that is pulling more at the edges and bottom, while also being constrained in the middle.

Now imagine this same process but for a 3d object and you can start to understand why it lifts at any sharp corners. This is actually a lot harder to visualise than I first thought and I doubt I will be able to fully understand it until I get to play with my own Mendel..

A number of solutions exit to this problem of warping that are currently being used and that I’m aware of. They include a heated bed that keeps the object and the air above it warm. This helps speed the movement of the polymers slipping past each other to remove the built up stresses internally. A heated work chamber also has the same effect.

Another way to reduce these stresses is to design objects with warping in mind. Any sharp changes in direction will be a strong point for warping. So using rounded edges could help. I wonder if changing the orientation or shape of the infill could also help? Maybe a honeycomb or triangular infill instead of the standard hash pattern?

For more info google injection moulding warping and there are plenty of guides that give good tips for low warping product design.

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Well I hope someone was able to glean some useful information out of this. If nothing else it cemented some of the ideas more deeply in my own mind.

When I get time for my next post I will hopefully shed some light on the theory behind hot melt adhesion, and more importantly, what properties could make the best printing bed.