MÜGE BAŞARAN 030090704 WEEK 11

DISTRIBUTION REQUIREMENTS PLANNING (DRP)

Group: production planning

There is no old definition

New definition:

Distribution requirements planning (ORP) is a time-phased finished-goods inventory replenishment plan in a distribution network. Distribution requirements planning is a logical extension of the MRP system, and its logic is analogous to MRP. Distribution requirements planning ties the physical distribution system to the manufacturing planning and control system by determining the aggregate time-phased net requirements of the finished goods, and provides demand information for adjusting the MPS. A major difference between MRP and DRP is that while MRP is driven by the production schedule specified in the MPS to compute the time-phased requirements of components. DRP is driven by customer demand of the finished goods. Hence, MRP operates in a dependent demand situation, whereas DRP operates in an independent demand setting. The result of MRP execution is the production of finished-goods inventory at the manufacturing site, whereas DRY time-phases the movements of finished goods inventory from the manufacturing site to the central supply warehouse and distribution centers.

An obvious advantage of the DRP system is that it extends manufacturing planning and control visibility into the distribution system, thus allowing the firm to adjust its production plans and to avoid stocking excessive finished goods inventory. By now it should be clear that excessive inventory is a major cause of the bullwhip effect. Distribution requirements planning provides time-phased demand informa-tion needed for the manufacturing and distribution systems to effectively allocate finished goods inventory and production capacity to improve customer service and inventory investment.

(Joel D Wisner, Principles of Supply Chain Management: A Balanced Approach, pg.187)

SCANNING ELECTRON MICROSCOP (SEM)

Group: Micro structure characterization

There is no ol definition

New definition:

It is useful to consider the scanning electron microscope (SEM) as an instrument that greatly extends the usefulness of the optical microscope for studying specimens that require higher magnifications and greater depths of field than can be attained optically. Many SEM specimens are normally polished and etched in the same manner as would be done for examination in an optical microscope. Thus, tin lengthy and tedious procedures required for the preparation of TEM foil specimens are not needed for SEM specimens. The scanning electron microscope is capable of greatly extending the limited magnification range of the optical microscope, which normally extends to only about 1500 X. to over 50,000x. In addition, with the SEM it is possible to obtain useful images of specimens that have a great deal of surface relief such as are found on deeply etched specimens or on fracture surfaces. The depth of field of the SEM can be as great as 300 times that of the optical microscope. This feature makes the SEM especially valuable for analyzing fractures.

On the other hand, at low magnifications, that is, below 300 to 400x, the image formed by the scanning electron microscope is normally inferior to that of an optical microscope. Thus. the optical and scanning microscopes can be viewed as complementing each other. The optical microscope is the superior instrument at low magnifications with relatively flat surfaces ancl the scanning microscope is superior at higher magnifications and with surfaces having strong relief.

The scanning electron microscope differs significantly from the transmission electron microscope in the way an image of the specimen is formed. First, the field of view in tlw TEM specimen is uniformly -illuminated" by the high-speed electrons of the incident beam. After passirig through the foil specimen, these electrons are focused by a magtwtie objective lens to form an image of the specimen that is analogous to an optical shadow picture of the structure in the foil. The contrast in this image is produced by the varying degree to which the electrons are diffracted as they pass through the specimen. The image is thus roughly similar to that formed by an optical slide projector. In the scanning electron microscope, on the other hand, the image is developed as in a television set. The specimen surface is scanned by a pointed electron beam over an area known as the raswr. The interaction of this sharply pointed beam with the specimen surface causes several types of energetic emissions, including backscattered electrons, secondary electrons, Auger electrons (a special form of secondary electrons), continuous X-rays, and characteristic X-rays. Most of these emissions can furnish useful information about the nature of the specimen at the spot under the beam. In a standard scanning electron microscope, one normally uses he secondary electrons to develop an image. The reason for this is that the secondary electron signal comes primarily From the area directly under the beam and thus furnishes an image with a very high resolution or one in which the detail is better resolved. The secondary electron detector is shown to the right of the electron beam in the schematic drawing of Fig. 2.22. The front of this detector contains a screen biased at +200 V. Since most of the secondary electrons have energies only of the order of 3 to 5 eV, these low-energy electrons tend to be easily drawn into the detector by its 200 V bias.

In the part of the specimen surface used to form the image, that is, the raster, the elec-tron beam is swept along a straight line over the entire width of the raster, as indicated in Fig. 2.23. As the beam moves across the line, the strength of the secondary electron emission from the surface is measured by the detector and is used to control the brightness of the synchronized spot on the cathode ray tube used to view or record the image. When the electron beam completes its line scan at the far end of the raster, it is returned quickly to the other side of the raster and to a point just below the start of the first line. During the time of its return, the beam of the cathode ray tube is turned off. By repeating this line scan-ning process, the entire surface of the raster can be surveyed. The typical SEM uses 1000 line scans to form a 10 x 10 cm image. A CRT screen with a long persistence phosphor is used so that the image will last long enough tor the eye to be able to see a complete picture without problems of fading. The complete scanning process is repeated every thirtieth of a second, which conforms well to the one-twenty-fourth of a second frame time of a inotion picture. To obtain a permanent photographic record of the image, on the other hand. a cathode ray tube with a shun persistence phosphor is used. This avoids overlapping of images Iron, adjacent lines.

