Mould Nylon

Nylon (moulded) Nylon (machined surface) Mild steel... 

The surface structure of a plastic may exert a considerable influence on the friction an example is given by injection moulded articles of PP, which show, in touch with each other, a coefficient of 0.7, while for sand-blasted surfaces a value of only 0.3 is found. For injection-moulded nylon // = 0.65, for machined surfaces 0.47. As a matter of fact, lubrication has a strong influence the value of 0.47 for nylon is reduced to 0.19 with water lubrication and to 0.08 with oil. 

Rus Russell, D. P., Beaumont, P. W. R. Structure and properties of injection-moulded nylon-6. Part 3. Yield and fracture of injection-moulded nylon-6. J. Mater. Sci. 15 (1980) 216-221. 

Finely divided red phosphorus is an effective flame retardant which is currently used in some polyethylene, polycarbonate, PVC and moulded nylon products [40]. It is widely used for electrical switches and moulded glass/nylon products where colour is unimportant. The potential toxicity of this material (due to the possible generation of P4 or PH3) can be reduced by microencapsulation in a thermosetting resin. Cellulose phosphate (Chapter 10.1) also has some flre-retardant properties. 

M0S2 is really only effective in nylon, where it acts as a nucleating agent, promoting the growth of fine crystalline structure at the surface of the moulding. Nylons are prone to form a skin of amorphous polymer, as they freeze rapidly at the surface this is less wear resistant than the crystalline structure. 

Residual strains in injection moulded Nylon-6 have been measured but the stress distribution is parabolic, with a compressive stress as much as 6 Mn m on the surface and a tensile one in the middle of 4 Mn m dimension in water reverses this distribution making it tensile on the surface. This has been attributed to recrystallization in presence of water and not simple to adsorption and volume changes. The yield and fracture toughness of the Nylon-6 is affected by water content, moulding conditions due to changes in yield stress. There is a brittle to ductile transition with temperature associated with cc-transition. The yield stress increases linearly with crystallinity, whereas the fracture toughness falls consistent with a move from plane strain to plane stress conditions. 

The two part design, utilising an injection-moulded nylon housing, has an assembly efficiency rating of 93%. 

New materials also emerged. Nylon, developed brilliantly by W. H. Carothers and his team of research workers for Du Pont as a fibre in the mid-1930s, was first used as a moulding material in 1941. Also in 1941 a patent taken out by Kinetic Chemical Inc. described how R. J. Plunkett had first discovered polytetrafluoroethylene. This happened when, on one occasion, it was found that on opening the valve of a supposedly full cylinder of the gas tetrafluoroethylene no gas issued out. On subsequently cutting up the cylinder it was found that a white solid, polytetrafluoroethylene (PTFE), had been deposited on the inner walls of the cylinder. The process was developed by Du Pont and, in 1943, a pilot plant to produce their product Teflon came on stream. 

Materials which reduce the friction of mouldings and other finished products when these are rubbed against adjacent materials which may or may not be of the same composition. The most well-known examples here are graphite and molybdenum disulphide used in quantities of the order of 1-2% in nylons and other thermoplastics used in gear and bearing applications. 

Injection moulding and extrusion may be carried out at temperatures in the range of 300-380°C. The polymer has a high melt viscosity and melt fracture occurs at a lower shear rate (about 10 s ) than with low-density polyethylene (about 10 s ) or nylon 66 (about 10 s ). Extruders should thus be designed to operate at low shear rates whilst large runners and gates are employed in injection moulds. 

Copolymers of chlorotrifluoroethylene and ethylene were introduced by Allied Chemicals under the trade name Halar in the early 1970s. This is essentially a 1 1 alternating copolymer compounded with stabilising additives. The polymer has mechanical properties more like those of nylon than of typical fluoroplastic, with low creep and very good impact strength. Furthermore the polymers have very good chemical resistance and electrical insulation properties and are resistant to burning. They may be injection moulded or formed into fibres. 

