Thermoplastics Overview

Thermoplastics Overview

Introduction • Aim • Review important concepts regarding polymers (molding materials) • Why do it • By understanding how properties and characteristics of polymers affect the processing and final product you may interpret better the inputs and the analysis results • Overview • Polymers’ definition & classification • Polymers’ properties of interest • Thermoplastic material families & abbreviations

Plastics are Polymers • The word “polymer” is a combination of the Greek words: “poly" which means "many“ and “mer” which means “part” • The basic structure of a polymer molecule can be visualized as a long chain of repeating units

Polymers’ Classification • Thermoplastics • The word “thermoplastic” is a combination of the Greek words: “therm" which means “heat“ and “plastikos ” which means “capable of being molded” • Once the polymer is formed it can be heated and reformed over and over again (allows recycling) • Thermosets • The word “thermoset” is a combination of the Greek words: “therm" which means “heat“ and “sets” irreversibly when heated • Therefore, can not be remelted. Once these polymers are formed, reheating will cause the material to degrade

Classification Based on Morphology • Amorphous • Same molecular structure throughout molding cycle • Semi-crystalline • Compact molecular structure when cool, amorphous when hot

Processing conditions Mold temperature Melt temperature Ejection temperature Maximum shear stress Maximum shear rate Rheological properties Viscosity Juncture loss method coefficients Transition temperature Melt flow rate (MFR) Thermal properties Specific heat Thermal conductivity Physical properties Melt density Solid density Specific volume (pvT diagram) Mechanical properties Elastic modulus Poisson’s ratio Shear modulus Transversely isotropic coefficient of thermal expansion (CTE) data Shrinkage properties Shrinkage model Observed nominal shrinkage Observed shrinkage Properties of Interest Typically Include:

Processing Conditions • Mold temperature • temperature of the mold where the resin touches the mold • Melt temperature • temperature of the resin, or melt, as it starts to flow into the cavity • Ejection temperature • temperature at which a material is rigid enough to withstand ejection • Maximum shear stress • maximum shear stress for the material beyond which degradation starts to occur • Maximum shear rate • maximum shear rate for the material beyond which degradation starts to occur

Rheological Properties • Viscosity () • Material’s resistance to flow • Juncture loss method coefficients • Calculation of the hydraulic loss that occurs when the melt passes through a large change in path diameter, such as from the end of the runner to the gate • Transition temperature • Temperature to where the polymer freezes, the melt-to-solid transition occurs • The transition temperature corresponds to the glass-transition temperature (Tg) for amorphous materials • The transition temperature is the crystallization temperature (Tc) for semi-crystalline polymers • Melt flow rate (MFR) • Industry standard which measure how easily the melt flows • Problem: single data point at low shear rates • It is measured in grams per 10 minutes (g/10 min)

High resistance to flow Material Viscosity Water 10-1 10 Polymer 10,000 Glass 1020 Increasing temperature Viscosity (Pa-s) Low resistance to flow Shear Rate (1/s) B.a Polymer Viscosity • Viscosity is effected by • Shear rate • Flows easier when sheared • Temperature • Viscosity decreases when temperature is increased • Important for “thin wall” applications

Specific heat (Cp) Ability to hold heat. Measure of a material's ability to convert heat input to an actual temperature increase Thermal conductivity (k) Ability to conduct heat. Measure of the rate at which a material can dissipate heat Thermal diffusivity () Ratio of thermal conductivity to heat capacity Thermal Properties

Change Cooling Time Thermal Conductivity (k) Specific Heat (Cp) Density (p) Impact of Material on Cooling Time • Cooling time based on the thermal conductivity, density, and specific heat of the resin • These combine to create the “thermal diffusivity” of the material

Physical Properties • Melt density (m) • Density of the selected material in the melt state • Solid density (s) • Density of the selected material in the solid state • Specific volume (pvT diagram) • Describes the temperature/pressure relationship for polymers over the entire processing range (pvT)

Pressure Increases Specific Volume Temperature D.c pvT Properties • p - pressure v - specific volume T - temperature • Describes how the plastic contracts and expands when changing pressure and temperature • During filling and packing, the plastic contracts due to increased pressure • During cooling the plastic contracts due to decreased temperature

pvT Data for Crystalline material pvT Data for Amorphous material 1.10 1.05 1.00 Specific Volume [cm^3/g] 0.95 P=0[MPa] 0.90 P=50[MPa] P=100[MPa] P=150[MPa] 0.85 P=200[MPa] 0.80 0 50 100 150 200 250 300 350 0 50 100 150 200 250 300 350 Temperature [ºC] Temperature [ºC] Pressure-Volume-Temperature (pvT) • Amorphous materials transition corresponds to the glass transition temperature (Tg) • Crystalline materials transition corresponds to the crystallinization temperature (Tc)

Mechanical Properties • Elastic modulus (E) • Force you need to provide to elongate the material • Shear modulus (G) • Ratio of shearing stress  to shearing strain within the proportional limit of a material • Poisson’s ratio () • Ratio of the lateral to axial strains. Theoretically, isotropic materials (identical in all directions) will have a value for Poisson’s ratio of 0.25. The maximum value of n is 0.5, which denotes no volume change during deformation • It is also used to relate shear and elastic modulus • Transversely isotropic coefficient of thermal expansion (CTE) data • Specifies thermal expansion properties of the material • Combined with the Mechanical Data model to account for variation in properties parallel and perpendicular to the direction of flow

Shrinkage Properties • Shrinkage model used for Midplane and Fusion • Shrinkage model used during the simulation. Best model set by default for material tested • Residual strain shrinkage prediction method • Needs shrinkage data • Residual stress shrinkage prediction method • Uncorrected • no shrinkage data • Corrected (CRIMS) • Needs shrinkage data • Shrinkage prediction method used in 3D

Shrinkage Properties • Observed nominal shrinkage • shows average shrinkage value, parallel and perpendicular to the direction of flow • Observed shrinkage • shows the maximum and minimum shrinkage values, parallel and perpendicular to the direction of flow

Thermoplastic Material Families & Abbreviations

QUESTIONS?

Thermoplastics Overview