Skema Answer Manufacturing Proces 1

FACULTI OF MECHANICAL ENGINEERING UNIVERSITI MALAYSIA PAHANG BMM3643 (SEM II_2012-13) Assignment #1 1. a) What metals are frequently cast into products? b) What materials are used to produce the expendable patterns for investment casting? c) Explain why a casting may have to be subjected to various heat treatments. (8 marks) Answer a) Cast parts can range in size from a fraction of an inch and a fraction of an ounce to over 30 feet and many tons.
Moreover, casting can incorporate complex shapes, hollow sections or internal cavities, and irregular curved surfaces. b) In investment casting a pattern is formed from a low melting temperature, low vaporization temperature material, often wax. The mold is produced by surrounding the pattern with the mold material. The mold cavity is produced when the pattern is removed by melting/vaporizing the pattern. In early process development with porous mold materials the melted wax from the pattern would migrate into the mold material and be lost. ) Heat treatments (described in Chapter 4) such as quenching and tempering, among others, are carried out to optimize the grain structure of metal castings, thereby controlling and enhancing mechanical properties. Heat treating can control microporosity, which is a main reason that castings are weak in tension. 2. a) What are some of the attractive features of die casting compared to alternative casting methods? b) For the cast metal wheel illustrated in Figure below, show how (a) riser placement, (b) core placement, and (c) chills may be used to help feed molten metal and eliminate porosity in the isolated hub boss. ) What are some of the general defects encountered in casting processes? Name and briefly describe three. (8 marks) Answer a) Die casting is characterized by extremely smooth surface finishes, excellent dimensional accuracy, and high production rates. A single set of dies can produce many thousand castings without significant changes in dimension. b) Solutions; i) Riser ii) Core iii) Chills c) General defects include; v) misruns, in which the casting solidifies before filling the mold cavity v) cold shuts, in which two portions of metal flow together but there is lack of fusion at the joint; vi) cold shots, where solid globules of cast metal become entrapped in the casting; vii) shrinkage cavity, which is a depression on the casting surface or an internal void in the casting caused by solidification shrinkage; viii) microporosity, which is a network of small voids throughout the casting caused by localized solidification shrinkage; and ix) hot tearing, which is a crack in the casting caused by a mold that does not ield to the metal during the early stages of solidification shrinkage. 3. a) How does the fabrication of a thermoplastic polymer differ from the processing of a thermosetting polymer? b) What are the significant differences in the equipment and operating procedures between injection mold- ing of thermoplastics and injection molding of thermosets? c) Can thermosetting plastics be used in injection molding? Explain. (8 marks) Answer a) Thermoplastic polymers can be heated to a temperature at or near the melting temperature so that the material becomes either a formable solid or a liquid.
The polymer can than be cast, injected into a mold, or forced through a die to produce the desired shape. With thermosetting polymers, once the polymerization has occurred, no further deformation can occur. Thus, the polymerization reaction and the shape-forming process must be accomplished simultaneously. b) The differences in injection molding of thermosets are (1) shorter barrel length, (2) lower temperatures in the barrel, these first two reasons to prevent premature curing; and (3) use of a heated mold to cause cross-linking of the TS polymer. c) Thermosetting plastics are suitable for injection molding.

The basic modification which must be made to the process is that the molds must be heated to allow polymerization and crosslinking to occur in the mold cavity. The major drawback associated with this change is that, because of the longer cycle times, the process will not have as high a production rate as injection molding of thermoplastics. 4. a) Identify one injection molding process could be used to inject a single part with two or more different material as shown below. b) Describe process mechanism c) List and explain THREE (3) advantages of this technology? (8 marks) Answer a) Multi-shot injection molding ) This process ables to to shoot two or more different materials into the same mold, into different locations, resulting in parts with increased functionality, improved cosmetics, and multiple mechanical properties. c) Advantages; i) Reduced cycle time * Compared to multiple molding cycles of separate components, molding multiple materials in the same cycle has obvious time and labor benefits. ii) Reduced part cost * Combine reduced cycle times, reduced labor times, and eliminated assembly operations, and the total cost of multi-shot molded parts becomes less, compared to alternative single-shot methods. ii) Improved Adhesion * With multi-shot molding we get a true physical bond, resulting in a much stronger, longer lasting bond, compared to more traditional “skin on skin” insert molding or post-molding assembly. 5. d) What are sheet-molding compounds (SMCs)? Bulk-molding compounds (BMCs)? e) What are some of the forms in which reinforcement fibers appear in composite materials? f) Describe the problems involved in recycling products made from reinforced plastics. (8 marks) Answer a) Sheet molding compounds are sheets composed of chopped fibers and resin, the sheets being about 0. inch in thickness. These can be press-formed in heated dies to provide an alternative to sheet metal where light weight, corrosion resistance and integral color are desired. Bulk-molding compounds are fiber-reinforced thermoset molding materials containing short fibers in random orientation. They are formed into products using processes like compression molding, transfer molding or injection molding. b) Fiber-reinforced composites use the strength of the fibers to impart additional strength to the fiber-matrix whole.
The use of fibers means that added strength will be in the fiber length direction. The commonly used fiber forms are; i) long, continuous fibers are their use results in increased strength in the fiber length direction, ii) fibers woven into fabric layers used in thin sheet composites and they add strength in the two in-plane fiber directions, iii) woven fabrics of fibers formed in three dimensions so that when embedded in the matrix strength in three dimensions is increased, iv) short, chopped fibers that can be oriented in a particular direction or randomly. ) The main problems are that recycling usually requires the use of a single type of material, and that some plastics (mainly hard and brittle polymers) are more difficult to chop into small pieces for further processing than others. With reinforced plastics, this requires that the reinforcement be separated from the matrix, a very difficult task and uneconomical task. Note that matrices are often thermosets, so it is not practical to melt the matrix and separate the fibers from a molten phase. 6. ) In the casting of steel under certain mold conditions, the mold constant in Chvorinov’s Rule is known to be 4. 0 min/cm2, based on previous experience. The casting is a flat plate whose length =30cm, width =10cm, and thickness =20 mm. Determine how long it will take for the casting to solidify. h) A round bar of 15-mm diameter is extruded from a single-screw extruder of 100 mm barrel diameter. The material is LDPE. Calculate; i) The approximate flow rate (kg/h), ii) Speed of emerging extrusion Given : Density LDPE = 0. 92 g/cm3) (10 marks) Answer a) Volume V = 30 x 10 x 2 = 600 cm3 Area A = 2(30 x 10 + 30 x 2 + 10 x 2) = 760 cm2 Chvorinov’s Rule: TTS = Cm (V/A)2 = 4(600/760)2 = 2. 493 min b) R i) Flow rate, qe= CeDscr = 0. 006(100)2. 3 = 238. 86 kg/h ii) Density, ? = 0. 92 g/cm3 Cross-sectional area = (152*? )/4 = 1. 767cm2 Volume = 238. 86 / 0. 92 = 259635cm3/h = 72. 12cm3/s Extrusion speed = 72. 12 / 1. 767 = 40. 8 cm/s ******************************************************

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