Not logged in - Login / Register

Fiberglass Pool Molds

See also: Categories: In Ground Pool, Fiberglass Pools


A fiberglass pool mold (or mould) refers to a reusable rigid framed matrix, counterpart to a the actual fiberglass shell (cast), built to duplicate the shape, size and pattern of the final pool shell

Mold Construction

The first step in mold making consists of plug construction and/or preparation. The plug may be constructed of nearly anything, as long as its surface can be finished well enough to give a suitable mold surface. As stated previously, the plug can either be an existing item or something fabricated specifically for the mold-making process. Some of the materials commonly used in plug construction include wood, plaster, metal and polyurethane foam. The latter comes either as pre-formed sheets or as a two-part "mix and pour" system that chemically reacts to form the foam. The "mix and pour" foam will conform to the shape of any cavity into which the ingredients are poured.

The surface of the plug must be finished at least as well as the desired surface on the part to be produced. In most applications, the preferred plug surface would be a perfectly smooth and polished class "A" finish. If a particular texture or pattern is desired on the finished part, it can be incorporated into the plug surface. A high quality, sandable surfacing primer such as the Duratec Grey Surfacing Primer (#1041-B), works well as the finish coat on the plug. Incorporate flanges and any necessary parting dams onto the plug at this point (see Special Mold Construction Considerations.)

Before beginning construction of the mold, a release agent must be applied to the plug. This is the most important step in the process, because if the release agent fails to perform, the mold can't be removed without damaging it and the plug. A little extra effort at this point is better than hours spent trying to correct damage to the plug and mold. The release agent can either be a combination of parting wax and PVA, or a one-step release agent such as FibRelease.

When using wax, apply four coats, waiting one hour between the second and third coats. After the final wax coat has been buffed, spray three thin mist coats of PVA and allow it to dry for 30-45 minutes. FibRelease can be wiped or misted onto the plug, and allowed to dry for 30 minutes. Be sure to apply the release agent to the surface of any flanges and parting dams.

For most molds, polyester resin and 1.5 oz/sq. ft chopped strand mat yield satisfactory results. Mold strength and thickness can be built up more rapidly by adding woven roving or tooling fabric. With polyester molds, the first step in making the mold is the application of a tooling gel coat, which is distinguishable by its bright orange or deep black color. Prior to its application, be sure to catalyze the gel coat at the proper ratio. For best results, the tooling gel coat should be sprayed onto the plug with a gel coat cup gun in three passes of seven to eight mils each, building to a total thickness of 20-25 mils.

The surface coat should be stabilized with an initial layer of mat within one and a half to five hours, in order to prevent the gel coat from shrinking or lifting off the plug surface. Apply a coat of resin to the surface and lay the mat into the resin. Using a bristle brush, apply the resin to the mat, coaxing the mat into the various contours of the plug. A dabbing motion is much more effective than a painting motion, as long strokes tend to pull the mat around.

All trapped air pockets must be worked out so that the mat is tight against the plug surface, and it must be uniformly saturated with resin. Air bubbles and dry areas will appear milky against the tooling gel coat . Use a bristle roller to work air pockets out of the mat and a grooved saturation roller to help compact the laminate. Watch for bridging (lifting) of the fibers across sharp corners and in textured areas. Any air bubbles remaining after the resin gels must be carefully cut out with a sharp utility knife and a match patch laminated in place. Once the initial layer has cured, lightly sand it in preparation for additional layers, following the same procedure as with the initial layer. Most molds utilize 8-10 layers, but do not apply more than three to four layers at a time to minimize heat generation (exotherm). After the third layer of mat, a layer of woven roving or tooling fabric can be added to more rapidly build thickness. In general, a mold should be a minimum of twice the thickness of the part it is to produce.

Allow the completed mold to cure for at least 24 hours before attempting removal. Any support structures should be laminated to the back of the mold prior to releasing it from the plug. Release wedges can be inserted around the perimeter of the mold, between the mold and the plug, and gently driven into place in a progressive fashion. Air injection wedges, which attach to an air compressor, can be used to coax stubborn sections apart. Once the mold is released, wash off any residue from the release agent with warm water and inspect the surface. Any imperfections must be ground out and repaired. You're then ready to begin prepping the mold for part production.

Mold Maintenance

Before any part can be made in a new mold, it must be wet sanded and polished to a Class "A" finish. Wet sand the mold in a progressive manner, using 400, 600, and finally 1000-grit sandpaper. Be sure to change the water in your bucket and rinse the mold surface when changing to a finer paper to insure none of the coarser grit remains. For polishing, Fibre Glast Development Co. recommends using a twostep polishing compound and a high-speed buffer. The first stage removes the sanding scratches, while the second polishes the surface to the desired finish.

