mold rust.

[mjh368]

possible sources of rust on stainless steel components?

Wear.
Gassing from material or poor vents.
electolosis from poor gronding?? for real?

Am i missing somthing. we are having a rust problem and we cant use any sprays beacuse of medical/FDA.

 

[patprimmer]

How can wear cause rust on a solid SS component.

As this is in the plastics area, I presume you are processing some type of plastic. The type of plastic may have some impact on gassing. Can you tell us which plastic it is.

Does the rust form during use or during storage.

How can you get electrolysis in a plastics mould. The circulating water should ground it if the bolts holding it in the platen do not. A hint about mould temperature might help avoid wild guessing on my part.

Are you sure it is rust, or could it be just a brown stain.



Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[mjh368]

hey pat
sorry about my lack of info

the material is PP so i don't think that is the issue. mold runs 70F water with proper back pressure and RPMs.

We have the problem more in the summer than the winter and we have even seen the "rust" during dry cycles.

I agree that it might not be rust but just a brown stain or start of a burn. The temps and vents are good and the injection speed is on the slow side.

Bad steel is in question. but it has been an unsolved issue for someime.


I could wrong on the wear issue but where is it coming from in the dry cycle?

 

[Pud]

It's not fretting corrosion is it? (see Google for articles ad nauseum..) Really, without seeing the tool it's gonna be difficult to say the least.

Is the brown from an ejector pin hole?


Cheers


Harry

 

[mjh368]

sorry i guess my information relay skill on friday are quite poor.

The parts are sleeve ejected and when the mold goes to the shop you can see the stains all the way back on the sleeves. So when the sleeve ejects it stains the core and rear cavity.

I will check out the fretting corrosion...

 

[patprimmer]

Pud makes a very good point.

Is the stain only on the pins or is it on other surfaces with no metal to metal contact. It might be carried along a pin where the pin or sleeve pass through a waterway and the water seal is not perfect.

PP should not out gas anything that could corrode SS, unless it is Flame Retardant, which it will not be if it is for biological contact.

It is quite possibly a mould deposit from the lubricants, mould release or stabilisers used in the compound.

Vents should have a short land at a depth small enough to control flash, they should be at end points in the flow path and once past the land, the should be so big that they could not possibly block from crud and material carried out of the cavity by the gas.

They must vent to atmosphere.

Sometimes the fix can be as simple as wiping the parting line with a clean rag or paper towel at a regular interval, so as to clean deposites of the vent land.

Regards

eng-tips, by professional engineers for professional engineers
Please see FAQ731-376 for tips on how to make the best use of Eng-Tips Fora.

 

[Pud]

You will most likely find that the ejector sleeves are not stainless. You could try:

a) Some PTFE spray dry lubricant.

b) Using Titanium Nitrided (TiN)pins/sleeves.

If you are in a high summer humidity area it may just be plain old condensation causing rust on the (non SS) pins. TiN coatings are corrosion resistant too.

I have never come across ss pins - even in ss tooling (UK)

Cheers


Harry