Types of non return valves

Visit the EMI web site www.emiplastics.com

Tel: 800-543-6169

Understanding What a Non-return Valve Does

Figure 1.
Recovery phase - As the screw rotates, it moves resin forward - at the same time, the screw moves backwards. This creates an increasingly larger chamber for the resin between the end cap and the non return valve.

When resin enters the barrel of an injection molding machine, it is picked up by a screw that fits closely within the barrel. A non-return valve is attached to the end of the screw. At the beginning of a cycle, the screw is all theway forward in the barrel. As the screw turns, it conveys the resin forward, compressing and heating it until it becomes molten. As the screw turns, it also moves backwards in the barrel creating a chamber between the end of the barrel and the non-return valve (figure 1). This is called the recovery phase of the cycle.

As the molten plastic (or melt) reaches the end of the screw it flows through the non-return valve into the chamber that has been created as the screw moves backwards. Once the correct amount of melt has filled into the chamber, the now-retracted screw stops rotating and then moves forward, injecting the melt through an end cap and nozzle into the mold. As the screw injects the melt into the mold, the non-return valve closes to prevent the plastic from flowing back to the screw (figure 2). This is known as the injection phase of the cycle.

Figure 2.
Injection phase - The screw stops rotating and moves forward forcing the resin through the nozzle into the mold. This action also causes the valve to shut off, preventing the material from flowing backwards.

Note: The plastic material being molded may be called by several different names, depending upon where it is in the process. When it enters into the barrel in the form of pellets it is called resin. As it melts it becomes molten plastic, or melt. As it collects at the end of the barrel, or as it is injected into the mold it is called a shot.

Since the amount of melt in the chamber is predetermined to be the correct amount to exactly fill the mold (plus a little extra which will remain as cushion in the chamber), it is important that the correct amount be injected. If the non-return valve does not efficiently shut-off as it should, some of the melted plastic may slip back past the seal of the valve, shorting the amount of melt to the mold (this is called a short shot). When parts are molded with too little material, they may have holes, thin spots or other inconsistencies. Note: If too much is injected, parts may be misshapen or may need to be trimmed. It can also increase material costs or cause mold or other tooling damage.

Different types of non return valves