Evaluate resin moldability with injection machine as rheometer.

Rising quality consciousness among plastics processors is drawing resin suppliers' attention to the importance of consistent processing. Some suppliers are working to ensure uniform molding of test specimens in order to obtain repeatable physical-property data. At Dow Plastics in Midland, Mich., this is combined with innovative methods of producing "realistic" q-c data on resin moldability characteristics, using the injection machine as a rheometer. Dow has developed methods that processors can use on their own, without special molds or equipment, to get more practically useful information on how a resin will run than is available from current data-sheet information.

Melt flow rate (MFR) is the conventional, easy-to-measure property used as a q-c variable and indicator of processability by resin producers and molders alike. "Melt flow rate is not a measure of moldability," declares John W. Bozzelli, the Dow plastics development specialist and injection molding "guru" who developed the new processability measures. The key problem is that MFR test conditions bear no relation to actual molding conditions - particularly with respect to shear rate. Bozzelli's solution (developed in collaboration with Rodney J. Groleau of RJG Technologies, Inc., Traverse City, Mich.) is to test the resin flow using an actual injection machine and mold.

INJECTION RATE IS CRITICAL

Bozzelli uses a standard ASTM test-specimen mold, which has been modified with the placement of a pressure transducer under each of two ejector pins located at opposite ends of a "dog-bone" tensile-bar cavity. (Such a mold is not strictly necessary, though helpful, as will be explained.) The pressure transducers are connected to the injection machine controls for cavity-pressure monitoring.

Dow's test-molding conditions are strictly regulated so as to leave resin viscosity as the only uncontrolled variable. Care is taken to ensure that mold and melt temperatures, screw rpm, plastic flow rate (fill time), and pressure in the mold are held uniform from shot to shot. Most important of all is ensuring consistent and reproducible injection speed or fill time. (Bozzelli defines fill time for his tests as the time to fill the mold 99% completely - i.e., without packing.) Repeatable injection speed means a repeatable shear rate on the melt, without which it would be impossible to separate shear-thinning effects from batch-to-batch viscosity variations inherent in the material.

To obtain consistent injection speed, it helps to have a precise and repeatable molding machine. For its testing program, Dow has installed over a dozen Mannesmann Demag 110-ton machines with NCIII microprocessor controls (supplied by Mannesmann Demag Corp., Plastics Machinery Div., Torrington, Conn.). Although some of these machines are now three to four years old, they still maintain [+ or -] 0.04 sec on a 1.36-sec injection time, Bozzelli reports.

The Demag machines maintain tight control of injection speed by means of a binary valve system (digital hydraulics), which controls and real-time monitoring of the volume of oil flow per unit time. Bozzelli says the test could be performed on all machines, both open-loop and closed-loop, with the aid of a fill-time clock to provide a manual check on consistency. But in either case, it is essential to inject in what he calls a "decoupled" mode - that is, allowing the machine to use up to 80-95% of its max. injection-pressure capability, if needed, in order to meet the injection-speed setpoint. The required pressure will vary according to the viscosity of the material at that speed (shear rate). But any change in fill time (detectable with a clock), whether with open-or closed-loop control, would result from the machine's hydraulic-system variability, not the resin.

DYNAMIC VS. STATIC VISCOSITY

Bozzelli recommends two different tests of "real-world" moldability with his injection molding rheometer. The first he calls dynamic flow, which relates to the high-shear conditions during mold fill. Ideally, he would like to record the integral under the hydraulic-pressure curve during fill as a measure of dynamic flow. With today's available equipment, a more practical and quite satisfactory solution is to record the peak hydraulic pressure achieved during the fill portion of the cycle (prior to packing). The easier the resin's dynamic flow, the lower will be the peak hydraulic pressure. Note that this test does not even require a mold, as an "air shot" would be sufficient. It is important, however, to eliminate consideration of any transient peak that may be characteristic of the machine's hydraulics right at the start of injection. And if flow tests performed on different machines are to be compared, it's also essential to obtain comparable plastic pressure data by multiplying the peak hydraulic pressure times the machine's intensification ratio, commonly around 10:1 but ranging anywhere from 8.3:1 to 24:1.

