Polyurethane Rim System

This application relates to the subject matter of co-pending application "Polyurethane Molding Process with Siloxane Internal Release Agent", filed Mar. 20, 1981, Ser. No. 246,103, now U.S. Pat. No. 4,379,100 by one of the present inventors, the teachings of which and the prior art cited therein are incorporated herein by reference.

This invention is a multi-station process for molding polyurethanes by reaction injection molding (hereinafter "RIM"). More particularly, this invention is a polyurethane molding system wherein the reaction ingredients are brought to a series of RIM mixheads from a common source as three separate streams and mixed independently in each mixhead in such proportions as may be desired to give molded products of different physical properties at each molding station. This permits the production from the same reactants of molded products of high physical properties and thus of higher cost simultaneously with the manufacture of products of lower physical properties and thus lower cost. The molding system offers considerable economy in that only a single source of supply need be used for each of the three reactant streams. This permits better control of the compositions and allows the economies of large scale batch preparation of each stream.

In brief compass, the present invention is a polyurethane RIM process having at least two molding stations consisting of a mixhead and mold cavity fed by the mixhead. The mold cavities are capable of rapid and repeated opening for demolding the product and closing to receive the next shot. The reactants include the customary long chain polyol, a chain extender, a crosslinker, the isocyanate and usually additives such as a mold release agent. The reactants are supplied to each mixhead from common sources as three streams. One stream is the isocyanate stream containing all of the isocyanate and free of any compound that would react with it, e.g. hydroxy and amino compounds The second stream, here called the Poly A stream, contains the chain extender and crosslinker and some of the long chain polyol but is free of isocyanate which would react with these ingredients. The third stream, the Polyol B stream, is also free of the isocyanate and is usually free of any of the chain extender and crosslinker but contains the balance of polyol necessary to give a stoichiometric reaction with the isocyanate to an isocyanate index of 90 to 110. At each mixhead, the proportions of ingredients are controlled to give the desired isocyanate index, preferably of about 100. The ratios of the Polyol A to Polyol B streams are varied as desired to control the final physical properties of the molded product produced at each station. If a product of high physical properties and performance is desired, it may be produced, recognizing that generally it will be more expensive. Conversely, a product of lesser physical properties can be produced at another station at the same time at lower cost.

For example, the flex modulus of the molded product may be controlled by controlling the ratio of the Polyol A to the Polyol B stream. If more high molecular weight polyol is used, the flex modulus decreases and the product is less expensive. The higher the flex modulus used, generally speaking, the more expensive the product, and one does not want to give the customer a product any tougher than the customer requires and is willing to pay for. On the other hand, a higher flex modulus permits the use of thinner cross-sections for the same product and characteristics, such that a lesser weight of the polymer can be used, which would make the product less expensive. Thin cross-sections are harder to fill, however, and do present mold design problems. Consequently, the multi-station system proposed in this invention offers the product designer a great deal of flexibility, while offering the manufacturer the economies of preparing but three mixes or batches of the reactants, rather than a series of much smaller batches for each of the mixheads producing different products. The flexibility allowed by the concept of this invention regarding product design, costs and performance has not heretofore been appreciated in the RIM type of polyurethane molding process.

The drawing is a schematic illustration of the multi-station RIM polyurethane molding process of this invention.

The following table gives an example of a three part polyurethane foaming composition which can be used in the practice of this invention, and gives the ranges of the various ingredients that may be used. The specific example shows the manufacture of three different products, X, Y and Z, having substantially different flex moduli by varying the ratio of Polyol A stream to that of the Polyol B stream. Product X, for example, could be a Ford Mustang fascia, Product Y could be a 1980 Chevrolet Corvette front fascia and Product Z could be a 1980 Chevrolet Corvette rear fascia.

While the above example is a urea urethane system, the teachings of this invention are applicable to any urethane system that will accept the three stream approach whether it be polyester or polyether based, and whether extended with amines and/or alcohols.

The suppliers above referred to are more completely identified as follows:

With reference to the drawing, illustrated is a three stream--three molding station polyurethane RIM process in accordance with the teachings of this invention, and following the example of the preceding table. The isocyanate stream, the MDI prepolymer, is prepared and stored at 10, the polyol stream is prepared and stored at 20 and the Polyol B stream is prepared and stored at 30. Each of these streams is conducted by manifolds 11, 21 and 31 to the feed lines to the various molding stations, only three of which are illustrated, which have mixheads 40, 50 and 60, respectively and mold cavities 45, 55 and 65, respectively, to produce the three products X, Y and Z.

For molding station 40, the Polyol B stream is introduced from line 31 by line 32 through control valve 42, and the Polyol A stream is introduced from line 21 by line 22 through control valve 43 in the proportion desired, in the case of Product X a ratio of Polyol A to Polyol B of 1.09. Sufficient isocyanate from manifold 11 is added via line 12 and control valve 44 to give an isocyanate index of 1.05.

Similarly, Polyol B is supplied to mixhead 50 via line 33 and control valve 52 and Polyol A is supplied by line 23 and control valve 53 to give, in the case of Product Y, a Polyol A to Polyol B ratio of 1.15. Isocyanate is added by line 13 and control valve 54 to give an isocyanate index of 1.05.

In mixhead 60 the Polyol B stream is supplied by line 34 through valve 62 and the Polyol A stream through line 24 via control valve 63 to give for Product Z a ratio of Polyol A to Polyol B of 1.25. The isocyanate is added by line 14 through control valve 64 to give an isocyanate index of 1.05.

The reaction injection molding process proceeds quite rapidly, particularly so in the present case as a large amount of aromatic polyamine is present. Usually with this type of RIM process, injection or mold filling is completed in less than 5 seconds, and the product is demolded in less than a minute thereafter. In the three cases of the example the times from the initial mixing of the three ingredients in the mixheads to complete filling of the mold in each cycle is less than 1.2 seconds. Cure times are quite rapid, and in all three cases the products are demolded from the molds within 40 seconds from the time of completion of injection into the molds.