Ch 11
11.26 How are the individual wax patterns attached on a "tree" in investment casting?
Heat is applied to the wax pattern and/or tree at the contact surface. The surface of the pattern and/or tree melts, at which time the pattern and tree are brought into contact and firmly held in place until the wax solidifies. This is repeated for each pattern until the "tree" is completed.
11.5) What is the function of a core? Cores are used in casting involving internal cavities or passages. The core allows for the formation of an interior surface to a casting. It is removed from the finished product before the cast is further processed.
In order to remain in the desired location, the core is anchored using core points. These recesses are added to the mold in order to support the core and allow for gas exchange/escape. To keep the cores in place without shifting chaplets may be used to anchor the core in place. And the cores used in casting should also possess some properties like strength, collapsibility and permeability.
11.38 Rank the casting processes described in this chapter in terms of their solidification rate. (That is, which processes extract heat the fastest from a given volume of metal?)
There is, as expected, some overlap between the various processes, and the rate of heat transfer can be modified when desired. However, a general ranking in terms of rate of heat extraction is as follows: Die casting (cold chamber), squeeze casting, centrifugal casting, slush casting, die casting (hot chamber), permanent mold casting, shell mold casting, investment casting, sand casting, lost foam, ceramic-mold casting, and plaster-mold casting.
11.20) why is the investment casting process capable of producing fine surface detail on castings?
They are made from wax patterns. The pattern itself has excellent properties; since it is usually made by die casting, rapid prototyping or machining. Since the wax mold is then coated with a layer of ceramic slurry, the first layer can contain ceramic particles that are extremely fine in size, which will result in a smooth surface on the finished casting.
11.34 Chocolate forms are available in hollow shapes. What process should be used to make these chocolates?
Thin shells are typically and easily made through slush casting (see Fig. 10.11 on p. 247, and also slush casting in Section 11.4.3 on p. 278), using split molds. This can be verified by obtaining such a chocolate and breaking it, and observing the interior surface is rather coarse and shows no evidence of having contacted a mold.
11.37 Can a chaplet also act as a chill? Explain.
While, in theory, a chaplet can serve as a chill, in practice chaplets rarely do so. Chaplets are intended to support a core or a section of mold. If they are placed in a position to support the core, they may not be in a location that requires a chill. Chaplets have a large footprint, and this helps to transfer heat to the core. However, heat transfer to the core is not an option for faster cooling of the casting; heat instead must be conducted outside of the mold. Therefore, the chaplet cannot usually be considered a chill.
11.33 Why does the die-casting machine shown in Fig. 11.20 have such a large mechanism to close the dies? Explain.
As discussed in the text, the molten metal in die casting is introduced into the mold cavity under great pressure. This pressure has thus a tendency to separate the mold halves, resulting in large flash and unacceptable parts. The large clamp is therefore needed to hold the mold together during the entire casting cycle.
11.11) List the advantages of pressure casting over other processes?
During die casting, molten metal is forced into the die using high pressure. This process is easily automated, significantly reducing potential labor costs. In die casting process post machining can be totally eliminated and the parts produced will have good dimensional accuracy and close tolerances. The cost of the die, however, is higher than other molds. This makes die casting economically sound only for the production of larger quantities of casts. The process is limited to high fluidity metals only.
11.8) Describe the features of plaster mold casting.
A plaster mold casting is made of plaster of paris, typically gypsum or calcium sulfate. In order to strengthen the mold and control setting time, and silica flour is often added to the mix. The plaster is then poured over the pattern. The pattern for the plaster mold casting can be made out of various compounds, including aluminum alloys, thermosetting plastics, brass, or zinc alloy. The primary benefit of plaster mold casting is the smooth surface finish and fine details. With the mold having a lower thermal conductivity than those made of other materials, a slower cooling time will result in uniform grain structure and less warping. Plaster mold casting is used only for aluminum, magnesium, zinc, and some copper based alloys.
11.12) What is the purpose of riser? A vent?
A riser represents a special piece of the mold designed to prevent shrinkage in the main cast. The riser supplies additional molten metal into the mold as necessary. This means that any shrinkage will form in the riser, rather than the resultant cast. A vent in the mold is designed to allow gas formed by a reaction of the molten metal hitting the sand. Any air that is in the mold at the time of pouring will also escape form vents.
