how to build a heating
so, i`ll try to explain how to calculate and build a heating by using carbon...the whole thing (the explanations, not building the heating
) is kind of difficult for me, i`ll have to use my dictionary a lot of times - i hope my head doesn`t explode
hopefully you even understand something
i also have to say that it`s not possible to build a 2m long heating-sheet with the carbon-fibreglass-hybrid-band i used! you wouldn`t get enough heat if you would use such a long sheet. that`s why i had to split my heating in to sheets of 1m each. this is a big disadvantage of my heating sheets. so for bladder-presses i don`t think that it would work to but the heatingsheets into the press because you would become a mess with the connections of the sheet in the middle of the press.
maybe this could be prevented by using one single, thinner roving instead of the heating sheet i used...
well, as i wrote before: for this kind of heating we use the characteristic of carbon, that it produces heat because of it`s electrical resistor.
why heat?
well, heating up your setup has several advantages. but the advantages depend on the system of epoxy you use...so i can just talk about the experiences i made with my stuff.
i use epoxy with which can be worked for 40minutes. after this time it gets to cure. for curing it need`s about 16-18 hours, fully cured after 24hours - so this is a pretty long time!
this time can be reduced by heating the whole thing up. with 60° it only lasts 3 to 5 hours until the epoxy is fully cured. so with a heating you save a lot of time.
an other point is a thing called "tempern"... i think the english word for this is "to anneal". the whole thing is about how to heat up your setup: you start at room temperature and then add every hour 10 to 20 degrees of temperature untill you reached the wanted maximum of temperature, then let the whole thing bake on this temperature for a set time and then cool down everything in steps, like you heated it. with this method of heating you can increase the strenght of the fibreglass.
but its very complicated because you have to use a system that heats up evenly and you have to controll the heating....
my setup described here is more the "freestyle-do it yourself-way to become a thing that more or less heats up the press something like evenly"
material
for my heating sheets i used a carbon-fibreglass-hybrid band. i tried to look up more informations about the carbon bands (as carvejunky asked in his post)...but i didn`t find much. i just can tell you that there are two widths aviable: 150mm and 250mm. both bands have a weigth of 225g/m2. and they are constructed like this: 2carbon rovings, 1 roving fibreglass, 2carbon rovings, 1 fibreglass etc. as you can see below there are 25 pairs of carbon rovings (from now on called carbon-double-roving)
calculations
and now for the funny part
i really love to type some forumlas on the computer, damn. well, let`s start:
first of all you have to calculate the
area, which gets heated from one carbon-double-roving (see above, 2 carbon rovings). for this you need the width, that get`s heated from such a double-roving:
this width you get by divide the total width through the number of carbondoublerovings.
Width_roving = total width / number of doublerovings
in my case:Width_roving = 150mm / 25 rovings = 6mm per doubleroving
for calculating the
area, that gets heated by one double-roving you have to multiply the width, that get`s heated from one doubleroving, with the length of such a roving.
Area_roving = Width_roving * lenght_roving
in my case: Area_roving = 6mm * 1m = 0.006m2
as i said on the beginning, i split the whole thing up into two sheets of 1m lenght each. so one doubleroving heats up 0.006 square-meters of the heatingsheet.
then you have to set a
area-heat-output. (flächenheizleistung for the germans) this area-heat-output is measured in Watt per squaremeter. so, by setting the wanted heat-output you determine the temperature you want to reach with your heating. i found somewhere a table with some heat-outputs and temperatures referred to it...so i used this table and appreciated the needed heat output:
area-heat-output (W/m2) temperature (°C)
224 -------------------------------- 38
449 -------------------------------- 55
673 -------------------------------- 70
898 -------------------------------- 83
1122 ------------------------------- 95
1347 ------------------------------ 104
well, as i said, i found this table somewhere. i think it was in an article about tempering from a professional manufacturer - that`s why there are such precise datas of heat-output and temperature.
but, as i said, this is a how-to for a risk-loving hobbyist - so the table can be used as a clue to appreciate the area-heat-output on your press.
