How to demagnetize a 1m^3 injection mold? 2

[JAFDias]

Hi there.

I'm currently dealing with an magnetic issue in one of my 700 plastic injection molds.

It's a huge mold (arroud 1 cubic meter volume).

Can you give me a "simple" and economically interesting way to release that amount of steel from magnetic forces?...it's getting difficult to deal with all the magnetic sensores inside it to manage the movement of the parts. They became very unstable and security has been compromised.

The is always the change to unmount the mold into pieces, although the 1mx1m section is always present in the plates of the mold.

Thanks.

Best regards

RDias

 

[MagMike]

Unfortunately, there is no simple or cheap way to elminate the magnetism in something that large. The only two ways to do it are by heat-treating or demagnetizing by an alternating current. The heat-treating will probably change the mechanical surface treatment of the mold, and it would expensive to demagnetize by alternating current.

Mike

 

[EdStainless]

How much would enough welding cable cost to wrap the entire mold (in one direction)? Setting up a decaying AC field isn't that difficult. It is a major project, but easier than taking the mold apart.

= = = = = = = = = = = = = = = = = = = =
Plymouth Tube

 

[JAFDias]

The mold has,round numbers, 1mx1mx1m (1m^3) dimentions. Even using only one layer, i should need days to wrap the cable arround the mold.

Indeed, it were a possible solution. Although it's a big mold, and apart from the natural difficulty to do that, how many Km of cable would i need to achive the necessary oposite magnetic field? Too many, right?

I'm considering changing the sensors type. Passing from magnetic sensors to mechanical actuation (switch) and still use a magnetized mold. The field is stabilized. Hasn't been improving power or so ever... so... worst shouldn't be (I hope). What do you think about that solution?

 

[IRstuff]

The objective would NOT be to "achi[e]ve the necessary op[p]osite magnetic field," but to DEmagnetize it by using an alternating current.

I'm a bit unclear why or how your mold got magnetized in the first place. The usual steels that are used normally would not be magnetizeable.

Rather than mechanical sensors, which will reduce the reliability of the overall system, you might look to see if there optical or ultrasonic proximity sensors that could be used.

TTFN

FAQ731-376

 

[JAFDias]

Thank you for the gramatical (is it well writen?)corrections in the 1st place. I don't write in english so much.

The objective is precisely what you said. Maybe i don't expressed the right way.

Plastic injection molds are made in standard steels, like CK45. In this case here are some more noble materials like AMPCO 83 but just in some parts. The main structure of the mold is in CK45 and pre-treated stell like PM300.

During milling the parts in CNC, electro-erosion, planar rectification or even in convencional machining, magnetism is induced to the steel. However, depending of the amount of removed steel from the gross plate, this efect appear more or less.

During injection process, due to the slide and movements of the mobile parts, the effect may be improved. Even due to the plastic it self, elecrostatic energy is seen regulary on the injected pars.


Refering to the sensors, optical sensors are not suitable to this particular aplication. They would have to be mounted in an agressive ambient (surrounded by lubrication grass, oil and plastic gases. Ultrasonic ones, i don't know their behaviour that much to tell if it would be possible to used them. I'll try to search something about it with my current suppliers.Appreciate the sugestion.

Best regards.

RDias

P.S. - I'll have to ask you for some pacience about my writing.

 

[BobM3]

During milling the parts in CNC, electro-erosion, planar rectification or even in convencional machining, magnetism is induced to the steel. However, depending of the amount of removed steel from the gross plate, this efect appear more or less.

I haven't heard of this. Do you know of anything on the web that talks about this?

 

[JAFDias]

There must be somewhere. I can't help you with that.

Is just something i know from experience.

 

[Clyde38]

JAFDias:
Maybe this company can help you, Diverse Technologies.

http://www.diverse-technologies.net/

[link

http://www.linkedin.com/in/clydehancock

]

[/url]

 

[Skogsgurra]

I need to understand one thing: Are the sensors you mention magnetic reed switches or are they proximity switches? If they are proximity switches, you should not have a problem with the steel being magnetic. If they are reed switches, yes perhaps. But the problem with reed switches are more often that they tend to weld together and stay closed if not protected by a series resistor.

