POLYESTER "PLUG" AND "MOLD" BUILDING
By Cecil Collum
[Editor's Note:
For those not familiar with the "Laying up" of a fiberglass part, it is first necessary to carve, build
or otherwise construct a "pattern" from which the part(s) are to be duplicated. This pattern is called
the "plug". Once the Plug has been constructed, then a "mold" is made from the Plug. It is inside the
Mold that the fiberglass is "layed up", and from which the finished part is removed.
The photos for this article were taken from the internet and not supplied by Cecil. Any errors in the
use of the photos are my responsibility and not Cecil's. The canopy depicted in these photos is for an
F-94C and the credit source given on the internet is vague.]
Have you ever imagined creating an exact duplicate of a model boat hull or model airplane fuselage?
Whether as a hobby or trade, making a mold from an existing part or from your own design and pulling a part
from it can be a rewarding and challenging skill.
This article is intended to familiarize you with the basic steps in "plug" and "mold" tooling and should
enable you to experience the art of fiberglass tooling, provided you follow this guide carefully. There
is no quick or easy way to produce a quality polyester "plug" or "mold". Good tooling is a painstaking craft,
starting with the careful preparation of the pattern (plug) to the final building of the mold (from which it
is possible to fabricate a finished part). The surface of the pattern must reflect the mirror finish desired
in the mold (ugly in, ugly out). The building of the pattern must be exact to eliminate any faults that will
ruin the mold.
A finished "Plug" to be used for the making of a fiberglass Canopy. This plug has been
carved from blue foam, sealed, and sanded. Then a coat of epoxy resin has been applied and finish
sanded then polished.
An original plug can be made from wood coated with polyester or epoxy resin. It can be made of plaster, RTV
silicone or foam. The choice of plug-making material is limited only by your imagination. Plaster, foam and
low-density materials seldom last more than one mold. If several molds are to be taken from the plug, a
polyester and fiberglass plug can be used to make several molds. The plug should be glossy and as defect-free
as possible to minimize the amount of sanding and buffing of the mold. If the mold requires some cosmetic
reworking, care must be taken to avoid changing its dimensions or features.
A WORD OF CAUTION: Do NOT use varnish or lacquer as the finish coating of a plug. The heat generated by
The photos for this article were taken from the internet and not supplied by Cecil. Any errors in the use of the photos are my responsibility and not Cecil's. The canopy depicted in these photos is for an F-94C and the credit source given on the internet is vague.]
Have you ever imagined creating an exact duplicate of a model boat hull or model airplane fuselage? Whether as a hobby or trade, making a mold from an existing part or from your own design and pulling a part from it can be a rewarding and challenging skill.
This article is intended to familiarize you with the basic steps in "plug" and "mold" tooling and should enable you to experience the art of fiberglass tooling, provided you follow this guide carefully. There is no quick or easy way to produce a quality polyester "plug" or "mold". Good tooling is a painstaking craft, starting with the careful preparation of the pattern (plug) to the final building of the mold (from which it is possible to fabricate a finished part). The surface of the pattern must reflect the mirror finish desired in the mold (ugly in, ugly out). The building of the pattern must be exact to eliminate any faults that will ruin the mold.
A finished "Plug" to be used for the making of a fiberglass Canopy. This plug has been
carved from blue foam, sealed, and sanded. Then a coat of epoxy resin has been applied and finish
sanded then polished.
An original plug can be made from wood coated with polyester or epoxy resin. It can be made of plaster, RTV silicone or foam. The choice of plug-making material is limited only by your imagination. Plaster, foam and low-density materials seldom last more than one mold. If several molds are to be taken from the plug, a polyester and fiberglass plug can be used to make several molds. The plug should be glossy and as defect-free as possible to minimize the amount of sanding and buffing of the mold. If the mold requires some cosmetic reworking, care must be taken to avoid changing its dimensions or features.
A WORD OF CAUTION: Do NOT use varnish or lacquer as the finish coating of a plug. The heat generated by
the mold laminate can soften and attack these coatings.
Once the plug has been prepared and is a mirror image of the desired part, it must be waxed with a mold release wax formulated for FRP (fiberglass reinforced plastic). It is important that the plug be thoroughly waxed to avoid any sticking when removing the mold. This can ruin your day very quickly! My first attempt to make a part utilizing this method resulted in both the mold and part discarded because I could not separate one from the other.
When breaking in a "green" (new) mold, ALWAYS apply 3-5 coats of mold release wax (use a non-silicone carnauba-based wax, polishing each coat to a high gloss. Use clean toweling, turning frequently while polishing. Each coat should be applied in alternate directions and allowed to cure at least one hour prior to applying following coats. On new molds, allow the last coat to stand for twenty-four hours before the application of gel coat to allow for the dissipation of all solvents/vapors.
To ensure a clean release of a mold from a plug, use a Poly Vinyl Alcohol (PVA) solution of water soluble film forming materials. When the waxing and polishing of the plug is completed, PVA is applied to assure proper release of the mold from the plug. PVA is best applied with a spray gun using high air pressure (80 to 100 PSI) and low output of material. Apply several thin coats followed be a heavier wet coat of approximately 2-4 mils. If spray equipment is not available, acceptable results can be obtained by using a poly foam brush, lightly wetted with PVA and a delicate one-way brush stroke. Allow the PVA to dry at least 30 minutes or until tack-free and glossy. PVA's cannot generally be applied over silicone-based waxes. Use Carnauba based mold waxes only.
The finished mold taken from the "F-94C Canopy Plug" shown earlier. This mold will be used for the "laying up" of a fiberglass canopy and also for the making of another "plug" to be used in the stretching and forming of a clear acetate canopy.