Page 350 - Centrifugal Pumps 2E

Chemical Pumps Metallic and Nonmetallic 315

Table 15-3
Resin Performance
Oxidizing
Resin Strong Alkalies Agents Organics Temp.
Type Acids (Caustic) (Bleaches) (Solvents) Limit
General Purpose Polyester Very Very
(Fiberglass Boats & Bathtubs) Poor Poor Poor Poor 160°F
Isophthalic Polyester
(Structural Applications) Fair Poor Poor Fair 190°F
Anhydride Polyester Excellent Poor Poor Good 275°F
Bisphenol A Polyester Good Good Fair Poor 250 °F
Epoxy Poor Excellent Poor Excellent 190-250 °F
Conventional Vinyl Ester Good Good Fair Fair 210°F
High-Performance Vinyl Ester
Dow Derakane 470
IR GRP Materials Excellent Good Good Excellent 300°F

When a piece is to be made, a portion of the batch for the piece is mea-
sured within an ounce of what is required. If there is too little, the die
will not be completely filled; if there is too much, the piece will have an
extra thick parting line and will not meet specifications. When the por-
tion of the batch is put into the die, the die closes and compresses the
batch at a temperature of approximately >300°F for 10 minutes.
The atmospheric condition to which the whole molding machine is sub-
jected should be controlled for temperature and humidity to obtain the
proper quality of the piece. The design engineer has to work closely with
the tooling engineer to make sure there is proper flow of material and the
path of glass or reinforcement is in the proper location. Experience has
shown that reinforcing ribs on casings can be detrimental to the strength
because the glass will form a continuous path within the rib producing a
knit line. (Knit lines are a result of material coming from two directions
and meeting.) This is usually a weak point. In many cases the piece will
be stronger by eliminating ribs where it was thought they would be bene-
ficial.
The pieces made from the compression molding process are consistent
from one piece to another, both in dimensions and in quality. Poor quality
from this process can be a result of (1) a bad mix of batch, (2) batch that
is too old, (3) temperature within the die that was not controlled, (4) tem-
perature of the atmospheric conditions that were not controlled, (5) ex-
cess humidity within the atmospheric conditions, (6) too little or too
much batch, (7) time under compression that was not held as specified.