Removal of faulty heating cartridges 1

[Speedy]

Folks,

I have a problem with the removal of faulty heating cartridges.

These have being in place for less than a year. The heated material is the core of an aluminium mould. There is no ‘through hole’. It is an experienced contract machine builder that is doing the work. They believe that it is a build up of aluminium oxide that is bonding onto the element.

They tried to drill out the centre of the element and tapped into its outer wall. When pulled, it simply pulled the top inch or so out. This got a good laugh at the time. We also tried heating the core but the difference in thermal expansions is only minute. One guy thought we might be able to heat the core and cool the element with liquid nitrogen. Drilling a through hole might be our only option but is very messy. We would have to back fill this every time we remove an element.

I am aware that there are conductivity enhancing pastes but the contractor has found these to bond also over time.


Any ideas as to how we could remove these and prevent this from happening in the future?

Appreciate any help!!

Speedy

 

[Metalguy]

What is the ID and material of the element?

If it's big enough to get a welding electrode inside, just run a bead lengthwise as far as you can. When that bead shrinks during cooling there won't be much holding it in.

Try coating the OD of the new one with milk-of-magnesia (Mg oxide).

 

[unclesyd]

We use several thousand of heating cartridges in all sizes of Al blocks. We operate at 600°F ± 25°F for periods up to a year or longer.

We do several things to facilitate removal.
The elements are wired to prevent them from over shooting the set point.
We drill thru if possible with a slightly larger hole than recommended.
We use a split heating element.
The most successful lubricant has been a very light coating of Therm-A-Plate 220.

We have trouble removing less than 5 % of the elements during overhaul. We have improved the block insulation to the point that we don’t remove the elements if they check good. The loss of several elements want change the temperature that much.

Them-A-Plate

http://www.socousa.com/showprod_detail.asp?pid=8O4&type=cat&id=2&prid=102

 

[ccw]

Hi Speedy,
Here are a couple of things we have used to remove pesky stuck cylinders in blind holes:

1. cross drill the aluminum making a small conduit into the bottom of the blind hole. Seal off the element at the top, making it a closed piston. Pump in and pressurize thick grease into the small hole and push the element out from the bottom using the hydraulic force. (The element becomes a piston in a cylinder.) A variation on this would be to pump the grease down to the bottom of the blind hole using the center bore of the heater element.

2. Drill out the center of the faulty element. Heat soak the element and aluminum up to elevated temp say a few hundred degree fahrenheit. Shock chill the center length of the remaining element structure with LN2. Note: Transfer of the fit between the element OD and the core ID between interference to clearance, back to interference, can be a few second phenomenon as the transient radial thermal wave crosses the faying surfaces, so keep a prevailing withdrawal force on the element while pouring in the LN2. Wear protective equipment as the LN2 will boil and spit back out the hole.