Water injection moulding

Researchers from Bayer [44] developed Durethan grades of PA specifically for use in the water injection moulding process. No hydrolytic degradation takes place when the hot melt comes into contact with water. These particular grades are particularly resistant to conventional engineering coolants and, thus, are used in applications in automotive under body applications. 

Water injection moulding (WIM) or water assisted injection moulding (WAIM) is much newer and less well established. 

The negligible water absorption avoids the need for predrying granules. The low specific heat (compared with polyethylene) enables the polymer to be rapidly heated in injection cylinders, which therefore have a higher plasticising capacity with polystyrene than with polyethylene. The setting-up rates in the injection moulds are also faster than with the polyolefins so that faster cycles are also possible. 

Unlike other water-soluble resins the poly(ethylene oxide)s may be injection moulded, extruded and calendered without difficulty. The viscosity is highly dependent on shear rate and to a lesser extent on temperature. Processing temperatures in the range 90-130°C may be used for polymers with an intrinsic viscosity of about 2.5. (The intrinsic viscosity is used as a measure of molecular weight.)... 

It was claimed that the maximum continuous operating temperature in most chemical environments was 120°C and even 140-150°C in some instances. The major chemical applications were in the form of pipe and tank linings and injection moulded valve and pump parts. Coatings could be applied to metals by means of fluidised bed, water suspension and organic dispersion techniques. 

A number of injection mouldings have been prepared from CAB with about 19% combined acetic acid and 44% combined butyric acid. Their principal end products have been for tabulator keys, automobile parts, toys and tool handles. In the United States CAB has been used for telephone housings. Extruded CAB piping has been extensively used in America for conveying water, oil and natural gas, while CAB sheet has been able to offer some competition to acrylic sheet for outdoor display signs. 

Another event which may occur is hydrolysis. This is a chemical reaction between the plastic and water. It occurs extremely slowly at room temperature but can be significant at moulding temperatures. Hydrolysis causes degradation, reduction in properties (such as impact strength) and it is irreversible. Table 4.3 indicates the sensitivity of plastics to moisture. Note that generally extrusion requires a lower moisture content than injection moulding to produce good quality products. 

Nylons 6,6 and 6 are the ones usually employed as textile fibres. Where individual monofilaments are used, such as in brushes, sports equipment, or surgical sutures, nylons 6,10 and 11 tend to be used, because of their greater flexibility and water resistance. All nylons can be injection moulded and the resulting articles have found widespread use in engineering applications, such as bearings and gears. 

Control of the temperature of rubber compound whilst being processed in extruders and injection moulding machines is vital if the product from these processes is to be uniform in quality. Failure to control temperatures in processing equipment can lead to scorching of the compound as it emerges from the extruder die or injection machine nozzle. Most modem items of processing equipment are fitted to a temperature control unit using either oil or water as the circulatory medium. 

ISO 8361-2 1991 Thermoplastics pipes and fittings - Water absorption - Part 2 Test conditions for unplasticized poly(vinyl chloride) (PVC-U) pipes and fittings ISO 9393-2 1997 Thermoplastics valves - Pressure test methods and requirements - Part 2 Test conditions and basic requirements for PE, PP, PVC-U and PVDF valves ISO 9852 1995 Unplasticized poly(vinyl chloride) (PVC-U) pipes - Dichloromethane resistance at specified temperature (DCMT) - Test method ISO 9853 1991 Injection-moulded unplasticized poly(vinyl chloride) (PVC-U) fittings for pressure pipe systems - Crushing test... 

Polyolefin foams are easier to model than polyurethane (PU) foams, since the polymer mechanical properties does not change with foam density. An increase in water content decreases the density of PU foams, but increases the hard block content of the PU, hence increasing its Young s modulus. However, the microstructure of semi-crystalline PE and PP in foams is not spherulitic, as in bulk mouldings. Rodriguez-Perez and co-workers (20) showed that the cell faces in PE foams contain oriented crystals. Consequently, their properties are anisotropic. Mechanical data for PE or PP injection mouldings should not be used for modelling foam properties. Ideally the mechanical properties of the PE/PP in the cell faces should be measured. However, as such data is not available, it is possible to use data for blown PE film, since this is also biaxially stretched, and the texture of the crystalline orientation is known to be similar to that in foam faces. 

Plastic closures can be made by injection moulding, where the mould has two parts - a core which has the inside features of the closure and a cavity which has the features of the outside of the closure. Molten plastic is injected into the mould, which is water-cooled. The plastic solidifies, and the mould opens and the completed closure is ejected. If the closure is a two-piece design (i.e. fitted with a loose or flowed in-liner), then this is added later. Another technology for moulding plastic closures for the beverage market is compression moulding. A hot pellet of PP is positioned in a mould, similar to the cavity of an injection mould. As the mould closes, the pellet is squashed and defoimed into the shape... 

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. 

In 2005, Husky working alongside BIOTA Brands of America, blow moulding equipment supplier SIG Corpoplast and Cargill Dow, which supplied its NatureWorks PLA material, introduced the first biodegradable water bottle onto the US market. Husky supplied BIOTA with the 24-cavity HyPET 120 injection moulding system. 

PLA permits manufacture of varied and complex bottle shapes and sizes. Monolayer bottles of NatureWorks PLA can be formed on the same injection moulding/stretch blow moulding equipment used for PET, with no sacrifice in production rate. PET has some properties that PLA does not have and so NatureWorks is targeting applications where it has a competitive edge such as fresh food packaging and products that don t require sophisticated barriers such as water, milk and juice products. 

In the above studies compression and injection moulding were carried out with native potato starch dried at room temperature and then mixed with 7% water. After processing, the samples were subjected to ambient atmosphere and investigated after a storage time of at least 4 weeks. 

Figure 7.10 shows the microhardnesses of several injection moulded starch samples processed at different temperatures with different initial water content. There is an increase in H with the injection moulding temperature in the range 80-110 °C, from 120 MPa to 140 MPa (Balta Calleja etal, 1999). These microhardness values are notably higher than those found for conventional injection moulded thermoplastic polymers like PE (50-60 MPa) (Rueda et al 1989 Balt Calleja et al., 1995). 

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