Heater Design

The power of the heater will determine how fast the plastic will melt and get to temp. The equation is

Q=mc(Tf-Ti)

Q is the heat given off

m is the mass of the plastic

c is the specific heat constant

Tf is the final temperature

Ti is the starting temperature

The specific heat constant can be found here. It changes based on temp but you use the average of you initial and final temperature.

m=1000g

c= 2.073 J/gK

Tf=200 C

Ti=30 C

Plug that into our equation we need 352410 Joules to melt the plastic.

A watt is a J/s

Say our heater is rated to 175W, it will take 33.56 minutes to heat the pot. This does not take into consideration losses through the walls.

Concept 2, the pot has a capacity of 2-3 kg of plastic. So the 175W heater would take 2-3 times longer to melt.

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2 responses to “Heater Design”

  1. voidwarranties says :

    what if we added some sort of an internal heater, ran a tube through the middle near the bottom or heated the piston? or could we reduce the size of the chamber? this has more to do with surface area than anything, could we make it long and skinny instead of short and fat?

  2. nommymanwich says :

    So these calculations assume perfect distribution and just gives you a ball park of the power you will need to heat it. The heater we plan to use is a rod that will be located in the bottom of the pot. Its rated to 175W and runs off 120AC. We will wrap the pot in some insulation to help prevent heat losses and use some mixing to help melt the plastic evenly. The surface area does not matter because we are not heating the pot itself then the liquid, but instead heating the plastic and trying to prevent heat form leaving the pot. If it takes to long to melt we can always add more heaters. This may need to happen but we will see how pricey the electronics are. The pot as it is speced now can hold 2-3 kg of plastic. On the BOM in the material page I have a second list for concept 2 that is the 3″ pipe. This would hold enough to make 1kg but only ends up saving us 25 bucks. I feel the goal of this contest is to manufacture plastic and not just make it on a 1kg batch.

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