The DNA of Rotomolding
by Roy Crawford, University of Waikato, New Zealand
The early years of the rotational molding industry were dogged by a low technology image. However, since the mid-1990’s there have been very significant advances through the development of internal air temperature measurement as a means of controlling all aspects of the manufacturing process. Today the “Rotolog” trace showing the variation of temperature inside the molding throughout the cycle is being used more and more as the primary means of controlling the quality of rotomolded parts.
When this technology was first developed, the temperature trace shown in Fig 1 was often referred to as the “fingerprint” of rotomolding. The rationale was that the general shape of this trace is unique to rotomoulding (just as fingerprints are unique to humans), and a specific trace for a particular molded part was unique to that part. and it could be used as the basis for quality control. Molders and designers became aware that consistent quality parts would be produced day after day if the same shape of the trace was maintained for a particular molded part.
Initially the main quality control parameter that was cited as being most important was the Peak Internal Air Temperature (PIAT). It was recognized that if this parameter was maintained for a particular resin and a particular molding then good quality parts would be achieved.
More recently, as the understanding of the technology of internal air temperature measurement has advanced, and the resins for rotomoulding have become more sophisticated, it is apparent that other features of the “Rotolog” trace are equally important.
This has led to the concept of the “DNA of rotomoulding”. The basis of this is that many aspects of the resin, the additive package, the powder quality, the manufacturing conditions and post-molding operations affect the characteristics of a particular molded part. This is similar to the situation in humans where the specific arrangement of a relatively small number of genes gives unique characteristics to every human being. Many of the above parameters affect the shape of the internal air temperature trace – such as the slopes of the graph during all of the key heating and cooling stages of the process.
Thus the temperature trace shown in Fig 2, with all of the specific slopes as indicated, represent effectively the DNA of a particular rotomolded part. Each of the slopes is influenced by resin, the additive package, the powder quality, the molding conditions and theses, in turn, give unique properties or characteristics to every rotomolded part.
Having recognized this phenomenon, the next stage in the development of rotomolding technology is to relate the shape of the internal air temperature trace to the properties of the end product. It is known how the input variables affect the shape of the trace, and it is known how the properties of a rotomolded part are affected by molding variables. Therefore on viewing an internal air temperature trace in real time as the part is molded, it should be possible to predict the performance of that molded part.
Just as the specific DNA of an individual human being can predict the propensity to certain diseases, it should be possible to predict if a particular rotomolded part is likely to have warpage problems, impact toughness problems, residual stress problems, etc based on it’s “DNA” trace.
And more importantly for rotomoulding, we can control the shape of the DNA trace. If the DNA trace produced for a particular molding has a shape that is linked to potential performance problems, then it is possible to alter the shape of the DNA trace for subsequent moldings to overcome, or alleviate, the potential problem.
This approach creates an exciting future for rotomolding and would allow this manufacturing process to leap ahead of other molding methods for plastic parts. Fig 3 illustrates some of the building blocks that contribute to the DNA of a rotomolded part. The challenge is to link these parameters directly to the DNA trace illustrated in Fig 2. Work on this is well underway in that many molders are gaining sufficient experience with internal air temperature traces to be able to alter the shape of the temperature trace as desired. For example, it is now straightforward to shift the trace to the left in order to reduce cycle times. It is also possible to control precisely the slope of the heating and cooling rates in order to maximize impact properties, and the rates of powder lay-up, heating rates and cooling rates can be defined within bands to create parts with generally acceptable properties.
Controlling an DNA of rotomolded parts is the exciting new frontier for the rotomolding industry. The challenge is to link the input parameters (the “building blocks”) to the shape of the internal temperature trace and then to link this to the quality of the molded part. The reward is that continual monitoring of the internal air temperature trace throughout the manufacturing process provides the DNA of the molded part at its “birth”, and the behavior of the part during its future life is therefore known at the time that the part is manufactured.