SPE Library

Analysis of the State of the Art of Technical Drawings of Plastic Molded Parts Regarding Tolerances

Anja Falke | Friedrich-Alexander | Martin Bohn | Tim A. Osswald, May 2021

The plastic-specific material properties are often not taken into account in the specification of technical drawings of injection molded parts. As a result, tolerance requirements are specified, that are too tight and sometimes even impossible to manufacture, which result in high production expenses. To avoid this, it is necessary to coordinate the functionally required accuracies of plastic components with the technical possibilities available for injection molding production.
In this paper a systematic analysis of drawings from practice is used, to show the current state of the art regarding geometric product specification and tolerance assignment of plastic molded parts. In addition to the quantification of the number of specified features, the unambiguousness of the product specification is assessed. Beyond that, the degree of accuracy of the tolerance requirements is quantified and the manufacturing feasibility is checked in accordance with ISO 20457 in order to then determine the resulting production expense that is necessary to achieve the required tolerances. It is proven that for almost a fifth of the plastic parts tolerance requirements are specified that are not feasible to be produced in the injection molding process. Additionally, it is found that all drawings examined do contain ambiguously specified features, that do not allow for an unambiguous verification.

The Most Frequent Design Flaw That Leads to Part Failure

Paul J. Gramann, Ph.D., P.E., May 2021

The topic presented in this paper is not new. There are numerous reasons why sharp transitions should not be present in a plastic part. However, the number of failures that are occurring at sharp transitions is still very common. In most cases, they can easily be avoided by simply removing metal from the mold to make a smooth transition. This paper will review where most of these transitions are being found, and why they are common in critical parts. A tensile testing study was performed to better understand the effect of geometric transitions. Two cases studies are given showing why the sharp corners can significantly reduce the lifetime of a plastic part.

Fused Filament Fabrication Feedstock Characterization via In-Line Rheology

A.R. Colon | D.O. Kazmer | A. Peterson, May 2021

An instrumented hot end has been developed to monitor the pressure in Fused Filament Fabrication, and is used as an in-line rheometer to characterize the viscosity of an acrylonitrile butadiene styrene (ABS) material. Additional analysis was performed on the transient pressure data to consider compressibility effects and nozzle drool. The range of flow rates was identified at which the pressure in the hot end was most stable. Stabilization time given compressibility effects was also evaluated.

Lifetime Prediction of Continuous Fiber-Reinforced Plastics Based on Nonlinear Damage Accumulation

Simon Rocker | Reinhard Schiffers | Lars Gerdes | Daniel Hülsbusch | Frank Walther, April 2021

Full Title: LIFETIME PREDICTION OF CONTINUOUS FIBER-REINFORCED PLASTICS BASED ON NONLINEAR DAMAGE ACCUMULATION AND FINITE ELEMENT SIMULATIONS Abstract: This paper presents an approach for lifetime prediction of fiber-reinforced plastics based on nonlinear damage accumulation. Already established damage accumulation laws, such as Palmgren-Miner, are to be modified with nonlinear parameters in order to characterize the damage evolution of fiber-reinforced plastics in a more accurate way. For this purpose, cyclic investigations were carried out on glass fiber-reinforced polyurethane with quasi-isotropic layer setup to determine basic mechanical characteristics. The stiffness-based characteristic values, recorded to develop the simulation model, are generated from hysteresis loops, which are also used to calibrate the material model. The experimentally determined stiffness degradation is converted into a damage curve by assigning the first measured value to degree of damage 0 and the failure value to degree of damage 1. Therefore, a hysteresis loop for each degree of damage between 0 and 1 is present, so that a damage dependent stress-strain ratio can be determined and transferred to the material model cali-bration. In addition, a characteristic damage development is derived from the damage curves, whereby the stress level and the influence of sequence can be taken into account for a nonlinear damage accumulation model on global level. Based on the global findings an algorithm is presented that transfers those to the local level in finite element simulations. This approach provides the fundamentals for a lifetime prediction of fiber-reinforced plastics with varying fiber orientations under cyclic loading.