11.7) What are composite molds> why are they used?
As their name suggests, a composite mold is a mold formed from a combination of two or more materials. They are often used in shell molding. Although they can be used in other casting processes. A composite mold will combine the advantage of its various materials and allow for complex casting to be made.
11.28 You have seen that, even though die casting produces thin parts, there is a limit to how thin they can be. Why can't even thinner parts be made by this process?
Because of the high thermal conductivity the metal dies exhibit, there is a limiting thickness below which the molten metal will solidify prematurely before completely filling the mold cavity.
11.25) Explain why squeeze casting produces parts with better mechanical properties, dimensional accuracy, and surface finish than do expendable-mold processes.
Because squeeze casting combines forging and casting, trapped gases remain in solution when the upper die or punch is applied due to the pressure exerted on the gas, and as result porosity is not present in squeeze cast products. Heat transfer occurs very quickly in squeeze cast parts. The rapid transfer of heat allows the metal's cooling with fine grain microstructure, ensuring excellent and homogeneous physical characteristics. The applied pressure from squeeze casting, combine with materials typically used for the process, produces parts with great accuracy dimensionally. Pressure imparted by squeeze casting, along with metals typically used, produce parts with a fine surface finish.
11.30 It was stated that the strength-to-weight ratio of diecast parts increases with decreasing wall thickness. Explain why.
Because the metal die acts as a chill for the molten metal, the molten metal chills rapidly, forming a fine-grained hard skin (see, for example, Fig. 10.3 on p. 239) with higher strength. Consequently, the strength-to-weight ratio of die-cast parts increases with decreasing wall thickness.
11.22) Would you recommend prehearing the molds used in permanent-mold casting? Would you remove the casting soon after it has solidified? Explain your reasons.
By preheating a mold to be used for a permanent mold casting, there is less likelihood that the metal will chill when toughes the mold surface, which could lead to low fluidity. Pre heating the molds for permanent mold casting also lowers the fatigue and shock imparted on the mold by repeated casting with molten metal. As for removing the casting quickly, the part must be given adequate time to cool in the mold so that there is no possibility of it being distorted or acquiring other defects during the shakeout process. Small parts require less time than large casting.
11.27 Describe the measures that you would take to reduce core shifting in sand casting.
Core shifting is reduced in a sand mold by core prints, chaplets, or both. Core prints (see Fig. 11.6 on p. 265) are recesses in the pattern to support the core inside the mold. If excessive shifting occurs, chaplets may be used. Chaplets are small metal supports which act both as a spacer for the core to assure proper core location and as an added support to resist shifting.
11.19) Why does die casting produce the smallest cast parts?
Die casting involves high pressures. So, it is possible to make cast pieces with lower wall thickness than those cast by other methods. High pressure in die casting allows the liquid metal velocity in the runner to be high compared to other processes, ensuring that the small parts cool before the runner do. Applying vacuum to the die adds an extra level of assurance that this will be the case.
11.10) What are the advantages and limitations of die casting.
During the pressure casting process, gas pressure is used to force molten metal into a graphite or metal mold. If gas is unavailable or undesirable, vacuum can also be used to force the molten metal into the mold. A primary advantage of the pressure casting process is reduction and removal of dissolved gases. It also produces a final casting that has a lower porosity.
11.9) Name the type of materials typically used for permanent mold casting process.
Each half of the mold can be made from a variety of materials. Typical materials for permanent mold casting are cast iron, steel, bronze, graphite, and refractory metal alloys. Cores made of metal and sand are placed in the mold prior to casting to produce casting with internal castings. Typical materials are oil bonded sand, resin bonded sand plaster, graphite, low carbon steel, hot work die steel or gray iron.
11.1) Describe the differences between expendable and permanent molds.
Expendable molds are often used from inexpensive, disposable materials, such as sand, plaster, ceramics, etc. These materials are usually mixed with a variety of bonding agents after casting, because of the nature of the materials, the molds can be broken in order to remove the casting. Permanent molds are made of more expensive, permanent materials, such as various metals, This molds will maintain their strength through multiple uses and at higher temperatures than disposable molds. They are designed to allow for easy casting removal with having to break the mold.
