Polymer Categories, Acetal (POM)

Acetal polymers are formed from the polymerization of formaldehyde. They are also known by the name polyoxymethylenes (POM). Polymers prepared from formaldehyde were studied by Staudinger in the 1920s, but thermally stable materials were not introduced until the 1950s when DuPont developed Delrin.1 Homopolymers are prepared from very pure formaldehyde by anionic polymerization, as shown in Fig. 1.4. Amines and the soluble salts of alkali metals catalyze the reaction.2 The polymer formed is insoluble and is removed as the reaction proceeds. Thermal degradation of the acetal resin occurs by unzipping with the release of formaldhyde. The thermal stability of the polymer is increased by esterification of the hydroxyl ends with acetic anhydride. An alternative method to improve the thermal stability is copoly merization with a second monomer such as ethylene oxide. The copolymer is prepared by cationic methods.3 This was developed by Celanese and marketed under the tradename Celcon. Hostaform is another copolymer marketed by Hoescht. The presence of the second monomer reduces the tendency for the polymer to degrade by unzipping.4 There are four processes for the thermal degradation of acetal resins. The first is thermal or base-catalyzed depolymerization from the chain, resulting in the release of formaldehyde. End capping the polymer chain will reduce this tendency. The second is oxidative attack at random positions, again leading to depolymerization. The use of antioxidants will reduce this degradation mechanism. Copolymerization is also helpful. The third mechanism is cleavage of the acetal linkage by acids. It is, therefore, important not to process acetals in equipment used for polyvinyl chloride (PVC), unless it has been cleaned, due to the possible presence of traces of HCl. The fourth degradation mechanism is thermal depolymerization at temperatures above 270°C. It is important that processing temperatures remain below this temperature to avoid degradation of the polymer.5 Acetals are highly crystalline, typically 75% crystalline, with a melting point of 180°C.6 Compared to polyethylene (PE), the chains pack closer together because of the shorter C O bond. As a result, the polymer has a higher melting point. It is also harder than PE. The high degree of crystallinity imparts good solvent resistance to acetal polymers. The polymer is essentially linear with molecular weights (Mn) in the range of 20,000 to 110,000.7 Acetal resins are strong and stiff thermoplastics with good fatigue properties and dimensional stability. They also have a low coefficient of friction and good heat resistance.8 Acetal resins are considered similar to nylons, but are better in fatigue, creep, stiffness, and water resistance.9 Acetal resins do not, however, have the creep resistance of polycarbonate. As mentioned previously, acetal resins have excellent solvent resistance with no organic solvents found below 70°C, however, swelling may occur in some solvents. Acetal resins are susceptible to strong acids and alkalis, as well as oxidizing agents. Although the C O bond is polar, it is balanced and much less polar than the carbonyl group present in nylon. As a result, acetal resins have relatively low water absorption. The small amount of moisture absorbed may cause swelling and dimensional changes, but will not degrade the polymer by hydrolysis.10 The effects of moisture are considerably less dramatic than for nylon polymers. Ultraviolet light may cause degradation, which can be reduced by the addition of carbon black. The copolymers generally have similar properties, but the homopolymer may have slightly better mechanical properties, and higher melting point, but poorer thermal stability and poorer alkali resistance.11 Along with both homopolymers and copolymers, there are also filled materials (glass, fluoropolymer, aramid fiber, and other fillers), toughened grades, and ultraviolet (UV) stabilized grades.12 Blends of acetal with polyurethane elastomers show improved toughness and are available commercially. Acetal resins are available for injection molding, blow molding, and extrusion. During processing it is important to avoid overheating or the production of formaldehyde may cause serious pressure buildup. The polymer should be purged from the machine before shutdown to avoid excessive heating during startup.13 Acetal resins should be stored in a dry place. The apparent viscosity of acetal resins is less dependent on shear stress and temperature than polyolefins, but the melt has low elasticity and melt strength. The low melt strength is a problem for blow molding applications. For blow molding applications, copolymers with branched structures are available. Crystallization occurs rapidly with postmold shrinkage complete within 48 h of molding. Because of the rapid crystallization it is difficult to obtain clear films.14 The market demand for acetal resins in the United States and Canada was 368 million pounds in 1997.15 Applications for acetal resins include gears, rollers, plumbing components, pump parts, fan blades, blow-molded aerosol containers, and molded sprockets and chains. They are often used as direct replacements for metal. Most of the acetal resins are processed by injection molding, with the remainder used in extruded sheet and rod. Their low coefficient of friction make acetal resins good for bearings.16 

Modern
Plastics
Handbook
Modern Plastics
and
Charles A. Harper Editor in Chief
Technology Seminars, Inc.
Lutherville, Maryland
McGraw-Hill
New York San Francisco Washington, D.C. Auckland Bogotá
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High-Molybdenum Alloys13

