Real-time temperature monitoring advances rotomolding control.

Real-Time Temperature Monitoring Advances Rotomolding Control

A system that senses air temperature inside the mold during the rotational molding process and feeds that information in real time to a remote monitor has been developed by researchers at The Queen's University of Belfast, Northern Ireland. Details of the device, called the Rotolog, were presented at the October meeting of the Association of Rotational Molders in Toronto.

The Rotolog has important implications for quality control in rotomolding, according to Professor R.J. Crawford and Paul Nugent, developers of the system. For the first time, they say, air temperature inside the mold is being monitored by thermocouples to provide information on the polymer melt; this data can then be compared with charts of desired polymer processing temperatures to optimize physical properties by more accurately controlling the molding cycle.

Crawford claims that the monitoring device can be easily adapted to the controls of a rotomolding machine. He and Nugent are presently seeking distribution arrangements with several U.S. and Canadian machinery manufacturers, and the unit may soon become available commercially.

REMOVES THE GUESSWORK

The Rotolog consists of a monitoring device, receiver, IBM-compatible PC, and software. The monitoring device attaches to the arm of the machine and measures what is happening to the mold. Air-temperature measurements inside the mold are taken via thermocouple through the vent port. There are no physical connections between the monitoring device and the outside of the oven. Instead, a radio transmitter sends signals from the monitoring device to a receiver. The receiving unit processes the signals and passes them on to the personal computer. The computer scans the data and presents it in a real-time, understandable format for machine operators and q-c personnel. Software analyzes information gathered from the mold. The Rotolog reportedly can operate with all types of machines and can monitor four different molds on any one arm.

The system promises to take much of the guesswork out of rotomolding. The real-time output of the Rotolog is said to enable the molder to identify four critical elements in the cycle: the time at which all powder has melted; the optimum maximum internal temperature inside the mold; when crystallization/solidification has occurred; and the point at which de-molding can begin. This means several tangible benefits for molders, say the developers:

* Fast set-up time--Trial and error during first runs is eliminated because the air temperature gives a precise, real-time indication of the temperature of the plastic.

* Control for double material layers--By tracking when the powder melts into the outside layer, it indicates the optimum time to introduce the second material.

* Balancing multiple molds--If there is a range of molds on one spider, each can be monitored to arrive at the best compromise for optimum conditions on all parts.

* Troubleshooting--Time/temperature profile graphs obtained from the computer can be used as a quality standard to compare data on molded parts.

* Minimized warpage--By keeping close track of the air temperature during cooling, warpage can be eliminated or at least minimized.

* Use with regrind--The Rotolog, coupled with experiments on taking parts out of a machine, may indicate when the less-stable regrind deteriorates properties of parts during the heating cycle, according to Dr. James White, director of the Rotational Molding Development Center at the University of Akron, Ohio.

SIMULATES PRODUCTION

A computer-simulation program called Rotosim, an essential part of the Rotolog system, was developed to anticipate the effects of processing variables prior to actual molding. The Rotosim program allows "what-if?" experiments on changing various parameters, such as mold geometry, oven temperature, rotational speed, type and amount of material, and type of cooling. With computer simulation, molders can reportedly achieve optimum molding conditions without costly and time-consuming trial and error.

The latest version of Rotosim software incorporates data from molding trials of a range of plastics, processing conditions, and mold materials, including the measured effects of processing conditions on final part properties. In these trials, thermocouples were placed inside the mold, away from the wall, to record the real-time temperature at various levels of the melt as well as the internal air temperature over time during the cycle.

By comparing what was happening to the powder inside the mold to the air temperature at various stages during the rotomolding process, Crawford and Nugent were able to follow the variation of the air temperature inside the mold as a function of the molding cycle. The temperature variation formed a curve of molding conditions, i.e., when the material melts, sticks to the mold wall, and forms a solid part.

Typically, the temperature curve builds up steadily until the powder begins to melt and stick to the wall of the mold. While still climbing, the slope changes as energy is absorbed to melt the plastic. Once all of the powder is melted, the rate of increase in air temperature again steepens.

It is along this portion of the curve that Crawford and Nugent have identified the point that indicates optimum "cook," when the part should be removed from the oven. (Until now, this has been a temperature at which rotomolders had to guess, based on time monitoring.) The Rotolog temperature curve then begins its descent as the mold proceeds to the cooling station, where air and sometimes water cool the mold to form a solid part. Again, the shape of the curve of air temperature indicates when the part is ready to demold.

Crawford says that this graph, which he refers to as a "fingerprint" of correct molding conditions, is unique to rotational molding and can be used as a control device. Although trial runs were initially based on PE, similar fingerprints reportedly can be developed for other materials, various quantities of powder in the mold, and different mold metals.

Because there is a narrow processing window in which to obtain optimum impact strength of the final part, holding the process to the optimum time-vs.-temperature fingerprint for a particular material reportedly ensures maximum part strength.

MORE REAL-TIME CONTROLS IN

THE WORKS

Other developments in real-time controls for rotomolders are under way. Esso Chemical Canada, Toronto, has a prototype monitoring unit based on technology that's apparently similar to Rotolog. Both units use a transmitter and an insulated box of electronics, according to Blair Graham, Esso research engineer. He would not say whether the Esso unit measures air temperature inside the mold. Graham says the Esso and Rotolog systems were developed simultaneously but independently, without any knowledge of each other's projects.

Patent applications have been filed on both systems. Graham says that Esso Canada and Queen's University have arrived at a tentative agreement for cooperative development. If such an agreement can be worked out, he foresees a commercial product becoming available next year.

FSP Machinery Co. of Winkler, Manitoba, and Ferry Industries of Kent, Ohio, also claimed to be working on real-time controls based on information gathered from inside the mold, but would not provide any details. (CIRCLE 7)