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This article is designed to help molders understand how a few analytical tools can help diagnose a part failure. Michael Sepe is our analyst and author. He is the technical director at Dickten & Masch Mfg., a molder of thermoset and thermoplastic materials in Nashotah, WI. Mike has provided analytical services to material suppliers, molders, and end users for 15-plus years.
Following systematic, step-by-step procedures when searching for the cause of part failure yields better results than a shotgun approach. （图片） We have spent a lot of time over the past few years discussing the importance of polymer molecular weight to performance and the correlation that can be drawn for most materials between average molecular weight and melt-flow rate (MFR). For most materials, comparing the MFR of molded parts to the related raw material provides a good gauge of the effect that processing has on the polymer.
However, while an excessive shift in MFR documents a problem, it does not automatically indicate the root cause. It only shows that somewhere between the bag or box of raw material and the part conveyor, something undesirable happened to the material.
To pinpoint the root cause, we can combine a systematic use of the MFR test with knowledge of processing and materials. There are several steps in the process that can contribute to degradation, and by monitoring the MFR of the material as it passes through these, it is possible to identify the source of the problem.
Overdrying or Oxidation?
The first step is drying the material prior to processing. If your material does not require drying or preheating, then this step can be omitted from consideration. If drying is a necessary step in the process, then the effect of drying on the polymer must be considered.
Many people in the industry refer to the “overdrying” of a material, a term that has no scientific basis since, strictly speaking, overdrying means removing too much moisture. There are no well-interpreted data to show that drying to a very low moisture level does any harm to a material. In fact, there are good data to indicate that the opposite is true.
However, it is true that drying a material for a prolonged period of time at an elevated temperature can cause oxidation, which in turn results in degradation. Nylons exhibit the greatest sensitivity; however, problems have also been documented with polyesters and polycarbonates. If a processor suspects that a material has been damaged by overly aggressive drying, comparing the MFR of as-received pellets to pellets from the dryer can rule this in or out as a possible problem.
Melt Temperature and Residence Time
The next step is the heating cylinder. A lot happens here and the concern common to all polymers is time and temperature. These work together and must be considered together. A melt temperature that may be appropriate for a residence time of 5 minutes may be disastrous for 10 minutes—a key consideration if a job is moved between presses with different barrel capacities.
The other concern during melt processing is the possibility of hydrolysis, or degradation through a chemical reaction with water. A lot of materials require drying to produce acceptable parts. But not all materials will degrade if molded wet. If you are processing polyesters, polycarbonates, nylons, polyurethanes, or blends containing any of these materials, then the contributions from excessive moisture content must be considered.
It is not possible to separate the effects of moisture from those of temperature, and time is a factor in both. The longer the material sits in the heating cylinder, the more opportunity both temperature and moisture have to damage the polymer. However, the combined effects of all these conditions can be captured by performing an MFR test on a purging from the nozzle. If the increase in MFR is excessive, then the effects of time, temperature, and (if applicable) moisture can be sorted out by measuring the melt temperature, residence time, and raw material moisture content, and comparing the results to the recommendations of the material supplier.
Do not guess at these parameters. Melt temperature measurement requires a calibrated pyrometer with a preheated probe and a large enough mass of material to prevent the probe from cooling during the measurement process. Residence time calculations are always lower than the actual values because they do not consider the mass of material in the screw flights.
Determine residence time by sliding the hopper aside while the process is on cycle and letting the material in the feedthroat run down until the screw is visible. Then drop a few pellets of a different color of the material into the feedthroat and count shots until you see the new color appear in the molded product. Multiply that number of shots by the average cycle time. You will be surprised to find that the real residence time is 1.5 to three times longer than the calculation based on shot size and shot capacity. Finally, moisture content must be measured with a device that actually measures moisture. Loss-in-weight systems are not adequate (see “Where Does the Moisture Go?” series in March, May, July, and September 2002 IMM).
Test Parts and Runners
Finally, we have the effects of the mold. These are often neglected because it is assumed that the worst is over once the polymer has exited the nozzle. However, the effects of shear stress are often highest in the mold because flow paths are the most restrictive and the polymer is beginning to cool. Because the gate usually represents the point of greatest shear stress, it is best to test the runner and parts separately.
Hot runners demand special consideration. A hot runner system is like a black box in the molding process. We can read the temperatures on the various zone controllers, but we do not truly know the temperature of the melt at all the points in the system. Well-designed systems have relatively uniform temperature within a zone, are free of dead spots, and strike a good balance between residence time, pressure drop, and shear.
Unfortunately, well-designed systems matched to the appropriate raw material are in the minority, and the effects of the conditions in a hot runner should never be taken for granted. It is also important to remember that the hot runner represents an extension of the conditions in the injection cylinder and that set temperatures in the runners and gates tend to be at least as high as those on the barrel controllers.
Isolating the Cause
The table above shows an application of this exercise to a product molded in PC. It is an eight-cavity tool fed by a hot runner that then enters four cold runners. Each of these runners feeds two cavities. Short shot studies show the flow of the system to be balanced to within 5%; therefore, for purposes of MFR testing, all of the runners are considered to be equivalent and all of the parts are also tested as a single group.
The total increase in the MFR from pellets to parts is in excess of the recommended maximum of 40%. However, if we only looked at the pellets and the parts we would be hard pressed to identify a root cause. But with this stepwise technique, we can see that the largest change occurs as the material passes through the subgates and enters the cavities. While this is not a common result, it does sometimes occur in materials that are more susceptible to degradation from high shear stresses.
An examination of the gates feeding each cavity showed that they were only .020 inch in diameter. Opening the gates to .035 inch reduced the MFR of the molded parts to 13.96 g/10 min or a total increase of 28%. Not only does this technique allow for isolation of the possible root causes, but it also saves the time of working on things that are not the problem—something anyone with a full plate in a manufacturing environment can certainly appreciate.
Contact information
Dickten & Masch Mfg., Nashotah, WI
Mike Sepe; (262) 369-5555, ext. 572
dmlab@execpc.com
www.dicktenplastics.com
2/16/2005
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