Prevent Failure by Understanding Why Plastic Parts Crack
Total Page：16
File Type：pdf, Size：1020Kb
CONSULTANTS CORNER Prevent Failure by Understanding Why Plastic Parts Crack A variety of factors can contribute to or cause part failure, but understanding the causes can minimize the risks By Jeffrey Jansen Senior Managing Engineer & Partner, The Madison Group aterial failures have an overall economic cost of Consideration should be given to determining and meas - billions of dollars annually, and can have a dev - uring the critical performance properties required for a astating effect on a product or even an entire successful product given the application. It is important, Mcompany. While the precise definition of failure can be some - however, to understand that whatever performance factor what elusive, a good working description is “a component is being measured — such as impact resistance, tensile or process that is not working the way it was intended.” Fail - strength or fatigue lifetime — the property will naturally vary ure is not always catastrophic, but it can be detrimental and across a distribution due to the inherent variability associ - have wide-reaching negative implications. ated with polymeric materials and their associated Certainly, manufacturers attempt to avoid failure, but manufacturing techniques. often unanticipated factors result in unexpected problems. Failure, regardless of the type, occurs when a portion of The chances for a successful application can be significant - the stress distribution exceeds the lower tail of the strength ly increased through preventative measures, including attribution. In many cases, a relatively large safety margin appropriate material selection, proper is thought to be in place. However, unan - mold design and process development ticipated factors produce a shift of either through simulation, chemical compati - the stress or strain distributions that bility screening, environmental testing, results in premature unexpected failure and a variety of mechanical evaluations. (see Fig. 1). Even when preventive actions are tak - Multiple factors affect the performance en, failures can still occur. If failure cannot of plastic components, and these are cat - be avoided altogether, then at least it can egorized as: provide a potential learning opportuni - • Material ty. • Design In conducting a failure analysis, the • Processing focus of the work is to ascertain the • Installation nature and cause of the failure. By doing • Service conditions this, information can be obtained to help These factors do not act independent - to avoid future failures. Knowledge of ly, and in most cases failure within plastic Failure of an overmolded plastic boss how and why plastics fail is imperative components is associated with multiple through environmental stress cracking in preventing failure. By avoiding com - causes. This article addresses five of the (ESC). The failure was caused by the mon mistakes it is possible to produce presence of residual corrosion most common contributing causes of plas - plastic parts that have a greater chance inhibitors on the brass insert. tic part failure, in the author’s experience. of successful performance. Courtesy of The Madison Group 48 | PLASTICS ENGINEERING | APRIL 2017 | www.4spe.org | www.plasticsengineering.org The Way the Part Was Designed The Actual Situation of the Part in Service Figure 1. Schematic representation of the anticipated safety margin and the actual overlap of applied stress and part strength of the product while in service. The change is often due to unanticipated factors that result in failure. Ignoring the effects of time ure will be manifested as brittle fracture, often referred to Thermoplastic materials exhibit viscoelastic properties as creep rupture (see Fig. 2). because of their unique molecular structure, including: Short-term mechanical properties, such as tensile or flex - • Relatively long polymer chain length associated ural strength, are inadequate to predict the long-term load- with high molecular weight bearing capabilities of a plastic material. Creep testing, either • Entanglement of the molecular chains using a creep stand or dynamic mechanical analysis (DMA), • Chain mobility because the molecules do not is required to predict the effective life of a continuously share chemical bonds with other molecules, loaded component. Taking time into account, and assess - instead held together by intermolecular forces ing the potential continuous stress placed on a plastic The viscoelastic nature of plastics leads not only to tem - component while in service can be effective in preventing perature dependence, but also to reliance on time. creep rupture failures, particularly at the design and mate - Specifically, the mechanical properties of a plastic material rial selection stages. will change if the material is under continuous stress. A marked decay in the apparent modulus will be observed for Chemical contact in service plastics that are under continuous stress at levels below the It is my experience that environmental stress cracking (ESC) yield point of the material for an extended period of time. is the leading mechanism of plastic component failure. I This reduction in modulus is the result of reorganization believe this results from a general widespread lack of aware - of the polymer chains, minimizing localized stress. Given ness and understanding of the interaction between plastic sufficient time, this reorganization will result in disentan - materials and chemicals throughout the plastics supply glement and subsequently, crack initiation. If the applied chain, coupled with the increasing use of plastic materials in stresses are below the yield point of the material, the fail - diverse, demanding applications. Figure 2. The equivalent effects of temperature and time are illustrated for a polyacetal resin. www.plasticsengineering.org | www.4spe.org | APRIL 2017 | PLASTICS ENGINEERING | 49 CONSULTANTS CORNER Environmental stress cracking is a failure mechanism • Exposure to an elevated temperature for an whereby a plastic material cracks due to contact with an extended period in the injection molding machine incompatible chemical agent while under tensile stress. It is barrel; a solvent-induced failure mode, in which the synergistic • Excessive shear during the injection process; effects of the chemical agent and mechanical stresses result • Overly aggressive sterilization conditions. in a brittle premature cracking. The contact with the chem - ical agent does not produce direct chemical attack or Molded parts in which molecular degradation has tak - molecular degradation. en place can look good and be in dimensional tolerance, Conversely, the chemical permeates into the molecular but may be inherently brittle and prone to premature fail - network and interferes with the intermolecular forces bind - ure. The reduction in molecular weight most often ing the polymer chains. This leads to accelerated brittle associated with molecular degradation reduces the inher - behavior and premature failure through molecular disen - ent ductility of the plastic material. tanglement. The ESC failure mechanism is similar to creep The material experiences a decrease in the level of entan - rupture, and in many circumstances, the plastic would under - glement associated with the shortening of the polymer go stress cracking in air given sufficient time. The chemical, chains. This reduces the energy required for disentangle - and its effect on the intermolecular forces binding the indi - ment to occur, and shifts the preferred mechanism from vidual molecular chains, accelerates the stress cracking. yielding to brittle fracture. Published chemical compatibility tables and general guide - Avoiding molecular degradation requires critical atten - lines can be useful in identifying potential problematic plastic tion to manufacturing conditions. This is not limited to and chemical combinations. However, the definitive ques - molding, but also to preliminary steps, such as com - tion of compatibility is addressed through environmental pounding and drying. Post-molding activities, including stress crack resistance testing. sterilization, machining, plating and welding also should Samples are placed under a tensile load and exposed to be considered. the chemical of interest for a period of time, while inspect - ed for the presence of cracks. A key aspect in the prevention Impact loading of ESC failures is the anticipation of chemicals that will con - Impact is rapid contact between a material of interest and tact the plastic part while in production, assembly, installation another object. Impact in regards to plastic performance and service. is normally associated with high speeds – high strain rates. The effects of high strain rate stress loading are not insignif - Molecular degradation icant. The viscoelastic nature of polymeric materials imparts Molecular degradation is the deleterious alteration of the strain rate dependency. The application of stress through molecular structure within a polymeric material due to a impact loading results in rapid deformation of the plastic chemical reaction. There are several molecular degradation material. mechanisms, including oxidation, hydrolysis, chain scission At increasingly elevated strain rates, the polymer mole - and destructive crosslinking, all of which involve permanent cules that make up the formed plastic component are molecular-weight changes. All these types of degradation precluded from having sufficient time to yield and deform, can occur throughout any point in the material life