High-molybdenum stainless and nickel alloys are welded with an overmatching filler metal. This is necessary to maintain corrosion resistance in the weld metal at least equal to the base metal. The reason is that molybdenum and chromium segregate as the weld metal solidifies from the melt. This leaves local areas with high and low molybdenum content. Pitting corrosion can start in the low-Mo areas, with the pits eventually growing even into metal with high molybdenum content. This occurs in alloys ranging from 316L to C-276, for the most part being more severe at higher alloy contents This matter began to receive attention when the 6% Mo stainless steels came on the market. If any of these 6% Mo grades are welded without filler metal, the result is a weld bead that may be as low as 3% Mo in areas. The end result can be that this weld has only the pitting corrosion resistance of 317L stainless. In the case of tubular products autogenously welded in production, a high-temperature anneal is used to homogenize the metal. In addition, a small amount of nitrogen, 3–5%, is added to the torch gas. Fabrications of thin sheet, which cannot be annealed after welding, should have this nitrogen addition to minimize the loss of corrosion. Even so, because thin-sheet welds solidify more quickly, the segregation is less severe. In normal fabrication of a 6% Mo grade, alloy 625 (ERNiCrMo-3) filler metal is used. The weld metal contains 9% Mo. After welding, segregation causes some areas to have as little as 6% Mo. The result is that the alloy 625 weld bead has approximately the same corrosion resistance as the 6% Mo base metal. Higher alloy weld fillers, such as ERNiCrMo- 10 or ERNiCrMo-14, may also be used, though the benefit may be more theoretical than real. ERNiCrMo-4 is not suggested, as it has 5% less chromium than does AL-6XN, for example. Since the mid-1980s nearly all of the 6% Mo alloy fabrications have been made, and put into service, using a 9% Mo weld filler. ERNiCrMo-3 weld filler is widely available and is appropriate for welding lower alloys such as 317L, 317LMN, and 904L for chloride service. The problem of reduced weld bead corrosion resistance from molybdenum and chromium segregation exists with most of the 13–16% Mo nickel alloys as well. Filler 686 CPT (ERNiCrMo-14) does appear to be markedly less susceptible to this effect than other high-molybdenum alloys. 

STAINLESS STEELS
James Kelly
Rochester, Michigan
Mechanical Engineers’ Handbook: Materials and Mechanical Design, Volume 1, Third Edition.
Edited by Myer Kutz
2006 by John Wiley & Sons, Inc.
 

Line types and thicknesses

The standard ISO 128:1982 gives 10 line types that are defined A to K (excluding the letter I). The table in Figure 3.4 shows these lines. The line types are 'thick', 'thin', 'continuous', 'straight', 'curved', 'zigzag', 'discontinuous dotted' and 'discontinuous chain dotted'. Each line type has clear meanings on the drawing and mixing up one type with another type is the equivalent of spelling something incorrectly in an essay. The line thickness categories 'thick' and 'thin' (sometimes called 'wide' and 'narrow') should be in the proportion 1:2. However, although the proportion needs to apply in all cases, the individual line thicknesses will vary depending upon the type, size and scale of the drawing used. The standard ISO 128:1982 states that the thickness of the 'thick' or 'wide' line should be chosen according to the size and type of the drawing from the following range: 0,18; 0,25; 0,35; 0,5; 0,7; 1; 1,4 and 2mm. However, in a direct contradiction of this the standard ISO 128-24:1999 states that the thicknesses should be 0,25; 0,35; 0,5; 0,7; 1; 1,4 and 2mm. Thus confusion reigns and the reader needs to beware! With reference to the table in Figure 3.4, the A-K line types are as follows. The ISO type 'A' lines are thick, straight and continuous, as shown in Figure 3.5. They are used for visible edges, visible outlines, crests of screw threads, limit of length of full thread and section viewing lines. The examples of all these can be seen in the vice assembly detailed drawings. These are by far the most common of the lines types since they define the artefact. The ISO type 'B' lines are thin, straight and continuous, as shown in Figure 3.6. They are used for dimension and extension lines, leader lines, cross hatching, outlines of revolved sections, short centre lines, thread routes and symmetry ('equals') signs. 

Engineering Drawing for Manufacture
by Brian Griffiths
· ISBN: 185718033X
· Pub. Date: February 2003
· Publisher: Elsevier Science & Technology Books


 

Manufacturing Data and Knowledge

Machining methods (also referred to as machining practices) provide CAPP with the knowledge, expertise, and procedures that a human process planner uses. These methods may be based on sound scientific principles, experimental results, experience, or preferences established within a particular machining context. They also may be generic and applicable over a wide range of machining problems or specific to a single one. The challenges in using machining methods within CAPP fall into the following categories: • Identification and retrieval • Implementation • Maintenance • Customization Identification and retrieval are concerned with understanding how a human process planner applies experience and techniques to make decisions when generating process plans: What decisions are being made? What characteristics of the situation are being recognized by the planner that trigger these decisions? The main challenge here stems from the fact that human planners do not necessarily follow a consistent strategy in applying methods. The process often requires complex trade-offs of information from several sources. When one of these sources is experience, the basis of the applied method can be difficult to verify. Thus, identification and retrieval of methods are not just a bookkeeping task. Rather, it requires the cultivation of an attitude toward process planning based on a sound methodology for applying machining methods. Methods implementation requires an approach that is general enough to capture information from very different sources while at the same time is simple enough to provide a maintainable, noncorruptible environment. Rule-based expert systems have been the most commonly adopted implementation strategy among CAPP system developers. Because the need to update or add new methods always exists as more information becomes available or as new methods are applied to more applications, maintenance of the knowledge base becomes a key concern. As changes are made, the integrity of the information needs to be preserved. One problem occurs when new methods are added that conflict with old ones. The system needs to include a strategy for resolving such conflicts. One approach that has been used extensively with expert systems is to place the onus on a knowledgable engineer to avert such problems. However, as the size of the knowledge base grows, the cost of employing dedicated personnel for this task becomes prohibitive. Finally, creating off-the-shelf CAPP systems with the methods included is a difficult if not impossible task. This is because it is unlikely that the system developer can capture all the desired methods from all potential users during system development. Thus, while a system may come with some generic, widely accepted methods, it must include a facility to allow new methods customized to each context to be added to the system.

THE MECHANICAL
SYSTEMS
DESIGN
HANDBOOK
Modeling, Measurement,
and Control
OSITA D. I. NWOKAH
YILDIRIM HURMUZLU
Southern Methodist University
Dallas, Texas
CRC PRESS
Boca Raton London New York Washington, D.C.