The latest on molding of polyurethane
Authors: Müller Henry , Küper B. , Maier U. , Pierkes L. 
Published: April 2003

The complexity of the counterflow injection process may be regarded as a prime reason for the reluctance to apply PUR. The fact of several reactions and physical processes taking place along side each other in a very short time causes considerable difficulties and leads to high reject rates or high finishing costs in practical operation. These difficulties include obtaining an adequate mixing quality for high‐grade components, transporting the ready‐mixed blend into the mold cavity, and maintaining absolutely constant the optimum process parameters for production.

The homogenizing effect of high‐pressure mixing heads was established by using transparent model liquids. The measuring systems we applied was a chemical decolorizing method and the so‐called random sampling method. The findings established with these methods and the model liquids have been summarized in the form of design and setting recommendations for counterflow injection mixers. We also compiled an energy description for the layout and operation of counterflow injection mixers, based on the correlation between energy input and mixing quality. This description is in the form of a dimensioning reference value and allows the user to achieve a rapid layout. We checked the findings obtained on real PUR systems. The dimensioning reference value makes it possible to establish a critical limiting blend.

The guarantee of a specific blend quality is an essential, though by no means adequate, criterion for the production of defect‐free moldings. While the ready‐mixed blend is flowing into or inside the mold cavity, incorrect flow guidance can cause bubbles to be enclosed in the blend, or several flow fronts meeting at an unfavorable point can lead to air pockets being trapped. We were able to observe and record the flow processes visually by using transparent molds. The effects of a series of throttle and gate systems on air bubble enclosure can now be assessed on the basis of these records. We also developed an easy‐to‐use method for the mold designer, which allows him to describe the flow processes in the mold. By using this method, the designer can establish a visual picture of the filling process right at the design stage. This makes it possible to eliminate serious errors early on, at little expense, by modification of the construction drawing. In the past, these errors could only be rectified by costly alterations to the finished mold, if at all.

One of the characteristics of the PUR manufacturing process is that the chemical reaction and the shaping of the reaction melt take place simultaneously within a very short time and mutually influence one another. The process parameters for a given mold geometry must be selected so as to ensure that both the process sequence for the chemical reaction and the flow and filling process are controlled in such a way as to give a defect‐free part which can be demolded in the shortest possible time. The trial and error method of process optimization applied today is time‐consuming and costly. In addition, it does not permit clear‐cut conclusions to be drawn about the machine setting from the molded part, on account of the complex chemical/thermodynamic interactions involved. Objective criteria for machine setting can be found by measuring the decisive process parameters in the mold itself. Automatic process monitoring is possible, or better, essential, for recognizing errors and deviations right at the manufacturing stage and implementing countermeasures automatically.

The control parameter is the reaction temperature in the mold. It is this which determines the reaction speed, the viscosity profile, the crosslinking and curing process and hence the flow in the mold, the foaming and filling process, and the attainable demolding time. The level and time profile of the reaction temperature is controlled via the liberated heat of reaction, the starting temperature of the components, and the mold wall temperature, which, in the case of thin‐walled parts, constitutes the chief influencing variable.

The pressure inside the mold is easy to measure and provides a valuable aid in setup and process monitoring. Its profile indicates the flow resistance, the filling and foaming pressure, and the reaction shrinkage. The pressure measurement can thus be used to monitor the viscosity profile of the reaction, and then the metering time, filling capacity, gas charge, and blowing agent aligned to give optimum mold filling without overinjection.

A modular unit concept is described for monitoring and controlling the metering unit. Precise operation of this unit marks a prerequisite for a reliable process sequence. This concept incorporates the potential of present‐day microcomputers and personal computers to log and evaluate data. The central feature here is the control of component temperatures and the flow measurement and metering control. It is essential to measure component pressures in the recirculation and mixing phase in order to detect and smooth out pressure differences which could otherwise lead to a change in the blend (dynamic reference value shift).

When it comes to the gas charge, the solution of the gas by the conveying pressure is decisive. The dissolution process in the mold is pressure‐ and time‐dependent. It is thus essential to measure the pressure inside the mold in order to check the actual impact of the gas charge for the foaming and holding pressure phase in the mold.