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Next, we evaluate the flow through up to 15 different channel geometries (though additional geometries can be run for special applications). The flow-channel geometries include both "cavity-like" and "runner-like" channels. Standard cavityflow-channel thicknesses range from 0.020 to 0.120 in. and runner channel diameters range from 0.040 to 0.180 in. Flow velocity and melt pressures are captured directly through the highly instrumented tooling using an integrated high-speed data-collection system developed by our controls team. Additional specialized test cartridges are currently being developed for thin-wall and micro-molding, including use with high-flow materials such as LCPs. In each one of these 15 geometries, the melt is injected at up to 10 speeds from very slow to very fast. When restriction through a channel at a given flow rate exceeds the machine's pressure limits, the data is automatically discarded. Only data captured at controlled flow rates is utilized. Figure 5 shows one form of a Therma-flo Moldometer data sheet developed for resin distributor Nexeo Solutions. This format includes two graphs; the one on top represents the cavity, and the one on the bottom would represent the runner channels. Notice that on each one of these you would have a curve that represents behavior of the material through different cross-sectional sizes (thickness and diameter). Figure 6 shows a Therma-flo cavity data sheet for a HDPE. Each of the curves shows the injection moldability of the melt though each of five different cavity wall thicknesses (from 0.030 to 0.100 in.). On the "Y" axis is an output of pressure per inch (psi/in.) of flow length. On the "X" axis you have velocity (in./ sec). This is very different from typical MFI or capillary rheometer data. There are no intangible units such as poise, Pascal- seconds, reciprocal seconds, or g/10 min, such as are normally reported. Therma-flo data is much more tangible and relevant to what designer or processor would want to see.
Note that data for the 0.020-in. wall thickness is not shown, as the melt was unable to flow through the measure- ment channel without exceeding machine pressure limits. Also data for flow through the 0.030-in.-thick channel is limited for the same reason, as pressure limits were exceeded at flow velocities higher than 27 in./sec.
To understand the use of moldometer data one needs to better understand how that data is obtained. During the molding characterization, the flow lengths of the test channels vary with wall thickness. Very thin walls have shorter flow lengths. The total measured pressure to flow the length of any given channel is then provided as pressure per inch of flow length (psi/in.) vs. injection velocity for each of the wall thicknesses and runner cross-sections. Flow velocity is used because it is the easiest and most direct way to relate to conditions in a mold. If you talk to anyone about filling a cavity or mold, they will most naturally express filling in seconds. ("We fill our cup molds in 0.2 sec") Never does anyone tell me they fill their cup molds at a shear rate of "X" sec '. And few would say they fill each of the cavities in the cup mold at a flow rate of "Y" in.Vsec.Though a volumetric filling-rate value may be desirable when documenting the process to run the actual molding machine, I would find myself converting this value to fill time to best visualize the process.
The simplest and most direct way to get a value for injection time is from injection velocity. The following example demonstrates this. Assume three different mold cavities, each having a wall thickness of 0.04 in. and a cavity flow length of 6 in. The first is a cup with a 1.5-in. radiused base and a depth of 4.5 in. The second is a thin, flat strip-like part that is 0.45 in. wide x 6 in. long. And the third part is a flat disk having a 6-in. radius.
If each part is filled in 0.25 sec, they will also fill at an aver- age fill velocity of 24 in./sec (6 in. pided by 0.25 in./sec). Using a Therma-flo moldometer data sheet, a velocity of 24 in./sec can easily be applied to each of these parts. Alternately, it would be easy to determine that a part with a 6-in. flow length filled at a velocity of 24 in./sec will fill in 0.25 sec.
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