The paper deals with studies of optimisation of joining a rubber sleeve of a packer to its cap. The current technology of manufacturing a packer from a rubber sleeve and two aluminium caps and reinforcing the joint by a Kevlar strip has been time-consuming and costly. The investigations were conducted to establish the influence of the type of material used for the cap on the strength of the joint between the sleeve and the cap. For this purpose peel testing and shear testing were applied. Results obtained show that the most suitable material for the packer cap is rubber, that the joint length is important for its strength, and that wall thickness of the cap exerts no particular influence on the strength of the joint between the sleeve and the cap.