A novel method for the prediction of adhesive strength for two-component injection molding of thermoplastics with thermoset rubbers

Two component injection molding is a widespread technique to produce polymer products that consist of two materials. This technique is commonly used to combine various material properties, or functionalities in one product. The ‘hard-soft’ combination where a stiff material is over-molded by a soft layer, is one example, often seen in valve like products where the soft part is used as a seal. Thermoplastic elastomers (TPE’s) are commonly used is this case. However, TPE’s have only limited properties for chemical and temperature resistance. For such applications it would be beneficial to use a thermoset rubber as EPDM, NBR, NR, etc. Although injection molding of thermoset rubbers is not that uncommon, using it in an over molding process is rather rare and not much is known about the final properties of the over molded product. One of the most important parameters is the adhesion between the two materials. Recommendations for good adhesion can be found in literature for specific material combinations, but data on predicting the strength of adhesion in thermoset rubber over molding in function of material and process settings does not exist. This paper presents a novel, empirical method to predict the strength of adhesion in function of material and process settings.