An introduction to heat transfer.

From it’s heated print bed to it’s heated extruder, a reprap is hot property. So having a sound understanding of what heat is and how this heat is transferred to other materials may help you in building better reprap related paraphernalia.

So consider this a short summary of all things hot and how it can be related to reprap’s.

First up, the basics.

What is ‘heat’ and why do things expand when they get hot?

Just like a rolling car, each atom in a material has a kinetic energy when its moving. Heat is simply the sum of the kinetic energy of every atom in a material. However unlike a rolling car atoms in a solid material are restricted and so vibrate when heated. The hotter something is the more its atoms, and the molecules they make up, vibrate.

So why does a material expand? To answer this it helps to imagine a weight on the end of a spring that you are bouncing up and down ever so slightly. During this gentle vibration the weight stays close to your hand. Now if you bounce the weight more violently and at the right frequency the weight will move over a larger area and so, on average, spend more time away from your hand. Replace the weight with an atom and the spring with inter-atomic forces and more or less the same idea applies. The hotter the material the more violent the vibrations, and so on average each atom spends more time away from its neighbour, which causes the material to expand slightly.

Still using the spring analogy, if you heat a solid material its kinetic energy builds until the spring ‘breaks’. At this point the material will melt. Materials with high melting temperatures have stronger intermolecular bonds which can be thought of stronger, stiffer springs. Its obviously a little more complicated than this in reality but that is the general idea.

So how does this atomic scale kinetic energy get transferred? There are four key ways in which heat is transferred to the environment around you, with only three applicable to reprap’s at this stage*. They are; convection, conduction and radiation.

Convection

There are two types of convection, free convection and forced convection. Free convection is simply air expanding as it warms and so becoming less dense forcing it to rise. The air will continue to rise until it either cools or reaches a level in the atmosphere that is of the same density. Forced convection, as the name suggests, is convection that is forced by some mechanical means. Using a fan to cool an object is an example of forced convection. In short, convection removes heat by transporting it away, as opposed to conducting it away. Its worth noting that if you replace the word ‘air’ above with the word ‘liquid’, every statement still remains true.

On a reprap, free convection is what keep your electronics cool and forced convection, such as by a fan, may help cool your parts during printing. If your extruder is well insulated then conduction plays a bigger role than convection. However, where convection is really important is if you have a heated print bed. Its large surface area and open structure allows for a lot of heat to be removed, reducing its efficiency. So how do you increase the efficiency of your heated bed? Some methods so far include:

• Using insulation below the bed to stop convection removing heat from below. This may be in the form of a floor tile, expanded foam or just some carpet underlay.
• Insulating the part of your bed that your not using for that print. This has been done with wooden slats which are added or removed depending on the size of the object.
• Use a full enclosure to stop the hot, convected air from escaping into the room.
• Use a removable lid to stop convection during warm up before a print or between prints.
• Using a shroud or shallow wall around the heated bed to stop cool air from being drawn in from the sides. Even a few cm high lip around the bed may make a dramatic difference. Experimenting with some form of air curtain could also have the same effect.

Conduction

Conduction is the transport of atomic vibration from one atom to the next. Like a ripple on the surface of a pond, the kinetic energy, which is heat, can be passed from one atom to the next with out any movement of mass. Thus conduction is the movement of heat through an object, or from one object to another, by by physical contact. In liquids or gases a similar effect conducts heat, but with collisions between atoms transference kinetic energy.

In a solid the density, types of atomic bonds and crystallographic structure all effect how fast these vibrations move. In electrically conductive materials electrons also play a large role in the transport of heat in a similar way to lattice vibration.

Conduction is what transports heat from a surface and into the air before its convected away. This conduction of heat into the air can be greatly minimised through the use of insulation. However a hot extruder barrel losses the majority of its heat through conduction upwards into the extruder housing. Heat is also lost doing its intended job of heating and then melting the plastic. So how can you reduce the heat lost from your extruder through conduction? A few ways include:

• Using a poor thermally conducting substance as a thermal barrier. Most often used is PTFE which has a very low thermal conductivity. Other examples are most ceramics and plastics. As a general rule of thumb, if its a poor electrical conductor then its a poor thermal conductor.
• Reduce the cross sectional area (CSA). A smaller CSA means less material to conduct heat.
• Increase the length that heat must travel. By making the path heat must travel longer (decreasing the thermal gradient) you increase the resistance to heat loss.
• Reducing the pressure applied between two materials. Its known that having a large force applied between a hot object and cold object increases the thermal conductivity. This is why a CPU heatsink is clamped down with great force.
• Reducing the surface area between two materials. Having a very rough or pitted surface decreases the surface area in contact and so decreases the thermal conductivity.Thermal grease (heat sinking compound) acts to increase the area in contact.
• Make the heater barrel electrically isolated from other parts. This reduces the movement electrons which can aid in heat transport. Although I wouldn’t stress over this too much as the effect is relatively small in this application.

Radiation

This is the least obvious form of heat loss. When you stand in front of an open fire you feel warmth on your face, this infrared radiation. This is the least important form of heat loss with regards to a reprap. Ways to reduce radiative heat loss include:

• Insulate a hot object with aluminium foil (‘tin foil’) which reflects infrared radiation. Just make sure you insulate with a none thermally conductive material as well.
• Make your object smooth and polished. Soother objects have a reduced surface area and thus, less area to emit infrared radiation.

So there you have it, a basic introduction for all things hot. If you have spotted something not quite right in all this by all means let me know.

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*Whats a forth way that heat can be transferred? Through the breaking of molecular bonds in an endothermic reaction. A good example is evaporation of water. Heat is consumed when water evaporate and although you may not notice it, is released when it rains in a process known as latent heating. So unless someone is has strapped a split system air-conditioner or evaporative cooler to their reprap its not really applicable, at least at this stage.