Page 122 - Engineering Plastics Handbook

96 Engineering Plastics

satisfy. Arange of colored decorative effects allows the designer to combine
functionality with consumer appeal, using in-mold decorating, painting
(IMD), foil stamping, and other techniques, according to DuPont. The
company developed etching technology and special paints to accommodate
®
Delrin Forming Solutions [8]. Ensinger/Penn Fiber Plastics supplies
thermoformable sheet as coil stock and precut sheet for FS technology [8].
Powder metal injection molding (MIM) is a mass-production, versatile
method to mold complex metal and ceramic shapes [17]. A 50 to 70%
(volume) metal or ceramic powder is mixed into a polyacetal resin at high
shear, and the resulting suspension is injection-molded [17]. Powder metal
injection molding requires polyacetal binder, which is added to the metal
powder by high-shear mixing. The polyacetal binder serves as a carrier
during injection molding of complex metal and ceramic shapes. Molten sus-
pension is injected into the mold cavity and sets. Postmolding requires two
steps: removing the polyacetal binder and sintering the metal or ceramic
shape with special furnaces, such as the BASF catalytic EBO 120 furnaces
for catalytic debinding [17]. This furnace uses the patented Advanced
Temperature Control System (ATC) with the following advantages: (1) It
is fully automatic with highest safety standards; (2) there is reduction of
nitric acid time, resulting in lower part cost; (3) equipment control deter-
mines the end of debinding; (4) there is online signal input and evaluation
(ATC); (5) a costly empirical series of evaluations is eliminated; and (6)
emissions are reduced [17]. The EBO 120 furnace has additional advan-
tages such as touch panel with SIMATIC control, automatic control of
sweep gas flow depending on nitric acid mass flow, and two-stage propane
flare [17].
Recent efforts are directed to improving removal (catalytic debinding)
of the polyacetal binder, especially reducing debinding time [17]. Atwo-step
process has required an initial 24 h to debond plus another 16 to 24 h for
sintering. Modified polyacetal allows a one-step debinding and sintering
process in a total of 24 h [18]. Catalytic debinding begins at the molded
product surface, gradually penetrating to the center of the part. Total
debinding time primarily depends on the size and shape of the molded part,
furnace loading, and amount of acid used [17]. Polyacetal binder decom-
poses mostly to formaldehyde in the presence of nitric acid vapor by an
interfacial reaction at the vapor/solid boundary which penetrates through
the polyacetal molding, leaving the metal powder form [17].
The rate of depolymerizing polyacetal to formaldehyde is the subject of
much research, because the rate of depolymerizing is a critical factor in
MIM costs, and there is not that much proven information on it. Removal
of binder is determined by various factors, namely, binder removal tem-
perature, about 120°C (248°F); catalyst; linear or random degradation of
polyacetal resin; fineness of powder; mold part shape and size; and cat-
alytic debinding furnace. The catalytic furnace developed for debinding