PERMANENT MOLD CASTING PROCESSES Many variations of the permanent mold process are well-suited for mass production of high-integrity light metal castings for automotive components. This article is based on “High Integrity Permanent Mold Casting Processes: Current and Future,” a presentation at the American Foundry Society’s 6th International Conference on Permanent Mold Casting of Aluminum and Magnesium. J. L. Jorstad* JLJ Technologies Inc. This Chrysler NS cross member was cast on a tilt permanent mold machine. Richmond, Virginia However, with the incorporation of ceramic-foam filters and their ability Permanent mold casting consists of several basic to smooth melt flow (Fig. 1), opportu- processes. In this article, key characteristics of nities become available to top-pour each will be considered in terms of their impact with significantly fewer entrapped ox- on high-integrity products. ides and other quality detractors. Turbulent flow Combining filters with down-sprue GRAVITY FILLING PROCESSES and runner designs proposed by Prof. Gravity pouring, whether manual, via auto- Campbell has made it possible to pour ladles, or robotic pouring, can be susceptible to reasonably high-integrity aluminum turbulence, which has a negative effect on high- castings, perhaps most suitable for a Pintegrity castings. It is nearly impossible to have variety of less-critical automotive molten aluminum free fall more than a few cen- applications. timeters without initially exceeding a safe flow Static top-pouring has another velocity of about 0.5 to 1 m/s. Note that a free fall downside too, an ever-diminishing of less than 0.1 m will accelerate to more than 1 effective metal head as fill progresses. m/s flow velocity. Professor John Campbell has Unless the pour height is specifically Smooth flow often pointed out the propensity for turbulent increased (adding to the turbulence flow at velocities above 1 m/s, and the likelihood issue), little pressure remains to assist that high velocities will lead to harmful oxide fill or shrinkage feeding the uppermost reaches Fig. 1 — folds. Still, by applying tilt pouring techniques to of the mold cavity. One solution is to top off upper The effect of better control velocity at the onset of flow, and/or risers with hot metal. A more effective means to ceramic foam offset both the turbulence and diminishing metal on smoothing by applying appropriate filters as flow control de- flow. vices and to remove oxide films close to the point head issues is tilt pouring. where melt enters the casting cavity, high quality gravity-poured aluminum castings are possible. Tilt pouring Tilt pouring essentially removes the effect of Static top-pouring free fall at the beginning of the pouring process. Static top-pouring is the permanent mold ver- With care, a cup or basin can be filled while in a sion that has been a mainstay since aluminum’s horizontal position with free fall limited to only commercial introduction at the turn of the 20th a few centimeters. When tilting at an appropriate century. The process remains standard for most rate is initiated, melt flows from the basin to far high-volume automotive pistons and cylinder extremes of the mold cavity, also with minimum heads. Still, static top-pour is also the method free fall turbulence. Those extremes ultimately most likely to contain turbulence-related defects. become the low points in the mold after tilting *Life Member of ASM International is complete (Fig. 2). The basin then usually be- 30 ADVANCED MATERIALS & PROCESSES/APRIL 2008 comes the reservoir of hot metal for feeding and melt is moved into the tool by the force of an shrinkage as the uppermost areas of the casting electromagnetic pump. solidify. Tilt pouring is applied to simple single- • Molten metal bottom-fills the mold cavity station devices, as well as to large multi-station through a riser tube by pressurizing the melt sur- carousels. face (or by applying an electromagnetic force), thus creating a pressure imbalance (P between LOW-PRESSURE PERMANENT MOLD melt and mold cavity) sufficient to force melt to Basic low-pressure process rise into the mold at a controlled flow velocity. The low-pressure permanent mold (LPPM) • Pressure is maintained on the melt surface process is, in fact, a style of permanent or semi- until the casting fully solidifies. permanent mold, thus the term LPPM, but it can • Pressure is then released, allowing excess melt also work with sand molds to make limited- in the riser tube to recede back into the furnace. production prototypes or high integrity parts for Cast-weight to trimmed-weight yield in LPPM aerospace and the like. The process is illustrated is typically 95 to 98%. This basic version of low in its basic form in Fig. 3 and is briefly described pressure was developed in the 1940s to cast beer as follows: barrels, and soon became a popular means to cast • In