Influence of melt mixer on injection moulding of thermoset elastomers

Abstract

Injection moulding is considered as one of the most important processing techniques for the thermoset elastomer industry. As the demand to improve efficiency continuously increases, optimization on the machine is done frequently to reduce as much as possible defects and high cycle times typically found during production of such products. One of the drawbacks in most injection moulding machines is due to a short reciprocating screw. Polymers having high concentrations of additives might not n::.ix homogeneously within the short period of time during the plastication step. Apart frcm that the material might suffer from deficient mechanical properties, thus obtaining product parts which offer a lower performance. To solve these problems, for the fi::-st time in the thermoset industry, a melt mixer was placed inside the nozzle chamber, together forming a mixing nozzle, to study its outcomes. Various state-of-the-art melt mixers were designed and manufactured. The influence of these melt mixers were investigated using three different materials in this study; namely nitrile butadiene rubber (NBR), ethylene propylene-diene monomer (EPDM) a::id fluorocarbon (FKM) rubber. In order to fulfil the requirements concerning melt homogeneity, pressure drop, mixi.1g and increase in melt temperature it was necessary to analyse the melt flow behaviour before and after it passed through the melt mixer. The previously designed melt mixers were analysed using Moldflow® Insight software simulator and the ideal melt mixer based on some of the aforementioned criteria was chosen for each material studied. The chosen melt mixers and nozzles were manufactured and tested on an injection moulding machine under real industrial conditions. The moulded parts were then tested to quantify their hardness, compression set, tensile, mixing performance and curing level, in order to conclude whether improvements occurred. These melt mixers gave better physical properties to the moulded parts, even though be curing time in the mould was reduced by 10 s. This was because the increase in sheari::ig introduced a higher rate of crosslinking formation in the moulded parts. Even though curing time was decreased, a successful final product was still achieved which retained all the material properties requested by TSSM.