The early development of the nylons is largely due to the work of W. H. Carothers and his colleagues, who first synthesised nylon 66 in 1935 after extensive and classical researches into condensation polymerisation. Commercial production of this polymer for subsequent conversion into fibres was commenced by the Du Pont Company in December 1939. The first nylon mouldings were produced in 1941 but the polymer did not become well known in this form until about 1950. 

The polymerisation casting of nylon 6 in situ in the mould has been developed in recent years. Anionic polymerisation is normally employed a typical system uses as a catalyst 0.1-1 mol.% of acetic caprolactam and 0.15-0.50 mol.% of the... 

Reaction injection moulding techniques, developed primarily for polyurethanes (see Chapter 27), have also been adapted for nylon 6 in what must be considered as a variation of the polymerisation casting technique. 

The properties of the nylons are considerably affected by the amount of crystallisation. Whereas in some polymers, e.g. the polyacetals and PCTFE, processing conditions have only a minor influence on crystallinity, in the case of the nylons the crystallinity of a given polymer may vary by as much as 40%. Thus a moulding of nylon 6, slowly cooled and subsequently annealed, may be 50-60% crystalline, while rapidly cooled thin-wall mouldings may be only 10% crystalline. 

The suppliers of nylon 46 have laid particular emphasis on the fact that this polymer, with its highly symmetrical chain structure, leads to both a high level of crystallinity and a high rate of nucleation. In turn the high nucleation rate leads to a fine crystalline structure which in this case is claimed to lead to a higher impact strength (dry as moulded) than with nylons 6 and 66. 

Laboratory tests and experience during use have demonstrated that the nylons have extremely good abrasion resistance. This may be further improved by addition of external lubricants and by processing under conditions which develop a highly crystalline hard surface e.g. by use of hot injection moulds and by annealing in a non-oxidising fluid at an elevated temperature (150-200°C for nylon 66). 

The major nylon moulding materials are each available in a number of grades. These may differ in molecular weight but they may also differ in the nature of additives which may be present. 

Unlike polyurethane-RIM processes, nylon-RIM reactions are endothermic and require temperatures of 130-140°C. In contrast to the polyurethane-RIM systems, this enables thick wall parts to be made. Cycle times of 2-3 minutes are comparable to those for polyurethane-RIM. In the development stage, current work is concerned with reducing moulding times and optimising moulding conditions. 

Sterilisable mouldings have found application in medicine and pharmacy. Because of their durability, nylon hair combs have found wide acceptance in spite of their higher cost. 

There is persisting interest in nylon-RIM materials as alternatives to polyurethane-RIM. Advantages of the nylon materials are the better shelf life and lower viscosity of the reaction components, ability to mould thick-walled articles, absence of a need for mould lubrication and the ability to avoid using isocyanates with their associated hazards. The main disadvantages of nylon-RIM are the need to have heated storage tanks and elevated temperature reactions, difficulties in catalyst handling and the high water absorption of the product. Possible markets include exterior car body components and appliance and business machine components. 

Other, more recently developed, uses include microwave oven parts, transparent pipelines, chemical plant pumps and coffee machine hot water dispensers. One exceptional use has been to produce, by an extrusion moulding process, very large rollers for textile finishing for use where cast nylons cannot meet the specification. Also of growing interest are medical equipment applications that may be repeatedly steam-sterilised at 134°C, filtration membranes and cartridges for ink-jet printers. 

The properties of the polyurethane moulding compositions are also very similar to nylon 66. The greatest difference in properties is in water absorption, the 6,4-polyurethane absorbing only about of that of nylon 66 under comparable conditions. This results in better dimensional stability and a good retention of electrical insulation properties in conditions of high humidity. Resistance to sulphuric acid is somewhat bettter than with nylon 66 but both types of polymer are dissolved by phenols and formic acid. 

A further approach is used by Bayer with their polyesteramide BAK resins. A film grade, with mechanical and thermal properties similar to those of polyethylene is marketed as BAK 1095. Based on caprolactam, adipic acid and butane diol it may be considered as a nylon 6-co-polyester. An injection moulding grade, BAK 2195, with a higher melting point and faster crystallisation is referred to as a nylon 66-co-polyester and thus presumably based on hexamethylene diamine, adipic acid and butane diol. 

---