Heated Bed

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Heated Bed

Release status: unknown

No image available.png
Description Prototype an heated bed which avoid warp and the use of raft
License GPL
Author RepRap dev team
Based-on Mendel
Categories Heated Bed
CAD Models none
External Link none


Contents


Introduction

A heated build platform HBP greatly alleviates bad printing quality, by heating the beds surface in order to keep the base of an extruded part from cooling (and shrinking) too quickly. Such shrinking leads to warping; internal stresses in RP parts. The most common result is corners of parts lifting off the build surface. Heated beds usually yield higher quality finished builds with materials such as ABS and PLA. A HBP also permits one to print without the use of rafts!


Flat heated bed.jpg

Here are some examples of HBP results:

You can verify all the information on this page, looking on the many blog and forum messages about this subject over Internet.

The current design of Heated Bed on this page is almost equal of the one made by James Glanville.

Also keep in mind that despite the title this page only describes one out of many heated bed designs.

Mechanical construction

The flat and heated part of the bed can be made off many materials - next we present a few materials.

The heated bed should be insulated on the bottom side to not melt any plastic parts of the RepRap. Heated bed on top of cardboard or raw natural wool on top of MDF seems to to be adequate; research with other insulator materials is ongoing.[1]

Wiring & high current

As a HBP craves significant current, the selected wire needs to handle such a load. One can often find automotive wiring that handles 15A or even "ordinary" speaker wiring that seems to come in ridiculously fat sizes.

Consequences of of using too thin a gauge of wire or having loose connections be damaging to your equipment by leading to fire or melted plastic!

Surface Material

Kapton Tape

We can 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 damaging either the piece or the Kapton tape. Only few people have had good results printing directly onto various metal surfaces like copper or roughened aluminium. The tested temperature values for the heated bed are 120°C to 220ºC for ABS plastic and 55ºC for PLA plastic. The bed should be heated to that temperature before starting printing and kept at that temp during printing. The first layer should be printed at a slower speed, like 3-14mm/s instead of 16-32mm/s.

MarcusWolschon reports that 70°C seems to be adequate for printing ABS plastic on Kapton tape.[2]

HBP Material

Glass

A heated bed can be build using glass (3mm thickness for example). Glass is easily found in local hardware shops. Casainho bought his 3mm thickness glass in a local store for only 1.5€, to cover entire bed area of his Mendel. The glass was cut in the shop to the required measurement without any additional cost - total cost was only 1.5€.

Advantages of using glass:

  • comes really flat from factory;
  • does not bend, which means the stores still sell them really flat. You will not be able to bend it, so it will always be flat.
  • commonly found on local hardware stores and CHEAP (200x225x3mm for 1.5€)!
  • it is not a good conductor like copper or aluminium, so takes more time to heat up evenly but it also takes longer for the stored heat to dissipate when cooling.

Disadvantages:

  • normal glass (=cheap glass) will shatter easily when not heated evenly.
  • because of the lower thermal conductivity of glass hotspots are more likely than in aluminium or steel. Hotspots will immediately cause cracks when normal glass is used.
  • glass is not easy to work with (drilling holes etc).

See Casainho message about using glass.

Glass is ideally used on top of an aluminium or copper plate, so the temperature is evenly spread. According to the glass supplier rkoeppl asked about using glass for the heated bed, normal glass should not be used for temperatures over 80-100 Celsius. Single pane safety glass is much more expensive, but can safely be used for temperatures of up to 200 Celsius. If higher temperatures are needed there is the option to use the glass that is used for oven doors, it is hugely expensive however. The next video (recorded by Casainho on 2010.08.18) shows the Mendel Heated Bed made using glass and Kapton tape, it may be useful for you understand how it performs:

View the high quality original vorbis video file.

Metal

A flat metal sheet can also be used (6mm thickness or less). If aluminium is strong and does not bend, a 2mm can be used. It needs to be flat for obvious reasons.

Also take care about thermal expansion. If your bed is not allowed to expand in XY-direction it will use the only offered direction and bulge. A aluminium 20cm bed heated by 100°C will expand about half a millimetre along its length as it warms up.

One researcher is experimenting with a quick release bed made of thin steel[3] that is magnetically held flat against a thicker non-magnetic heated bed. (Curie temperature of neodymium magnets is around 300ºC -- low for a magnet, but safely above the 220ºC maximum build plate temperature).

Ceramic

On the RepMan a ceramic wall tile has shown good results.
The bed could also or additionally use insulating stand-offs.

HBC (heated build chamber)

Plastic bag

Once you have any kind of heated bed, a plastic turkey bag (aka oven bag) may make it work even better.[4]

That gives three advantages:

  • makes a bubble of warm air around all the parts you are printing; blocking cold air from cooling the sides and tops of your parts.
  • it significantly(??) reduces the power required to run the system (enough to stay below the "60 watts of electrical power" Gada Prize requirement?)
  • "the slightly acrid smell of molten ABS was ... eliminated"

Is double-bagging any better than single bagging?

