Last month I visited molding suppliers in an Asian country (not China). One of the visits unfolded in a manner that is all too familiar and raises questions about how the qualification process is performed when selecting suppliers in low-cost locations.
The molder was running a relatively large part in a polycarbonate-based alloy. An inspection of the drying equipment showed that it was a hot-air system as opposed to a dehumidifying desiccant dryer. The parts exhibited splay at various locations on their surface that are consistent with the presence of excess moisture in the material. However, the parts were being painted and the paint in this case masked the cosmetic effects of the splay. But with this type of resin, the cosmetic effects are the least of our worries. When this material is molded with excess moisture, the polymer undergoes degradation. The resulting decline in molecular weight causes the material to lose impact strength, a quality that is a primary reason for using this material in the first place.
This scenario plays out repeatedly in low-cost manufacturing locations. Fundamental aspects of material preparation such as drying are carried out using inadequate equipment. Not all materials suffer damage at the molecular level if they are dried improperly. Many resins are dried prior to processing strictly for cosmetic purposes—like ABS, SAN, and acrylic. However, materials such as polycarbonate, polyesters, nylons, and polyurethanes undergo hydrolysis when they are exposed to elevated levels of moisture at high temperatures. This chemical reaction can take hundreds of hours if the exposure occurs at end-use application conditions.
This is shown in the accompanying table as a progressive increase in melt flow rate (MFR) with time. Increases in MFR that exceed 40% are considered to be associated with excessive changes in average molecular weight. The data in the table show that this benchmark is reached in approximately 675 hr, or four weeks. In the barrel of an injection molding machine at a temperature of 300 to 320 C, this process takes just a few minutes.
To make matters worse, hydrolysis is not always accompanied by visual defects. All materials have a range of moisture contents where molecular damage can occur without producing any cosmetic evidence for the presence of moisture. PET polyester, for example, can become extremely brittle without showing any cosmetic signs of wet material. This is the reason that manufacturers of hydrolytically sensitive polymers instruct processors to use dehumidifying desiccant dryers capable of achieving a dewpoint of -40 F (-40 C). In order for the heated air passing through the hopper to remove moisture from the pellets, it must be extremely dry. If it is not, there is no tendency for the moisture in the raw material to migrate to the heated air. In fact, hot, humid air can actually add moisture to a resin.
Those are exactly the conditions found in many processing plants in South Asia. With hot-air dryers, the dewpoint of the air is essentially the same as the dewpoint of the ambient air in the plant. This will vary over the course of the year. In the cooler months the dewpoint may fall as low as 10 to 15 F (-13 to -10 C).
While far from ideal, it is frequently adequate in practical terms when coupled with the fact that the initial moisture content of the pellets in these cooler, dryer months is lower and therefore less moisture must be removed. However, the summer months in that part of the world bring oppressive heat and humidity, and most processing plants are not climate-controlled. The climate inside the plant is the same as the climate outdoors.
During these times of year, dewpoints can be as high as 95 F (35 C), and hot-air dryers are not suitable for drying hydrolytically sensitive engineering polymers under such conditions. The parts may look satisfactory, but they are a ticking time bomb waiting for application conditions such as an abrupt impact event, chemical exposure, or a sustained load to produce premature failure.
Because the problems associated with improper drying are seasonal, and because the logistics of product flowing from thousands of miles away can make traceability very challenging, fluctuating product quality can be very difficult to relate to the root cause in the plant environment. The incidence of failed parts in the field rises and falls in a pattern that is very difficult to decipher, particularly when the OEM selecting the molder is not aware of the inadequacies within the overseas processing plant.
Who is signing off on these processors? How is it that companies making products where proper functioning is critical do not possess the proper equipment for sustained levels of successful processing? Who conducts reviews and audits of prospective suppliers and according to what criteria do they award the business?
Polymer degradation remains one of the primary causes for plastic part failure. It is not a coincidence that a disproportionately large number of these failures occur in the types of materials that we have mentioned. It is ironic and more than a little bit frustrating that in a world where the documentation process associated with quality assurance has grown by orders of magnitude over the last 20 years, our procurement practices have left OEMs barely treading water in the process of quality improvement, simply because no one knows what to look for in selecting a qualified supplier.
Even more distressing is the behavior that often takes place once the realization has dawned and it is time to take corrective action. The first step is to educate the part supplier as to the need for the proper equipment. When we informed the molder of the polycarbonate-alloy panels that he was not drying the material properly, he was initially stunned. As far as he was concerned, drying meant passing heated air of a particular temperature over the pellets in the hopper for a prescribed period of time. It took some time to explain the need for a more sophisticated approach. Dehumidifying desiccant dryers are more expensive and require more attention to detail in their operation and maintenance.
Once this reality has dawned on the molder, the price of the part typically goes up. In other words, it costs a little more to make good parts that it does to make junk, at least in the short term. But short term tends to be the focus these days, especially when we are looking for the best “deal.” Warranty costs, customer complaints, and even the occasional litigation are problems for other departments.
Some time ago, business author Tom Peters observed that you cannot cut your way to prosperity. Cost reduction is an important function in any organization, but it should be recognized that it is a self-limiting activity. At some point the product simply cannot be made for any less without some quantum leap provided by innovation. Continuing to move the responsibility for manufacturing to increasingly less capable venues for the sake of a lower piece price is just plain bad economics.
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