Wet lay-ups

This method is sometimes combined with bag molding to enhance the properties. This procedure can be called bag molding. Because it is difficult to wet out dry fibers with too little resin, initial volumetric fraction ratios of resin to fiber are seldom less than 2 1. On a weight basis the ratio is about 1 1. Liquid catalyzed resin is hand-worked or automatically worked into the fibers to ensure wet-out of fibers and reduce or eliminate entrapped air. 

As an example glass RP boats in use are visually almost indistinguishable from traditional, but more expensive, wooden craft. Slightly higher tech is the Raptor Sportjet, an advanced personal watercraft (PWC) produced in North America. Although hand laid in glass fiber molds and room temperature cured, the all-glass prototype incorporates 0°/90 woven fabric and stitched triaxial reinforcement as well as chopped strand mat. Hull cavities are filled with two-part polyurethane foam for buoyancy and noise reduction. 

Conyplex in the Netherlands, like many series production boatbuilders uses glass fiber constructions, including in its latest Contest 44 and 50 yachts, to achieve easily driven hulls of modest displacement. Performance of its new-generation boats is further enhanced by wing keels, an RP wet lay-up appendage. The company has been transitioning to resin injection molding techniques. 

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The laminates are employed mainly where an intermediate degree of heat stability is required which does not Justify the use of the more expensive silicone and other laminates considered in Chapter 29. They have additional advantages over the silicones in their ease of forming by wet lay-up techniques and the greater strength of the laminates. 

Part Design Blow Molding Casting Compression Extrusion Filament Winding Injection Matched Die Molding Rotational Thermo- forming Transfer Compression Wet lay-up (Contact Molding)... 

A boiler lay-up is provided when the plant is out of service for some period. The lay-up may be a wet lay-up, wherein water remains in the boiler and is intended only for short periods or a dry lay-up, which is for extended periods. 

This view of the corrosion process is, however, more often than not too simplified an explanation. First, even when general corrosion take place (as in an idle or wet lay-up boiler), the reaction mechanisms tend to occur at many localized points on the boiler metal surface, typically where cracks and other imperfections in the magnetite film exist. Second, such processes almost always lead to derived forms of localized corrosion, which often result in severe metal wastage through the formation of deep pits. 

Wet lay-up programs are therefore used to protect standby boilers. These are essentially the same programs used to protect idle boilers that may be out of service for perhaps 30 days. (Boilers having outages of longer than 30-day periods generally are subjected to dry lay-up programs.)... 

The procedure for wet lay-up of idle boilers over a longer term is the same... 

A further longer term wet lay-up alternative is through the use of volatile corrosion inhibitors (VCIs) such as dicyclohexylamine acetate. These are dissolved in the water at a temperature below 60 °C, and the water is circulated for 4 to 5 hours. The boiler does not need to be completely filled because the VCI migrates to all parts of the boiler and reaches equilibrium in each of the void spaces. With traditional lay-up chemicals, the oxygen scavenger may become depleted easily (which is why the reserve usually is so high) and corrosion protection is quickly lost however, with VCI programs, there is always a volatile buffer available that maintains equilibrium and hence corrosion protection. 

The proper procedures for wet lay-up of boiler auxiliaries are equally important and tend to parallel those methods used for storing boilers. 

Compression Molding Flexible Plunger Flexible Bag Molding Laminate Hand Lay-Up Vacuum Bag Molding Vacuum Bag Molding and Pressure Pressure Bag Molding Autoclave Molding Autoclave Press Clave Wet Lay-Up... 

One way to apply such moderate pressure is to enclose the wet-liquid resin material and mold in a flexible membrane or bag, and draw a vacuum inside the enclosure. Atmospheric pressure on the outside then presses the bag or membrane uniformly against the wet lay-up. An effective pressure of 69-283 kPa (10 to 14 psi) is applied to the product. Air is mechanically worked out of the lay-up by hand usually using serrated rollers. The vacuum direcdy helps to remove air in the wet lay-up via techniques such as using bleeder channels within the bag (using material such as jute, glass wool, etc.) to aid in the removal of air and also to permit drainage of any excess resin. This layup is than exposed to heat using an oven or heat lamp. 

Pari design Casting Compression Filament winding Injection Matched die molding Rotational Transfer compression Wet lay-up (contact molding)... 

Use In pressure-sensitive wet lay-up and hot-melt adhesives, dip coating, spraying, and spreading applications. 

The several methods to convert resins and reinforcing fibers into composites are classified as either "wet lay-up" or "dry layup" methods. These processes are as follows ... 

Wet lay-up" refers to a process in which liquid resin systems of low viscosity are used to impregnate the reinforcements, either before or after the reinforcements have been laid in place. The liquid resin penetrates the fibers and displaces the air. The distinctive feature of this method is that the composite object is shaped into its final configuration while the resin component of the lay-up is still liquid (uncured). Cure is effected after the lay-up is completely in place and conforms exactly to the mold. 

Typically, a low molecular weight solid epoxy resin and latent curing agent, such as dicyandiamide dissolved in an appropriate solvent, are widely used in dry lay-up formulations for electrical laminates for computers, aerospace applications, and communications equipment. Wet lay-up systems are almost exclusively based on low-viscosity resins cured with aliphatic amines. These ambient cure resin systems are used primarily for manufacture of large chemical-resistant tanks, ducting, and scrubbers. Filament winding is used... 

DJE.R. n>ow] Epoxy resins for adhesive, civil engineering, casting, potting, encapsulation, filament winding, powd. coatings, and wet lay-up tq>plics. 

Wet Lay-Up Process. Formulations are impregnated by hand or sprayed into fibers that are in the form of woven, knitted, stitched, or bonded fabrics. This is usually accomplished by rollers or brushes. Spray-up is a partly mechanized version of the hand lay-up process. The thermoset precursor is applied by means of a spray gun to the mold surface simultaneously with the chopped glass-fiber roving. Laminates are left to cure under standard atmospheric conditions. 

In pressure-bag molding the reinforcement and the resin mixed with catalyst are placed in a mold, and a flexible bag is placed over the wet lay-up after a separating sheet (such as cellophane) is laid down. The bag is then inflated with an air pressure of 20-50 psi (1.4—3.5 kg/cm ). The resin and reinforcement follow the contours of the mold (Figure 2.47). After the part is hardened, the bag is deflated and the part is removed. The technique has been used to make radomes, small cases, and helmets. 

The use of prepreg in the manufacture of a composite components offers several advantages over the conventional wet lay-up formulations ... 

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