Gas injection moulding

In recent years major developments have been made in the use of an inert gas to act as the core in an injection moulded plastic product. This offers many advantages including greater stiffness/weight ratios and reduced moulded-in stresses and distortion. 

The first stage of the cycle is the flow of molten polymer into the mould cavity through a standard feed system. Before this flow of polymer is complete, the injection of a predetermined quantity of gas into the melt begins through a special nozzle located within the cavity or feed system as shown in Fig. 4.45. The timing, pressure and speed of the gas injection is critical. 

7 Shear Controlled Orientation in Iitjection Moulding (SCORIM) 

One of the majOT innovations in recent years is the use of pulsed pressure through the gates to introduce and control the orientation of die structure (or fillers) in injection moulded products. A special manifold is attached to the machine nozzle as illustrated in Fig. 4.46. This diagram relates to the double live feed of melt although up to four pistons, capable of applying oscillating pressure may be used. 

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Fig. 4.4S Stages in the gas injection moulding of an automotive handle (courtesy of Cinpress Ltd)...

During gas injection moulding a gas such as nitrogen is injected into the melt in such a way that it stays in discrete pockets in the thicker sections and as the material cools these pockets of gas help to keep it in contact with the walls of the mould. Moulding of polypropylene in thick section may be done by this... 

The characteristic feature of gas injection moulding technology is the filling of a form with two different materials. Plastic materials form the first component. The second component consists of a gas, generally nitrogen (N2). The two components do not mix. All standard injection moulding machines which are equipped with a device for introducing gas are suitable for the GIT process. The injection of the gas may be performed by a machine nozzle or by a separate injection module in the mould. 

Figure 10.17 The gas injection moulding process 10.9.1.2 Patent Situation...

The most significant disadvantages of gas injection moulding technology are ... 

Process Variations in the Application of Gas Injection Moulding Technology... 

Sanchez-Soto, M., et al. Optimising the gas-injection moulding of an automobile plastic cover using and experimental design procedure, Journal of Materials Processing Technology 178 (2006) 369-378. 

Paste injection moulding processes have also been developed. In one technique used for applying PVC soles to shoe uppers the paste is injected by gas under pressure into a hot mould, the last and shoe upper forming the top half of the mould. The paste gels in the mould, adheres to the upper in the presence of a suitable adhesive and is stripped hot from the mould. 

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. 

Gas-assisted injection moulding is a variant of injection moulding suitable for producing parts with internal cavities or hollow parts. It is used for the manufacture of products such as household appliances and automobile parts. 

Apart from the injection step itself, which is identical to common injection moulding, there are three steps for mould filling and gas introduction ... 

Figure 5.5 schematizes the principle of gas-assisted injection moulding. Adding to the advantages of injection are ... 

Olefins or alkenes are defined as unsaturated aliphatic hydrocarbons. Ethylene and propylene are the main monomers for polyolefin foams, but dienes such as polyisoprene should also be included. The copolymers of ethylene and propylene (PP) will be included, but not polyvinyl chloride (PVC), which is usually treated as a separate polymer class. The majority of these foams have densities <100 kg m, and their microstructure consists of closed, polygonal cells with thin faces (Figure la). The review will not consider structural foam injection mouldings of PP, which have solid skins and cores of density in the range 400 to 700 kg m, and have distinct production methods and properties (456). The microstructure of these foams consists of isolated gas bubbles, often elongated by the flow of thermoplastic. However, elastomeric and microcellular foams of relative density in the range 0.3 to 0.5, which also have isolated spherical bubbles (Figure lb), will be included. The relative density of a foam is defined as the foam density divided by the polymer density. It is the inverse of the expansion ratio . 

INCREASING THE DIMENSIONAL STABILITY OF PRODUCTS MADE FROM FOAMED AND FILLED THERMOPLASTICS BY THE USE OF GAS COUNTERPRESSURE INJECTION MOULDING Piperov N L... 

The manufacture of products, e.g. containers and lids, made from filled foamed PE by gas counter pressure injection moulding was studied with special reference to the effect of production conditions on dimensional stability. Results obtained are discussed and evaluated. 7 refs. 

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