Breaker Plates and Screens By Peter Stewart-Hay
Breaker Plates
On the drawing, there is a breaker plate at the exit end of the extruder and this is a circular piece of machined tool steel with a number of holes drilled through it. The breaker plate performs three separate functions; 1 It turns the rotating molten plastic to a linear flow. 2 It is a holder for the screen packs which fit into an increasing taper on the screw side of the breaker plate. This taper prevents the screen packs from backing out of position when the extruder is running. 3 The breaker plate provides the seal for containing the high pressure plastic between the throat of the extruder crosshead and the extruder barrel. A properly cleaned breaker plate fits into machined grooves in the throat of the extruder crosshead and the extruder barrel and is mechanically but tightly clamped. Plastic leaks are not acceptable. A damaged or a rusty breaker plate or a breaker plate with holes blocked with cold plastic must never be inserted in an extruder during set up. It is good practice that, upon shut down, the operator immediately remove the breaker plate from the machine, discard the old screen pack and manually pull the hot plastic from the holes in the breaker plate. ( All without scratching or marking the breaker plate.) Do not torch clean the breaker plates to prevent warping and other heat damage. Finally the operator should then send the breaker plate over to the machine shop for evaluation and proper cleaning. After that, they should be stored in the tool and die shop. A new or refurbished breaker plate should also immediately be supplied for the next set-up. Like extruder tooling, breaker plates should never be left out at extruders where they will rust or perhaps be accidentally damaged.
Screens
In the wire and cable industry, all extruders employ screen packs to provide back pressure at the extruder screw and thus improve uniformity of the melt. Filtering is not a necessity because the plastics are virgin or perhaps mixed with a small percentage of clean regrind. The mesh is mild steel, stainless steel for plastics that corrode mild steel or nickel alloy screens to avoid corrosion by fluoropolymers and PVDC. The screens are die cut and have no loose wires on or in them. A picture of a typical screen is given above.
 The wire mesh number is the number of wires per inch in the mesh and they typically vary between # 10 and # 200. Simple screens are commonly used because they have the same number and size of wires in each direction. To prevent corrosion, screens should be stored in the die room or the maintenance shop and stored between leaves of rust protected paper or in plastic bags. Never use a rusty screen to prevent possible breakage and serious contamination of the melt. Screens are only used once and then discarded.
Screen Packs Screens are bundled into sets with the coarsest one in direct contact with the breaker plate to mechanically protect the integrity of the finer screens from being pushed through the breaker plate holes. Likewise a coarse screen is placed on the extruder screw side to protect the finer screens from being twisted by the rotation of the melt. Finally the packs are mirror imaged to prevent them from being inserted backwards. Typical thermoplastic materials such as PE, PE co-polymers, polypropylene or PVC typically employ a screen pack such as 14/40/100/40/14. Other Extruder Tooling
All operator tooling that touches the inside surfaces of extruder and the extruder tool surfaces must be manufactured from copper, brass or bronze. This includes knives, wrenches, extraction tools and copper wool for cleaning.