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moldmakingtechnology.com 27 machined channels. These channels must be as straight as pos- sible to maximize the venting performance of the valve. The valve itself can be located in different positions in the mold to achieve various benefits and mold performance. The main benefit of external mold venting at runner side is the ability to vent the gases during molding at the subrun- ner. Gases are usually pushed into the cavity with the plastic material, and with an external venting approach, the gases naturally flow where the drag is lower. For example, resis- tance to the flow at the end of the cold runner is very low compared to the resistance inside the cavity, where the num- ber of vents are limited. Venting at runner side can also reduce air counter pres- sure, because the air in the sprue and the cold runner is completely ejected instead of compressed into the cavity. Bernoulli's principle states that an increase in the speed of a fluid occurs simultaneously with a decrease in pressure. According to this principle, the speed of the air flowing through the cold runner (no counter pressure) toward the valve results in a depression and air suction from cavity to cold runner. The benefits of this can significantly change depending on cold runner and sprue dimensions in relation to the volume of the injected part (see Figure 2). The ability to vent gases during molding is the main advantage of external mold venting at runner side. In an example such as this, gases are pushed into the vents and not into the mold cavity along with the plastic material. FIGURE 2 to the mold cavity. They are designed to allow the air and resin gases to rapidly leave the cavity via a venting channel that is a few square millimeters—large enough to overcome the geometrical limits of standard venting grooves. The external mold-venting valve con- sists of a body that houses a guide for a slide. Both the body and the guide are designed to guarantee a conical closure when the sliding element moves. A spring keeps the slide in an open posi- tion to create an increased venting volume for the mold cavity. The body also contains wide venting channels connected to the venting area between the body and slide (see Figure 1a,b). The venting valve is connected to the cavity or cold runner by a groove that allows the gas to flow from the mold cavity to the valve. When the injected plastic reaches the vent, it applies pressure on the slide, compress- ing the spring, causing it to perform the conical closure and preventing the plastic from flowing into the valve. The venting grooves on the valve body are connected to the open venting space by HAIMER The only true solution for roughing and trochoidal milling operations in the industry. Tooling Technology Measuring Instruments Balancing Technology Shrinking Technology Haimer USA, LLC | 134 E. Hill Street | Villa Park, IL 60181 Phone: +1-630-833-1500 | Email: haimer@haimer-usa.com | www.haimer-usa.com September 12 - 17, 2016 Come visit us in Chicago! Booth #W-1452