Air Set Sand Mold

Surface quality of the parts being produced are affected by this tensile test along with the voids that are being produced by the gating structure of the mold. I would discuss this issue with the molding/tooling engineer at the location where your having parts made. I don't think I would be too concerned unless you are seeing issues with products coming from this location.

Also I might ask if there was a specification given on this at the time orders were placed by purchasing agents. Most of the time engineering and purchasing work together to form specification before orders are placed.

One of the biggest issues is with dimensional stability. Often times the machine shop is scrapping parts becasue the cast features are not located properly and I was wondering if the sand is soft that they may be distorting the sand when they remove the pattern.

I'm not sure if low tensile strength of the sand equates to a softer mold. Is there any correlation there?

Dimensional stability is very much related to a weaker sand. Mold wall movement or collapse will result from weak sand. In green sand the green compression gives the relative strength of the sand whereas in non-bake or air set systems the dog bone is utilized. You should require the foundry to do dimensional sampling of your product. Take 4 critical dimensions and insist that they be measured prior to shipment.

That large a difference in the tensile strength is indicative of either poor calibration of the pumps or severely worn mixer arms resulting in poor mixing of the sand, binder and catalyst. Sand temperature would affect the stripping and set up times but shouldn't affect the tensile unless they test before the mix is completely set up.

A "soft" mold would only occur when there are areas that have little or no binder or catalyst but a "weak" mold can exist even if the mold has completely set up since it requires a sufficient amount of binder to provide the strength whereas it needs enough catalyst to set up within a predetermined amount of time. Insufficient catalyst or cold sand generally just means a longer set-up.

Thanks, this is what I suspected but needed to confirm. I asked them if there was a number that they were looking to hit in regards to tensile strength of the sand but they could not provide one. Is there an industry standard for tensile strength of no-bake sand? Is higher better or do you run into other problems if strength is too high?

It sounds like measuring the hardness of the sand prior to pour will not provide the control that is needed, correct?

A good resource is the American Foundry Society.

I am attaching a thread to a study done:

http://www.cerp-us.org/reports/PDFs/1413-145%20Inorganic%20Properties%20testing%20PUBLIC.pdf

As this shows, a lot depends on the binder being used, the time the test is time after the dog bone is made and the humidity which has a major effect on the strength if it sits for a while.

Different metals will require different strength characteristics. Less binder is used for aluminum than for bronze which would use less than iron or steel. The 30 seems way too low. If the 100 produces dimensionally stable molds then that should be the minimum allowed.

The industry standard is basically the minimum amount of binder required to produce a dimensionally stable(no mold wall movement), non-porous casting without burn in, veining, scabs, buckles or other expansion defects.

Thanks all for your input. Greatly appreciated.