Cold moulding process

There are also two variants of the direct foam moulding process the so called hot moulding process and the cold moulding process. 

Lay-up is a cold moulding process and therefore cure is achieved by the use of a catalyst (e.g. a peroxide) and also an accelerator (typically cobalt napthanate). This may be assisted by the application of heat, generally by the use of an oven. When the laminate is sufficiently cured, it may be released from the mould and excess material at the edge trimmed back. Often the released part is post-cured at elevated temperature to reach the temperature performance or mechanical properties required for the application. 

As this is a cold moulding process there is no heat to promote the cure and consequently the resin mix requires both catalyst and accelerator. This must be mixed immediately prior to each shot (a batch would immediately start to cure). Once thoroughly mixed the resin is poured on to the middle of the reinforcement. 

Resin injection moulding is a cold moulding process applied at medium pressures (approximately 450 kPa), where mould surfaces are enriched with release agents and gel coat before GF reinforcement is placed on the bottom of the mould, allowing the plastic to extend beyond the sides of the mould. Then the upper mould is placed in its place and it is clamped to stop followed hy injection of activated resin under pressure into the mould cavity (Figure 9.11). By using this technique, it is possible to obtain a fibre/matrix ratio of 65 wt%. 

Therefore the reaction rate for an in situ surface modification during a moulding process has to be very fast, as can be concluded from the model assumption in Fig. 18. The chemical coupling of substances has to be finished after a very short time because at the moment of contact of the hot melt front with the tempered or rather "cold" mould surface the temperature drops rapidly and as a result an exponential decrease of the reaction rate should be observed (Arrhenius equation). 

The polyether triols are the most important class of polyether polyols and they are used in flexible PU foam fabrication. The majority of polyether triols used in flexible foams are copolymers of PO-EO. Random copolymers are used in continuous slabstock flexible foams and block copolymers (PO-EO), with terminal poly[EO] block, are used in moulded foams (hot moulding and cold cure moulding processes). 

PHD polyols are successfully used for high resilience flexible PU foams (made by the cold cure process), for continuous slabstock flexible PU foams and for elastomers obtained especially by reaction injection moulding (RIM) technology. 

PIPA polyols, in spite of some disadvantages (tendency to foam shrinkage, and scorching), are used successfully for continuous slabstock flexible PU foams and high resilience foams (cold cure moulding process). 

In the blow moulding and thermoforming processes, solidification commences only when the melt contacts the cold mould wall, and flow stops. The time available for melt stress relaxation depend on the thickness of the product, and how close the melt is to the cold mould. There is likely to be high orientation in the thin side walls of a thermoformed disposable cup, made from a highly viscous glassy polymer that has solidified rapidly (hence the heat reversion shown in Fig. 1.16). Hence, if an empty cup is squeezed flat, vertical cracks tend to occur in the side walls, in the direction of orientation. 

Plastic bottles are manufactured by extrusion followed by blowing (extrusion blow moulding), as shown in Fig. 7.33. A certain length of tube is extruded (the parison) and a mould applied. The latter obturates the bottom end of the parison. Air pressure is supplied inside the parison, forcing the melt onto the cold mould and hence giving it its final shape. In this process, the orien-... 

Under normal processing conditions the surface of the moulded part always has a swirl pattern. This occurs on mould filling and is due to the collapse of the foam cells as they come into contact with the cold mould surface. 

The basic requirements of the extrusion blow-moulding process are to extrude a molten plastic tube (the parison), clamp it in a cold split mould, expand the parison by means of compressed air, cool the shaped moulding, remove from the mould and trim as necessary. Interest in the present work centres on the example of the Acitainer, which is produced by this process. 

For a bottle, the process is somewhat similar, except for the machinery which should be different, since the melted polymer is injected into a cold mould shaping the bottle. Thus the two polymers (or the three) are injected into the mould, in succession. The second polymer is injected while the surface of the previous one is still melted. 

Several engineering plastics have been used as the matrix in both of these GMT systems however the versatile and modestly priced polypropylene is by far the most popular. Both types of GMT are fabricated by a process known as hot flow stamping . This is similar to sheet moulding, but uses preheated blanks pressed in cold moulds, with a cycle time of less than one minute, comparable with injection moulding. (SMC uses heated press tools to mould cold blanks, with rather longer cycle times.)... 

Fig. 19 Processing route to directly form simple geometries from woven polymer fibre plies. Fabric plies (or a piecmisolidated fabric plate) (a), may be placed in a constraining Irame (b) to prevent shrinkage if preheating is required and also to prevent the fabric plies being pulled excessively into the mould during forming. The clamped fabric plies may then be preheated in an overt, and then stamped in a cold mould (c), or placed cold in a mould and heated in situ. The part may he consolidated in the mould, cooled and removed (d). Adapted from [22]...

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