Developing Photopolymerizable Acrylate Resin Formulation for Impact Modified 3D Printed Thermosets

Chinmay Saraf | Amy Niu | Alan J. Lesser,, April 2021

This contribution focuses on engineering photopolymerizable acrylate resin formulations for a superior fracture energy absorption of 3D printed acrylate thermosets. Herein, we report a polydimethyl siloxane-based block copolymer as an impact modifier, compatible with the UV curing process, which undergoes reaction induced phase-separation during the 3D printing process to form a rubbery phase sufficient for enhanced impact properties. A systematic investigation of the effect of concentration of the impact modifier on the morphology of rubbery domains and fracture toughness was conducted. Results show that at an optimum concentration of 15 wt.% and particle size of 57 nm, an order of magnitude improvement in the fracture energy release rate is realized. Fractographic analysis of the impact modified thermosets using optical microscopy indicates the presence of significant plastic deformation in an otherwise brittle material. Notably, the engineered acrylate thermosets, at an optimum concentration, exhibit similar improvements in the impact properties irrespective to the print layer thickness and independent of the crack orientation with respect to the printed interphase. Detailed investigation of the failure mechanisms for impact modified thermosets show that the block copolymer diffuses to the interphase during the 3D printing process, resulting in preferential localization of the impact modifier near the print interphase resulting in an isotropic enhancement of the fracture toughness.

Thermal Analysis of Plastics

Jeffrey A. Jansen, March 2021

Thermal analysis is an important group of tests used in the analysis of plastics and other polymeric materials. It consists of a family of well-established techniques that evaluate material properties as they change with temperature, time, and ambient environment under conditions of thermal programming. The results of thermal analysis tests provide qualitative and quantitative information about the material being evaluated. In particular, this information is important to address plastic failures or in characterization of the material composition and physical properties. The upcoming webinar on thermal analysis will introduce the four primary techniques:

• Differential Scanning Calorimetry (DSC)
• Thermogravimetric Analysis (TGA)
• Thermomechancial Analysis (TMA)
• Dynamic Mechanical Analysis (DMA)

The webinar is designed to introduce the techniques to the attendees so that they may get a better understanding of how the techniques can be used to evaluate plastic materials and solve problems. No single thermal analysis technique is best suited universally, but together they provide essential data for the characterization of plastics materials. This presentation will review thermal analysis techniques and their application to plastic problem solving through case studies. The webinar will be a practical treatment of the techniques, and the focus will be on how the techniques can be utilized to better understand polymeric materials. At the end of this presentation you will:

• Gain insight into the different types of thermal analysis techniques
• Recognize which technique is best suited to obtain the information you need
• Understand how thermal analysis can be used to characterize the composition and properties of plastic

Environmental Stress Cracking

Jeffrey A. Jansen, February 2021

Environmental stress cracking (ESC) is the leading cause of plastic component failure, and a recent study suggests that 25% of plastic part failures are related to ESC. If you deal with plastic components, then “Environmental Stress Cracking of Plastics” will provide you with information that will enhance your understanding of the interaction between chemicals and plastic resins, with the goal of preventing part failure. ESC is a solvent-induced failure mode in which a plastics crack through contact with a chemical agent while under stress. The webinar will be presented from a practical viewpoint with actual case studies to illustrate the ESC mechanism and explain plastic performance. Topics covered during this session include:

• Introduction to ESC
• How plastics fail
• Explanation of the ESC failure mechanism
• Generalizations related to chemical interaction with plastics
• ESC resistance testing used to evaluate plastic/chemical compatibility
• Case Illustrations of some common solvent-based failure modes

At the end of this presentation you will:

• Have a better understanding how chemicals effect plastic components.
• Recognize potential situations where ESC failure could occur in your parts.
• Be able to design a chemical resistance program to evaluate plastic/chemical combinations.

