Downstream Equipment

As the strands leave the extruder die they enter a conveyor system. The conveying system is called the downstream equipment. The downstream equipment must perform many tasks. Down stream equipment maintains the strands in the extruded condition until pelletized and bagged. It must cool, chop into pellets, separate the pellets by size, and load in bags, drums, or boxes. The process completes many function and is similar to a conveyor line.

After the plastic and additives are mixed into the extruder they leave the die head in a melted condition. When the melted strands drop from the die, they are transported to a pelletizer, about 24 feet down stream. The conveyer is a continuously moving belt that picks up the extruded material and transports it to a cold water spray; then the pellets are subjected to drying fans. The speed of the conveyor is adjustable to match the speed of the extruding strands as they exit the die head. If the conveyor moves too fast, the strands thin out and may break. If the conveyor moves too slowly, the stands build up into globs at the die head.

The most difficult part in monitoring the compounding process is the synchronization of the conveyor speed with the extrudent speed. Surging in the extruder, caused by voids in the melt stream and build up in the vent area, will cause a strand to elongate, thin out, break, bunch up, and thicken. If the strands touch each other prior to cooling, they fuse into a strand that is twice as large as required. Any of these problems can produced plastic pellets of irregular shape and sizes. This directly affects the eventual consistency of the compounded material. This is an especially tricky endeavor since large pellets require a long time to melt and small pellets decompose under the same condition.
 

 Pelletizer

After the strands are cooled they are feed into a pelletizer where they are chopped into 1/4" lengths. The pelletizer, illustrated in Figure 4-15, pulls the cooled and hardened strands into a cutter head that chops them into uniform lengths. The pellets then fall onto the first of three vibrating screen trays called classifiers. The first tray has the large holes which allow all he acceptably sized pellets to drop through to the second tray. All oversized pellets, or broken strands remain trapped in the first tray. The second tray allows the fines (particles smaller than 1/4") to fall to the bottom tray while capturing the acceptable pellets. The rejected oversized pellets and fines are returned to the extruder's hopper. At the hopper the rejected materials are added in amounts of less than 4% and compounded a second time. On runs of 500 pounds or less the rejected materials are discarded. A continuing problem in compounding is that some longer pellets turn on their end and drop through the screen holes in the classifiers. Unfortunately, without effective quality control procedures, these over sized pellets are often shipped along with acceptable materials.
 

Quality Control

Samples are drawn from each run roughly midway through production. A sample of 25 pounds is injection molded into a master unit die (MUD mold) to produce tensile, impact, color, and flow test specimens. Approximately a pound of pellets is retained. One third of test specimens are used to measure the material's physical and chemical properties, moisture content, and color retention. The remaining two thirds of specimens and plastic pellets, along with the test results, are reserved into two sample bags. One bag is retained in the compounder's archives and the other is shipped to the customer with the materials. The moisture content of plastic pellet is determined at the end of the process to insure quality control. The compounder depends on the efficiency of the extruder vent and conveyor drying fans for the removal of interior and exterior pellet moisture. Most materials are not dried fully after compounding. Polycarbonate is the lone material that requires further drying.