Friday, November 22, 2013

Installing the Heat Bed and the Thermistor and Assembling the Hot End (Videos 11, 12 and 13)

There is a helpful page on the NWRepRap site that has hi-res photos of all of the components.

Why do we even need a heat bed?  Why can't we just print directly to a smooth surface at room temperature?  According to this page, printed parts tend to curl as they cool from the bottom up.  So the idea I guess is to keep the entire part warm until it's complete, kind of like a heat lamp is used at a restaurant to keep the entire order warm while it is assembled.

The NWRepRap kit comes with an MK1 heat bed.  Information about the MK1 appears here.  There is also a heat bed called the MK2.  The MK2 is not necessarily "an improved MK1", but the two designs were developed in parallel.  I read somewhere that the MK2 has a hole in the center which makes it easier to level because it is mounted on just 3 points (unlike the MK1 which is mounted on 4 points), but the presence of the center hole results in a cold spot in the center so from that standpoint I have read that the MK1 can be considered superior to the MK2.

You prepare the MK1 heat bed for installation by soldering the 18 gauge wire (mine came in nice red-black pairs) to the heat bed and optionally soldering 2 surface-mount LEDs and a 1 KOhm surface mount resistor.  The LEDs are supposed to light up when current is applied to the heat bed.  Since an LED lights up only when current passes through it in a particular direction, you are advised to mount the two LEDs in opposite directions so that either way you run current through the heat bed, at least one LED will light up.  The surface mount components are tiny and require tweezers to manipulate into place.  If you have any doubt about your soldering abilities, skip this optional step.

There are several options presented in the video for mounting the heat bed on the four M3 screws.  I decided that it would be best to the the springs between the MDF and the heat bed, but to hold the heat bed down, I used M3 locknuts and a washer.  This would make adjusting the height easier than using a dual nut configuration as suggested in the video.

There are two thermistors provided in the NWRepRap kit. They are of the 100k NTC type. One is to be used to measure the temperature of the heat bed and I assume the other will be used to measure the heat of the hot end.

The thermistor for the heat bed is prepared for installation by sliding the PTFE insulator wrapping over the bare leads. Then you place two pieces of skinny heat shrink over the two individual leads and one fatter heat shrink over both stranded wires. Then, you solder the leads to the stranded wires. After the soldering is complete, you heat the skinny heat shrink so it shrinks over the two solder joints and then you slide the fatter heat shrink over the skinny heat shrink and apply heat.

After doing all that it's good to test the resistance of the thermistor at the ends of the stranded wire with a DVM to make sure you don't have a short or an open circuit. Touching your finger to the thermistor should change the resistance.


Testing the resistance of the
thermistor at room temperature
Testing the resistance of the
thermistor at body temperature
Installing the thermistor requires you to physically attach the thermistor to the heat bed or to something heated by the heat bed and to run the thermistor wires to in a similar manner as the heat bed wires.  The video suggests that you kind of push the thermistor up through the hole in the center of the MDF against the bottom of the heat bed and tape the wire with masking tape such that the thermistor stays in contact with the heat bed.  I wasn't so comfortable with this approach because I thought that regular operation would cause vibration that might cause the thermistor to lose physical contact with the heat bed so I researched some alternatives.

I found two suggestions for mounting the thermistor from this blog entry written by Josef Prusa:
  • Attach the thermistor on top of the glass using kapton tape
  • Glue the thermistor on the underside of the heat bed using epoxy
Another opinion is given here (see the post from xiando on Aug 24, 2012):
  • Use kapton tape to attach the thermistor to the underside of the heat bed
  • Small amount of thermal grease between the heat bed and the thermistor encased in some RTV silicone.
I found a product called Arctic Alumina Thermal Adhesive.  Basically it combines the functionality of thermal paste for maximum heat transfer and RTV silicone for adhesion.  NWRepRap advised me to purchase an RTV silicone called Permatex Ultra Copper.  This RTV silicone is to be used in assembling the hot end.

Permatex Ultra Copper comes in a toothpaste tube with a plastic tip that can be attached to it and it's ready to use out of the tube.  Arctic Alumina is epoxy-like and comes in two tubes whose contents must be mixed with a little plastic spatula.


Arctic Alumina and Permatex Ultra Copper

I wanted to compare the thermal conductivity for the Ultra Copper and the Arctic Alumina but neither manufacturers was able to provide thermal conductivity measurements.  However, the technical support person for Arctic Alumina seemed very knowledgeable and explained that for the smallest gaps on the order of the thickness of a piece of paper, Arctic Alumina would perform better with respect to heat conductivity.  It terms of adhesion he claimed that Arctic Alumina would be permanent, while RTV silicone might crack after a while.  So I put my faith in his opinion and went with Arctic Alumina.

I found Arctic Alumina pretty easy to work with.  I mixed it on a piece of Aluminum foil with the provided spatula.  It's a pretty thick paste at that point.  You get about 5 minutes until it transitions from paste to rubber-like semi-solid.  At this point you can use an X-acto knife to scrape any excess.  After about an hour the bond is very strong - I was able to pick up the heat bed by pulling on the wires attached to the thermistor.



Thermistor has been attached to bottom of heat bed using Arctic Alumina 

Heat bed has been mounted.  Heat bed wires are visible on the left and thermistor wires on the right.

The last step in mounting the heat bed is to attach the wires to MDF.  I decided not to drill the two large holes in the MDF as was done in the video and pass the wires through them. The reason is that once you pass the wires through the two holes in the MDF, you have to pull them out through the same holes to remove the heat bed. So instead I drilled two 3mm holes and attached the wires to the MDF with tie wrap. This way, if I ever have to remove the heat bed, I just snip the tie wrap and remove the four lock nuts to free the heat bed.

Heat bed and thermistor wires have been attached to the MDF with two tie wraps

I went on to assemble the hot end.  This requires preparing another thermistor and the heat resistor by soldering wires onto both of them.  Then you attach both of them to the hot end.  Instead of using Permatex Ultra Copper as suggested in the video, I decided to use Arctic Alumina.

Hot end with thermistor and heat resistor installed

Finally I bent the wires and attached them with Kapton tape.

Hot end with heat resistor and thermistor wires neatly attached with Kapton tape.

This is how my Prusa Mendel looks at the end of video 13.


No comments:

Post a Comment