Abstract: The prerequisites for the injection moulding from thermoplastic material of container lids with a tear-up opening arrangement which is to function well on opening as well as on reclosing are improved through the use of a modified thermoplastic containing inert filler in a quantity of 10-15% of the weight of the mixture. Such a modified thermoplastic is manufactured by heating a thermoplastic, known in itself, e.g. polyethylene, to melting temperature with simultaneous admixture of the filler. The mixture is mechanically processed for a good distribution of the filler in the melted plastic mass which thereafter is cooled to form the modified thermoplastics.