Plastics Technology

JAN 2018

Plastics Technology - Dedicated to improving Plastics Processing.

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When I ask processors about their most important responsibility, the answers usually focus on factors relating to productivity. These typically include some conversation about cycle time, machine utilization, making parts to print, maintaining process capability indices (C pk ) of 1.33 or greater, etc. It is rare to hear any mention of molecular weight, crystallinity, preservation of additives, or minimizing molded-in stress. While these factors are the foun- dation for the success or failure of a product, processors often do not appre- ciate their own role in controlling them. But if these characteristics are not given the proper attention, then all the aspects associated with productivity matter very little. The first material characteristic we will address here is molecular weight. It has been recognized for almost 100 years that the unique characteristics exhibited by polymers are due to the large size of the molecules and their extended-chain configura- tion. This combination of characteristics gives rise to something called chain entanglement. It is the foundation of mechanical performance. Short-term properties such as impact resistance are particularly sensitive to changes in molecular weight. But long- Many processors don't realize that preserving material characteristics is crucial to product success and failure. The focus here is on molecular weight. term properties such as fatigue resistance and environmental stress-crack resistance are also closely correlated with the length of the chains that make up the material. The manufacturers of plastics raw materials pay close attention to the molecular weight of the products they produce, and they capture this with a measurement of proper- ties such as melt flow rate and intrinsic viscosity. It is then up to the processor converting the raw material to a molded article to preserve that molecular weight. This does not happen automatically. The elevated temperatures associated with melt processing, combined with the time that the material spends in the barrel, can have a significant effect on the molecular weight of the compound. The higher the melt temperature and the longer the barrel residence time, the greater the chances are that the molecular weight of the polymer will be negatively affected. The response in most materials is a process called chain scission, where the length of the chain is shortened by the thermal stress of processing. In addition to the dual influ- ences of time and temperature, the processing of some materials involves the potential hazard of degradation due to the presence of excess moisture in the material at the time of processing, a process known as hydrolysis. Condensation polymers such as polyesters, poly- carbonate, nylon, and polyurethane are particularly susceptible to this problem. Drying these materials down to a moisture content that prevents hydrolysis is essen- tial. Proper control over melt temperature, residence time, and—where appropriate—moisture content ensures that the molecular weight of the polymer will be properly maintained as the material undergoes conversion from pellets to parts. Not a week goes by when I do not work on a failed product where polymer degradation is at least a factor. A Processor's Most Important Job Get more insights on Materials from our expert author: short.ptonline.com/materialsKH Learn more at PTonline.com KNOW HOW MATERIALS By Mike Sepe PART 1 Preserving the molecular weight of a polymer—in this case, nylon—through appropriate processing conditions is a primary responsibility of the molder or extruder. The problem is that technology for assessing the molecular weight of processed materials is not often found in a typical molding plant. (Image: Malvern Instruments) 30 JANUARY 2018 Plastics Technology PTonline.com K now How MATERIALS

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