So, my questions are:
1. What type of sensors do you have?
2. How do your sensors fail? Do they stay closed or do they stay open?
3. Do you know that the steel is magnetized - or is it an assumption by you or a colleague?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[JAFDias]

Hi there.
Thank you for your post.

I don't know for sure what kind of sensors do i have, i mean, i never gaved my self the work to search for the characteristics.

The sensors remain open during all movement, but when i remove the cilinder from the mold and make it move outside.... they work fine.

Yes. i know the steel is magnetized. A screw drive was the test part for knowing it. It glues to a vertical wall.

Regards. RDias

 

[Skogsgurra]

Quite often, the screwdriver is the magnetic part. Does an ordinary nail stick to the wall also? All nails?

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[JAFDias]

Nail? I supose you don't want to refer to "finger nail".

The screwdriver wasn't magnetized. We tested it in other metalic parts, away from the mold.

 

[Skogsgurra]

I hope that you understand what I mean. In a case like this, it is important not to use the biggest gun you can find before finding out what the real problem is.

The big gun would be a huge coil with a low-frequency high-amp decaying AC current flowing through it. But, if your problem is something else, I think it is a good idea to find that out before building the big gun.

One step towards a better understanding is to know what type of sensors you have. If they are proximity sensors, then they are not influenced by surrounding magnetic fields.

I have done demagnetization of objects that size. You need to apply a heavy current at a low frequency. 50 Hz does not penetrate the steel very well because of eddy currents counteracting the field. We use a four quadrant thyristor rectifier and control it from a function generator set to around 1 Hz. We start with high current, usually around 150 A peak and reduce it linearly down to 0 A over 20 - 30 seconds.

It works quite well, but it does cost and it is not easy to find someone to do it (if you are not close to us).

Discharging a capacitor in a coil wound around a piece of steel that big does not work very well. Mainly because it is very difficult (need a very big capacitor) to get down to the right frequency and also because the energy is consumed by the eddy currents before the steel has been completely demagnetized.

Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[Skogsgurra]

"when i remove the cilinder from the mold and make it move outside" Does this mean that your sensors are fitted to pneumatic or hydraulic cylinders?

How close to the mould are they when the cylinders are fitted to (or in) the mould?

Gunnar Englund

http://www.gke.org

--------------------------------------
100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[JAFDias]

Hi Skogsgurra. Once more, many thanks for this important replies.

Answering to your last question: Yes the sensor(s) are mounted in hydraulic cilinder(s).

They are actually mounted inside the mold with steel all arround and behind. In the front is screwed to 4 cilinders a steel plate to do the extraction of the injected plastic part.
_ | | _
| _|_|_ |
| | | |
| | | |
|_|_____|_|

_________
| _____ |
| |º º| |
| | 0 | |
| |º___º| |
|_________|

Something like this graffity.

 

[Skogsgurra]

OK. Then you will probably not have a problem with your mould being magnetized. If your cylinders are surrounded by steel, there will be no magnetic field from the steel that can influence the sensors.

The reason is that the iron in itself has a much higher permeability than air and even if your mould is highly magnetized, the interior magnetic field will be 'conducted' around any cavity inside the iron and will not be able to influence your sensors.

If your sensors, however, are situated close to the iron, the iron in itself will shunt some of the field coming from the inside piston and your sensors will remain open.

It is usually possible to readjust the sensors so they are close to the cylinder wall and away from the mould's steel. That will reduce the shunting effect of the iron in the mould and increase the influence from the internal magnet. I recommend you to try that.



Gunnar Englund

http://www.gke.org

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100 % recycled posting: Electrons, ideas, finger-tips have been used over and over again...

 

[JAFDias]

OK. Understood.

However, the problem is precisely the gap between stell and cilinders. They are quite closeto walls, almost touching. The effect produced is exactly what you mention. There is influence of the magnectic field of the steel with the one created by the internal magnetic of the cylinder.

In a first approach to the problema we reduced the gap from that internal magnet to the sensors by machining the cylinder wall in order to create a lower slide zone where the sensors are mounted, but now...we cannot reduce it any more. It is thin enough.