Gardner Business Media

Hot Runner Technology OCT 2017

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October 2017 5 a magnitude of the fourth power. This means that a small change in radius will result in a rather large difference in pressure drop from gate tip to gate tip. The common perception is that if the gate tips are all within tolerance, this will not matter. However, it is important to ask what the acceptable tolerance is along with the gate tip dimensions. Just because they are within tolerance does not mean there will not be a difference in pressure drop. For example, a tolerance of +/- 0.002 inches on a gate diameter of 0.300 inches may be too large of a tolerance range to achieve consistent molding. Of course, a processor may adjust individual tip temperatures to try overcoming some of these variations. It is important to understand that by doing this, the processor is changing the material condi- tions from which the parts are being molded. If the processor runs semi-crystalline materials, then he or she may create further issues with crystallinity differences near the gate versus other areas of the part. Additionally, the resultant variations in ther- mal expansion of nozzle and manifold steel that occur because of different temperature settings could ultimately lead to other filling variations, leaking or breakage issues. For example, if a cavity is filling behind other cavi- ties, a processor may increase the temperature of the nozzle feeding that cavity. However, that temperature increase could drive the gate tip further into the gate orifice because of Photo courtesy of Incoe Corporation. FIGURE 2: Checking the uniformity of the stack height across all nozzle tips is important to avoid points of tolerance stack-up issues shown here. Images courtesy of Beaumont Technologies and the AIM Instititute. FIGURE 1: The hot tip's natural position with expected thermal expansion is shown on the right. The image on the left shows the location after additional thermal expansion. Notice the effective flow channel is less on the image on the left.

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