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Plastics Testing

11.1 Introduction

Although the terms “testing” and “characterization” are used inter- changeably to describe evaluation of various properties of plastics, there is a subtle difference between the two terms. Characterization often refers to evaluation of the molecular or structural characteristics of plastics while testing is used to refer to evaluation of behavior of plastics in response to the applied external loads, environment, etc.
The testing is not limited to plastics resin form only. Besides the resin itself, very often testing the fabricated part in its final form needs to be an essential part of the design validation step to ensure that end-use performance requirements are adequately realized. Such part tests will have to be application specific and often need to involve testing under actual or simulated service conditions employing spe- cialized and nonstandard methods/procedures.

11.2 The Need for Testing Plastics

It is simple to understand the rationale for testing plastics. For a resin supplier, plastics testing is an integral part of new product develop- ment, application development, and quality control. Specifically, the key reasons are

1. Assessment of the material’s behavior in order to determine its suit- ability for desired application(s)

11.1

2. Evaluation of whether it meets various compliance requirements, for example, U.S. FDA and UL
3. Evaluation of lot-to-lot quality and consistency of the product being manufactured for quality control purposes
4. Specific evaluations of the material for customer support

The assessment of material’s behavior is inclusive of understanding structure-property relationships as well as comparison of the perfor- mance characteristics of one product against another. In the case of new product development, understanding the structure-property rela- tionship, thereby enabling the performance envelope of the product decisions for further development effort to be made. Such understand- ing is crucial to facilitate material design and predict how the product may perform under actual service conditions during use. Intermaterial comparison is essential in exploring substitution opportunities in existing applications.
Quality control of the resins supplied to the customer is an important aspect that the plastics manufacturer/supplier addresses on a regular basis to ensure the quality of the product and to demonstrate lot-to-lot consistency for compliance with the sales specifications. Since it is not practical to test every property for each lot of the material produced, spe- cific properties that are more sensitive to swings in production fluctua- tions are usually chosen for quality control purposes. These could include

■ Melt flow rate (MFR)
■ Chemical composition by infrared spectroscopy
■ Molecular weight and molecular-weight distribution (MWD) by gel permeation chromatography
■ Tensile properties
■ Notched impact strength, etc.

It is not too uncommon for internally developed, nonstandard test procedures to be employed for quality control purposes. Both resin and part testing are often carried out by the resin supplier.
The end user’s reasons for testing plastics are essentially twofold. The primary reason is to verify the quality of the incoming material to assure that it is within the specifications. The second reason is to validate the part design, which may involve testing the resin as well as the final part.

11.3 Diverse Types of Testing

Testing of plastics is directly linked to the type of data that needs to be evaluated, and the data fall under several categories:

1. Analytical data associated with structural features
2. Data needed to aid in material preselection
3. Data needed for computer-aided engineering (CAE) and computer- aided design (CAD)
4. Data needed to understand processing behavior
5. Data needed for regulatory compliance/approval
6. Data needed to satisfy original equipment manufacturer (OEM)
specification
7. Miscellaneous data

11.3.1 Analytical data associated with structural features

Analytical data associated with structural features includes molecular parameters like molecular weight, molecular weight distribution, tac- ticity, branching or chemical heterogeneity, morphological features like degree of crystallinity, etc. Such data are intended for material design and customer support and typically not shared broadly due to the proprietary nature of the data. Characterization of the analytical data is essential for establishing the structure-property relationships, which is useful in tailoring products designed to yield desired perfor- mance characteristics.

11.3.2 Data needed to aid in material preselection

Invariably every resin supplier provides a technical datasheet for each product that is commercially offered. These datasheets typically list a general set of 10 to 15 properties that are intended to represent the types of characteristics in each product, as illustrated in Fig. 11.1.
Since the primary purpose of the datasheet is a sales tool, the data reported in the datasheets generally focus on single-point data only, which is adequate for initial screening or intermaterial comparison. By their very nature, such data are inadequate for design and engineering analysis, as they bear little relevance to the end-use performance of the product and provide very little insight into how well the plastics will perform in service.1–3

11.3.3 Data needed for CAE and CAD

When plastics are considered for load-bearing applications involving complex shapes, CAD/CAE, utilizing finite element analysis (FEA) tech- niques, become powerful tools for design engineers in performing engi- neering analysis to predict the performance.