11.3) What are the major types of sand molds? What are their characteristics?
Green molding sand is the most common material for casting molds. Green sand is a mixture of sand, clay, and water. It is so called because the mold is damp during pouring. Its primary benefit is it is lowest cost option for molds. These molds are used for large casting because of their high strength. Second type of sand molds is called a cold box mold. This type of mold combines organic and inorganic binders with the sand in order to provide greater strength by strengthening the chemical bonds in the grains. While more expensive than the green sand option. Cold box molds can be made significantly more accurate. Finally, a no bake mold can be used. In order to manufacture a no bake mold, a liquid resin is combined with the sand. This composite will then harden at room temperature in order to be used as a mold.
11.18) why can blind risers be smaller than open top risers?
Have to account for the shrinkage in the slower cooling areas of the casting. So risers have to be big enough to be the last part of the casing to solidify. If the riser solidifies before the mold cavity is full it serves no purpose, and because the open ton risers exposed to cool air on the surface have to be made large enough to not be the first part of the csting to solidify. Blind risers can be made smaller since unlike the open top riser which is exposed to air, they contact the on all side, reducing their susceptibility to this phenomenon.
11.17) Describe the drawbacks to having risers that is (a) too large, (b) too small.
Having to large of a riser will add cost to the reclaim process. Also, once the riser is cut off, the resultant are will require more machining, again adding to the cost. Having a riser that is too small will mainly affect the finishing casting, either in form of sufficient material to compensate for shrinkage in the mold, or the creation of shrinkage pores caused by a non uniform solidification front.
11.29 How are hollow parts with various cavities made by die casting? Are cores used? If so, how? Explain.
Hollow parts and cavities are generally made using unit dies (see Fig. 11.21d on p. 281), although cores also can be used. Core setting occurs mechanically, e.g., for an aluminum tube, as the die closes. A rod, which extends the length of the cavity, is pushed into the mold and the molten metal is then injected. This "core" must be coated with an appropriate parting agent or lubricant to ensure easy ejection of the part without damaging it.
11.6) What is the difference between sand mold and shell mold casting?
In sand casting, a mold in the desirable shape is imprinted in sand. A gating system is then added and molten metal is poured into the mold. The metal is then allowed to cool and solidify. The sand model can then be broken away and the cast is removed. A shell mold is made of ferrous metal or aluminum. It is heated to 175 to 370C and coated with an agent such as silicon to induce parting. It is then secured to a box or other chamber. This method produces low cost and dimensionally accurate molds with a good surface finish.
11.32 In shell-mold casting, the curing process is critical to the quality of the finished mold. In this stage of the process, the shell-mold assembly and cores are placed in an oven for a short period of time to complete the curing of the resin binder. List probable causes of unevenly cured cores or of uneven core thicknesses.
In the production of shell molds and cores, lack of temperature control is often the most probable cause of problems. Unevenly cured cores or uneven core thicknesses are usually caused by furnace- or temperature-control related problems, such as: (a) Insufficient number of burners or inoperative burners in the curing furnace. (b) One-half of the core box is higher in temperature that the other half. (c) Mixture of low- and high-temperature melting-points ands that were improperly blended, thus causing different parts of the core to cure differently. (d) Temperature controllers not functioning properly. (e) The core was removed too slowly from the furnace, allowing some of it to be heated longer.
11.23) Give reason for and examples of using die inserts.
Insert lower the cost of production for casting by making them modular. Different parts can be cast using a common die when an insert is installed.Inserts are used to mark a part with a part number, lot, date, Etc. the die special feature in it which allow for a die insert to be used for markings.
11.14) Lost are the advantages of the lost foam casting.
Lost foam casting is a casting process in which foam is used as the pattern as opposed to the normal materials, such as wax. The primary advantage to lost foam casting is the ability to simple evaporate the pattern out of the mold rather than applying high heat and pouring the pattern out. This allows for a tendency to greater detail and a lower probability of defects. The elimination of cores makes complex casting designs easy and the process has minimal finishing and cleaning operations.