Bozzelli considers this simple measure of dynamic flow to be much more valuable than MFR as a "real-world" indicator of the energy required to fill a mold and what sort of length of flow can be obtained with different resins, with different lots of the same resin, or with differing proportions of regrind. He says it correlates very well with laboratory capillary rheometer tests.

However, there's a second important flow characteristic that relates to how easy it is to pack out a mold. Sink marks and weld lines are also related to this second parameter, which Bozzelli calls static flow. "Just because a material is easy to flow into a mold doesn't necessarily mean it's easy to pack out," he says. In his view, static flow is a particularly relevant indicator of moldability differences between amorphous and crystalline resins.

Static flow requires a test mold with at least one cavity-pressure sensor at the far end of the flow path in the cavity. During the holding portion of the cycle, Bozzelli measures the peak cavity pressure and the peak hydraulic pressure during hold - which he says is equivalent to the hold pressure at the nozzle. The hydraulic pressure at the ram is converted to plastic pressure at the nozzle by multiplying by the machine's intensifying ratio. Then it is possible to calculate the total pressure drop in the mold during holding by subtracting the cavity-pressure value sensed at the end-of-fill location from the nozzle plastic pressure. The remainder is Bozzelli's indicator of static flow, and a lower number means easier packing.

ENHANCING RESIN Q-C

Dow has now purchased 17 of the Demag machines, placing them in quality-assurance labs at virtually all of its domestic ABS, polycarbonate, and ABS/PC plants, as well as some polystyrene facilities and its Midland R&D lab. Some of Dow's European PS plants are starting to follow this approach, too.

Bozzelli believes firmly that dynamic and static flow tests represent important advances in characterizing resin moldability. "Every time I mold, I use them," he affirms. He uses these tests in development of new resins to meet particular processability goals. "They are also used to help confirm acceptability of a resin for a given customer when we know MFR is not telling us the whole story. It can happen that two lots with identical MFR do not run the same way for the customer."

However, Dow is not yet using Bozzelli's dynamic and static flow tests for routine q-c of resin batches, partly because of the relatively short time Dow has been working with this technology. Both Bozzelli and Randy Howard, director of Dow's laboratory for engineering thermoplastics Technical Service & Development, do believe that as this technology evolves it will be incorporated into q-c in order to produce material with superior processing consistency. For the present, Bozzelli is concentrating on educating molders to the value of the tests and their superiority to MFR numbers.

At present, the Demag machines are used at the resin plants primarily to produce highly uniform test specimens so that any physical-property variations can be attributed to conditions of resin manufacture, not specimen molding. Bozzelli notes that his use of a "decoupled" molding process in order to obtain consistent shear rates is vitally important, because molding shear conditions have important melt-orientation effects. Fast injection (higher shear) induces more initial orientation, but most of it has time to relax before the melt freezes. Slow injection (lower shear) produces less initial orientation, but more is retained, for a net increase in molecular orientation in the part. This is evident from Dow lab tests showing that notched Izod impact strength of 3.3 ft-lb/in. for an ABS resin when molded with a 1-sec fill time increases to 4.4 ft-lb/in. with a 4.3-sec fill time, and to 5.1 ft-lb/in. with a 10.62-sec fill.

To further ensure consistent treatment of the melt during molding, Bozzelli developed another simple test, related to his static-flow concept, with which to establish molding conditions for a given range of similar materials - e.g., all ABSs. He averages the peak cavity pressures sensed at the two ejector-pin transducers during hold. He then adjusts the holding pressure in order to obtain consistent average pressures for that range of materials. The purpose is to ensure a consistent degree of packing of the part, which affects strength properties.

One other domestic resin producer is using 110-ton Demag machines in an effort to produce material test bars under exactly reproducible molding conditions. Hoechst Celanese Corp., Chatham, N.J., has installed fully automated molding cells, in which molded test specimens are rejected if either the computer-monitored injection conditions or part weight (robotically tested) vary outside of SPC limits (see PT, May '91, p. 31). Bozzelli says Dow is likewise planning to move toward automated cells for molding test specimens, based on cavity pressure as the key SPC variable.