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Proper Mold Design for Permanent Mold Production

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Proper Mold Design for Permanent Mold Production SIMPLE SOLUTIONS THAT WORK! PROPER MOLD DESIGN FOR PERMANENT MOLD PRODUCTION higher degree of reliability with John Hall regard to pressure applications of President www.cmhmfg.com fluids and gases. • The tilt pour process allows the Article Takeaways: molten metal to flow to the bottom 1. Advantages and disadvantages of tilt pour permanent of the mold, forcing the air out molding the top. As the molten aluminum 2. Understanding mold design requirements flows through the runner a static 3. Types of tilt pour gating skin of aluminum oxide forms which allows clean metal to enter The demand for permanent mold machining allowances, which will the mold cavity. castings has steadily increased with lower the downstream cost. • Automatic pour machines automotive parts leading the way: • Ferrous and nonferrous inserts eliminate many of the variables suspension links, intake manifolds, can be accurately cast in place. found in hand pouring. pistons, and other functional parts Typical insert materials can be • Permanent mold castings have a of internal-combustion engines are iron, steel, stainless steel, or smoother as cast surface finish typical applications. Other applica- copper base alloys. In some cases, than sand casting and finishes tions include aviation engines, mis- threaded inserts can be cast in approaching 100rms can be siles, motor housings, nozzles, fan place, eliminating the need for achieved. In many cases casting cases, outdoor lighting standards, machining and related costs. buyers can use as cast surfaces lawn mower chassis, barbeque grills, • Permanent mold castings are without subsequent finishing for and kitchen pots and pans. chilled castings, they are generally cooking utensils, hardware items Advantages and Disadvantages sounder than sand castings. and automotive parts. The decision to use tilt pour per- They are generally stronger than • Studs, nuts, bushings, pipes and manent molding should be based sand or die-castings and are less other inserts may be cast as integral on thorough engineering and pro- porous than die-castings. Castings parts of the casting. The inserts duction cost studies. Properly made produced in the permanent mold must be held in positive position tilt pour castings have the following process have finer dendritic arm in the mold to prevent movement advantages: spacing (DAS) and grain structure. during casting process. The inserts • Dimensional accuracy is superior The finer structure displays better should be scored, knurled, or under to sand or shell mold castings, strength properties than those cast cut to provide a locking surface. because the mold is rigid and in similar alloys in sand castings. • Size limitations - Most castings does not allow mold wall Permanent mold castings have weigh less than twenty pounds, movement during solidification fewer inclusion defects than sand however castings as large as 350 and dimensional repeatability is castings. Therefore, the casting pounds have been poured. The improved. The reduction in casting designer has the freedom to use casting design may be so complex variations allows a reduction in thinner sections and lighter weight that it is not practical in permanent designs. These castings have a mold processes. 46 Mold Design Due to the wide variation in cross outlets, such as the parting line, When designing molds for per- sectional area of commercial cast- and clearances around ejector pins, manent molds casting, the most ings it may be necessary to use highly usually provide adequate venting. common mistake foundry engineers conductive chills, air cool, water A properly designed gating system make is not understanding the tilt cool or take advantage of varying in the tilt pour process can reduce pour process or understanding the the mold wall thickness in order to the venting necessity. The molten individual foundry’s requirements. promote directional solidification. metal can be taken to the bottom In permanent mold casting, solidi- Adequate gravity head should be of the mold, thereby forcing the air fication occurs much more rapidly provided to ensure filling of all parts out the top as the mold is tilted. In than in sand casting. However tilt of the cavity. Ample flat area should some cases, supplemental venting pouring allows better filling of the be allowed to seal against metal must be added. mold with minimum turbulence leakage at the parting line. Two Commonly used venting methods and controlled thermal gradients inches at the bottom and one and include: to establish directional solidification one half inches at the sides is nor- 1. Slot or “scratch” vents usually towards a riser. The rigidity of a per- mally enough to seal molds up to .005 to .010 inches deep, cut manent mold necessitates some dif- thirty inches square. Caution must