■ Isothermal stress-strain curves at temperatures and strain rates reflective of service conditions
■ Temperature dependency of dynamic modulus
■ Isochronous creep curves at ambient and elevated temperatures and several stress levels
■ Impact behavior at ambient and subambient temperatures
■ Effect of exposure on the behavior of plastics to environments the product is typically exposed to during its service life
■ Effect of anisotropy on the material performance
■ Viscosity-shear rate data

With operating conditions varying over a rather broad range of tem- peratures, loading history and environments over the life cycle of the product, using the instantaneous material properties at 23°C and 50% relative humidity reported in the datasheet for design would be a gross mistake. Yet design engineers rely heavily on the datasheet properties because the datasheet is readily available.
The specific material properties relevant for plastics product design are adequately addressed4–6 and a comprehensive new ISO guide for design data on plastics is currently under development.7 In the inter- est of making the information readily available to readers, relevant information is culled here from these works.
The testing involved for generating design data naturally depends on the type of CAE analysis being carried out.

1. Structural analysis
2. Manufacturability assessment
3. Part assembly design

Structural analysis is employed in assessing the structural integri- ty of the designed part over its useful life or in determining the required geometry of the part to ensure part functionality. As func- tional requirements are often specific to each application, the materi- al properties essential for structural analysis can be classified into two categories: those that are somewhat application-specific and those that are generic in nature. Whether the individual property is application-specific or generic, certain properties are directly employed in design calculations while others are employed more or less for verification of design limits. For example, although parts may fail in service under multiaxial impact loading conditions, the impact energy data can only be used in design verification, at best. Additional examples of properties that are useful only for design verification include fatigue (S-N) curves, wear factor, PV limit, retention of prop- erties following exposure to chemicals and solvents, and accelerated aging or ultraviolet (UV) exposure/outdoor weathering.
The essential properties needed for various types of structural design calculations (beam or plate, buckling, pipe, bearings analysis, or some combination thereof) are compiled in Table 11.1. The key design parameters in all the previous analyses, except in the case of bearings, are maximum deflection and maximum or critical stress, in order to determine appropriate part thickness to provide sufficient rigidity and strength to the part. The specific material properties employed in the calculations of each design parameter involved in these analyses are summarized in Table 11.2. The required modulus type for these calculations is determined by the stress type and dura- tion. Tensile or secant modulus is adequate where loading is limited to short duration, with secant modulus justified when the stress-strain behavior exhibited by the plastic is nonlinear. Tensile creep modulus is required where stress is encountered over extended periods of time. Shear modulus is needed where torsional loads are involved.
Processing simulations are employed in assessing the manufactura- bility of parts from the plastics. Among the various plastics processing methods, injection molding is the most dominant method in practice. As such, the CAE tools for simulation of the injection-molding process are more advanced in terms of number of CAE programs available and their sophistication. Recently, greater emphasis seems to be given to the development of CAE tools for simulation of other processing meth- ods such as extrusion, blow molding, and thermoforming.
Most of the commercially available injection-molding simulation programs allow a two-dimensional (2-D) analysis, incorporating tem- perature distribution through the thickness dimension. Enhancements touting full three-dimensional (3-D) simulation have only been introduced recently. Both types of simulation programs are rather complex in nature, utilizing a quite rigorous definition of the part geometry and incorporating various viscosity models to describe the flow behavior of polymer melts. Some expertise is required to use these programs. Simple two-dimensional programs, which do not involve such rigorous analysis, are also currently on the market.
The main objective of these methods is to simulate the part filling and post filling steps in order to optimize the manufacturability of the part.

TABLE 11.1 Material Properties Needed for Structural Design Calculations

Plate or beam analysis Pipe analysis Buckling analysis Bearing analysis
Tensile modulus Tensile modulus Compressive modulus PV limit*
Secant modulus Tensile creep modulus Secant modulus Wear factor
Tensile creep Poisson’s ratio Tensile creep modulus Coefficient of friction
modulus Critical stress Shear modulus Coefficient of
Shear modulus
Poisson’s ratio intensity factor K1c
Tensile stress at yield Poisson’s ratio
Compression thermal expansion
Tensile stress Tensile creep strength
at yield rupture stress Tensile stress at yield
Tensile creep rupture stress
Shear strength

*No ASTM or ISO standards exist today.

TABLE 11.2 Relevance of Material Properties in Structural Design Calculations

Type of analysis Design parameter Relevant properties needed

Plate analysis Maximum deflection Modulus (tensile, secant, tensile creep)
Poisson’s ratio
Maximum stress Tensile stress at yield
Tensile creep rupture stress
Beam analysis Maximum deflection Modulus (tensile, secant, creep, shear)
Poisson’s ratio
Maximum stress Tensile stress at yield
Tensile creep rupture stress
Shear strength
Buckling analysis Critical stress Modulus (compressive, tensile, secant)
Poisson’s ratio
Compression strength
Tensile stress at yield
Pipe analysis Hydrostatic design stress Tensile stress at yield
Tensile creep rupture stress
Compression strength

Radial displacement Critical stress intensity factor K1c
Modulus (tensile, tensile creep)
Poisson’s ratio
Bearings analysis PV value PV limit
Coefficient of friction
Volumetric wear Wear factor
Coefficient of thermal expansion

There are three main types of analysis in injection-molding simulation:

1. Simple mold filling analysis to determine the ability to fill the mold cavity and to assess the pressure requirements.
2. Advanced mold filling, packing, and cooling analysis, which is carried out to optimize the processing conditions or to evaluate design alter- natives such as number of gates, proper gate size, its location, etc.

