Characterization of Splits in Rubber Parts – Cryogenic Verses Melt Flow Fronts

Purpose

Characterization of splits in molded rubber parts is a new and innovative diagnostic application of root cause analysis designed to help trouble shoot split related defects in molded rubber articles, such that splits caused by typical cryogenic processing are identified to exhibit different physical characteristics at the split compared to the physical characteristics of flow front problems, during filling of the mold cavity, that might be exposed by the same cryogenic process.

Design/Methodology/Approach

A specialized mold is designed and built to mold a rubber part in a precision controlled press so as to isolate various process parameters that either promote or inhibit a controlled melt flow front of the rubber in the molded article, resulting in test samples that are known to have poor melt flow fronts compared to other samples known to have good melt flow fronts.  Three different compounds with different ranges of glass transition characteristics are used for the comparative test matrix.  These test parts are subjected to a series of cryogenic processing temperatures and deliberately broken during the frozen state in a controlled test fixture and protocol so that the break faces of each sample can be evaluated and compared to each other.

Findings

Comparative analysis under magnification of the cryogenically broken faces of all the test samples reveals that the surface appearance, texture, and fracture planes through rubber portions of the samples wherein good melt flow of the rubber existed, exhibit one set of characteristics, while cryogenically broken faces of test samples wherein melt flow front problems existed, exhibit a different set of characteristics.

Research Implications

Manufacturers of rubber products can make use of the knowledge of the characterization of splits in rubber parts to help determine whether a split defect is “caused” by cryogenic processing or simply “exposed” by cryogenic processing.  Knowing this difference will dictate a completely different set of problem solving techniques and/or focus for the rubber part manufacturer as they set about efforts to eliminate the split defect.   Further research and study is warranted on other thermoset rubber elastomers, thermoplastic rubber elastomers, and selected plastic products.  Furthermore, additional research is warranted for different types of product shapes such as tubes, sheets, dual hardness products, et cetera.

Originality/Value

Characterization of splits in a rubber product, differentiating whether the split defect is caused by or exposed by cryogenics, is a new and innovative diagnostic method for molders of rubber products to apply root cause analysis to split defects.

Keywords

Cryogenic, temperature, liquid nitrogen, deflashing, splits, flow, knit lines, melt flow, flow fronts, shot blasting, tumbling, polycarbonate media, throw wheel speed, -320 degrees F