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A heated bed helps greatly to avoid the warp problems and permits the print without using raft! Examples:
There had been some good results using ABS and PLA on it. Here are some links of tests made with results:
- The heated bed of NopHead;
- NopHead tests on ABS using heated bed - (Casainho follow the same steps and got the same results);
- NopHead tests on PLA, PCL and HDPE, using heated bed;
Please verify all the information on this page, looking on the many blog messages about this subject over Internet.
We should use kapton tape on top of the heated bed, since hot extruded plastic sticks very well to it and also it's easy to peel off the piece from it in the end of print, without damage the piece and the kapton tape.
The tested temperature values for the heated bed are 220ºC for ABS plastic and 55ºC for PLA plastic. Bed should be heated to that temperature values before starting printing and keep that values during printing. The first layer should be printed at a slower speed, like 3mm/s.
A heated bed can be build using a flat aluminium sheet of 6mm. It needs to be flat and be aluminium because it is a very good conductor, so the heat will be even across the whole bed with only a few heating elements.
It should be insulated on bottom side to not melt plastic parts of the RepRap. On the RepMan a ceramic wall tile has shown good results. It should also use insulating stand-offs.
Be careful as ABS does not stick well to aluminium. Copper is known to work better as is covering the bed with (expensive) Kapton Tape.
Also take care about thermal expansion. If yous bed is not allowed to expand in XY-direction it will use the only offered direction and bulge. Acrylic is known to do this, as are all metals. In the case of aluminium, a 20cm bed heated by 100 degrees C will expand about half a millimetre along its length as it warms up.
For power the bed, you should use a PC PSU that can output at least 10A @ 12V. If you need more power on your bed than you need more power input and vice-versa.
The heated elements can be nichrome wire or power resistors. Nichrome wire is cheaper and takes less space.
There electronic control circuit [NOT TESTED YET] can be the following one. It uses a Arduino to read temperature from a sensor e turn on/off one power mosfet. Target and current temperature values can be seen on a LCD. Target temperature can be selected using three buttons.
The TSIC101 is supplied with +5V from the Arduino board. It outputs a linear voltage between 0 and 1 volt. 0 volt for -50ºC and 1 volt for 150ºC.
Ardunio can be configured to have an ADC voltage reference of 1.1V, which means it can read steps of 1.1V/10bits ~= 1mV.
Since TSIC101 outputs 5mV for each 1ºC, Arduino will be able to measure each step of 0.2ºC (TSIC101 resolution is 0.1ºC).
We may not let the TSIC101 going over 140ºC for his safety, since 150ºC is the maximum.
The power mosfet FDB8880 is rated for maximum VDss of 30V, which means it can cut at least 24V, however we will be using 12V.
It can cut also as maximum 11A if have a good dissipation.
It is controlled by Arduino digital 6 which can output a PWM signal of +5V.
The diode D1 is a protection against any possible voltage surges because of commuting the heater element.
(TODO: include scematics and board-layout here)
The three buttons let user select the target temperature. More functionalities can be added to firmware and use these three buttons to navigate on menus, for example.
There should be a MODE button, UP button and DOWN button;
The display must be a minimum of 4 characters (although 8 would be the useful minimum). 10 would be pure luxury and 2 lines would be overkill (unless it can be purchased for less than a one line display).
The mode button when pressed toggles between: On - Set - Off and then back to On.
The display appears as: "(Bed|Set|Off)[@: =][ 1-9][ 0-9][0-9][°]*[CF]*" when expressed as a regular expression.
In On and Off mode the Bed temperature is constantly displayed with a one second update. In Set mode we see the temperature that the bed should be heated to. This set temperature should be saved in EEPROM so it is available on the next start.
The Arduino have pull-up resistors that should be enable for the buttons working correctly.
The LED can be used to signal something to user. It is a 20mA LED with 2V Vf.
- There is no need to implement PID control on firmware, but it can be done.
- Last temperature values inputed by user should be saved on EEPROM, so user do not need to input it again.
- There could be a few profiles (with temperature value changed by user), one for each kind of plastic, like for ABS target temperature = 120ºC and PLA target temperature = 55ºC.
- Maybe a cool down slope is important, like let user define how much time the bed should take to go from target temperature to final one.
Skeinforge options for heated bed
Skeinforge have an option to control heated bed and chamber. Since we are not controlling the heated bed using RepRap main board or extruder board, we don't need that Skeinforge option.
We don't need raft and we need to print the first layer at a lower speed so the plastic can stick to katon tape. Skeinforge options for it on "Raft" -> "Object First Layer". Here the main description taken from Skeinforge Manual:
Object First Layer
The first layer of your object is printed "differently" from the rest of the object. In some cases (especially if you are printing without raft) it is needed to print the first layer of your object slower or to deploy bit more filament.