The Consequences of Ductile-To-Brittle Transitions in Plastics

Jeffrey A. Jansen, December 2020

Thermoplastic resins are utilized in many applications because of their unique property set, including their ductile response to applied stress. This ductility is associated with the viscoelastic nature of polymers and is attributed to their unique molecular structure. In spite of that inherent ductility, most plastic components fail through one of the many brittle fracture modes. Experience through conducting thousands of plastic component failure analyses has shown that less than 5% were associated with ductile overload. The remainder represent brittle fractures of normally ductile materials. Thus, within evaluations of plastic component failures, the focus of the investigation frequently turns to identifying the nature of the ductile to brittle transition. This relatively brittle response to stress is evident through the examination and characterization of the fracture surface morphology. There are numerous factors, associated with material, processing, design, and service conditions that influence a ductile-to-brittle transition within plastic materials. These include:

• Temperature
• Stress Concentration
• Chemical Contact
• Molecular Weight
• Degradation
• Filler Content
• Contamination
• Poor Fusion
• Strain Rate
• Time Under Load
• Crystallinity
• Plasticizer Content

Navigating Plastic Material Selection

Jeffrey A. Jansen, November 2020

Material selection is one of the fundamental aspects that will determine the success or failure of a product. With so many choices available today regarding plastic materials, it is imperative that anyone involved in product design or material selection understand resin properties and how they will affect end product performance as well as part design and manufacturability. While plastic material selection is a frequent topic of discussion, it is not as simple as it may first appear. A thorough understanding of the short-term and long-term properties of the potential plastic resins is essential. To help make the best plastic resin choice, is also essential to have a basic knowledge of polymer chemistry. This webinar will address some of the considerations that need to be made when selecting a plastic resin, and outline the challenges and benefits of selecting an appropriate material. The presentation will introduce a method of systematic selection that will optimize the plastics material selection process.

Impact Failure of Plastics

Jeffrey A. Jansen, June 2020

While in service, plastic materials are subjected to many different types of mechanical stress. One common type of stress that is typically severe on plastics is rapid impact loading. The rate at which loading is applied, otherwise known as the strain rate, is a very important factor in the performance of a plastic component. Impact, together with snap fit assembly, and rapid pressurization are the most common forms of rapid loading or high strain rate mechanisms.

The response of plastics to impact and the ability of a plastic part to withstand the stress through absorption of the applied energy is dependent on many aspects, including the material, design, processing and the service conditions.

Topics covered as part of this presentation will include:

• Failure Mechanism of Plastics
• Strain Rate as a Ductile-to-Brittle Transition
• Impact Failure
• Factors Effecting Impact Resistance
• Impact Testing
• Case Studies

Impact loads are among the most challenging stresses that plastic component designers and manufactures must deal with. In many cases impact stress is not adequately accounted for. In may cases this leads to unnecessary premature or unexpected failure.

Plastic Failures Associated with Metal Fasteners

Jeff Jansen, May 2020

The need to secure plastic components is prevalent in the manufacture of assemblies in many industries. Joining plastic components to other plastic parts or metal parts often involves the use of mechanical fasteners, such as screws, inserts, or rivets. The joining of plastic parts is inherently more complicated than assembling two metal components because of the fundamental differences in physical properties, including strength, chemical resistance and susceptibility to creep and stress relaxation. Case Studies will be presented to illustrate failures associated with the interaction between plastic components and metal fasteners. The presented cases will illustrate how the failure analysis process was used to identify the failure mechanism as well as the primary factors responsible for the failures. The cases depict representative failures involving varied designs and service conditions.