(Reza Abbaschian,Lara Abbaschian,Robert E. Reed-Hill, Physical Metallurgy Principles, PG.48)

POLYMER ADDITIVES

Group: material

There is no old definition

New definition:

A polymer seldom is sold as a pure material. More often a polymer contains several additives to aid during processing, add color, or enhance the mechanical properties.

Plasticizers

Solvents, commonly called plasticizers, am sometimes mixed into a polymer to dramatically alter its theological or mechanical properties. Plasticizers are used as processing aids since they have the same effect as raising the temperature of the material. The resulting lowered viscosities reduce the risk of thermal degradation during processing. For example. cellulose nitrite thermally degrades if it is processed without a plasticizer. Plasticizers are more commonly used to alter a polymer's mechanical properties such as stiffness, toughness, and strength. For example, adding a plasticizer such as dioctylphthalate (DOP) to PVC can reduce its stiffness by three orders of magnitude and can lower its glass transition temperature by 35 °C. In fact, highly plasticized PVC is rubbery at room temperature.

Flame Retardants

Since polymers are organic materials, most of them arc flammable. The flammability of polymers has always been a serious technical problem. However. some additives that contain halogens, such as bromine or chlorine or phosphorous. reduce the possibility of either ignition within a polymer component or once ignited, flame spread. Bromine is more effective flame retardant than chlorine.

Stabilizers

The combination of heat and oxygen can result in thermal degradation in a polymer. Heat or energy produce free radicals which react with oxygen to form carbonyl compounds, giving rise to yellow or brown discolorations in the final product. Thermal degradation can be suppressed by adding stabilizers, such as antioxidants or peroxide decomposers. These additives do not eliminate thermal degradation but slow it down. Once the stabilizer has been consumed by reaction with oxygen, the polymer is no longer protected against thermal degradation. Polyvinyl chloride is probably the polymer most vulnerable to thermal degradation. In polyvinyl chloride, scission of the C-CI bond occurs in the weakest point of the molecule. The chlorine radicals react with their nearest CH group, forming HCI and creating new weak C-CI bonds. A stabilizer must therefore he used to neutralize HCI and stop the autocatalytic reaction, as well as preventing corrosion of processing equipment.

Antistatic Agents

Since polymers have such low electrical conductivity, they can easily build-up electric charges. The amount of charge build-up is controlled by the rate at which the charge is generated compared to the charge decay. The rate of charge generation at the surface of the component can be reduced by reducing the intimacy of contact. whereas the rate of charge decay is increased through surface conductivity. }fence, a good antistatic agent should be an ionizable additive that allows the charge to migrate to the surface. At the same time it should be creating bridges to the atmosphere through moisture in the surroundings. Typical antistatic agents are nitrogen compounds, such as long chain amines, and amides and polyhydric alcohols.

Fillers

Fillers can be classified three ways: those that reinforce the polymer and improve its mechanical performance: those used to take-up space and so reduce the amount of resin to produce a part - sometimes referred to as extenders: and those, less common, that are dispersed through the polymer to improve its electric conductivity. Polymers that contain fillers that improve its mechanical performance are often referred to as reinforced plastics or composites. Composites can be furthermore divided into composites with high performance reinforcements, and composites with low pelfOrmance reinforcements. The high performance composites are those where the reinforcement is placed inside the polymer so that optimal mechanical behavior is achieved, such as unidirectional glass fibers in an epoxy resin. High performance composites usually have 50 to 80% reinforcement by volume and usually have a laminated tubular shape containing braided reinforcements. The low performance composites are those where the reinforcement is small enough that it can be well dispersed into the matrix. These materials can be processed the same way as their unreinforced counterparts. The most common filler used to reinforce polymeric materials is glass fiber. However, wood fiber, which is commonly used as an extender, also increases the stiffness and mechanical performance of some thermoplastics. To improve the bonding between the polymer matrix and the reinforcement, coupling agents such as silanes and liteuzates are often added. Extenders, used to reduce the cost of the component, are often particulate fillers. The most common of these arc calcium carbonate, silica flour, clay, and wood flour or fiber. As mentioned earlier, some fillers also slightly reinforce the polymer matrix, such as clay, silica flour, and wood fiber. Polymers with extenders often have significantly lower toughness than when unfilled.

Blowing Agents

The task of blowing or foaming agents is to produce cellular polymers,  also called expanded plastics. The cells can he completely enclosed (closed cell) or can he interconnected (open cell). Polymer foams are produced with densities between 1.6 kg/m3 and 960 kg/m3. There are many reasons for using polymer foams, such as their high strength to weight ratio, excellent insulating and acoustic properties, and high energy and vibration absorbing properties.

Polymer foams can be made by mechanically whipping gases into the polymer, or by either chemical or physical means. The basic steps of the foaming process arc (1) cell nucleation. (2) expansion or growth of the cells and (3) stabilization of the cells. Cell nucleation occurs when, at a given temperature and pressure, the solubility of a gas is reduced, leading to saturation, and expelling the excess gas to form bubbles. Nucleating agents are used for initial formation of a bubble. The bubble reaches an equilibrium shape when the pressure in the bubble balances the surface tension.

(Tim A. Osswald, Polymer Processing Fundamentals, pg.15-17)