along the parting line leading to ferences in the application of these be used when designing molds not the outside of the mold principles. It is essential that the to make them too rigid. The parting 2. Very small holes drilled through entire casting and its gating system line is the hottest portion of a mold, the mold in areas where they will be removed with a simple parting and each face of the mold/platen not affect the casting surface or from the mold. Removal must be assembly will run progressively the ability to strip the casting possible without excessive mechan- cooler (see fig. 1). Different heating 3. Plug vents, which are holes that ical force on the casting or without of the mold will cause the mold to are drilled in the mold and filled excessive abrasion of the mold open at the parting line. In order to with a slotted plug coating. A front ejector will ensure prevent mold warping at the parting Gating/risering – As the mold is that the casting draws straight and line, over all mold thickness should tilted, molten aluminum enters the pulls with the movable mold half. be held to a minimum and stiffen- permanent mold and loses heat Heavy sections are generally placed ing ribs should not be used. rapidly compared to sand molds. The on the parting line to permit feeding. Mold design can dramatically affect rapid cooling also necessitates rapid Sprues, runners, gates and risers are casting quality as well. When design- filling. In general, the gating/riser- also placed on the parting line so ing a mold, the following factors ing system in the tilt pour process they can be stripped with the casting. must be considered. should accomplish the following: The casting and gating system must • Venting • Fill the mold cavity in a tranquil be arranged to promote directional • Gating & Risering manner reducing turbulence and solidification starting in the remote • Chills the formation of dross areas and progressing towards the Venting – All the air in the mold must • Feed the casting during liquid riser. escape as the mold is filling. Natural contraction Figure 1 Figure 2 47 • Provide fast solidification to therefore the mold fill rate, as nec- casting in the mold so that solidifi- increase production by reducing essary. In such cases multi ingates cation starts in the thinner sections cycle time may be placed at various levels to and progresses to • Promote progressive solidification allow metal to flow at the most the heavier sections. Due to the to the riser desirable rate. For large castings, the wide variation in castings, this is not • Minimize further downstream gating system might be placed on always possible and a hot spot will processing (decrease finishing both sides. In applications when a form. Some relief may be obtained time for gate removal) runner/riser is being used, a runner by adding ribs to a boss to introduce Figure two illustrates three types extension should be used to prevent more feed metal into the heavy of tilt pour gating. The multi ingate the backwashing of dross contam- section. Ribbing is not always effec- system has lower finishing cost, but inated first metal of the pour. tive or the casting may not be mod- can cause turbulence and dross The actual dimensions of the mold ifiable. In such cases, defects. If high quality levels are and gating system will depend on it is prudent to cool the heavy section required, the continuous ingate the weight and dimensions of the of the mold so that the casting will might be desirable. This system casting to be produced. Figure four solidify quickly. could be used with a top riser and/ is given as a guideline. All the dimen- or shrink bobs as necessary. sions are based on the thickness of Localized chilling can be obtained by installing copper inserts (fig. 4). Direct gating can be used very the casting which is referred to as “T”. In permanent mold casting riser Extending them outside the mold effectively in tilt pouring because and cutting cooling fins into the chill the automatic tilting of the mold sizing is critical. The riser must be large enough to eliminate shrinkage can increase their effects. Air chills eliminates human variability in the are holes drilled into the mold and a pour rate. Additionally, as the liquid defects. In some cases an oversize riser van superheat the mold and blast of air is blown into to the relief. aluminum enters the mold it flows Moderate control of solidifications through a static skin of aluminum actually cause a shrinkage defect in the casting. Furthermore, a riser in can be accomplished by varying the oxide. The oxide acts as a barrier thickness of the mold coating. allowing only clean metal to enter which metal freezes too slowly may the mold cavity. delay the opening of the mold until The importance of properly excessive contraction stresses have designed permanent molds cannot The use of a side riser permits more developed in the casting. An over- be over looked.
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