Casting Porosity and how much should be "expected"
Casting Porosity and how much should be "expected"
(OP)
I have a sealing system that has a casted (ductile iron) housing. This housing has one area that has always had porosity problems. Lately it seems (to me) that it's getting worse. The foundry that we deal with claims that it is the nature of the part and that they cannot improve on it. My question is, does anyone else have porosity issues and how do you "gage" the levels to ensure the quantity is not getting worse. Also, my part is not complex at all. Should I be sourcing other suppliers or is casting porosity really that hard to control?
Thanks,
Loosehead.
Thanks,
Loosehead.





RE: Casting Porosity and how much should be "expected"
Casting-quality has many variables which contribute to the quality of a part. The type of casting (sand, investment, etc.), the type of material being cast, the foundries own internal QC program (or lack thereof), etc. can all play into answering your post.
The foundries QC program may help do determine if the raw materials going into the mold are per your specifications and do not exceed any ranges you may have specified.
In a nutshell, gating and their locations within the mold often contribute to the the presence (or lack) of porosity. Well designed molds will reduce the presence and probability of pores. "Dead ends" or locations in the mold that confine or trap hot gases, air or sand often contain unwanted pores.
RE: Casting Porosity and how much should be "expected"
RE: Casting Porosity and how much should be "expected"
RE: Casting Porosity and how much should be "expected"
These can be detected by radiographing those regions which will indicate the extent and nature of defects.
I suppose you are needing hydraulic soundness in your casting. This is your requirement and the vendor must satisfy this. It is not possible to say that accept a leaky castings. No it is not done.
If there are no good ductile iron founders in your region,then convert this into a steel casting,you will have controlled some of these defects.
RE: Casting Porosity and how much should be "expected"
Is the porosity shrinkage related or is it gas porosity ?
Should you decide to source other suppliers, you may wish to seek out any foundries that utilise 'solidification simulation' software. MAGMAsoft is one such package - and if in the hands of an experienced operator should be able to identify and therefore remedy your porosity problem.
The foundries who do use this type of software will probably also use 3D modelling software eg Pro-Engineer, Solidworks etc. They may then be able offer suitable design change options for your casting, that may facilitate easier castability.
Good Luck
RE: Casting Porosity and how much should be "expected"
RE: Casting Porosity and how much should be "expected"
Porsosity control is a common problem and as metman says that an experienced caster with gut feeling can control this defect.
RE: Casting Porosity and how much should be "expected"
RE: Casting Porosity and how much should be "expected"
I would consider the reference radiographs noted by TVP as your reference standard. Define the maximum size pore, how much porosity is acceptable in an area (clusters) then define how much total (accumulated) porosity is acceptable. This acceptance criteria can be implemented on a random/percentile basis, then increase the frequency of testing if rejectable parts are discovered by your QC personnel. Any through-pores would be rejected.
Also, it could help those replying - to help you if the specific type of discontinuity is known such as porosity, shrink, etc. noted by others who have replied. Voids are not always porosity with castings, but I won't preach to choir.
IMHO:
I do agree with some of the statements made previously by some of the others, it sounds like your casting house is coming up with excuses when they should be trying to figure a solution as to keeping a valued customer satisfied.
On the other hand, if there was no acceptance criteria standard specified when the project was awarded to this casting house, then they have a righteous beef regarding "...that we changed the rules..."
RE: Casting Porosity and how much should be "expected"
RE: Casting Porosity and how much should be "expected"
I have just met with the foundry. He is stating that they could make a porosity "free" part but would require new tooling to do so. He claims that the cost of our part would substantially increase. I have a hard time believing and have decided to persue other foundries while working with existing to get us through until we start a steady supply from somewhere else.
I apprieciate all your help.
Loosehead.
RE: Casting Porosity and how much should be "expected"
A company I was working for switched from machined shaft couplings to casted couplings due to the cost savings. However, not long after the change, several couplings were breaking. It was attributed to a poor batch of couplings due to high porosity.
I simple porosity check that I did was as folows: a representative part, that was of know quality... obtained through x-ray... was weighed. The part was also placed it in a water jug, to measure the displaced volume. These numbers were then used as our benchmark. After all this, it was sectioned it to verify its quality. All subsequent parts recieved from the foundry were then weighed.. and if a significant discrepancy on the light side was noted.. its volume was checked. In some cases, we found parts with nearly equivalent volumes.. but 5-10% less weight... these we chose to x-ray... and found unacceptable porosity... similar to those that had failed in service.
Hope this helps with your problem.
jetmaker