Check ring valve

An alternative to the check ring valve is a smear head (Fig. 6.33), which is based on the use of a small clearance in the diameter over significant land length. This design restricts the back flow of the melt during... 

Figure 6.32 Schematic of a combination of a check ring valve and an adapter. ...

A screw melt seal or valve, similar to a check ring valve, but designed differently. Uses a roundball to seal off the melt so it does not flow back over the screw flights during the injectirm cycle. 

For LC resins having very low viscosity, the check ring valve should be ensured for proper functioning. Due to malfunctioning of check rings, inconsistent parts, short shots and poorly formed weld lines are observed. 

They include melt stagnation, less probability of overpacking, less chance of streak formation in the part, and reduced abrasion due to the absence of metal-to-metal contact. Lower viscosity melts may require the use of the check ring valve. 

Use is made of screw tip valves, popularly called non-return valve, ball check valve, or sliding ring valve. They are used in reciprocating injection and injection blow molding machines (IMM and IBMM) to control the melt flow in one direction (Chapter 4). There is also the smearhead for IMM, IBMM, and extruder. Back flow will not occur... 

Front discharge ball check less expensive than sliding ring valve. 

As plastic is pushed forward (overcomingbackpressure) the screw is forced to retract. There is now a cylinder of plastic in front of the screw tip and nonreturn valve. Upon injection, the screw moves forward seating the nonreturn valve or check ring, which acts like a plimger, piston, or ram to push the plastic out the nozzle tip into the sprue hushing of the mold. 

During the injection part of the cycle, backflow up the screw channel by the molten polymer is prevented by some form of non-return valve such as a check ring (Figure 10.1). The check ring moves axially with the screw and rotates at a slower speed than the screw due to drag from a thin melt film between it and the barrel surface. 

By transferring the check ring mixer concept to extrusion, the mixer can be accommodated within the barrel as part of the screw. Being without the non-return valve function required for injection moulding it can be described as a floating... 

Many materials require the use of a valve with a check ring to be fitted to the end of the screw to prevent backflow. They also help to ensure that a constant cavity pressure is maintained. The most important design consideration is that they should avoid flow restrictions or hold up of the melt flow. Non-return valves are more prone to wear than other components, so it must be ensured that suitably toughened materials are used in manufacture. Various designs of non-return valves are shown in Figure 3.8. 

Bowl does not open Check 1. That tank contains water 2. That cock in conduit from tank is open 3. Control valve 310-1 4. Paring disc device 310-2 5. Control means of bowl Rubber ring C20 for operating 300-4 slide... 

Spring-loaded check valve —Screw cap Rubber sealing ring... 

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