Composites Manufacturing Processes Icon

Gel Coated Cultured Stone

Gel coat is a specialized polyester resin that is formulated to provide a cosmetic outer surface on a composite product and to provide weatherability for outdoor products.

Several variations of cultured stone products are manufactured using a gel coated surface and a resin-matrix casting process. In this process a gel coat film (usually clear) is sprayed on the mold surface. Once the gel coat is sufficiently cured, a polyester resin matrix is blended by adding various types of fillers to the resin. Pigments for both a solid background color and the look of veins found in natural stone can be added. The resin matrix is then transferred to the mold, where vibration is applied to level and compact the matrix. Following the cure, the part is removed from the mold.

Gel Coated Cultured Stone Molding Diagram

Gel coat is not paint. Paint contains solvents that must evaporate for the paint to dry. The ‘solvent’ in gel coat is styrene monomer and/or methylmethacrylate (acrylic), which cross-links during curing. The monomer does not have to leave the system for the gel coat to cure; in fact, it is beneficial to reduce monomer loss.

The appearance of the cultured stone products is determined by the type of filler used and by the application of colorants to the matrix. Fillers come in a variety of materials. Many of the fillers used in the composites industry are mineral substances. Mineral fillers have distinctive shapes that relate to their chemical structure.

  • Marble: The natural marble look is reproduced by formulating a matrix using calcium carbonate filler. In some cases, other fillers or combinations of fillers may be used. Resin, initiator, filler, and pigment are mixed to form a solid-color matrix. The marble veining effect is created by adding a second pigment to the matrix and partially mixing it to produce the desired look.
  • Onyx: The process of manufacturing cultured onyx is similar to that of cultured marble, except alumina trihydrate (ATH) filler is used. The cultured onyx matrix generally has a higher resin content compared with cultured marble, and the combination of materials creates a translucent appearance. Background and veining pigments are added to the matrix to produce an onyx stone look.
  • Granite: The cultured granite appearance is created by blending colored chips into the resin matrix. These chips can be made from cultured marble castings, thermoplastics, or even actual granite stone that has been ground into particles. The cultured granite matrix usually consists of polyester resin, initiator, colored chips, and ATH filler.