Discussion

We're organized in this RepRap-Forum with gathering ideas, troubleshooting, and designing the heated bed. Please join us and remember you're welcome to log in and edit this page as well!

If you've come to this page, you may well want to look at this heated bed design too.

Heater types

Various designs for heating the beds (be the actual bed made of whatever) have been used.

Nichrome

[5] uninsulated nichrome wire attached to the bottom of the bed with kapton tape.

Aluminium Clad Resistors

[6] [7]

Pros:

  • very cheap,
  • can handle ABS and PLA easily,
  • Mains or 12V powered

TO220 Resistors

[8] [9]

"Don't use TO220 resistors. All mine failed eventually so I use AL clad now, which have never failed."

--nophead

Hot Plate

"Hot plates" are normally used for cooking, and take mains voltage

[10]

Grip Heater

Motorcycle grip heaters work off of 12V, and deliver about 35W to 2 flexible pads of about 100x50. See http://open3dp.me.washington.edu/2012/03/of-hbp-and-other-hot-things/ or http://www.google.com/?q=grip%20heater .

Etched PCB as a heating Element

Main Article: PCB_Heatbed Ultimachine sells a 214mm x214mm board 10A @ 12v (120W) board: [11]

Pros:

  • Even Heat Distribution
  • Simple, clean implementation
  • Very little space required
  • Lightweight

Cons:

  • Relatively expensive (if you use a professionally manufactured board)

Stripboard PCB as a heating Element

Ghetto HBP on thingiverse is an example, with a link to one supplier of large (10x4 inch)stripboard for $2.65 each. So about $5 plus S&H for a HPB circuit board.

Pros:

  • Cheap
  • Even Heat Distribution
  • Simple, clean implementation
  • Very little space required
  • Lightweight

Cons:

  • Must be soldered into the desired configuration
  • Large stripboards are relatively hard to find


Milled PCB as a heating Element

Pros:

  • Less expensive choice
  • Fun to make ;-)

Cons :

  • 20x20cm copper-clad blank plates are hard to find. Exploring whether 2 15x20 plates side by side will fit the intended use.


Note: Cheap pcb blank plates from eBay often have way less than the usual 35 microns copper thickness. If no data on thickness is available, estimate experimentally by measuring the resistance of a trace of known length and width.

Spreadsheet to calculate trace resistance: Media:trace_resistance.ods

A great online [PCB trace resistance calculator] with thermal correction.

Clothes Iron

Some have used a cheap (in every sense of that word) clothes iron. These can be purchased locally anywhere in the world, and will require some modification.

[12]

Pros:

  • Very inexpensive ($14 + fixings)
  • Very fast heat up (1600W)
  • Integrated thermostat (just turn the dial to select the temperature).
  • Mains voltage. No need for a power supply.
  • Most irons come with a little indicator light that shows when it is heating.
  • The knobs and buttons on the iron come with little springs that look like they will be good for extruder idler.
  • Glass can be removed and a spare one used for quick swapover between prints.

Cons:

  • Heavy
  • Needs a bit of space under the bed.
  • Unless you also use a solid state relay, temperature is not regulated by the firmware and you need to manually turn it off (con for unattended printing).
  • Mains voltage. Danger if not installed safely.
  • I had to cut the tip off to get it to fit under the bed properly.

Silicone Heating Mat

See the Heated_WolfBed (in German)

Pros:

  • Fast heating
  • Reliable
  • Mains voltage - DOESN'T require any amps from the 12 volt supply.
  • Very low height
  • Easy to install (adhesive backed)

Cons:

  • Expensive - silicon mat (50€) plus solid state relay (16€).
  • Mains voltage, but can be purchased for 12v. Need GROUND line to aluminum bed for safety.
  • Relatively high temperatures possible (safety problem if thermistor dies/falls off).

Sources:

Peltier Element

There have been reports here & there on utilizing the Peltier effect through often cheap of the shelf Peltier elements. They are commonly used amongst other things for thermoelectric cooling.

They can be sourced, e.g eBay.


Electric construction

To power the bed, you should use a PC PSU or universal power supply that can output at least 10A @ 12V.
You may be able to get universal laptop-PSUs with adjustable voltage for a range of 15-24V at 80-180W.

The heating elements can be nichrome wire, power resistors for higher temperatures or ready-made, flat heating-pads for lower temperatures.
nichrome wire is cheaper and takes less space then power resistors.

(Note that for first tests a thermometer for the apropriate range and manually controlling the PSU-voltage can be enough to print. So you can postpone the electronics for later if this is not your strong point.) [suggested improvement: have two separate heaters spacially interleaving, one under-powered all the time, the other controlled by PWM. This would ease the current burden on the PWM power circuit. It would also improve temperature accuracy.]

Electronics Design #1

One possible electronic control circuit (NOT TESTED YET) can be the following one. It uses an Arduino to read the temperature from an appropriate sensor and PWM one power MOSFET. Target and current temperature values can be seen on a LCD. Target temperature can be selected using three buttons.

