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Melting hot plate connector 2.5 upgrade  

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stephen.f2
(@stephen-f2)
Eminent Member
Melting hot plate connector 2.5 upgrade

Hi all,

I was doing some testing after doing the 2.5 upgrade. I noticed the hotbed wasn’t heating so I moved the cable and all was well. I should have stopped then and investigated the fault. However about 2 hours into a print I noticed the hotbed connector was melting

On investigation I discovered that when I connected the hotbed to the cable I mistakenly installed the washer between the cable connector and the hotbed PCB on the VCC line

This caused two problems, firstly there was insufficient contact between the PCB and the connector which came loose and caused an intermittent contact which eventually came loose. Secondly there was insufficient thermal connection between the connector and the plate. So the excess heat melted the cover

To fix it I took it apart and noticed that the washer had caused some corrosion on the pad

To clean it I used a cotton bud and some PCB cleaner. Then I put some thermal paste on the pads to ensure a better contact and replaced the connectors properly.

It just goes to show that one must pay close attention to the build instructions.

Napsal : 16/12/2018 8:36 pm
stephen.f2
(@stephen-f2)
Eminent Member
Topic starter answered:
Re: Melting hot plate connector 2.5 upgrade

Apologies for the repeated photos, I’m using an iPad and have yet to learn how to use it properly with this app

Napsal : 16/12/2018 8:39 pm
thrawn86
(@thrawn86)
Honorable Member
Re: Melting hot plate connector 2.5 upgrade


Apologies for the repeated photos, I’m using an iPad and have yet to learn how to use it properly with this app

I might be misunderstanding, whats the point of the thermal paste? even a "conductive" paste like AS would be worse than metal to metal contact.

I'm not really a huge fan of their newer design and trying to use a ring terminal clamped on to the pcb.

Napsal : 16/12/2018 8:55 pm
stephen.f2
(@stephen-f2)
Eminent Member
Topic starter answered:
Re: Melting hot plate connector 2.5 upgrade

Hi Curtis,

I used thermal paste because it eliminates any small airgaps between the faces and inhibits corrosion. Given the crimp and screw method of connection there will be gaps. When I get round to it I’ll remove it and solder the connectors directly or use silver loaded epoxy. I really don’t like the use of crimp connectors and screws. The movement of the print bed is bound to work them use eventually.

Regards,

Steve

Napsal : 16/12/2018 10:15 pm
Olef
 Olef
(@olef)
Prominent Member
Re: Melting hot plate connector 2.5 upgrade

Remove the thermal paste and clean all surfaces with isoprop. This is not an area which should have any form of contamination.

Napsal : 17/12/2018 8:35 am
Bunny Science
(@bunny-science)
Noble Member
Re: Melting hot plate connector 2.5 upgrade

Lest the eleictrically naive get into trouble following the OP's advice.

DO clean the connector pads and terminals to remove all oxidation

DO torque the bolts to make tight, solid connections

DO NOT add thermal paste or conductive epoxy - those have no role here. You're trying to make a good metal to metal electrical contact, not transfer heat from bed to wire. Neither thermal paste nor conductive expose will assist in making a better electrical contact. They just contaminate the connection.

At most - you can use some DeOxit on the contact surfaces to prevent oxidation, but even that is dubious. Clean metal to metal contact with enough force to prevent oxidation is what you want.

DO NOT solder the wires. Prusa intentional changed to the crimp/bolt connector because soldered on wires suffered fatigue failures with the constant motion. A soldered wire has a short flex length right at the end of the solder. The crimps yield a longer flex length because the strands are not rigidly locked to each other.

Napsal : 17/12/2018 5:18 pm
JoanTabb
(@joantabb)
Veteran Member Moderator
Re: Melting hot plate connector 2.5 upgrade

Maybe I am seeing things, but the discolouration on the pad appears to be perfectly round, suggesting that the stainless washer was between the copper lug and the copper pad on the Heatbed...

Stainless steel is a poor conductor. it should have been between the socket screw head and the lug, to spread clamping pressure.

regards Joan

I try to make safe suggestions,You should understand the context and ensure you are happy that they are safe before attempting to apply my suggestions, what you do, is YOUR responsibility. Location Halifax UK

Napsal : 17/12/2018 5:49 pm
stephen.f2
(@stephen-f2)
Eminent Member
Topic starter answered:
Re: Melting hot plate connector 2.5 upgrade

I’m sorry if my original post mislead people. The fundamental point is to make sure that the washer is above each connector not below. If you don’t do this you risk the same failure as I experienced. I made a mistake in suggesting the use of thermal paste. I confused good thermal contact with good electrical contact, the latter being what’s required. I apologise.

However I’m really not happy with the arrangement for the connection. The forward and backward motion of the hot plate and the vibration of the plate will eventually work the bolts (screws in America) loose. It might be best to install anti vibration washers to prevent this and/or to check the connections are tight as part of regular maintenance. Or use locktite.

Secondly, in my experience, using crimped connectors in joints subject to constant movement will eventually cause the joint between the cable and the connector to fail. It’s worth periodically checking the crimps on the cable,

Regards,

Steve

Napsal : 18/12/2018 10:05 am
LA 3D Printer Repair
(@la-3d-printer-repair)
Member
Re: Melting hot plate connector 2.5 upgrade


However I’m really not happy with the arrangement for the connection. The forward and backward motion of the hot plate and the vibration of the plate will eventually work the bolts (screws in America) loose. It might be best to install anti vibration washers to prevent this and/or to check the connections are tight as part of regular maintenance. Or use locktite.

Secondly, in my experience, using crimped connectors in joints subject to constant movement will eventually cause the joint between the cable and the connector to fail. It’s worth periodically checking the crimps on the cable.

The moving portion of the bed harness is retained on both sides by the "clamps", there is no cable motion beyond that point - the crimped connections are not under any mechanical stress if the machine is assembled correctly.

We just had a customer this week that also assembled their bed incorrectly and had a melted connector, although in their case they simply installed the crimped lugs on the bottom, and the stainless fasteners were insufficient conductors and generated enough heat to melt the printed part.

Napsal : 21/12/2018 12:53 am
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