Search Results for Molding resins

Molded plastic couplings used in an industrial application exhibited abnormally brittle properties, as compared to previously produced components. The couplings were specified to be molded from a custom-compounded glass-filled nylon 6/12 resin. An inspection of the molding resin used to produce the discrepant parts revealed that the pellets were of two general types, neither of which matched the pellets from a retained resin lot. Investigation included visual inspection, micro-FTIR in the ATR mode, and analysis using DSC. The thermograms supported the conclusion that the brittle couplings contained a significant level of contamination, polypropylene and nylon 6/6. The source of the polypropylene was likely the purging compound used to clean the compounding extruder. The origin of the nylon 6/6 resin was unknown but may represent a previously compounded resin.

The handle from a consumer product exhibited an apparent surface defect. The handle had been injection molded from a medium viscosity grade ABS resin. The anomalous appearance was objectionable to the assembler of the final product and resulted in a production lot being placed on quality-control hold. Investigation included visual inspection, 24x micrographs, and FTIR in the reflectance mode. The spectrum obtained on the included material was characteristic of polybutadiene, the rubber-modifying agent present in ABS. This supported the conclusion that the inclusion's most likely source was an undispersed gel particle formed during the production of the molding resin.

Cracking occurred within the plastic jacket (injection molded from an impact-modified, 15% glass-fiber-reinforced PET resin.) of several assemblies used in a transportation application during an engineering testing regimen which involved cyclic thermal shock (exposing the parts to alternating temperatures of -40 and 180 deg C (-40 and 360 deg F)). Prior to molding, the resin had reportedly been dried at 135 deg C (275 deg F). The drying process usually lasted 6 h, but occasionally, the material was dried overnight. Comparison investigation (visual inspection, 20x SEM views, micro-FTIR, and analysis using DSC and TGA) with non-failed parts supported the conclusion that that the failure was via brittle fracture associated with the exertion of stresses that exceeded the strength of the resin as-molded caused by the disparity in the CTEs of the PET jacket and the mating steel sleeve. The drying process had exposed the resin to relatively high temperatures, which caused substantial molecular degradation, thus limiting the part's ability to withstand the stresses. The drying temperature was found to be significantly higher than the recommendation for the PET resin, and the testing itself exposed the parts to temperatures above the recognized limits for PET.

A production lot of mechanical hinges used in an automotive application had failed during incoming quality-control routine actuation testing. A change in part supplier had taken place between the approval of the prototype parts that performed acceptably and the receipt of the first lot of production parts. The mechanical hinges were specified to be injection molded from an impact-modified, 13% glass-fiber-reinforced nylon 6/6 resin. Investigation of samples representing the failed components and the original prototype parts included visual inspection, 118x SEM images, micro-FTIR, DSC analysis, and TGA. It supported the conclusion that the hinge assemblies failed through brittle fracture associated with stress overload during the actuation of the parts. The failed part material was found to be degraded, most likely occurring during the compounding of the resin or during the actual molding of the parts. While resins for both failed and non-failed parts produced results characteristic of a 13% glass-fiber-reinforced, impact-modified nylon 6/6, the failed part material, contained a significantly lower level of rubber, which rendered the parts less impact resistant and subsequently lowered the ductility of the molded hinge assemblies. No recommendations were made.

A failure analysis was conducted on a flow-sensing device that had cracked while in service. The polysulfone sensor body cracked radially, adjacent to a molded-in steel insert. This article describes the investigative methods used to conduct the failure analysis. The techniques utilized included scanning electron microscopy, Fourier transform infrared spectroscopy, differential scanning calorimetry, thermomechanical analysis, and melt flow rate determination. It was the conclusion of the investigation that the part failed via brittle fracture, with evidence also indicating low cycle fatigue associated with cyclic temperature changes from normal service. The design of the part and the material selection were significant contributing factors because of stresses induced during molding, physical aging of the amorphous polysulfone resin, and the substantial differential in coefficients of thermal expansion between the polysulfone and the mating steel insert.

Numerous protective covers, used in conjunction with an electrical appliance, failed during assembly with the mating components. The failures were traced to a particular production lot of the covers and occurred during insertion of the screws into the corresponding bosses. The parts had been injection molded from an ABS resin to which regrind was routinely added. Inspection of both the failed covers and retained parts, which exhibited normal behavior during assembly, included visual inspection, micro-FTIR in the ATR mode, and analysis using DSC. The FTIR results indicated the presence of contaminant material exclusively within the ABS resin used to mold the failed covers, and the thermograms suggested contamination with a PBT resin. Further TGA analysis showed the contamination was estimated to account for approximately 23% of the failed cover material. The conclusion was that the appliance covers failed via brittle fracture associated with stress overload. The failures, which occurred under normal assembly conditions, were attributed to embrittlement of the molded parts, due to contamination of the ABS resin with a high level of PBT. The source of the PBT resin was not positively identified, but a likely source appeared to be the use of improper regrind.

