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1、Nowadays, injection molders are under increasing pressure (pun intended). Demands for tight tolerances, near-zero rejects, and ever-lower cost that were once limited to automotive and medical jobs are now common even for molders of consumer products. Molders have often responded by limiting themselv
2、es to molds of low cavitation because they are easiest to balance naturally. But low cavitation requires more molds, more machines, more floor space for the machines, and more people to run them. To remain competitive in a global marketplace, cost can only be reduced by producing parts faster with m
3、ore consistency and less scrap. If higher-quality parts can be produced from an eight-cavity mold rather than a four-cavity tool, or from 16 cavities instead of eight, cost savings and greater customer satisfaction will follow. The question is how to get there. The basis of the solution came to me i
4、n 1997 when I was wrestling with the issue of mold-filling imbalance. For years, I had observed imbalances in what have always ben called “naturally balanced“ runners. A member of mycomputer-aided-engineering consortium at Penn State Erie was encountering just such a problem. His eight-cavity mold w
5、as “naturally balanced,“ and yet the parts from the molds inner cavities were consistently heavier than those made in cavities farther from the sprue. Tight weight tolerances for this part didnt allow for such variation, and at nearly $20/lb, material waste had to be held to a minimum. I first blame
6、d the two usual suspects: cooling variations created by higher thermal load of the plastic parts near the center of the mold, and plate deflection within the mold. These standard scapegoats had served industry well for years. But this mold had a well-designed cooling system that provided increased c
7、ooling to the inner cavities. One look at the molds rigid support columns and the parts thick walls told me plate deflection wasnt the problem either. MeltFlipper inserts use changes in runner elevation to “flip“ the melt orientation by 90, eliminating asymmetrical distribution of hotter and cooler
8、melt to diverging runner branches.Fig. 1High shear and frictional heating near the outer wall of a runner channel lower the viscosity in this region. This is the root cause of filling imbalances. I started thinking about runner systems. I started thinking about the laws of physics and everything I k
9、new about laminar flow. Then I started retracing the runner from the gate back down to its first branch. The end of symmetryPlastics exhibit a laminar (or streamline) flow through a runner, so the melt is divided into many concentric layers or “laminates“ that have different shear and temperature co
10、nditions. Regardless of flow rate, shear is greatest near the wall of a runner channel and lowestor zeroat the center (see Fig. 1). Melt viscosity will be significantly lower in these high-shear laminates than in the middle of the runner flow. Additionally, high shear near the runner wall can create
11、 significant frictional heating in the outer laminates. Sophisticated 3D mold-filling simulations have shown these outer laminates to be as much as 100 C hotter than the laminates in the center of the runner channel. Laminar flow and the low rate of heat transfer between laminates maintain the disti
12、nct layered structure as the melt proceeds along the runner. Highly sheared, hotter, less viscous material flows in an annular ring along the channel walls surrounding low-shear, cooler, more viscous laminates flowing in the center of the channel. Sometimes unusually low-shear conditions in a cold-r
13、unner mold can cause the outer laminates to be cooler than the inner laminates. But under any circumstances, there will be a difference in the melts condition between these laminates. Looked at in cross-section, the pattern of flow laminates across the runner is initially symmetrical. But when the f
14、low splits at a branch, that symmetry is lost, and filling imbalance begins. The computer doesnt lieI asked Dr. Jack Young, a mechanical-engineering colleague at Penn State Erie, to help me make a computer analysis of an eight-cavity test mold I was having built. With assistance from a student resea
15、rcher and Fluent Inc.s FIDAP fluid-dynamics analysis software, we needed more than three months to develop the required model and complete an analysis. Fig. 2FIDAP analysis output shows the asymmetrical temperature distribution in a secondary runner branch as a result of frictional shear heating and
16、 laminar flow. Fig. 3Distribution of melt We could have modeled a runner in five minutes using any of the commercial injection molding software packages. But we needed to start from scratch in order to overcome the limitation of the one-dimensional solution techniques used for runner analysis in these programs. Without recognizing the shear-induced imbalance created in a branching runner, developers of injection molding simulation software have used one-dimensional analysis to sim
