Self-release

Self-Releasing Urethane Molding Systems Productivity Study... 

This review identifies the primary reason for utilizing self-releasing urethane molding systems increased productivity. In achieving this goal, however, the practical issues of various performance features, which may be critical to overall results, must be recognized, understood, and dealt with. 

For example, the need to paint parts is most often a very real and critcal issue which must be effectively worked out when using self-releasing systems. 

To date self-releasing systems have not provided infinite cycles of release, yet the number of consecutive releases for molds of differing degrees of complexities result in significantly improved productivity - the range being between 25% to 140% increased production yield. 

The technology of self-releasing systems based on IMR agents is both new and Incomplete in total development. Probably much needs to be done in terms of refinement and further extension of use, yet the overall benefits of a system which can be termed "effective" are of considerable practical value, even now. 

The problems surrounding part quality, while not always identified in a quantitive way, are certainly inherent in the use of external mold release sprays. A self-releasing system, made possible by the incorporation of an internal mold release agent in polyurethane chemicals offers a possible way of eliminating, or at least, minimizing the problems akin to external release methods. 

The gains which can be realized are distinct and real. The benefits derived from of a self-releasing system based on effective internal mold release agents are ... 

A self-releasing system based on an amine modified polyol to which had been added a proprietary IMR agent when formulated with the aromatic amine chain extender diethyltoluendlamine (DETDA) and MDI quasi-prepolymers demonstrated exceptional process and molded product attributes. The formulated systems Is shown In "Table I". 

In addition to release per se, there are other features which are often crucial to the adaptability and usefulness of a fully formulated self-releasing system. A brief review of these characteristics are Important to assessing the value or worthiness of the self-releasing system as a whole. 

Physical Property Retention IMR agents in self-releasing systems... 

For self-releasing systems the presence of an IMR agent on the surface of the part, which makes It self-releasing, also lowers the value of the solid s critical surface tension, making It less wettable by the paint, than If It were free of the IMR surface coating. This problem, however, can be solved by either one of two ways (1) by the addition of solvent to the paint formulation that... 

From each of the foregoing topics, It Is quite obvious that the ease by which IMR agents are selected Is greatly curtailed. Finding a compound which can be "dumped" Into any and all systems to provide the property of self-release Is perhaps realistically Impossible. Each and every system must have Its own "right" formulation - a task so far accomplished only by a limited number of suppliers of urethane raw material chemicals. 

It has generally been observed that urethane systems, which are cured on the surface of some low energy materials, are also free from adhering to them, i.e. they are self-releasing. Materials which show this characteristic relative to urethanes in a very effective way include at least three different types of plastics polytetrafluoroethylene, polyethylene, and polypropylene. These materials all have a defined critical surface tension sc less than about 30 dynes/cm. Assuming that this value is near, the liquid vapor surface tension lv value of an effective IMR urethane systems, then the work of adhesion as given by Equation 4 is as follows ... 

Cycle Efficiency The number of consecutive self-releasing cycles... 

In a paper by Taylor (13), et. al. this type of relationship has been extended to reflect productivity Improvements in a similar manner. Increased cycle efficiency alone, however, is not the whole answer to Improved productivity, yet it is fundamental to the reason for wanting systems containing IMR agents. Ideally, a self-releasing system which would provide an infinite numbers of releases (n = o) is a valid but difficult target. On a more practical level, however n releases between 30 and 300 are attainable. 

Productivity The primary reason for wanting a self-releasing system Is Increased productivity. In Its simplest form productivity Is a function of three factors cycle time, machine utilization,... 

To this productivity gain, must be added the gains of Improved machine utilization and of reduced scrap rate. The actual results of these combined effects as determined by an extensive production trial of fascia manufacture Is tabulated In "Table V" The total gain realized (60.5%) Is significant. It Is typical of the Increased productivity an effective self-releasing system can provide to a RIM process manufacturing operation. 

Self-releasing systems based on "effective IMR agents can typically Increase productivity by about 50% to 75%. In conjunction with this Increase, a cost reduction of between 12% to 20% should be expected. 

There are two major disadvantages often cited in the use of isocyanate wood binders. The first is a higher raw material cost as compared to conventional binders and the second is a tendency to adhere to metal transfer plates or press platens. The latter problem has been solved on a commercial scale by producing a multilayer product with solid veneers or phenolic-bound particles as face layers. All isocyanate-bound non-veneered board can be made by treating the metal surfaces with a release material or, more recently, using a self-releasable isocyanate. 

Perforated release films are made from modified polypropylene and can be rigid (recommended for use on flat or uncomplicated mold surfaces) or elastic (thin and stretchable, for large complex molds without need for tucks and folds). The films have good mechanical properties and performance at elevated temperature and are naturally self-releasing, treated on one side to assist adhesion to the breather fabric or a protective film. 

Self-release mould surfaces can be obtained to some extent by coating the mould metal with a liquid-castable liquid silicone rubberj which is then cured. Or a resin-bonded PTFE (as supplied by Du Pont, for example) can be sprayed on to the mould and using a special heat-treatment cycle this is baked in place to give a permanent self-release film. 

Matched metal moulding techniques have limitations when used to process composite materials of this form. In addition, a complicated and costly split mould would be required to remove the male section from the Inside diameter of the bellows after moulding and curing the epoxy resin. A flexible, self-releasing silicone rubber mould core offers a simple solution. 

ACS, Polymeric Materials Science Engineering Fall Meeting 1999. Volume 81. Conferenee proceedings. New Orleans, La., 22nd-26th Aug. 1999, p. 108-9 DYNAMICS OF SELF-RELEASE POLYMERIC DERIVATIVES... 

An unaccelerated isophthalic for RTM application with self-release characteristics. 

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