Temperature sensor

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.

Power MOSFET

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 with no heatsink.
Any MOSFET with an Rds(on) of less than about 20 milliohms at Vgs=4.5v,Tj=175c will be suitable, such as IRL3803 and STP55NF06.
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)

LCD

More info on Arduino LCD page.

Buttons

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.

LED

The LED can be used to signal something to user. It is a 20mA LED with 2V Vf.

Firmware

  • 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.

Schematic

Schematic source files for KiCad.

Schematic:

Reprap-heated bed.png

Electronics Design #2

This design uses a simple analog control circuit and is easy and cheap to build (20$US for the electronics).

Bi-metal temperature switch

NC Temperature switch

A simple bimetal NC temperature switch controls the temperature. These switches are on (=connected) when the temperature is below the value of the switch, when the temperature gets above the rated value of the switch it turns off (disconnects). When the temperature of the switch has dropped below a certain point, say 10C cooler than the rated temperature the switch turns on again. For printing ABS a switch with a rating of 140C works and keeps the temperature of the bed between 125-140 degrees. The switches can be bought for a few dollars on the internet for (search on ebay: "NC Thermostat Temperature Switch Bimetal Disc").

Thermal fuse

To protect the bed from over-heating in case of a problem a thermal fuse is used. A rating of around 200C should work.

Fuse

For safety a fuse is placed in the circuit for safety in case of a short. The rating of the fuse is dependant of the voltage and wattage you are using.

Power resistors

The bed is heated with power resistors. In my (North90ty) case for a 220mm by 230mm bed made out of 4.6mm aluminium around 150W of power is needed to heat the bed to 125C, i use no insulation under the bed and the printer is in a cool room. I use 12 10W resistors which are wired so they equal 70 Ohms. My bed is powered on 115V AC. Power resistors can be glued to the bed with epoxy steel, look at the temperature rating of the epoxy steel before you buy it, a rating around 500C is recommended.

Grounding of the bed and printer

For your safety it is important to ground the bed and the printer, especially when using a high voltage power supply or power straight from the net (110/220AC).

Circuit

Heated bed 2.png

SAFETY

It is very important to use:

  • grounding of the bed to prevent electrical shocks [citation needed: shocks from a 12-24V supply?]
  • a thermal fuse to protect against over heating
  • fuses for safety in case of a short (almost all power supplies have these built in, but extra safety isn't a bad thing).

Operational considerations

Initial heating

Due to the nature of things, the heated bed radiates heat. Loss of radiated heat from the bed results in poor efficiency when heating, which results in the need for more power to reach the desired temperature. A suggestion was made to try using a lid over the heated bed, to keep the heat trapped above the bed. Initial tests have proved successful at improving the initial heating time, though no detailed data on how much faster this improves heating is currently available.
The test lid was acrylic and approximately the same size as the bed. The piece of acrylic was on spacers that provided approximately 10mm clearance between the bed and the acrylic lid. Removal of the lid (to allow extruding) resulted in an almost immediate 5ºC temp drop (at 110ºC), which was then easily compensated for by the heating circuit in a minute or two.
Further suggestions have been made along the following lines:

  • Thermal blank: Effectively a plate to cover the entire plate except for the area to be printed. Some suggestions are to use the printer to print this thermal plate, however there is the issue of the waste of plastic and/or re-use of these blanks over multiple prints yet to be addressed.
  • Sliding plates blank: Same basic principle as the above, using sliding plates that can be moved into place around the bed to cover the non-printing area.

None of these ideas have yet been tested.

Removing the final printed piece

You should let the Heated Bed and piece cool down to avoid warping.

The printed piece may be very dificult to release from the Heated Bed. A good way to release it is using a hair dryer, blowing hot air against the piece when both Heated Bed and piece are cold (blowing approximated 20 seconds). If you want to release the piece when both Heated Bed and piece are hot, you may instead blow cold air, or use a wet towel and squeeze water around the piece and into the holes.

You can also use a pen knife to release the piece but be careful to not hurt yourself.

Skeinforge options for heated bed

Skeinforge has an option to control a heated bed and chamber.
Since we are not controlling the heated bed via gcode using the RepRap main board or the RepRap extruder board, we don't need that Skeinforge option.
We don't need a raft if the bed is flat enough but we need to print the first layer at a lower speed so the plastic can stick to kapton tape.
Skeinforge options for this are in "Raft" -> "Object First Layer".
Keep in mind that skeinforge often renames options in newer versions or moves them to a different tab.
Here the main description taken from Skeinforge Manual at the RepMan-wiki:

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.

Links

  • RepRap: Blog: "No-curl hair dryer" 2008-12-11 : rather than buy nicrome wire and manually attach it to the bed and add some safety features around it, it reduces assembly time and may reduce net cost to buy a cheap off-the-shelf hair dryer (which already includes nicrome wire and some safety features) and clamp it in place.