11.21) What differences, if any, would you expect in the properties of casting made for permanent mold versus said casting processes.
One acceptable answer is that permanent mold castings have closer dimensional tolerance, more uniform physical properties, can develop a finer finished surface and reproducible thin walled parts as compared to sand casting. Sand casting is more adept for producing very intricate, large pieces at lower cost than those of permanent mold casting. The actual cost variance will depend on the alloy that must be used for the sand casting.
11.16) Why are risers not as useful in die casting as they are in sand casting.
Remember that in sand casting, risers are located and sized to act as pools of molten metal to account for shrinkage of the metal. Since the cooling rate of the sand cast product is slower, placing risers appropriately can control the cooling rate and shrinkage rate. Another reason is that, in order to make die casting economically feasible from tooling and material standpoint, its cooling rates must be relatively fast. Using risers is this case would slow the cooling time and affect production rate.
11.31 How are risers and sprues placed in sand molds? Explain, with appropriate sketches.
Risers and sprues are usually created from plastic or metal shapes which are produced specifically for this purpose. Thus, a metal sprue is machined to duplicate the desired shape in the mold. This sprue model is then affixed to the pattern plate before the flask is filled with sand. The sand mold is prepared as discussed in the chapter (see Fig. 11.8 on p. 267). When the pattern plate is removed, the riser and sprue patterns are removed at the same time.
11.2) Name the important factors in selecting sand for molds.
Sands used in molds can vary widely on a number of characteristics. Each of the various types of sands will impact the casting process in different way. Sands with round, fine grains will pack closely, This allows it to form a smoother surface on the mold. Find grain sands will generally enhance the strength of a mold. They do, however, lower mold permeability, which can negatively impact the ability of gases and steam escaping during solidification.
11.4) List important considerations when selecting pattern materials.
Selecting pattern materials requires considerations of a few key characteristics of the desired product. First and foremost is the size/shape of the desired casting. Once size and shape are established, one must consider how accurate each casting must be. Lower variation tolerance requires a mold that can be more dimensionally accurate. Two final consideration are the number of castings required and the process molding. A relatively large number of casting will likely require a more permanent pattern material in order to keep costs and manufacturing time down.
11.36 The "slushy" state of alloys refers to that state between the solidus and liquidus temperatures, as described in Section 10.2.2. Pure metals do not have such a slushy state. Does this mean that pure metals cannot be slush cast? Explain.
The "slushy" state in alloy solidification refers to an intermediate state between liquid and solid. Slush casting involves casting an alloy where the molten metal is poured into the mold, allowed to begin to solidify. The molten portion of the metal is then poured out of the mold, leaving a shell behind. This can be done using pure metals as well as alloys. foo
11.35 What are the benefits to heating the mold in investment casting before pouring in the molten metal? Are there any drawbacks? Explain.
The benefits to heating the mold include: Greater fluidity for detailed parts (in that the molten metal will not solidify as quickly), a possible reduction in surface tension and in viscous friction in the mold, and slower cooling. The main drawbacks to heating the mold are that the mold may not have as high a strength at the elevated temperature, and the metal may be less viscid and becomes turbulent as discussed in Chapter 10. Also, the solidification time will be larger with increased mold preheat, and this can adversely affect production time and process economics as a result.
11.15) What are the reasons for the large variety of casting processes that have been developed over the years? Explain with specific examples.
The interpretation could be based on analysis on a materials basis, application basis or economic basis. Example is a comparison of investment casting and sand casting. While investing casting is more expensive, it is possible to achieve closer final tolerance and as a result may be a beneficial offset in some cases. Another acceptable response is the comparison of hot chamber and cold camber process of permanent mold casting. Choosing to use hot chamber casting because it can be more completely automated thereby reducing the costs is reasonable, but it also has some inherent disadvantages that must be taken into account before concluding it is the best possible option.
11.13) What is squeeze casting? What are its advantages?
The primary advantage of squeeze casting is a significant decrease in the amount of pressure required than that used in traditional forging. This process also overcomes the feeding difficulties. In addition, more complex and intricate details can be produced during squeeze casting.
