|We begin a new series of articles 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.|
A customer called one day to ask for help in solving a problem with some polycarbonate housings that appeared brittle. Bosses on the inside of the part were fracturing easily during assembly and testing after the part had performed successfully for more than two years. Parts were sent in for evaluation. As is often the case when the customer is not the molder, there were no good parts or raw material available for comparison, only the parts that were failing.
There are two major causes for embrittlement in plastic materials. The first is contamination. Few materials can be mixed together at the press without causing a wide variety of problems. The methods for detecting contamination will be covered in a future article. The second cause is molecular weight reduction. All plastic materials consist of polymer chains, long strand-like molecules that are created by stringing together a large number of small molecules until the chains grow long enough to become entangled with each other. It is this entanglement that is primarily responsible for the toughness that we expect from most of the plastic materials we use.
Understanding Molecular Weight
Longer polymer chains mean higher molecular weight, increased chain entanglement, and improved properties. The property that benefits the most from higher molecular weight is impact strength. Any time a polymer is heated and subjected to the stresses of melt processing, there is the potential for these polymer chains to break. If enough of the chains break, the molecular weight is reduced, and beyond a certain point, we begin to see a decline in toughness. This is especially noticeable in materials like polycarbonate where toughness is the primary reason for selecting the material in the first place.
Injection molding is particularly rough on polymers because in addition to the barrel temperatures required to soften the material, there can be significant shearing forces applied to the material to move it rapidly through the machine nozzle and into the mold cavities. In addition, some materials react negatively when they are heated to melt processing temperatures in the presence of excess moisture. The polymer chains that make up polycarbonate will break up rapidly if the material contains more than .02 percent moisture when heated to processing temperatures.
While molecular weight sounds like a high-tech property that may be difficult to measure, in most materials, it is related to a property that is easy to measure and that most molders relate to very well, the property of viscosity or melt flow rate. Any processor who molds different grades of polyethylene or polypropylene is accustomed to referring to the materials by their melt flow rate. These processors also understand that a 15 melt polypropylene flows more easily than a 5 melt polypropylene. So where do the numbers come from and what do they mean?
|Figure 1. A melt indexer, for measuring melt flow.
Figure 1 shows a melt flow testing machine. It is a simple device consisting of a heated chamber with a cylindrical hole for loading raw material. At the bottom of this hole is an insert with a smaller hole. For a given material, a specific temperature is set and material is loaded into the heated cylinder. A specified weight is then placed on the molten material and a timer measures the length of time required to push a certain amount of material through the orifice at the bottom of the cylinder. Alternatively, the clock can be run for a set amount of time and the weight of the material can be measured at the end of that time.
The flow rate is converted to a standard of grams/ 10 minutes and this is the melt flow rate. While high melt flow rate materials are desired for ease in processing, they also contain shorter polymer chains that will not endure abusive environments as well as higher molecular weight materials that do not flow as well.
While the melt flow test has its limitations, it can be very useful in measuring the degree to which a material has been altered by processing. Reground molded parts can be melted and the viscosity can be tested. This result can be compared to the value for the raw material. As a rule of thumb that works for almost any unfilled material, the melt flow rate of pellets should not increase by more than 30 percent during processing. Thus, if a molder starts with a 10 melt flow material, the reground part should have a melt flow no higher than 13. If it exceeds this limit, then either heat or moisture (if it is a moisture-sensitive material) has degraded the material and performance problems are more likely to occur.
The parts sent in by our customer were made of a 10 percent glass-reinforced polycarbonate. The rules for allowable melt flow changes in filled materials are a little more generous because some of the change in viscosity will be due to changes in the length of
the glass fibers. Glass-fiber breakage is an unavoidable consequence of high-shear processing. For a 10 percent glass-filled polycarbonate, an increase in melt flow rate of as much as 45 to 50 percent is permissible without causing a serious reduction in performance.
Because we had no raw material in this case, we had to rely on the specification limits issued by the material supplier. The melt flow rate limits for the raw material were 6 to 10 g/10 minutes. We were then looking for values in the molded parts to be no higher than 15.
The actual melt flow rate for the material in the molded parts was 52 to 54. This clearly showed that a severe drop in molecular weight had occurred, and the brittle behavior was easily explained. It is important to note that the test does not determine the cause of the degradation, only that it exists. However, in this case the parts were of a light pastel color. If excessive heat had been the cause, the color of the product would have almost certainly shifted to a darker hue. The color match was good, so the logical conclusion in this case was insufficient drying. The splay that normally appears when wet polycarbonate is molded was masked by the light color of the part and the presence of the glass fiber.
The vast majority of impact problems with polymers result from molecular weight reduction. This is easily checked with a simple melt flow test, and often no further testing is required.