In-line color blending trims waste in plastisol rotomolding.
OLD WAYS ARE WASTEFUL
Traditionally, pigment paste is batch mixed with plastisol in a 55-gal drum then fed into a rotational mold using a standard pneumatic pump. A color change would require an exchange of mixing drum and pump. Furthermore, the batch process is prone to uneven mixing, which then requires an additional painting step to correct defective part appearance. Color repeatability from batch to batch is also unreliable.
According to Bran+Luebbe, the typical mold-filling procedure is also wasteful. It is reportedly a common industry procedure to overfill the mold by up to 15% to ensure a minimum acceptable piece weight. Also, an inadequate pump or dispensing nozzle often results in leaks and further material losses.
NEW IN-LINE SYSTEM
Bran+Luebbe addressed the deficiencies of plastisol batch-mixing methods by applying metering and dispensing technology that it has used successfully in other industries. Each pigment paste is contained in a separate 5-gal tank and has an individual dispensing line with a dedicated reciprocating-plunger pump, static mixer, leak-free dispensing nozzle, and control. Changing colors involves simply turning off one piston pump on the pigment line and turning on another.
Bran+Luebbe can build machines that have up to eight color-metering heads and pumps. Each machine also has one common plastisol head and pump. A valve in the plastisol line directs material to the selected pigment head.
Pigment and plastisol are continuously and automatically proportioned and blended by an in-line static mixer on their way to the mold. The mold is filled by a handheld, leak-free dispensing nozzle developed by Bran+Luebbe.
System controls determine how much material is delivered by each stroke of the pistons in the pigment and plastisol pumps. An operator can dial in pump-stroke adjustments to regulate pigment concentration and total amount of material dispensed into the mold. Other dials at the control panel select the pigment to be used for a given shot. Uniform and repeatable mixing reduces the need for painting, and control of dispensing avoids overfilling the molds.
Although Bran+Luebbe will custom design a plastisol pigmenting/dispensing system to meet specific needs, a standard system costs approximately $20,000-25,000 per color, based on a two-operator station with three colors per operator.