USG Casting Fireplace Surrounds - Manual (Page 16)

is achieved (at least 3/8”). In some systems, the slurry is retarded so it will run through the equipment without
setting, and is accelerated at the nozzle to facilitate maximum production. USG recommends spray equipment
from RimCraft Technologies, Inc. (www.rimcraft.com).
The fiber reinforcement does tend to “bridge over” in certain mold areas (highly detailed areas and sharp cor-
ners), so pressing the sprayed material down will prevent any voids in the piece. Spray equipment may be specially
designed to fit your production needs, or it may be off-the-shelf components linked together in a workable fashion.
Either way, the spray equipment must be matched to the design of the pieces and the required daily production.
Until this has been fully analyzed, the spray system, which at first may appear to be the best option, might not
perform as needed when the final details have been worked out. As in the hand-layup system, the goal is to have
5%-6% of fiber based on the weight of the mix.
Molds designed with deep and narrow openings are not good choices for spray processes.
Solid-casting Liquid slurry is prepared and poured into the mold and allowed to set. When solid-casting, use a displacement
plug and/or framing in the back of the part. A displacement plug is typically made of wood or sometimes foam, and
is inserted into the mantel part of the surround to make it lighter. The displacement plug can also be part of the
mechanism that anchors the surround to the building structure. USG highly recommends the use of a displacement
plug when solid-casting to reduce the weight of the surround and to allow for a cavity that can be used in conjunc-
tion with a wall-mounted ledger or other means of mechanical attachment to the building structure. Also, the
displacement plug can be designed to be left in the surround as a reinforcement and to help attach it to the building
structure. Other methods may include a built-in frame when the surround is cast. The frame will not only reinforce
the surround but can also be used to attach the surround to the structure. Treat displacement plugs and reinforce-
ments made from wood with a non-flammable sealer before use to prevent water penetration. Metal framing
materials need to be epoxy-coated to prevent reactions with the gypsum cement. Regardless of how carefully the
plug or frame is developed, cast pieces are always heavier than hand-layup or sprayed pieces. Large solid-cast
fireplace surrounds are not recommended due to excess weight, amount of material required to fabricate, difficulty
of installation, life-safety considerations due to weight, and dissimilar movements from thermal forces which can
result in cracking when the surround is placed closer to the firebox than a combustible material should be positioned.
Production
Manufacturing
16 Casting Fireplace Surrounds