060070103-Rifat Yılmaz-4th Week Definitions

Injection molding of Thermoplastics (Previous)-GROUP: Manufacturing Method

When thermoplastics are heated, they experience a change of state; they turn soft and melt, becoming flowable. When cooled down, they solidify again. This is the reason that plasticating units are operated hot and molds are operated cold when working with thermoplastics. Generally, the temperature difference is more than 100 C. The thermoplastic materials developed for injection molding generally constitute relatively low-viscosity melts result that injection times are short and low clamping forces are needed.

The injection mold should remove the heat from the material fast and steadily. Therefore, the cooling system has to be carefuuly designed. The coolant-usually water, provided the mold temperature is below 100 C - flows through channels around the cavity. For reasons of economics, such as the quality of the molded parts, which depends heavily on uniform heat flow in the mold, the cooling circuit is monitored very precisely and cooling equipment is used to ensure that the coolant is always at the same temperature.

Molded parts requiring no machining can only be produced if all joints and mold parting lines are so well scaled that is unable to penetrate and harden there. Otherwise, flash would be formed and machining become necessary. To this end, all joint gaps must remain smaller than 0.03 mm even during full injection pressure, until the melt has solidified. These requirements are particularly demanding where large molded parts and large injection molding machines are involved as the molds must be extremely rigid and the clamping units must function very precisely; the rigidity of clamp plates is particularly critical.

(How to make injection molds, Georg Menges, Walter Michaeli, Paul Mohren, 3rd Edition, p.107)

Injection molding of Thermoplastics (New)

The vast majority of injection molding is applied to thermoplastic polymers. This class of materials consists of polymers that always remain capable of being softened by heat and hardening on cooling, even after repeated cycling. This is because the long-chain molecules always remain separate entities and do not form chemical bonds with one another. An analogy can lie made to a block of ice that can be softened (i.e., turned back to liquid), poured into any shape cavity, and then cooled to become a solid again. Thermosetting polymers are generally more expensive to mold than thermoplastics and represent only about 5% of plastics processing.

The injection molding process cycle for thermoplastics consists of three major stages, as shown in Figure 8.1: (1) injection or filling, (2) cooling, and (3) ejection and resetting. During the first stage of the process cycle, the material in the molten state is a highly nonlinear viscous fluid. It flows through the complex mold passages and is subject to rapid cooling from the mold wall on one hand, and internal shear heating on the other. The polymer melt then undergoes solidification inside the cooled mold, under the high packing and holding pressure of the injection system. Finally, the mold is opened, the part is ejected, and the machine is reset for the next cycle to begin.

(Geoffrey Boothroyd, Peter Dewhurst, Winston A. Knight, Product Design for Manufacture and Assembly, Third Edition, pages 332-333)

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Gun Drills (Previous)-GROUP: Manufacturing Method

The gun drill is a single point end cutting tool used for drilling deep holes. Theoretically, the hole depth has no limit, but the torsional rigidity of the shank is the major limiting factor. The single-tip design of the gun drill, incorporating bearing pads to support and guide the tool, forces the cutting edge to cut in a true circular pattern and maintains its position throughout the length of the hole. The pressure on the bearing areas (wear pads) and the lifting forces generated by the coolant pressure act against the radial and tangential cutting forces. To initially guide the tool, an accurate pilot hole or a pilot bush is necessary.

(Production Technology, Tata McGraw-Hill Education, p.161)

Gun Drills (New)

Deep, straight, holes are possible with the gun drill process which was originally developed for manufacture of gun barrels. Hole depths of over 250 times diameter are possible gun drilling is shown in Figure 3B6. A rotating single-flute drill, normally carbide-tipped, is guided by a bushing at the start of the drilled hole and is self-guided thereafter by a bearing surface opposite the cutting edge. A hole through the whole length of the drill provides a means for oil coolant to flow at high pressure to the cutting edge and to flush chips from the hole. Figure 3B6- 1 shows various applications of gun drilling. Gun drilling is a critical operation for barrels because the hole must be straight and centered in the barrel over about 30 in (76 cm). Figure G9 shows a common gun drilling machine for making the bore holes in two barrels simultaneously.