TABLE 11.3 Data Needed for Injection-Molding Simulation—Simple Mold Filling
Analysis of Thermoplastics and Thermoplastic Elastomers

Property Variables

Melt viscosity Temperature, shear rate
Melt density
Thermal conductivity
Specific heat
Solidification temperature* Ejection temperature*

*Reference temperatures defined by simulation software.

TABLE 11.4 Data Needed for Injection-Molding Simulation—Advanced Mold Filling, Packing, and Cooling Analysis of Thermoplastics and Thermoplastic Elastomers

Property Variables

Melt viscosity Temperature, shear rate
Specific volume Pressure, temperature, cooling rate* Thermal conductivity Temperature
Specific heat Temperature Solidification temperature† Pressure, cooling rate Ejection temperature†
Crystallization temperature
(semicrystalline materials) Pressure, cooling rate
Enthalpy of crystallization
(semicrystalline materials) Cooling rate

*Predicted by compensating for crystallization kinetics at different cooling rates using DSC.
†Reference temperatures defined by simulation software.

3. Shrinkage and warpage analysis to satisfy tolerances and predict dimensional stability of the manufactured part.

The material properties needed for simple mold filling simulation are listed in Table 11.3. The material properties essential for advanced fill- ing, packing, and cooling simulations are listed in Tables 11.4 through
11.6. The requirements are essentially the same for thermoplastics and thermoplastic elastomers, while in the case of reactive materials, such as thermosets, the main differences are the reactive polymer viscosity in place of melt viscosity data and reaction kinetics data.
The simulation of extrusion generally includes consideration of the melting of the polymer in the barrel, flow of the melt in the die, and the cooling of the extruded shape. There are several simulation pack- ages on the market, employing different viscosity models to describe the flow characteristics of the polymer melt. The material properties needed for simulation of extrusion process are listed in Table 11.7.
The material properties needed for simulation of blow molding, blown film extrusion, and thermoforming are listed in Table 11.8.

TABLE 11.5 Data Needed for Molding Simulation—Mold Filling, Packing, and Cooling
Analysis of Reactive Materials Including Thermosets

Property Variables

Reactive viscosity Temperature, time, shear rate
Density, reacted
Thermal conductivity Temperature Specific heat Temperature Heat of reaction Temperature Isothermal induction time Temperature Gelation conversion Temperature
Reaction kinetics Temperature, conversion, heating rate

TABLE 11.6 Data Needed for Injection-Molding Simulation—Additional Data for
Shrinkage and Warpage Analysis

Property Variables

Molding shrinkage, parallel Thickness, processing parameters* pH, tH
Molding shrinkage, normal Thickness, processing parameters*
pH, tH Crystallinity (semicrystalline materials) Cooling rate Crystallization kinetics
(semicrystalline materials) Temperature, cooling rate
Tensile modulus, parallel Tensile modulus, normal Poisson’s ratio
In-plane shear modulus
Coefficient of linear thermal
expansion, parallel Temperature, thickness
Coefficient of linear thermal
expansion, normal Temperature, thickness

*pH = cavity pressure and tH = hold time.

common denominator in these processes is the biaxial orientation step involved. While in blow molding and blown film extrusion the biaxial orientation is induced in the melt state, in thermoforming it is induced in the softened state.
The part assembly design addresses the ability to join/assemble the component parts. Where the components are assembled with adhesives, it is important to know the compatibility and strength of adhesion to dissimilar substrates, in addition to the chemical compatibility of the plastic with the specific adhesive and its constituents. If melt bonding methods, like ultrasonic, vibration, or spin-welding processes, are employed, thermal compatibility aspects have to be taken into account. The broad possibilities of mechanical assembly methods include snap fits, press fits, bolts, and threads. The material properties needed for each of these design calculations are listed in Table 11.9.

