KEYS TO SUCCESS IN SEMI-PERMANENT MOLD CASTING
Sand cores in permanent mold aluminum castings create challenges that call for experience, persistence and creativity. BOB BRAUN, WISCONSIN ALUMINUM FOUNDRY CO., MANITOWOC, WISCONSIN; RANDY OEHRLEIN, CARLEY FOUNDRY INC., BLAINE, MINNESOTA; DAVID WEISS, ECK INDUSTRIES, MANITOWOC, WISCONSIN
D
eveloping a success- ful, repeatable, reliable process for pouring aluminum into a perma-
nent mold is no simple task. Te many variables include metal and mold tem- peratures, shrink factors, mold coatings, casting cycle times and melt quality. Considering the numerous factors that influence quality and quantity, intro- ducing sand cores to permanent molds only complicates matters. Beyond the obvious issues of cost
and binder material, a sand core can slow the heat transfer (and extend solidification time for the casting), change shrinkage factors for the cast- ing, introduce potential core coatings and require additional venting for gases in semi-permanent molds—so- called because of the disposable core. Additional issues arise in handling cores, such as trimming, insertion and removal from the metal mold. Sand cores also require added tooling, which includes validation and maintenance.
Te corebox itself is only one concern. Tooling includes trimming methods, core assembly fixtures, transport mate- rials, material handling components and verification fixtures. Te core’s effect on dimensional
accuracy is a critical consideration for components with tight dimensional requirements. One determination for any given casting part is whether the mold or the sand core will have the more significant influence on dimen- sional shrinkage. Among the deter- mining factors are the core material, resin percentage, casting size, geom- etry, wall thickness and whether the core is knocked out after solidification or remains in the casting during cool- ing. Additionally, some castings may include axis-specific shrink factors.
Gas and Venting
Figs. 1a-b: A metal ball detent pin (right) and bushing can help lift and set heavy cores (left).
36 | MODERN CASTING September 2013
All sand cores generate gas due to binder decomposition when contact- ing molten metal; the amount depends on the binder system and coating. Various core binders not only generate different amounts of gas, but also show different gas evolution profiles (i.e., when and at what rate gas is released). Te gas amount and its evolution rate are affected by binder type, melt tem- perature, core geometry and process-
Page 1 |
Page 2 |
Page 3 |
Page 4 |
Page 5 |
Page 6 |
Page 7 |
Page 8 |
Page 9 |
Page 10 |
Page 11 |
Page 12 |
Page 13 |
Page 14 |
Page 15 |
Page 16 |
Page 17 |
Page 18 |
Page 19 |
Page 20 |
Page 21 |
Page 22 |
Page 23 |
Page 24 |
Page 25 |
Page 26 |
Page 27 |
Page 28 |
Page 29 |
Page 30 |
Page 31 |
Page 32 |
Page 33 |
Page 34 |
Page 35 |
Page 36 |
Page 37 |
Page 38 |
Page 39 |
Page 40 |
Page 41 |
Page 42 |
Page 43 |
Page 44 |
Page 45 |
Page 46 |
Page 47 |
Page 48 |
Page 49 |
Page 50 |
Page 51 |
Page 52 |
Page 53 |
Page 54 |
Page 55 |
Page 56 |
Page 57 |
Page 58 |
Page 59 |
Page 60 |
Page 61 |
Page 62 |
Page 63 |
Page 64 |
Page 65 |
Page 66 |
Page 67 |
Page 68