Question

Discuss the advantages and drawbacks of the open molding method in thermoset composite processing.

Answer 1

OPEN MOLDING, also known as contact molding, open laminating, and wet lay-up, is the method used longest in the polymer-matrix composites industry to make thermoset composite products, and it is still the selected production process for a wide range of composite products. It is a basic process that provides many of the advantages of composites processing, using relatively basic materials technology and processing methods. The molding method involves placing reinforcements and liquid resin onto the surface of an open mold (which may or may not be precoated with gel coat), or onto other substrates, as, for example, when making a one-off sandwich construction, when making on-site repairs by applying a reinforcing vacuum-formed acrylic, corrosion-resistant lining on steel, on-site repairs of tanks and pipes, and so on. The hand lay-up version involves applying the reinforcements and the resin by hand, while the spray-up version uses tailored spray equipment to deposit both reinforcements and resin on the mold or an alternative substrate. Open molding is a process typically used for low- to medium-sized series (from a few to 200-300 parts/yr), offering a number of process and product advantages. Producing large, complicated shapes as well as smaller and simpler composite products is possible. The hand lay-up process involves low investment costs and little prior working knowledge of the process, while spray-up involves some investment in tailored spray-up machines and spray guns. Well-trained operators and dedicated facilities are required to produce components and products having high quality.

ADVANTAGES

(i) Accumulated experience as the process

has been widely used for many years.

(ii) Standardized training exists (e.g.,

City

and Guilds Hand Laminators Certificate

).

(iii) Short lead times for component produc-

tion.

(iv) Minimal equipment and tooling costs.

(v) Relatively inexpensive materials.

(vi) Wide choice of suppliers and material

types.

(vii) Versatility of part shape, size, and lami-

nate configuration.

(viii) Design changes can be easily effected.

(ix) Cost-competitive for individual items

and short production runs of complicated

shape.

(x) Fiber volume fractions are potentially

higher than for spray lamination.

(xi) Scope for a wide range of physical and

mechanical properties in the laminate.

(xii) Resin-rich composites can produce

high-quality corrosion-resistant components.

(xiii) Sandwich construction (foam or balsa-

wood core materials) and inserts are readily

incorporated.

More information on possibilities and limita-

tions can be found in the following references

(Wittman and Shook, 1982; Kelly, 1999; Mur-

phy, 1994, 1998).

DISADVANTAGES

(i) Health and safety legislation/regulation/

voluntary codes.

(ii) Cost of extraction and treatment of

VOCs.

(iii) High labor content.

(iv) Product quality and variability is depen-

dent on operator expertise (especially in mixing

resin).

(v) Low production rates due to cure times

of room temperature resins.

(vi) Resins need to be of low viscosity (low

molecular weight) and are hence volatile (espe-

ciallythe styrene in unsaturated polyester resin).

(vii) Dimensional inconsistency.

(viii) High shrinkage and potential exotherm

in resin-rich areas, especially for thick compo-

nents.

(ix) Draining of resin from vertical surfaces

may require the addition of thixotropic agents.

(x) Only one molded surface.

(xi) High levels of voids inherent in the

process.

(xii) Low molecular weight resin compro-

mises thermal and mechanical properties.

(xiii) Low fiber volume fraction (maxima

typically 20% for chopped strand mat or 35%

for woven fabrics (Eckold, 1994)) yields poor

mechanical properties.

(xiv) High levels of waste.