Collapsible mandrel

The assembly for inhibition by thread-winding technique is shown in Figure 4.13 except that a propellant grain is used in place of a collapsible mandrel. 

A collapsible mandrel is prepared by applying a very thin coat of silicone grease on it followed by application of aluminum foil of suitable thickness. This prepared mandrel is then held in the lathe machine. A thin uniform coat of resin (novolac epoxy resin plus hardener) is applied to the prepared collapsible mandrel revolving at a speed of 40 rpm and is allowed to cure partially. Then the dried rayon thread on a spool is passed through the resin formulation and is wound on to the mandrel... 

Filament-Winding. This requires a mandrel to shape the desired finished product. Continuous filament or woven tape is fed through a liquid resin bath to impregnate it, and then wound onto the mandrel in a calculated pattern to optimize the final properties (Table 15.27). The assembly is oven-cured. A collapsible mandrel can then be removed from the plastic product or the mandrel can be left as a part of the finished product. These are the strongest plastic products ever made. Typical products are pipes, tanks, and pressure bottles. Other suggested products... 

Filament winding of continuous roving for making pressure vessels and pipes has also shown many advances. These advances are concerned with fiber payout and the use of collapsible mandrels for continuous pipe production. 

The mandrel (mold) on which filament-wound products are produced is essentially light, but solid, in construction and (usually) near cylindrical. The most common product is pipe, when the mandrel can be made simply of aluminum sheet on a lightweight framework. RP can also be used. An important aspect of the mandrel is that it must be possible to extract it easily from the wound lay up, after curing. This may mean use of a tapered or collapsible mandrel (Figure 5.77), or even a sacrificial mandrel (made possibly of plaster) which is broken up inside the cured product. 

Figure 5.77 Basic configuration of a mandrel for filament winding, and the principle of a collapsing mandrel...

The FW mandrel must be strong enough to withstand rather high accumulated tension loads due to the filament winding, and must be stiff enough not to sag between end supports. At the same time, it must be possible to remove the mandrel from the finished part after curing, which may require the use of an intricate collapsible mandrel. 

After cure the component is removed from the mandrel either by collapsing the mandrel or by a mechanical puller device, typically hydraulic to generate sufficient force. Complex shapes may not allow the mandrel to be withdrawn. This problem may be overcome by the use of a collapsible mandrel or by producing the mandrel from a meltable material such as low melting point alloys or wax. 

The geometrical definition of the wound part is primarily done by the mandrel, which has to be removed from the part after consolidation of the composite material. In terms of tubes or cones with no undercut, this removal is relatively easily performed using a mandrel-extractor. The mandrel can then be reused. Mandrels of other geometries such as that required for vessels cannot be removed mechanically and have to stay inside the part. They are called expendable mandrels and may be of additional service for corrosion protection or as a diffusion barrier. Collapsible mandrels can be apphed for large part diameters with undercuts [28,31]. 

In the past a limitation on this process was that it tended to be restricted to shapes which were symmetrical about an axis of rotation and from which the mandrel could be easily extracted. However, in recent years there have been major advances through the use of collapsible or expendable cores and in particular through the development of computer-controlled winding equipment. The latter has opened the door to a whole new range of products which can be filament wound - for example, space-frame structures. Braiding machines for complex shapes are shown in Fig. 4.76. 

To ease part removal, mandrels may be constructed from water-soluble materials (sand), plaster, or an assemblage of metal shells that is collapsible or segmented [6], Tube mandrels constructed with a high-quality surface finish and a slight taper are often used for cylindrical parts. 

Fig. 3 Schematic of spiral mandrel blown film die operation (1) ring-shaped melt distribution (2) die body (3) spiral flow mandrel (4) sizing ring (5) spreader (6) film bubble (7) frost line (8) solidified film (9) bubble collapsing rollers (10) nip rollers (11) external bubble cooling air (12) internal bubble cooling air inlet (13) internal bubble cooling pipe and (14) heated internal bubble air return.

Most blown film operations extrude the resin in an upward direction. However, blown polypropylene film is generally extruded downwards and water or mandrel quenched. The extruded tube is then reheated, to a point still below its melt temperature, before it is blown. The collapsed bubble can be fed over a series of heated rollers to reheat it and relieve thermal stresses if a heat-stabilized film is wanted or it can be heated and reinflated in what is known as the double bubble process, which will be discussed in Section 7.3.7. In either case, the film is restrained until cooling is complete, to keep it from shrinking. 

The final wound structure is removed from the winding machine, and the mandrel/ workpiece is placed in an oven for curing. Various mandrels are used and can be made of hard chrome plated and polished ground steel, which may be of a segmented collapsible construction, but must have no concave curvature. If the part has complex geometry, then fusible low melting point alloys, a soluble plaster that can be removed with hot water after curing, or an inflatable bag can be used. In some instances, such as compressed natural gas (CNG) tanks, the mandrel is left in position as a liner. 

A visible exudation or efflorescence on the surface of a plastic - it may be caused by lubricant, plasticizer, etc. Techniques for making film by extruding the plastic through a circular die, followed by expansion (by the pressure of internal air admitted though the center of the mandrel), cooling, and collapsing the bubble. 

Some mandrels must be made collapsible to facilitate their removal because of geometry of the part or excessive shrinkage of the resin system. Other tools are made of breakaway plasters or of materials that can be dissolved or melted out. Inflatable mandrels have also been used for these purposes. 

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