most versions, a mold is positioned above other products, including aluminum cylinder a hermetically sealed furnace. However, in at least heads. LPPM has been used extensively since the one version the mold resides adjacent to the open early 1970s to cast styled aluminum wheels and well of an unsealed and unpressurized furnace, other automotive products such as cross mem- Contact bers and engine blocks. LPPM and its variations information Movable mold half quite easily accommodate disposable internal for the sand cores. American Pouring The key advantages of LPPM are that it is a nat- cup Foundry ural bottom-filling process, thus fill turbulence is Society: Molten minimized; and it provides a strong natural direc- Stationary Mold cavity aluminum tional solidification from cavity extremes back to Readers may mold half contact the hot riser fill positions. Another advantage in estoreservices terms of productivity is that, by using either mul- @afsinc.org to tiple riser tubes or a feeding manifold, multi-cavity place an order Plane parallel to floor casting of smaller parts such as knuckles and con- for the complete trol arms is quite feasible. The process also does conference not have the projected-area restrictions that apply proceedings, to the high-pressure processes often chosen for or visit our high-integrity chassis and suspension components. website, A downside to LPPM is the repeated rise and www.afsinc.org Fig. 2 — The tilt pouring permanent mold concept. Melt fall of melt in the riser tubes, which create oppor- /estore. is poured into a cup or basin while in the horizontal tunities aside from fill turbulence for the intro- position, and then it flows to lower mold extremities as the mold tilts into a more vertical position. duction of oxides. This is offset or overcome by some practitioners by maintaining sufficient pres- sure on the melt between shots to prevent melt 1. Moving die from receding more than a centimeter or so below 2. Fixed die the fill level; and/or inserting a filter between the 3. Moving platen riser tube and casting cavity to prevent the passing 4. Ejection platen of whatever oxides are formed due to melt level 5. Eccentric platen 6. Die height adjustment changes in the tubes. 7. Nozzle heating element 8. Furnace heating elements Vacuum riserless/pressure riserless casting 9. Pressurizing air or gas inlet The vacuum riserless/pressure riserless casting 10. Crucible 11. Thermocouple (VRC/PRC) process is illustrated in Fig. 4. This 12. Safety diaphragm variation on low-pressure casting was developed 13. Fixed platen 14. Casting table Hydraulic 15. Metal inlet cylinder Upper (movable) mold Ejector pins Side core Die cavity Lower Fill tubes (stationary) mold Molten Water-cooled aluminum bottom plate Electric SiC crucible holding furnace Fig. 3 — Schematic of the low-pressure permanent mold process Fig. 4 — Process schematic for the vacuum riserless/ (LPPM) for high-integrity castings. pressure riserless casting machine. Diagram courtesy Alcoa. 32 ADVANCED MATERIALS & PROCESSES/APRIL 2008 Casting process comparisons Mechanical Casting Piece countCapital Tooling Total Capabilities properties size Net shape per hour costs costs relative cost Permanent mold Static gravity 2 4 32553 Tilt3 443443 LPPM 3 5 45435 VRC/PRC 4 5 45324 CPC/PCPC 4 4 45324 High pressure Squeeze 4 2 53112 Rheocast 5 3 54135 High-vacuum 4 4 54123 In the context of the table: regarding capabilities, 5 means best, largest, or highest, whereas 1 means worst, lowest, or smallest; regarding costs, 5 means best or lowest, whereas 1 means worst or highest. by Alcoa and re-designed in the 1990s by CMI/A- • melt level in the furnace is kept constant each CMI as a robust technology in the high-volume casting cycle by replenishing the furnace with a automotive foundry environment. As in the basic quantity of new melt equivalent to the weight of low pressure process, the just-cast shot. • a mold is located above a hermetically-sealed Casting yield is typically 95+%. At Madison furnace, but Precision Products Inc., VRC/PRC casts struc- • the mold cavity communicates with the melt tural components such as knuckles, link arms, through not one but several riser tubes; and sub-frames. • the bottom mold half (the drag or cover half) The key advantages of VRC/PRC are forms the top-plate of the furnace, so • The close coupling between melt and casting • the coupling distance between the melt and allows cooler melt temperatures without danger mold cavity is quite short; and of miss-fills. Continued Scratch More Than the Surface. ASM International and the North American Die Get practical information Casting Association present this intensive you can use. one-day workshop bringing together some of • Reduce down time the world’s foremost experts in the fields of • Reduce soldering dies, coatings, surface treatments, and failure • Extend die life mechanisms.