Sandwich process

The sandwich process extends the scope of injection moulding and (as an instance of this) makes it possible to provide fibre-filled mouldings with surface finish of very high gloss. It is applied to produce reflectors for headlights in motor vehicles—where the substrate is heavily filled and resistant to distortion by heat yet the surface is mirror-like. 

The cycle starts with the plastification of the core component in the injection unit. Then the extruder moves to the bottom position, the injection unit moves forward to the extruder nozzle to link the nozzles of the extruder and the injection unit. The extruder starts plastification of the skin component and extrudes the melted skin component into the screw antechamber of the injection unit. Thus the skin and core components are located one after the other in the screw antechamber. After the extruder moved back to the top position, the injection unit moves forward to the mold followed by a conventional filling phase. Due to the fountain flow effect the first injected material forms the skin layer followed by the second component forming the core. Compared to the standard sandwich process the injection phase of the monosandwich process is less complicated as it is identical to the conventional injection molding process. 

The co-injection moulding of PVC-U with other thermoplastics (glass fibre reinforced PVC, polypropylene, ABS and polycarbonate), was investigated using the mono-sandwich process and the properties determined. Polypropylene was the only polymer not to exhibit good adhesion. The mechanical properties of the other samples were intermediate between those of the constituent polymers (104). 

This PET recyelate is used mainly in the nonfood sector, because of hygienic concerns. Small amounts of recyclate are used in bottle-to-bottle applications. In the sandwich process a core of recyclate is coated inside and outside by virgin PET. In this case up to 40% recyclate can be used. 

Surface of the sandwich Processing temperature Viscosity at RT Viscosity at 160°C Molding pressure Pressing force per m Curing time Cycle time... 

Materials to be inspected are, for example Concrete with lest frequencies of about 50 kHz, sandwich components with 500 kHz, metallic materials up to 10 MHz and thin components up to 35 MHz. Apart from the difference of materials, variable thicknesses and attenuations require different amplifications and signal processings. 

Ion Flotation and Foam Separation. Ions and dissolved surfactant molecules can be removed from solutions by the agency of foam. In this case ions are sandwiched in foam films. The scientific basis of these processes is weU understood and successes of metal ion recovery from solutions including U, Pt, Au, as weU as different surfactants (detergents) have been reported in the Hterature. 

Several manufacturiag processes can be used to produce phenohc foams (59,79) continuous production of free-rising foam for slabs and slab stock similar to that for polyurethane foam (61,80) foam-ia-place batch process (61,81) sandwich paneling (63,82,83) and sprayiag (70,84). 

The sandwich-type stmcture of polyurethanes with a smooth integral skin produced by the reaction injection molding process provides a high degree of stiffness as weU as exceUent thermal and acoustical properties necessary for its use in housing and load-bearing stmctural components for the automotive, business machine, electrical, furniture, and materials-handling industry. 

E. Greg Shinskey, B.S.Ch.E., Consultant (retired from Foxboro Co.), North Sandwich, New Hampsliire (Section 8, Process Control)... 

A variation of thermoforming which does not involve gas pressure or vacuum is matched die forming. The concept is very simple and is illustrated in Fig. 4.53. The plastic sheet is heated as described previously and is then sandwiched between two halves of a mould. Very precise detail can be reproduced using this thermoforming method but the moulds need to be more robust than for the more conventional process involving gas pressure or vacuum. 

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