A mystery change in the processing behavior of a new batch of nylon 6 shows the critical role additives play in nucleated materials.
More than 20 years ago I encountered a sudden upset in a very robust molding process. The details of the problem underscore an important aspect of material consistency and its relationship to processing and final part properties.
The part in question was running in a 10-cavity mold out of a nucleated, unfilled nylon 6. The part had some reasonably thick sections but ran on an automatic cycle time of only 22 seconds. No one in the plant gave much thought to the particulars of how this was possible. It simply ran that way. And then one day a new lot of material came out to the floor and as it fed through the dryer, the ejector pins began to push into the part, deforming the surface. By the time the lot change was complete, pins were pushing completely through the parts, leaving them in the mold and causing downtime due to process interruptions and tool damage.
The processor's usual response to this type of occurrence is to adjust the process. But the analytically minded molder will still want to know why such a robust process suddenly requires tweaking. A call to the material supplier brought the response that the material was within specification, and the only long-term solution that prevented constant tending of the job was a lengthening of the cycle time from 22 seconds to 27 seconds.
This is clearly an undesirable outcome not only for the profitability of the job but also for supply chain logistics. This increase in cycle time reduced the mold's output by 18.5%. When running at full efficiency, the customer required 80% machine utilization just to keep up with production demands. With the reduction in output, the machine now had to operate at an effective efficiency of 98.2%. This meant that the slightest hiccup shut down the customer's line.
Fortunately, the cycle time extension only lasted as long as the new lot. With the arrival of the next lot of material, the cycle time returned to normal despite the protestations of the material supplier that all the lots were the same.
CRYSTALLIZATION ACCELERATION
So what happened? The answer can be explained in part by the fact that the grade of material being used was nucleated. Nucleation speeds up the crystallization process in semicrystalline polymers such as nylon and polypropylene. Cycle time is governed by how rapidly a material increases in modulus as it solidifies, and the rate of this process in semicrystalline resins is determined to a large degree by how quickly the crystalline structure forms.
All semicrystalline polymers exhibit a particular behavior when it comes to the development of structure while cooling. In addition, the rate of crystallization is governed by the mold temperature and the part's wall thickness, since these determine the rate of cooling. Crystals begin to form once the polymer temperature drops below the melting point.
However, the process can be accelerated by introducing chemistry into the polymer that provides more sites where crystallization can begin. This technique is known as nucleation and the effects are significant because they change the crystal structure of the compound. While the overall degree of crystallinity in the material may not change significantly, the distribution and size of the crystals will be altered. In nucleated materials there are more crystals of smaller size instead of fewer larger crystals.
One of the practical considerations of this change is that nucleated materials tend to shrink less than their non-nucleated counterparts. By creating a more even distribution of crystals in the matrix, shrinkage-related problems such as warpage may be improved. Smaller crystal sizes also change the way light interacts with the polymer. Crystals scatter light, making the material appear translucent or even opaque to the observer. Smaller crystals interfere less with light transmittance, making the material appear clearer. This is the technology behind many of the clarified polypropylene materials.
Other additives cause nucleation either intentionally or unintentionally. Some colorants act as nucleating agents in certain materials. In addition, fillers such as glass fiber and talc can nucleate a polymer. And even small changes in the amount or type of the nucleating agent cause the crystallization rate to change. Other notable changes that occur when a material is nucleated involve productivity and final properties. Nucleated materials tend to be stiffer and stronger but less ductile than equivalent non-nucleated products. And, as suggested by the above discussion, the more rapid formation of crystals translates to a faster cycle time.
JUST A WEE BIT OF TALC
At the time that we encountered the problem with the nylon, we had no analytical tools to prove or disprove our belief that the material had changed. We could only surmise after the fact that something was different between the lots and endure the resulting loss in productivity and profit that came with it. However, if the same problem were encountered today, we would know how to diagnose the problem scientifically.
The technique of choice for detecting differences in crystallization rate is differential scanning calorimetry (DSC). We have discussed this technique in past articles as a tool for determining polymer composition and detecting differences in the degree of crystallinity. By observing the temperature range over which the material recrystallizes while it is being cooled, it is possible to see differences that can translate to the abovementioned considerations of shrinkage, cycle time, and properties.
Recently, this technique was applied to a problem of sudden cycle time change for a thin-walled polypropylene part. The product runs on a very fast cycle and has a long, thin shape that requires the attainment of a high modulus before the part can be ejected. Periodically, a batch of material comes in that exhibits sticking and requires a lengthening of the cooling time. Even with these changes, controlling changes in critical dimensions on this high-cavitation application can be a nightmare.
A DSC evaluation of parts produced from a good batch of material and a batch that cycles slowly clearly reveals the problem. Figure 1 above shows the behavior of the good polypropylene as it cools down from the melt state to the solid state at a constant rate of 10 deg C/min. The peak temperature of recrystallization is just above 117°C (242.6°F). Figure 2 below shows the behavior of the problem lot with a peak recrystallization temperature of 112.2°C (234°F), almost 5 deg C lower.
Now, the cooling rate of a polymer in a real injection molding process is much faster than that in a DSC instrument. If we assume an entry temperature of the polymer into the mold of 240°C (464°F), an ejection temperature of 80°C (176°F),
and a cooling time of 10 seconds, we calculate a cooling rate of 960 deg C/min, a process that cannot be duplicated in a lab instrument. However, it turns out that there is a relationship between the peak temperatures obtained in the DSC tests and cycle time.
To illustrate how even small amounts of an additive can change the crystallization process, the molder added less than .5% by weight of a fine talc to the problem lot. The processing difficulties associated with this lot immediately vanished and a DSC test on a part molded from this modified material is shown in Figure 3 below. Note that the recrystallization temperature has increased by more than 9 deg C to 121.5°C (250.7°F). Even if this type of at-the-press modification is not an acceptable alternative to the end user, it can be used to illustrate the effect of composition variation on the processability of the resin.
Resin suppliers rarely perform this type of a test on outgoing lots of material. Their assumption is that if the material formulation is put together within certain prescribed limits, the material will perform to the desired specifications. Therefore, when processing problems arise related to differences in nucleation, there is rarely a database available that can be referred to in order to determine what is normal and what represents a deviation. Fortunately, in this case the molder kept retains from past lots and was able to readily compare good and bad product. The key is to understand the relationship between the analytical output and the practical aspects of maintaining a competitive cycle time from lot to lot. Without that understanding, the results from the lab are just numbers.
These same types of issues can be encountered when molders change suppliers or alternate between suppliers of “equivalent” resins due to considerations of cost or availability. If your processing people periodically complain of long cycle times in a semicrystalline resin that arise for no apparent reason, variations in crystallization rate should be considered as a possible cause. Often these variations cannot or should not be “molded out”; they are part of the package that comes in your door.
About
Michael Sepe Michael Sepe has worked in the plastics industry since 1975 in a variety of roles involving both manufacturing and research and development. He is an independent consultant based in Arizona with clients throughout North America. He assists clients with material selection, designing for manufacturability, process optimization, troubleshooting, and failure analysis. Learn
more about Michael Sepe.
while it is being cooled, it is possible to see differences that can translate to the abovementioned considerations of shrinkage, cycle time, and properties.
Figure 2 below shows the behavior of the problem lot with a peak recrystallization temperature of 112.2°C (234°F), almost 5 deg C lower.
Fortunately, in this case the molder kept retains from past lots and was able to readily compare good and bad product. The key is to understand the relationship between the analytical output and the practical aspects of maintaining a competitive cycle time from lot to lot. Without that understanding, the results from the lab are just numbers.