for my press i took an
area-heat-output of 700 W/m2. this would result in about 70 degrees. this also depends on the system of epoxy you use - mine gets destroyed if heaten up on more than 80 degrees, so you have to pay attention to this!
for further calculations i`ll use the letter
"P" as heat-output, so P_area will be the area-heat-output:
P_area = 700 W/m2
after having set the wanted area-heat-output you have to transform this to receive the
needed heat-output for a carbon-double-roving.
so the heat-output of a doubleroving results in the multiblication of the area,heated by such a roving, with the total area-heat-output:
P_roving = Area_roving * P_area
in my case i got: P_roving = 0.006m2 * 700 W/m2 = 4.2 W
the next step is about electricity and power: in physics they have a forumla for the correlation of electricity, power and electrical resistance:
power = resistance * (electricity)2 <- this means... quadrilateral? hu, i have some difficulties with translating the whole physical expressions ...
P = R * I2
so if you transform this you become the needed
current flow of electricity (=I) through one double-roving:
current flow = square root of (P_roving / Resistance of a roving)
the
electrical resistance of a double-roving i got from a publication. there they made some heatings with this method too, and the measured the resistance of the carbon-bands, so i just adopted their results:
R_roving = 79.3 ohm
for my heating: I_roving = square root (4.2 W / 79.3 Ohm) = 0.2301 Ampere
the next step is to calculate the
total power of the heatingsheet. for this you have to multiply the total area with the area-heat-output:
P_total = P_area * Area_total
in my case: P_total = 700 * (0.15m*1m) = 105 W
so, now you have the total power of your heating...and we`re taking the last few steps!
now that you have the total power you have to calculate the
total current flow of electricity, called I_total.
I_total = P_total / U_total
"U" stands for the tension. this may vary from country to country... here in europe we have continuous current with 220 V. so my calculations where:
I_total = 105 W / 220 V = 0.4773A
and now for the last step: now you have to calculate how many double-rovings you have to connect parallel. with these parallel connections you determine, how "much" current flow of electricity flows through each double-roving. (in parallel connections the current flow kind of splits up and goes ...split up through the several rovings which are connected together. so by connecting 2 rovings together you reduce the current flow throug each one by half in comparison to one sinle roving)
to become this
number of rovings, connected parallel, you have to distribue the total current flow through the needed current flow of a double-roving:
number of rovings = I_total / I_roving
in my case: # of rovings = 0.4773 / 0.2301 = 2.0743 rovings
so you have to round the result you become, because you can`t connect 2.07 rovings. in this case it isn`t a big deal to round the whole thing... but also here you have to be careful! by rounding up/down the number of connected rovings you also change the current flow through the rovings and with this you also influence retroactively the heat-output of your heating and the temperature!!!
and now:
how to connect the carbon rovings
there are several possabilities. i just made the cheapest one: i took some copper wire, wrapped it around the rovings and connected them like this.
after you connected everything properly you have to put your heating sheet the first time under power. this is a very important step for following reason: the carbon-glass-hybrid band is a good thing, but also there some small little carbonfibres sometimes stick out and hang over the fibreglass, which should isolate the several carbonstrings. this creates small little short-circuits. so if you put the whole thing under power, these little fibres, which creat the short-circuits, will burn away because they get too much current flow.
if you wouldn`t do this, you had some short-circuits after laminating the heating-sheet. but with one big difference: after laminating these little fibres won`t burn away because they are covered with epoxy - they are in an isolating, airthight area. there they just become very hot, maybe the even glow and because of this they destroy your laminate in their environment.
so after this step you can laminate the heatingsheet. you have to be very careful and prevent unnecessary movement of the sheet, because they could create new fibres which stick out and which create new short-circuits. but as this is nearly impossible you have to check your heating after laminating: if you find some new short-circuits you have to cut them with a knife.
well... and then your heating is finished!
the last one i made for my skateboards worked for a year and lasted for about 40 skateboards. but on the connection-points of the carbon with the copper wire resulted a higher temperature (because of some conection-resistors between the different materials) and i think this is the reason why it doesn`t work anymore
but 10CHF (for a sheet with 15cm*1m) for one year of heating, gaining time and improving the strenght of my boards was totally worth it!
