The Influence of Processing on Structural Behaviour: An Interface Between Moldflow and ABAQUS

Abstract

It is well known that the injection molding process has an influence on the structural behaviour of the final plastic part. Due to shrinkage- and warpage effects, the final part does not have the same geometry as the mold, and it contains stresses. Nowadays cost and weight restrictions force part designers to use materials up to their mechanical limits. This puts very stringent requirements on the computer aided engineering of the parts and it becomes more and more important to account for the influence of the injection molding process on the part performance. Moldflow is a versatile processing simulation program which, among others, is capable of predicting the warped shape and the molded-in stresses in a part. ABAQUS is a general purpose structural finite element program that is used to predict the behaviour of the part under a specific load. This paper describes an interface between the two programs which transforms the warped shape and the internal stresses as calculated by Moldflow to ABAQUS. A structural analysis can be performed based on the warped shape and taking into account the stresses. The interface has been used to predict the shape of an injection molded thermoplastic fender. For this application, tight gap and flush requirements put strict requirements on the shape of the fender after mounting it on the vehicle. To be able to judge this shape, a special measurement method has been developed by RENAULT. In this method the plastic fender is placed on a measurement jig and fixed in a prescribed order. Afterwards the fender is measured and based on the results of these measurements the shape of the fender can be judged. This procedure is simulated using the interface and taking into account warpage and molded-in stresses. For reasons of comparison, this simulation is also done without taking into account the molded-in stresses. The results of both simulations are compared with actual test data supplied by RENAULT and it can be concluded that accounting for the actual warpage results in a more accurate prediction of the fender shape. The use of the interface can help in finding the optimal fender shape and the best molding conditions at a stage in the design phase where the final fixing system is not yet decided and tools still have to be made. In this way it helps to shorten the design cycle considerably.