Page 351 - Centrifugal Pumps 2E

316 Centrifugal Pumps: Design and Application

Resin Transfer

The tooling for resin transfer can be less costly than that of compres-
sion modeling, but the number of pieces that can be obtained from the die
will be less. In this process, the two halves of the die are separated and
reinforced cloth is cut to shape and put into the upper and lower envelope
portions of the die. Core made of beeswax is then set within the die. The
die is closed and vacuumed and brought up to temperature. A valve is
then opened to allow the resin to flow into the die. When the die is filled,
it is allowed to cool from 12 to 24 hours. The piece is then removed and
set into an oven. The beeswax is then melted and is recovered. The re-
sulting cavity gives the desired shape of the core.
The disadvantage of the resin transfer pieces is that there is a knit line
where the two halves of the die meet; therefore, the way the reinforced
cloth is put into the die is extremely important to obtain the proper
strength of the piece. Pieces made from this process usually do not have
the strength of a comparable compression molded piece. Pieces also do
not have the consistency of the compression molded piece due to the hand
lay up of the cloth. This process is usually used for larger types of pieces
where the allowable tolerances are greater than with the compression
molded piece. The internal finish for hydraulic passages with this pro-
cess is not as smooth as obtained with the compression molded piece.
The problems of quality in this process are:

• The quality of tooling that is used to substantiate life of the part for
consistency.
The type of reinforcing glass.
The method in which the glass is put in the die.
The amount of glass cloth that is put in the die.
The quality of resin.
The control of vacuum to allow the resin to come into the die.
The quality of the beeswax that is reused from one piece to another.
The length of time that the piece is allowed to solidify.
The temperature that is used to melt out the core.

Design Stresses
Designers should be made aware that the stresses advertised in the
sample ASTM bars will not necessarily be equivalent to the stresses of
the molded piece. This will be verified by the molder as well as the mate-
rial supplier. When designing with metals, a designer can use the same
stress throughout the piece; however, this is not the case with nonmetailic
parts, When designing the casing, it is advisable to use different stress