Failure Analysis of an Outdoor Instrument Housing

Jeffrey Jansen, May 2020

Cracking occurred within the housing for a piece of weather monitoring instrumentation being used as part of field service trial. The cracking was observed within the bosses used to secure the housing section to the mounting hardware. The focus of this investigation was the determination of the nature and cause of the failure. The results obtained during the evaluation of the failed housing indicated that the cracking occurred through three separate mechanisms. Significant factors in the failure included aspects of design, manufacturing, and the service conditions. This paper will review the testing performed to characterize the failure modes and identify the causes of the cracking, while demonstrating the analytical procedures used in the investigation.

Failure Analysis of Polymer Coating Systems

Gaurav Nagalia, May 2020

Failure analysis of polymer coating systems can be challenging due to the fact that coating systems typically involve multiple and generally very thin layered components. The root-cause for the failure of a polymer coating can be attributed to many factors. Thus, it cannot be easily determined by inspection or observations, and significant amount of testing is often required to determine the root cause for the failure. Typically, failures can be caused by selection of improper coating system, or it can be caused by insufficient surface preparation, or it can be caused by application related issues. This paper attempts to provide a guide to performing failure analyses of polymer coatings by discussing two separate coating systems that utilized a polyvinylidene fluoride (PVDF) top coat and evaluates the fundamental root causes of failure. The importance of reviewing background information, performing site-inspections, conducting relevant laboratory and field testing, and utilizing published literature to reach a root-cause for the failure is high-lighted. In both cases, laboratory examinations revealed that while high performance coatings were utilized, their compatibility within the system and their susceptibility to hazards within their respective applications, were not accounted for, leading to poorly designed coating systems that eventually failed.

How Poor Selection of Materials, Design, Tooling and Design Errors Affect the Aesthetics of Plastic Parts and What Designers Need to Know About the Science of Color and Appearance - Part 2

Vikram Bhargava, May 2020

Most engineers and designers come from the metal world. Therefore, many of them make assumptions on the predicted performance of plastic properties based on their metals background. Unlike metals, the knowledge of color and appearance is extremely important in the case of plastics. Most plastic parts have dual functions— physical performance and aesthetics. Aesthetics are important since very few of the parts need to be painted or otherwise decorated if designed and manufactured with due diligence. On the other hand, even if we are designing the most aesthetically critical metal components such as exterior automotive parts, we mostly choose the metals and alloys based on the physical properties, weight, and cost. The aesthetics are left to the paint specialist, who will in most cases find a paint system (primer, paint, and application method) that will meet the cost, durability, and cosmetic requirements. In other words, aesthetics and physical properties are quite independent of each other. A vast majority of metal parts meet their aesthetic and environmental requirements just by getting brushed, plated, chromate conversion coated or anodized. Plastic parts not only need to meet the short-term color and appearance requirements, but also need to be resistant to long term color shift and fading. This paper is in two parts. Part 1 - Appearance and Color Factors - Material - Design - Tooling and Processing Part 2 –The fundamentals of Color and Appearance, Specifications, Measurement and Tolerances

Transition Metal-Catalyzed Degradation of Polymers: Review and Future Perspectives

Andrew Worthen, May 2020

In many instances, failure of polymer-based articles is attributed to chemical interaction with metals or metallic compounds. Indeed, the stability of polymers is often modified by these species; however, their effects on the degradation of polymers are complex and influenced by many factors. This paper reviews known polymer degradation mechanisms and how metals may influence them, discusses deactivators and their role use as stabilizers in polymer formulations, provides literature-based vignettes describing example scenarios where metal-accelerated degradation of plastics may contribute to failures, and provides commentary regarding potential future areas of work in the field.

