PVC Extrusion Die Chrome Delamination

You say it's burning. Or is the surface coming out scorched or discolored as it wears the base metal.

The burn is observed as chunks of hardened (not quite burned) material coming out with the plastic specks to chunks of black burned plastic coming out with the plastic. Also when we open the die for cleaning we can see the burned plastic exactly matches the section of missing chrome.

I am thinking it is burning because of the heat transfer through the base metal is higher than the chrome and there is a much thicker non slip layer due the greater friction of the base metal.

The cause of the burn is irrelevant (but interesting) what we need is a temporary fix for the die...I am just afraid there may not be one.

Drew K

whats the temperature on the die set to

We vary the temperature at the die around 305 to 315 depending upon conditions, other sections of the extrusion system are set in the 330's. I am pretty confident the burn is happening due to the excessive friction of the base metal and disruption of flow at that point.

Could I possibly coat the delaminated section with a high temp polymer (epoxy) to decrease friction and insulate the section?

Drew K

You need to check these temperatures they are too high for PVC. PVC starts to soften at about 200°F. We don't extrude here but thermally weld. Plates over 250°C and the material starts to scorch. What I know about extrusion dies that the die temperature is usually hotter then melt temperature.

Could try a ceramic or silicone coating. Don't know how it will hold up PVC can be pretty abrasive.

Chrome is usually spray welded on. The thickness can be in thousands of an inch. Will usually wear smooth unless poorly done.

Can you check the surface texture- is it possible to polish the bare metal?

With the heat sensitivity of PVC, I wonder if the base metal has a rougher texture and is creating a lower flow zone, that exposes the adjacent flow layer to a longer heat history (and more surface area with the roughness). Unless you have a significant difference between the melt temperature and the die temperature it doesn't seem right to me that the loss of a chrome layer would allow such a difference.

Lyn might have dealt with this in the past.

It's a new one on me too. Sounds like heat or friction. Surface roughness or different heat in the un-plated areas sound plausible.

Don't think an epoxy will hold up. Polishing may help.

Do these areas of the die run hotter when things are normal?

I don't see much hope.

Different heats/extrusion rates probably won't help??

I don't have any experience extruding plastics, all my experience is with rubber extrusion but maybe some of my experiences can help?

You already know that you will need to re-chrome your die.

Here are some thing to think about:

1. In my experience, when the extrusion material touches the die in an area that has lost its chrome, it has a tendency to stick on the base metal where it sits and scorches and burns and then transfers to the moving extrusion.
2. Polishing the area may help, however it wont last long!
3. What is the cost of building a second die that can be swapped out when the chrome de-laminates on the first die?
4. I suggest taking a little time to closely inspect the area where the chrome is being damaged, is there a sharp corner or edge that is "starting" the de-lamination?

Best of luck. Bryan

http://www2.dupont.com/Teflon_Industrial/en_US/products/product_by_type/coatings/index.html

Several products available.

http://dixiepackingandseal.com/productdetails.aspx?id=262

I'd contact Dupont directly and ask them.

For a very temporary "fix" I have had some success with Teflon Dry Lube available in an aerosol spray can from Dupont, CRC and others. It probably won't stand up very long but it is easy to reapply. I use it as a release coating for thermo-forming dies running over 400º F. Of course, clean and polish the problem areas first.

And Product Item Detail | Miller Stephenson MS-122AD / MS ...

But with the extrusion process I see no chance of this working.

They'll send you a sample can for free.

Here is a link to a picture of a similar die. The chrome is gone from the left side corner and an irregular shape about the size of an American dollar bill just inside the landing (flat area near the lip) on the left side.http://www.exportersindia.com/technology-plastomech/extrusion-dies-india-625223.htm

The melt temperatures are similar to what we use on the other two machines and consistent with procedures that do not result in burning product. The more I read, the more I am convinced that the burning is happening because of the friction slowing the flow. I have not found any articles or information on the difference of heat transfer through base metal vs through base and chrome.

Drew K

What is the base metal? Why did the chrome delaminate? Was it friction, a pinch point, wear or a bad chrome job to the wrong type of base of base metal?

I seriously think I know of a fix to your problem but I have to ask my boss if I can tell you.

And the base metal material type is important, what is it?

I just started here so I am not certain why it delaminated. I don't think it is a pinch point, it could have been debris in the plastic or due to improper shut down leaving pvc to degrade and emit corrosives.

The base metal is either carbon or stainless steel.

Drew K

There are many types of mold steel. It's probably not stainless.

There is always a possibility of poor initial adhesion.

No silver bullets here.

I was really hoping for a spray or epoxy that I could apply so we could get some orders out before sending the die out for repair. I did contact someone about teflon coating company, but that would take as long as getting it repaired. At the power plant we used polyurea to repair cooling tower pipes, it would be nice if there were a silver bullet for this one too.

Drew K

H-e-a-t.

A-b-r-a-s-i-o-n.

P-r-e-s-s-u-r-e.

PVC does not evolve benign gasses when processed either.

Polish it and see what happens.

Yeah, it makes my nose hair shorter every time we purge out and start up. Not a mental image you wanted but there ya go : )

Drew K

These type of dies I have used very extensively. There are several reasons that one might apply chrome to the inside surfaces of the hanger area. One is to repair worn area and the other is to try and supply a smother run out area with out the normal die preparations it takes to supply a quality die surface.

I would suggest the following be completed before doing anything to the die. Check the controllers on the barrel heaters, the down spout, screen changer and the neck to the die. I would then after confirming that the temperatures are at set point. Check the heaters elements in the die to make sure they are correct wattages in all sections on the die. You can have a larger wattage heater in the die, in and area that is away from a thermocouple well and cause this type of issue as well. Then move on to the die gap and gas entrapment issues.

These issues can be hard to find on these sheet dies, but I would guess that this is not the first time your company has seen them. Dig back into the records from operations and see what the history shows.

You PM Me and I can try to help you further.

Fixit.

There is a product made from nano ceramics called "Cermalube" or something to the like. It penetrates and coats the metal and reduces friction but more important nothing can build up on it so it is claimed. Cerma Industries is the name I just looked it up. I do not endorse but I do have it in my car. I also know some gun users that make use of it and no feed back either way. Supposedly it is technology from NASA and space program.. Take that for what it is worth.

I also doubt your base metal is SS it is probably a heat treated hardened alloy or at least it should be. And different base metals bond differently with different coatings.

It should not be difficult to find out what your die is made of if you know where it was purchased from. Or who ordered it originally.