TABLE 11.7 Material Properties Needed for Simulation of Extrusion

Property Variables

Melting temperature
(semicrystalline materials) Glass transition temperature
(amorhous materials) Enthalpy of fusion
(semicrystalline materials)
Coefficient of friction Pressure, temperature, slip velocity
Thermal conductivity Temperature
Specific heat Temperature
Melt viscosity Temperature, shear rate pressure
First normal stress difference Temperature, shear rate Uniaxial extensional viscosity Temperature, time, strain rate Crystallization temperature
(semicrystalline materials) Pressure, cooling rate
Enthalpy of crystallization
(semicrystalline materials) Cooling rate
Crystallization kinetics
(semicrystalline materials) Temperature, cooling rate

TABLE 11.8 Material Properties Needed for Simulation of Blow Molding, Blown Film
Extrusion, and Thermoforming

Property Variables

Uniaxial extensional viscosity Temperature, time, strain rate Biaxial extensional viscosity Temperature, time, strain rate First normal stress difference Temperature, shear rate Thermal conductivity Temperature
Specific heat Temperature
Crystallization temperature
(semicrystalline materials) Pressure, cooling rate
Enthalpy of crystallization
(semicrystalline materials) Cooling rate
Crystallization kinetics
(semicrystalline materials) Temperature, cooling rate

The specific material properties employed in the computation of each design parameter involved in these analyses are summarized in Table 11.10. Similar to design calculations in the case of structural analyses, the modulus type suitable for these calculations is deter- mined by the applied stress type and duration.
It is worth noting that, although the exhaustive compilation of data identified here will suffice in most cases, commercial CAE tools often insist on a significantly large volume of additional data that are not

TABLE 11.9 Material Properties Needed for Part Assembly Design

Snap fit analysis: Tensile modulus Secant modulus Creep modulus Shear modulus Poisson’s ratio
Tensile strength at yield
Coefficient of friction
Press fit analysis: Tensile modulus Compression modulus Creep modulus Poisson’s ratio Compression strength Coefficient of friction
Tensile strength at yield Tensile creep rupture stress Stress relaxation*

Bolt analysis: Compressive strength Tensile creep rupture stress Tensile creep strain
Stress relaxation*
Thread analysis: Shear strength Coefficient of friction Tensile strength at yield
Weldability: Shear strength Density
Coefficient of friction Thermal conductivity Specific heat
Crystalline melting temperature

*No ASTM or ISO standards exist today.

covered here. This is attributed to many of the models available in the software tools for structural analysis like ABAQUS, MSC/PATRAN, DYNA3D, etc., and processing simulation software tools like MOLD- FLOW, C-MOLD, CADMOULD, TM Concept, POLYFLOW, etc.

11.3.4 Data needed to understand processing behavior

In order to assist customers in fabrication of useful articles from plas- tics, it is important to understand the processability of the resin in terms of maximum processing temperature to avoid degradation and any decomposition products, that is, off gases given off to ensure safe- ty during processing. In addition, processing simulation tools require information on maximum shear stress and shear rate allowable.

11.3.5 Data needed for regulatory compliance

In many applications, where consumer safety is of paramount impor- tance, the product performance criteria may stipulate compliance with various regulatory requirements. The specific requirements are, of course, dependent on the targeted application and should be deter- mined for each application.

TABLE 11.10 Relevance of Material Properties in Structural Design Calculations

Type of analysis Design parameter Relevant material properties

Snap fit analysis:
Cantilever Maximum deflection Modulus (tensile, secant, tensile creep) Poisson’s ratio
Deflection force Modulus (tensile, secant, tensile creep) Mating force Modulus (tensile, secant, tensile creep)
Tensile stress at yield
Coefficient of friction
Cylindrical Maximum interference Modulus (tensile, secant, tensile creep) Poisson’s ratio
Interference stress Tensile stress at yield
Deflection force Modulus (tensile, secant, tensile creep) Poisson’s ratio
Mating force Modulus (tensile, secant, tensile creep) Coefficient of friction
Torsional Deflection force Modulus (tensile, secant, shear) Poisson’s ratio
Permissible shear Poisson’s ratio
Press fit analysis Allowable interference Modulus (tensile, secant, compression) Poisson’s ratio
Coefficient of linear thermal expansion
Maximum stress in hub Tensile stress at yield Tensile creep rupture stress Compression strength
Thread analysis Stripping torque Shear strength Tensile stress at yield Coefficient of friction
Stripping stress Shear strength
Pullout force Tensile stress at yield
Shear strength
Bolt analysis Preload stress Tensile stress at yield Tensile creep rupture stress Compression strength Stress relaxation
Torque Tensile stress at yield
Coefficient of friction

Targeted application Industry focus Regulatory body Material property

Electrical components Information UL, CE Relative thermal (connectors, housings, technology index (RTI) lighting components) Lighting
Computer and Glow wire temperature business
equipment
Nondisposable Healthcare U.S. FDA Biocompatibility
products (medical
devices) Extractables
Applications involving Packaging U.S. FDA Extractables direct food contact

11.3.6 Data needed to satisfy OEM
specification

Contractual agreements with major OEMs often require that the material under transaction meet the specifications set forth by the OEM for the incoming material, as part of the quality assurance effort. The resin supplier is required to demonstrate that the shipped mater- ial is within the specification. The testing involved is dependent on the OEM and targeted application, to provide proof of compliance.

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