Validation of the Virtual Lifetime Prediction Method for Elastomer Components

Simon Rocker, May 2020

In the field of mechanical engineering technical elastomers are indispensable due to their material properties. They are often used to avoid load peaks and to influence the vibration behavior of dynamically loaded systems, because of their damping characteristics. Therefore, one field of research constitutes the damage accumulation and lifetime prediction. This paper presents the validation of the virtual lifetime prediction model method, which was developed at the institute of product engineering at the University of Duisburg-Essen. The lifetime is defined as the number of load cycles till the global damage reaches the value 1. This damage is calculated by a failure criterion based on the change of a characteristic value like the dynamic stiffness degradation from a finite-element (FE) simulation. The virtual lifetime prediction method uses a combination of a damage-dependent material model (Yeoh-Model) and a nonlinear damage accumulation model (nlSAM). Both models are calibrated by means of experimental data from dynamically loaded elastomer components. The nlSAM computes the local damage for each finite element depending on material stresses and pre-damage. The dynamic stiffness degradation is a result of locally changed material properties in the FE simulation due to the damage of each element. Finally, the lifetime prediction for unknown loads and different component geometries of the elastomer is carried out, which shows good agreement with the experimental data of the same material batch.

UV Effects on Plastics

Jeffrey A. Jansen, March 2020

If you work with plastic components that include outdoor exposure, then "Ultraviolet (UV) Effects on Plastic Materials" will provide you with information that will enhance your understanding of the interaction between UV radiation-based weathering and plastic resins, and help prevent premature failure. Topics covered during this session include an introduction to UV degradation and an explanation of the failure mechanism characteristic of UV radiation/plastic interaction. Case studies associated with UV radiation exposure will be presented.
You will learn…

• The mechanism of UV degradation
• The materials susceptible to and most affected by UV degradation
• The effects of UV degradation on plastic materials
• How the use of stabilizers can improve UV resistance of plastic materials
• How testing can be used to determine whether plastic materials are susceptible to UV degradation.

Fourier Transform Infrared Spectroscopy in Failure & Compositional Analysis

Jeff Jansen, January 2020

Fourier transform infrared spectroscopy (FTIR) is a fundamental analytical technique for the analysis of organic materials. It provides critical information in the evaluation of polymeric materials, including material identification, contamination, and degradation. The webinar will present a fundamental understanding of the technique and the following topics will be covered:

1. Theory of Infrared Spectroscopy
2. Test Result Interpretation
3. Application to Polymeric Materials
   1. Material Identification
   2. Contamination
   3. Degradation
4. Sample Preparation
5. Supplementing FTIR With Other Techniques
6. Cases Studies

Failure Associated With Injection Molding

Jeff Jansen, December 2019

The injection molding process is one of the key characteristics that determines how a plastic part will perform in service. Manufacturers certainly attempt to avoid failure, but often unanticipated factors result in unexpected problems. The chances for a successful application can be significantly increased through preventative measures, including appropriate material selection, proper mold design, and process development. Even when appropriate actions are taken, failures can still occur. The evaluation of these failures provides an opportunity for learning. By understanding how and why a plastic component is failed, steps can be taken to prevent future occurrences. Case Studies will be presented to illustrate failures associated with the deficiencies from the injection molding process. The presented cases will illustrate how the failure analysis process was used to identify the failure mechanism as well as the primary factors responsible for the failures.

Understanding Failure Rate in Plastic Components

Jeff Jansen, September 2019

When a plastic part fails, a tough question is often asked, “Why are a limited number of parts failing?”. This is particularly true with seemingly random failures at significant, but low, failure rates. Two aspects are generally linked to such low failure rates, multiple factor concurrency and the statistical nature of plastic failures. Failure often only takes place when two or more factors take effect concurrently. Absent one of these factors, failure will not occur. Plastic resins and the associated forming processes produce parts with a statistical distribution of performance properties, such as strength and ductility. Likewise, environmental conditions, including stress and temperature, to which the resin is exposed through its life cycle is also a statistical distribution. Failure occurs when a portion of the distribution of stress on the parts exceeds a portion of the distribution of strength of the parts. This webinar will illustrate how the combination of multiple factor concurrency and the inherent statistical nature of plastic materials can result in seemingly random failures.