Mould structure

INJECTION MOULDED STRUCTURAL FOAM MADE FROM POLYPROPYLENE... 

Fully cured unsaturated polyester resins used for glassfibre laminating are crosslinked thermosetting materials. Moulded structures will, therefore, be dimensionally stable, even at elevated temperatures. Rigid PU foam also has a crosslinked structure and is stable. The effect of dimensional changes resulting from thermal expansion of the foam can be ignored because the modulus of PU foam is low compared with that of the GRP skins. 

Function Creating a cellular structure during processing, usually by formation of an inert gas at the processing temperature used for foams and expanded materials, injection moulded structural foam. 

Materials/characteristics Physical blowing agents low-boil hydrocarbons. Chemical blowing agents fluorocarbons (for polyurethane and PS foams) sodium bicarbonate/citric acid or azo compounds, etc., for moulded structural foams. 

It is common practice to test the mould for vacuum tightness before its first use, or after repairing it. Insufficient mould tightness may result in material defects (such as pores), or insufficient compaction (and, thus, a low fibre fraction) in the final component. Typical sources of leaks are defective joints in the mould structure, such as weld lines or bonded joints. 

Sensors For process control, thermocouples and pressnre probes are often integrated directly into the vacuum set-up (or are already in the mould structure itself). 

It is worth mentioning that in some cases, sonrces of pressure leaks cannot be easily repaired. This is especially the case if the leak occurs in the mould structure, which is why vacuum leak tests on the empty mould, as described before, are so important. 

Hybrid polyester/polyurethane structurat foam - a new reaction spray moulded structural foam core support. 

Moving the central nozzle away from the mould axis causes an asymmetrical mould structure, asymmetrical loading of the ejection system and an asymmetrical stress distribution in the injection machine columns. The effective mould clamping force is diminished. 

Criens, R.M. and Moste, H.G., On the Influence of Knit Lines on the Mechanical Behaviour of Injection Moulded Structured Elements. S.P.E. Antec. 28 22-24 (1982). 

Polymerisation casting involves mixing monomer or low molecular weight polymer with a polymerisation initiator, pouring the mix into the mould and allowing polymerisation to occur in situ. A variation is to impregnate fibres with initiated monomer or other low molecular weight material and polymerise to produce composite structures. The main problem is due to the heat of polymerisation. Unless heat transfer distances are kept short or unless the reaction is carried out very slowly it can easily get out of hand. 

High molecular weight HDPE (HMW-HDPE) is used for blown film and for demanding moulding and structural uses. Examples are blow moulded drums for packing dangerous chemicals and pressure piping. 

The main application of poly (vinyl formal) is as a wire enamel in conjunction with a phenolic resin. For this purpose, polymers with low hydroxyl (5-6%) and acetate (9.5-13%) content are used. Similar grades are used in structural adhesive (e.g. Redux) which are also used in conjunction with phenolic resin. Poly(vinyl formal) finds some use as a can coating and with wash primers. Injection mouldings have no commercial significance since they have no features justifying their use at current commercial prices. 

Tbe term structural foam was originally coined by Union Carbide to describe an injection moulded thermoplastic cellular material with a core of relatively low density and a high-density skin. The term has also been used to describe rigid foams that are load bearing. Today it is commonly taken to imply both of the above requirements, i.e. it should be load bearing and with a core of lower density than the skin. In this section the broader load-bearing definition will be used. Whilst structural foams are frequently made from polymers other than polystyrene, this polymer is strongly associated with such products and it is convenient to deal with the topic here. 

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. 

Structurally viscous grades are based on branched polymers (branching being effected by the use of tri- or higher functional phenols). These polymers exhibit a sharp decrease in viscosity with increasing shear rate which makes them particularly suitable for extrusion and blow moulding and also, it is claimed, in reducing drip in case of fire. 

Glass-reinforced grades have widely replaced metals in pumps and other functional parts in washing equipment and central heating systems. In the manufacture of business machine and computer housings structural foam materials have found some use. Mouldings weighing as much as 50 kg have been reported. 

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