Affixing a heated bed

[codemaven 0]

Hi all,

I'm sure this has come up before.... But I can't seem to find anything. My question for you folks with heated beds is how do you attach them to your UM?

I'm planning to use the existing method of springs and screws into the delrin nuts, but I'm worried about the durability of the delrin parts with a heated bed. I'm using teflon washers and bushing sleeves on the bed to avoid heat migrating down the screws as much as possible, but I still expect that during long prints the screws going into the delrin will heat up. The listed melting point for Delrin is 175 degrees C. I don't expect to ever go that high on my bed and consequently the screws should keep much cooler... but will they potentially soften the delrin to the point that the springs can push them out?

I've been planning to put together a heated bed for some time... but I just got around to ordering parts. I'm going to go the route of a 220v mains heater on an aluminium bed. I'm experienced enough with mains electronics to handle it, but I still recommend others to take the low voltage approach. Instead of a silicone heating pad though I've opted for a 400W mica heater plate. It will be controlled by a 5A ssr driven from the UM hot bed mosfet. I'm planning to have a 200 degree NC bi-metallic cutoff switch in series with the heating element as a safety feature. I'm also going to connect an LED to a NO 50 degree bi-metallic switch so that the the LED is lit whenever the bed is over 50 degrees C. My plan is to power it from a 3F ultracaps (memory backup cap) that is kept charged from the +5v on the thermocouple output. The idea then is that the HOT BED warning LED will still work if the power is turned off. If the ultra-caps and +5v thermocouple supply don't work out as planned I'll fall back on a battery backup for the LED.

Cheers,

Troy.

[Daid 306]

You don't have to fear for your derlin parts, the screws are stainless steel which is a bad heat conductor.

Putting a ultracap in direct series with the 5V might cause a huge current spike on startup, which could be bad for the electronics, better to put a resistor in front of it to limit the initial current to the ultracap.

Other then this, your idea sounds fine!

[codemaven 0]

Thanks Daid,

Ultracaps have pretty high esr, so the current spike when they're fully discharged should be small and very short. None the less, I plan on putting a small resistor in series with it to limit the maximum current draw.

Regards,

Troy.

[joergen 2]

Instead of a silicone heating pad though I've opted for a 400W mica heater plate. It will be controlled by a 5A ssr driven from the UM hot bed mosfet. I'm planning to have a 200 degree NC bi-metallic cutoff switch in series with the heating element as a safety feature.

I would suggest to use a 145-160C bimetal cutoff switch. 400W may or may not reach 200C in a runaway event, but even if it reaches 200C it is too uncomfortable high for the machine (wood starts to suffer, oil in the bearings dries up, the grease on the Z rod melts or bakes in). Cutting it off much earlier (i.e. 145C) prevents some of the nasty side effects. My 180W heater will reach 135C uncontrolled, which is 20C over the max temp you need, and already feels uncomfortable hot, especially if you want to leave you bot unsupervised. I personally would feel very uncomfortable having a bot baking at 200C, and all the PLA/ABS melting on the HBP and flowing down like a slow volcano of molten plastic :-) although it would make a great art installation: printing a part, and destroying it afterwards right away: Joris?

[codemaven 0]

Thanks for your input Joergen... You're right. I originally was going to get a 160C cutoff switch, but opted for the higher one instead to give more potential operating range... but as you point out 200 is actually way too high, I would never need more than 110-120 in reality. I think I will get a lower temp switch.

These things do have quite a bit of hysteresis built in. It switches the power off at 200, but will not switch it back on until it cools down to about 150 I think..

Cheers,

Troy.