After polishing the mold, apply a release agent to it, following the procedures outlined for prepping the plug. A new mold is often given an extra coat of the release agent as added insurance. In the event a part doesn't release properly and damages the mold, repair will be necessary. Any loose or damaged material must be removed by sanding or grinding, and new tooling gel coat should be applied to that area. A coat of PVA or wax paper placed over the repair will be necessary for proper curing. Once cured, the repair can be sanded and buffed as previously described.

Special Mold Construction Considerations

Multiple Piece Molds

In some instances, the shape of the plug may require a multiple-piece mold so that the mold can be removed from the plug and subsequent parts removed from the mold. When making a multiple piece mold, start by constructing a temporary dam on the plug, along the desired parting line. This dam may be constructed of Masonite or a similar material, and held in place with clay. A sharp corner without a radius must be maintained on the portion to be molded first. Any locating keys or dowels for realignment of the mold pieces should be added to the parting dam. With multiple piece molds, construct the entire mold before releasing any part of the mold, in order to avoid realignment problems. After the first portion of the mold cures, remove the temporary dam and use the completed portion of the flange to form the parting dam for the next half. Apply a release agent to this surface before continuing the mold construction.

Alternate Construction Methods

If durability and dimensional stability are important factors in mold construction, epoxy resin can be used in place of polyester resin. The procedure for this is much the same as with polyester resin, except that mat cannot be used with epoxy, as the binder that holds the mat together is not compatible with epoxy resins. Start with two or four ounce fabric to minimize prints through of the weave pattern. Then switch to a 7-10 ounce fabric. Be sure to place some layers on a 45-degree angle for good stiffness. Epoxy surface coats should be brushed onto the plug for best results. Because epoxies are less prone to shrinkage than polyester materials, immediate application of a stabilizing reinforcement layer over the surface coat isn't critical.

If exceptionally rigid molds are required, carbon fiber can be used in place of fiberglass cloth. We recommend using epoxy resin with carbon fiber, and a flexible rubber squeegee works best for distributing resin through the fabric.

Selecting Materials to Mold Parts

Once the mold has been properly polished and coated with a release agent, you can begin making parts! The first stage in the process of molding parts is determining which resin and reinforcements will be used. Having previously discussed the merits of the three main resins, we will concentrate here on the specifics of reinforcement selection.

After choosing the type of reinforcement to be used, the biggest factors are choosing the style (weave) and weight of fabric best suited to a given application. The three main fabric styles are plain weave, twill weave, and satin weave. In addition, fiberglass is available in ounces per square yard, with the exception of mat, which is expressed in ounces per square foot.

When fabrics are woven, the fibers are bundled into yarns running a 0 (warp yarn) and 90 (fill yarn) degrees. Plain weaves use an "over-under" pattern, while in a satin weave one filling yarn floats over three to seven warp threads before being stitched under another warp thread, and twill weaves are a "2x2" pattern. Plain weaves are the least expensive and are good general purpose fabrics, but they don't offer the strength of satin and twill weave fabrics, but are equally strong in all directions.

The lighter the fabric weight, the easier it will drape over contours and the less resin it will take to wet it out. Lightweight fabrics are most commonly used for surfacing and radio-control (R/C) hobby applications. Medium weight fabrics are most commonly used in repair and fabrication work. The heaviest fabrics are generally used for rapid thickness build up, such as in boat hulls and mold making. Fabrics are sold by the running yard, generally in widths of 38, 50 and 60 inches, although not every fabric will be available in all of those widths. For a given project, choose a width that most closely approximates the width of the part to be made. The idea is to use as few separate pieces of fabric as possible per layer. The amount of resin needed will depend on the weight of the fabric selected. Fabric to resin ratios for most woven fiberglass and Kevlar® are about 50:50, while carbon fiber is 60:40. Fiberglass mat will require about twice as much resin as woven fiberglass for proper saturation. Extra strength can be built into parts by means of sandwich core construction. This process involves utilizing a core material, such as end grain balsa wood, polyurethane foam, vinyl foam, or honeycomb, between two laminate skins. Some core materials come in a variety of thickness, depending on the needs of a particular application. The strength and stiffness of a part can be increased significantly, with very little extra weight added to the part.

More fiberglass pools topics

Categories: In Ground Pool | [[Fiberglass Pool]

How it works

PoolsWiki has been created for the community to collaboratively organize and share information about swimming pool industry. Anyone who can access this site can edit most of its articles and create new ones.

Only registered users may create a new article. Some particularly controversial, sensitive and/or vandalism-prone pages are protected to some degree from public alteration. A particularly contentious article may be locked so that only administrators are able to make changes.

In certain cases, all editors are allowed to submit modifications, but review is required for some editors, depending on certain conditions.

WDS Editorial Contacts

  • Michelle Angela Blank

    Co-Founder & Lead Editor

  • Igor Golovach

    Co-Founder & Lead Editor