(James G. Bralla, Handbook of Manufacturing Process, pages  93-96-688)

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Upset Forging(Previous)-GROUP: Manufacturing method

Upset Forging is a metal shaping process in which a heated piece of uniform thickness is gripped between split female dies while a heading die is forced against the workpiece deforming and enlarging the end of the workpiece.This is a rapid coldforming process.

(Manufacturing Processes reference guide Robert H: Todd ,Dell K: Allen,Leo Alting)

Upset Forging(New)

Upset forging - is an operation that increases the diameter of a workpiece while shortening its length. It is the hot-material equivalent of cold heading. The workpiece material is normally in the form of a bar, on which a head or other larger diameter portion is produced by the process. One part of the die holds the bar while another part of the die is forced against it axially, causing the metal to flow and fill the die cavity. Normally, only the part of the bar to be upset needs to be heated. Making bolt heads is the most common application.

(James G. Bralla, Handbook of Manufacturing Process, page 38)

New definition is more understandable and obvious than previous one. It is better one.

Calendering(Previous)-GROUP: Manufacturing Method

In its simplest form, calendering equipment consist of two heated parallel rolls and the equipment is often called a two-roll mill or a batch mill. Polimer powder and plasticating and foaming ingredients are introduced into the nip redgions of these rolls. The polimer melts against the rolls and typically adheres to one of the two rolls, sometimes called the lead roll. The adhetives are dosed into the nip region and squeezed into the polymer. The plasticating temperature must be below the chemical foaming agent decomposition temperature to minimize premature foaming.

(J. L. Throne, Thermoplastic foam extrusion: an introduction, p.73)

Calendering(New)

Calendering is a method for producing plastic sheet and film. A heated, softened, plastic is forced between two heated rollers (“nip” rollers), with fixed spacing. The rollers form the plastic into a thick, continuous sheet. Additional rollers reduce the thickness and, if wanted, emboss the sheet. The sheet may be further reduced in thickness by stretching it. The process is used to make various sheet plastic components such as flooring and tape, and to provide material for further operations. Sheet thickness ranges from 0.002 to 0.060 in (0.05 to 1.5 mm). ABS and PVC are the most frequently processed plastics. Upholstery sheet, rainwear, shower curtains, and tape are made with the process. By calendaring two sheets of plastic with one or more layers of paper or other materials, such objects as credit cards, wallpaper, and playing cards can be made. Fig. 4J illustrates the calendering operation.

(James G. Bralla, Handbook of Manufacturing Process, pages 188-189)

New definition is more understandable and obvious than previous one. It is better one.

Centrifugal casting (Previous)-GROUP: Manufacturing Method

Centrifugal casting, due to inherent nature of the procedure itself, is only suitable for producing rotationally symmetric castings. In the centrifugal casting procedure, a chill mold is put into horizontal or vertical rotation and is filled with liquid material. The wall thickness or inside diameter of the solidifying tube-shaped casting is determined by the amount metal added. The solidification of melted metal takes place very rapidly and can be increased by additional cooling. A dense fine-grain microstructure is achieved by the influence of centrifugal force and extremely fast solidifying. This casting procedure is excellently suited to produce plain bearings and together with continuous casting is of great importance for producing stock material for plain bearings.

(H. Lipowsky,E.Arpaci, Copper in the automotive industry, p.49)

Centrifugal casting (New)

In centrifugal casting process, molten metal is poured into a revolving mold and allowed to solidify molten metal by pressure of centrifugal force. It is employed for mass production of circular casting as the castings produced by this process are free from impurities. Due to centrifugal force, the castings produced will be of high density type and of good strength. The castings produced promote directional solidification as the colder metal (less temperature molten metal) is thrown to outside of casting and molten metal near the axis or rotation. The cylindrical parts and pipes for handling gases are most adoptable to this process. Centrifugal casting processes are mainly of three types which are given as under.

(1) True centrifugal casting

(2) Semi-centrifugal casting and

(3) Centrifuged casting

(Rajender Singh, Introduction to Basic Manufacturing Processes and Workshop Technology, pages 249-250)

New definition is more understandable than previous one. It is better.