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September 3, 1998

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The Materials Analyst, Part 11: Finding the culprit in plugged subgates

This series of articles is designed to help molders understand how a few analytical tools can help diagnose a part failure problem. 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. He has provided analytical services to material suppliers, molders, and end users for the last 10 years. He can be reached at (414) 369-5555, Ext. 572.

Generally when people think of polymer analysis, they envision sophisticated processes designed to probe the most obscure details of molecular structure. Very few imagine that the same tools that researchers use to develop new materials and study fundamental structure-property relationships can also be used to solve a mundane production problem like plugged subgates. But this was the problem we were faced with when a client presented us with samples of subgates that contained an ingredient that was shutting off cavities at random in a high-production 32-cavity tool. Some of the obstructions were partial and resulted in short shots that had to be sorted out of the production.


Figure 1. DSC of a polypropylene copolymer.

     However, most of the time the gates were completely plugged. If the mold ran unattended for any length of time, the technician would return to find the mold running only 29 or 30 parts. The remaining cavities tended to be overpacked which led to additional problems with poor dimensional control and sticking of parts in the mold. The parts were being molded in an unfilled polypropylene copolymer to which the molder was adding a black concentrate. The molder had managed to collect several samples of the material that he was finding in the subgates, but a visual inspection revealed nothing. Along with these fragments, he also sent samples of the virgin material and the color concentrate.
     In past articles we have illustrated the use of differential scanning calorimetry (DSC) to identify the contamination of one plastic material with another. Transition temperatures such as melting points and glass transitions tend to be distinctive to specific materials, and with a good library, matches can usually be made. Since polypropylene runs at a relatively low stock temperature, there are many materials with higher softening temperatures that can cause flow problems if they are mixed with polypropylene. Usually these are semicrystalline materials with high melting points such as PET polyester, nylon 6/6, and PPS.


Figure 2. TGA on sample from plugged subgate.

     We first ran DSC tests on a sample of the virgin material and the concentrate. Figure 1 shows the test result obtained on the virgin material. The large melting endotherm at 167C, accompanied by a much smaller melt at 114C, confirmed the composition as polypropylene copolymer. The concentrate gave the same result. The first subgate fragment we tested also checked out as pure polypropylene. We heated the sample to 330C and found no trace of any foreign material in the sample.
     While the DSC is an excellent tool for finding contamination from semicrystalline polymers, detecting amorphous contaminants can be more difficult. Amorphous materials do not have a well-defined melting point; they possess only a glass transition. This is a relatively small-scale event when compared to a crystal melt and can therefore be missed. Most amorphous materials will be soft enough to at least pass through a subgate when the barrel temperatures reach 425F, but a few high-heat materials like polyetherimide (PEI) and polyethersulfone (PES) can pose problems. For these types of contaminants we often turn to thermogravimetric analysis (TGA). TGA identifies materials by their degradation temperature. The material is heated in an inert atmosphere like nitrogen until all of the polymer burns away.


Figure 3. TGA on plugged subgate showing weight gain above 600C.

     In this test, a material like polypropylene decomposes completely during this first step, but high-heat amorphous materials will leave behind a carbon deposit that will not burn away until oxygen is supplied to the instrument. This fingerprint difference allows us to detect contaminants that do not have a melting point. Figure 2, shows the result of a TGA run on one of the subgate fragments. Again the result revealed no contamination. The decomposition accounted for 99.2 percent of the sample, and no carbon was created during the test. A very small amount of residue was left behind, but this was not unusual.
     However, a close look at the high temperature part of the scan revealed that this residue was not an inert filler. When we expand the plot above 500C in Figure 3, it is clear that this residue is actually gaining weight. If this were a filler like glass or talc, it would either lose a small amount of additional weight or it would be unchanged. The only substances that will typically take on weight at these temperatures are metals. When the gas stream is switched from nitrogen to air, the oxygen in the air reacts with some metals.


Figure 4. TGA on second subgate plug showing 20 percent residue.

     This particular sample had almost doubled in weight between 500 to 850C and was still on the way up when the test concluded. A second test on another fragment was even more revealing. Figure 4, shows that this sample contained a residue of more than 20 percent. A call to the customer confirmed that the polypropylene being used was supposed to be unfilled. So what was the mystery material? Once again, an expanded view of the high-temperature portion of this test, shown in Figure 5, revealed a distinctive weight gain.
     At this point we knew we had a foreign material that was not a polymer. We also knew that it was inorganic and that it was capable of taking on oxygen. Unfortunately, this was as far as we could go with the combined tools of DSC and TGA. Fortunately, the residue from this second sample weighed almost a full milligram. This meant that we would be able to perform some additional tests in order to determine composition.
     A favorite method for identifying inorganic materials is a technique known as x-ray fluorescence (XRF). It works by irradiating the material with x-rays. These x-rays eject electrons from inner shells of the atoms in the material, creating an unstable situation.


Figure 5. Second subgate plug showing weight gain.

Outer shell electrons drop to the vacancies in the inner shells, and in the process x-rays are emitted. The wavelength of the emitted x-ray radiation is a fingerprint for the elements present in the sample. A further advantage of the test is its nondestructive nature. If sample is in short supply and may be needed for additional work, XRF will preserve the material.
     The results from the XRF test showed the residue to be made primarily of iron. The question then was, where did the iron come from? One possibility was tramp metal getting into the raw material or the concentrate. However, iron is easily picked up by a good hopper magnet, even when it is encapsulated in a plastic. This molder was using very strong magnets that would have picked up anything coming into the hopper. A check of the magnets revealed nothing.
     This left the screw and barrel. An inspection of the internal components revealed some wear and erosion of the screw tip. While the fragments were small enough to pass easily through the nozzle tip and runner system, they were large enough to become caught in the very small subgates of the tool.
     This application provides an excellent example of the benefits that come from combining practical molding experience with analytical methods. Analysis is not just for research and development. Used properly, it can even solve the everyday problems encountered on the production floor.

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