Housings (being tested as part of a material conversion) from an electrical appliance failed during an engineering evaluation. They had been injection molded from a commercial polycarbonate/PET blend. Parts produced from the previous material, a nylon 6/6 resin, had consistently passed the testing regimen. Grease was applied liberally within the housing assembly during production. Investigation included visual inspection, 24x SEM images, micro-FTIR in the ATR mode, and analysis using DSC. No signs of material contamination were found, but the thermograms showed a crystallization of the PET resin. The grease present within the housing assembly, analyzed using micro-FTIR, was composed of a hydrocarbon-based oil, a phthalate-based oil, lithium stearate, and an amide-based additive. The conclusion was that the appliance housings failed through environmental stress cracking caused by a phthalate-based oil that was not compatible with the PC portion of the resin blend. Thus, the resin conversion was the root cause of the failures. Additionally, during the injection molding process the molded parts had been undercrystallized, reducing their mechanical strength. More importantly, the resin had been degraded, producing a reduction in the molecular weight and reducing both the mechanical integrity and chemical-resistance properties of the parts.

This article reviews analytical techniques that are most often used in plastic component failure analysis. The description of the techniques is intended to familiarize the reader with the general principles and benefits of the methodologies, namely Fourier transform infrared spectroscopy, energy-dispersive x-ray spectroscopy, differential scanning calorimetry, thermogravimetric analysis, and dynamic mechanical analysis. The article describes the methods for molecular weight assessment and mechanical testing to evaluate plastics and polymers. The descriptions of the analytical techniques are supplemented by a series of case studies to illustrate the significance of each method. The case studies also include pertinent visual examination results and the corresponding images that aided in the characterization of the failures.

A chemical storage vessel failed while in service. The failure occurred as cracking through the vessel wall, resulting in leakage of the fluid. The tank had been molded from a high-density polyethylene (HDPE) resin. The material held within the vessel was an aromatic hydrocarbon-based solvent. Investigation (visual inspection, stereomicroscopic examination, 20x/100x SEM images, micro-FTIR in the ATR mode, and analysis using DSC and TGA) supported the conclusion that the chemical storage vessel failed via a creep mechanism associated with the exertion of relatively low stresses. The source of the stress was thought to be molded-in residual stresses associated with uneven shrinkage. This was suggested by obvious distortion evident on cutting the vessel. Relatively high specific gravity and the elevated heat of fusion indicated that the material had a high level of crystallinity. In general, increased levels of crystallinity result in higher levels of molded-in stress and the corresponding warpage. The significant reduction in the modulus of the HDPE material, which accompanied the saturation of the resin with the aromatic hydrocarbon-based solvent, substantially decreased the creep resistance of the material and accelerated the failure.

A production lot of plastic wire clips was failing after limited service. The failures were characterized by excessive relaxation of the clips, such that the corresponding wires were no longer adequately secured in the parts. No catastrophic failures had been encountered. Parts representing an older lot, which exhibited satisfactory performance properties, were also available for reference purposes. The clips were specified to be injection molded from an impact-modified grade of nylon 6/6. However, the part drawing did not indicate a specific resin. Investigation included visual inspection, micro-FTIR in the ATR mode, and analysis using DSC. The spectrum representing the reference parts showed a relatively higher level of a hydrocarbon-based impact modifier, while the results obtained on the failed parts showed the presence of an acrylic-based modifier. Also, the reference clip thermogram showed a melting transition attributed to a hydrocarbon-based impact modifier. The conclusion was that the control and failed clips had been produced from two distinctly different resins. It appeared that the material used to produce the failed clips had different viscoelastic properties, which produced a greater predisposition for stress relaxation.

This article reviews the analytical techniques most commonly used in plastic component failure analysis. These include the Fourier transform infrared spectroscopy, differential scanning calorimetry, thermogravimetric analysis, thermomechanical analysis, and dynamic mechanical analysis. The descriptions of the analytical techniques are supplemented by a series of case studies that include pertinent visual examination results and the corresponding images that aid in the characterization of the failures. The article describes the methods used for determining the molecular weight of a plastic resin. It explains the use of mechanical testing in failure analysis and also describes the considerations in the selection and use of test methods.

A number of plastic sleeves used in an automotive application cracked after assembly but prior to installation into the mating components. The sleeves were specified to be injection molded from a 20% glass-fiber-reinforced polybutylene terephthalate (PBT) resin. After molding, electronic components are inserted into the sleeves, and the assembly is filled with a potting compound. Investigation of the cracked parts and some reference parts available for testing included visual inspection, micro-FTIR in the ATR mode, and analysis using DSC. Subtle spectrum differences suggested degradation of the failed part material, and the thermograms supported this. The conclusion was that the failed sleeves had cracked due to embrittlement associated with severe degradation and the corresponding molecular weight reduction. The reduction in molecular weight significantly reduced the mechanical properties of the sleeves. The cause of the degradation was not evident, but the likely source appears to be the molding operation and exposure to elevated temperature for an extended period of time.