RepRap Heating Issue Continued
The RepRap was finished now in so far (image 1) that it was possible to get it working when the heated bed was covered with an insulating oven glove in order to prevent heat escaping into the environment. The oven glove had to stay on the bed until the set temperature of 110°C was reached and than -when the RepRap started printing- the oven glove had to be grabbed away rapidly. Printing was performed well with a heated bed that dropped its temperature to about 103°C but the problem appeared now after printing when it was tried to remove the printed object. It was really impossible to remove it from the bed!
This called for a more drastic approach.
1 shows the RepRap in the completed state. The RAMPS controller system
unit (black top cover with ventilator shown at the center right in the
image) has been mounted on a transparant polycarbonate support.
For the now chosen more drastic approach it was decided to review the heated bed setup totally by changing the heating power to a 24V at 20A system. Apparently more RepRap builders had suffered from heating problems with the bed because recently Mk2b heating beds had become available that would permit operation at either 12V or 24V. The 24V was obtained by doubling the etched circuit (that was in the Mk2a version at one side of the board) to both sides of the Mk2b circuit board.
It was quite sure that changing to 24V would resolve the heating problem but because the heated bed assembly had to be taken apart now, it was decided to implement a few more improvements:
The images below show the different components to be added or changed:
shows the pre-drilled and threaded aluminium support board and image 4
shows the support board mounted in a RepRap in a four point adjustment
position (this is an example picture and not our project).
The actual aluminium heat bed support that arrived from a supplier in China had to be adapted because it had the following shortcomings:
Image 4: Aluminium support board with extra holes needed (© FRS 2014)
Image 5: Aluminium support board with wrong distance for mounting sliding bearing holder (© FRS 2014)
The red and green marked holes in image 4 had to be drilled as an extra job. The too narrow distances between the four sets of two holes for attaching the sliding bearing holders (image 5) were left as they were, because the size of each of the holes was 0.5 mm larger than required and this just permitted (with some forcing) to mount the bearing holders.
Apart from these little problems, which had to be resolved, the aluminium support board was considered a useful improvement of the RepRap.
|Image 8 (large picture) shows the aluminium print bed with the plastic coated top surface. The
cutout in the print bed at the left of the large picture is for leaving sufficient
space for connecting the wires to the heated bed. The smaller insert
shows the bottom side of the print bed with the milled groove ending in
a well shaped space for guiding the thermistor cables and for placing the thermistor bedded in thermoconducting paste.
|The solid state relay in image 9 has been chosen to protect the power switching FET on the RAMPS controller board to be blown
because of the higher voltage and amps that will be drawn by the heated
bed. The RAMPS will switch the relay with its usual 12V output on the
RAMPS D8 pins. The secundary on the relay deals with the 24V at max.
47A from the additional power supply of images 10, 11 and 12 and connects
this power to the heated bed.
|The 24V power supply will
be constructed fromn two 12V HP server power supplies (images 10, 11 and
102. The HP power supplies are very reliable and they are sold at
prices ranging from € 12 to € 20 each. Instructions how to convert
these power supplies and how to combine them in series for obtaining
24V can be found here and here.
The choice for these powerful power supplies has been made because they will also be used in a different project for feeding a high voltage power supply. More details about this projet can be found here.
When all new parts had been collected it was decided to replace them with the old parts in the RepRap piece by piece and to test the RepRap after each change. This would permit to observe if these part by part upgrading had the desired effect.
The first part that was exchanged was the wooden bed by replacing it with the new aluminium support board. Then it was decided to retain the old heated bed but to exchange the glass plate on top of it by the new aluminium print bed and also by now installing a cork plate underneath the heated bed for better insulation against bottom heat loss.
After re-levelling of the print bed the RepRap was tested for heating up the bed to a set point of 110ºC. To my surprise it appeared that the exchange of parts so far did not make a significant difference. After a heating period of 20 minutes the bed had heated up to a deltaT of 70ºC, i.e. from ambient 20ºC to a temperature of 90ºC, and after 15 minutes more the temperature reached a temperature of 102ºC and remained at that temperature with a variation of ± 0.5ºC. The bed output was all the time at 100%, i.e. the PID algorithm did not cut in and full power was on the heater bed permanently, or with other words, the very slow speed of heating up (the slope angle of the temperature curve) did not require the PID algorithm to start working. Apparently the heating power of the bed heater was insufficient to compensate for heat loss at the surface of the aluminium print bed, similar to the glass plate! Moreover, the cork insulation underneath the heater plate did not (sufficiently) reduce heat loss from the bottom of the heater bed. This was tested by measuring the temperature of the aluminium support board, which is located 24 mm below the heater plate (20 mm air gap and 4 mm thick cork plate). At 100ºC of the aluminium print plate, the aluminium support board was found to have a temperature of 44ºC.
A quick conclusion that can be drawn from this experiment is that with the same heater, an aluminium coated glass plate of 2 mm thick and a surface of 400 cm2 has the same heat loss as an aluminium print bed of 3.5 mm thick and the same surface!
Next steps will be to add more insulation under the heater, to exchange the Mk2a heater for a MK2b heater and to operate the RepRap at 24V.
|Last Updated on: Mon Nov 10 22:19:03 2014|