Hi,
I am developing an automotive cylinderhead for which I am doing all the foundry work for the prototype. Iam having some pretty serous issues with core blows mainly from thewater jacket. I have developed casting experience/processes which Ihave used to produce other simpler, cored castings, with excellentresults.
1st Casting
The casting alloy used was scrapre-melted cylinder heads (so probably alloy 319 maybe 356). A quality319 ingot will be used once I get the process down. Plenty a flux wasused and degassing was preformed by bubbling argon gas injected tothe bottom of the crucible. I used a powder bed 3-D printer to makemolds/cores with a home brew plaster based powder concoction and hadoverall disastrous results partly because the molds did not staytogether.
2nd Casting
I then used traditional patterns/coreboxes and # 85 silica sand for both the molds and most of the coresusing Pep Set three part resin for the binder. For the water jacketcore I tried 3-D printing again to avoid making a core box do to itscomplexity. The results were disastrous due to extreme core blowsfrom the binders I used in the plaster. Here is a picture
3rd - 4th Casting
In these iterations I made the waterjacket core using a traditional pattern and #85 sand and sodiumsilicate binder set with CO2 gas. I used sodium silicate for thewater jacket core in hopes of an easier core shakeout as it somewhatdissolves with water. For the other mold and core pieces I used thesame #85 sand and Pep Set binder. The results were still disastrousas it was evident after inspection that the top vent holes andsurfaces solidified first as expected but the core gasses in thewater jacket continued to build displacing the still molten metal.Here is a picture of the water jacket core and the casting aftermachining.
5th Casting
This time I tried using sodium silicateas the binder for all the mold and core pieces setting it with CO2gas. I had a little better results but still unacceptable core blows.The pour temperature was 1350 deg F. Here is a picture of thecasting.
6th Casting
This last casting to date I used #30silica sand for the water jacket core to improve gas permeability andmade a gasket out of ceramic insulating blanket and placed it on thecore prints to help prevent metal from blocking their ventingproperties. I also poured at 1200 deg F to give less time for coregasses to build.
This time I did not get any core blowdefects on the top surface (as seen by non-destructive visualinspection). However, this time I did get core blows on the bottom ofthe casting right were the bottom feeding gates are, which wasunusual and did not happen with previous castings.
During this casting as well as some ofthe others, there were some gasses that burbled up from the pouringsprue during and after pouring. One theory I have is that it did notresult in any core blows in the bottom portion of the casting forprevious castings because the higher pour temperature for the othersallowed the metal to remain molten and gasses to rise to the top.
Here is a picture of the casting withthe runner and sprue attached. The cross section of the runner andgates are about 2" x 1.25" and 1" x 1.25"respectively.
Since the bottom of the head is thethickest the idea was to feed the casting from the bottom so as tosetup a top down directional solidification with a large crosssection runner that would solidify last and act as a feeder. This hasapparently been working as I have not had any issues with shrinkageor hot tearing (except one area that has some slight shrinkage andtearing which I am confident can easily be remedied by a simpledesign change - reducing cross sectional area in that spot). I amopen to other gating ideas. I placed ziped .stp file of the moldassembly on my website here [linkLink]404 Not Found[/link]. There is aslight difference in that the cross section of the runner in the CADmodel is oval and the actual pattern I made is rectangular.Additionally the water jacket core in the .stp assembly is notcorrect. The correct one is the waterjecket_sculpted_final.stl.
Any advise on what to try next orresources to learn from are welcomed. I am stumped as to what to trynext. Here are some ideas I have.
-Bake the water jacket core to get ridof any residual moisture.
-Use another type of sand binder. If sowhat?
-Use a courser sand for the outer moldsfor additional gas permeability. Size suggestions?
-Add additional core prints for thewater jacket core on the sides to aid in venting. The designconstraints will not allow for larger or additional core prints onthe top.
Thanks,
Adam