Polymer and Composite Manufacturing

Introduction and Context

Polymers and polymer-matrix composites occupy an important position in modern mechanical engineering because they combine low density, corrosion resistance, design flexibility, electrical insulation, and relatively low manufacturing cost. They are used in packaging, consumer products, medical devices, automotive components, aircraft structures, wind-turbine blades, sporting goods, and industrial equipment.

Polymer manufacturing differs fundamentally from metal manufacturing. Many polymers are processed above their glass-transition temperature or melting temperature, and their viscosity depends strongly on temperature, shear rate, molecular weight, and time. Consequently, successful production requires simultaneous attention to material selection, heating, flow, mold design, cooling, shrinkage, and post-processing.

Composites extend these possibilities by combining a continuous polymer matrix with reinforcing fibers, particles, or other phases. The matrix transfers load, protects the reinforcement, and provides shape; the reinforcement supplies much of the stiffness and strength. Manufacturing quality determines whether the theoretical properties of the composite are achieved in practice.

Learning Objectives

After studying this chapter, the student should be able to:

1. Explain the thermal, rheological, and structural behavior relevant to polymer processing.
2. Compare extrusion, injection molding, blow molding, thermoforming, rotational molding, and compression molding.
3. Relate mold design and polymer flow to product quality.
4. Identify common polymer-processing defects and recommend corrective actions.
5. Explain polymer recycling routes and their limitations.
6. Describe composite lay-up, molding, curing, and quality-control methods.
7. Select a suitable process for a product based on geometry, material, volume, performance, and cost.

1. Polymer Behavior Relevant to Manufacturing

1.1 Thermoplastics, thermosets, and elastomers

Thermoplastics soften when heated and solidify when cooled. The transition is generally reversible, so thermoplastics can often be remelted and reprocessed. Examples include polyethylene (PE), polypropylene (PP), polyvinyl chloride (PVC), polystyrene (PS), polyamide (PA), polycarbonate (PC), and polyether ether ketone (PEEK).

Thermosetting polymers undergo an irreversible chemical reaction called curing or cross-linking. Once cured, they do not melt on reheating; excessive heating causes degradation. Epoxy, phenolic, unsaturated polyester, and many polyurethane systems are thermosets.

Elastomers are lightly cross-linked polymers capable of large reversible deformation. Natural rubber, silicone rubber, and many synthetic rubbers are processed by methods such as compression molding, transfer molding, and injection molding.

1.2 Thermal transitions

The glass-transition temperature, T_g, is the approximate temperature range at which an amorphous polymer changes from a hard, glassy state to a softer, rubbery state. A semicrystalline polymer also has a melting temperature, T_m, associated with the melting of its crystalline regions. Processing conditions must provide sufficient mobility for flow while avoiding thermal degradation.

Cooling rate affects crystallinity, shrinkage, stiffness, and dimensional stability. Rapid cooling generally produces less crystallinity in many semicrystalline polymers, whereas slower cooling permits greater crystal growth. Processing conditions should therefore be specified with the material grade, not merely with the polymer name.

1.3 Viscosity and non-Newtonian flow

Polymer melts are usually shear-thinning: their apparent viscosity decreases as shear rate increases. A useful engineering representation is the power-law model:

where τ is shear stress in pascals, γ̇ is shear rate in s⁻¹, K is the consistency index, and n is the flow-behavior index. For a shear-thinning material, 0 < n < 1. This model is an approximation over a limited range of shear rates and does not represent all polymer effects, especially elasticity and temperature dependence.

A common temperature relationship for viscosity is the Arrhenius-type approximation:

Here, η is viscosity, T is absolute temperature in kelvin, E_a is an apparent activation energy, R is the gas constant, and η₀ is a material constant. The relation applies over a restricted temperature range and should not be used across phase transitions without suitable material data.

Flow is also affected by molecular orientation. Chains tend to align in the direction of flow, producing anisotropic shrinkage and mechanical properties. Fiber-filled polymers exhibit even stronger directional behavior because the fibers orient according to the local flow field.

1.4 Shrinkage and warpage

Polymer shrinkage arises from thermal contraction, crystallization, and molecular relaxation. A simple linear shrinkage estimate is:

where S is mold or process shrinkage, L_m is the reference mold dimension, and L_p is the cooled-part dimension. The equation is a definition and is useful for basic estimation; actual shrinkage depends on direction, pressure history, cooling rate, geometry, and material grade.

Unequal cooling or orientation creates differential contraction. The resulting warpage is reduced by uniform wall thickness, balanced filling, appropriate packing, controlled mold temperature, and adequate cooling-channel design.

2. Major Polymer Manufacturing Processes

2.1 Extrusion

In extrusion, polymer pellets are conveyed, melted, mixed, and forced continuously through a die. A typical single-screw extruder contains a feed zone, compression or transition zone, metering zone, heater bands, screw, barrel, screen pack, breaker plate, and die. The extrudate is then cooled and cut, wound, or calibrated.

Extrusion is suitable for pipes, films, sheets, rods, profiles, wire coatings, and filaments. It is a continuous process and is therefore economical for long products with constant cross-section. Die design must provide balanced flow and sufficient pressure control. A screen pack filters contamination and increases back pressure, but excessive pressure can reduce output or raise melt temperature.

Important process variables include screw speed, barrel temperature, die temperature, residence time, melt pressure, cooling rate, and haul-off speed. Output is governed by the balance between drag flow caused by screw rotation and pressure flow caused by resistance at the die. Increasing screw speed usually increases throughput, but may also increase shear heating and melt fracture.

Common extrusion defects include surface roughness, die lines, melt fracture, dimensional variation, bubbles, and distortion during cooling. Remedies include cleaning or redesigning the die, reducing excessive shear, improving drying, stabilizing temperature, and matching haul-off speed to extrusion rate.

2.2 Injection molding

Injection molding produces discrete parts by injecting molten polymer into a closed mold. The machine performs four principal operations: plasticizing, mold closing, filling and packing, and cooling followed by ejection.

The mold usually consists of a fixed half and a moving half. Its important features include sprue, runners, gates, cavity, core, vents, cooling channels, ejector pins, and alignment components. The gate controls the entry of material and strongly influences weld lines, orientation, packing, and appearance.

The filling stage should be fast enough to avoid premature freezing but not so fast that excessive shear, air entrapment, or burn marks occur. During packing and holding, additional material compensates for volumetric contraction. Cooling must continue until the part is sufficiently rigid for ejection.

A simplified cycle-time expression is:

The terms represent filling, packing, cooling, and mold-opening/ejection times. In many thermoplastic applications, cooling is the largest component. The expression is a process estimate, not a universal physical law; overlapping machine operations and mold-specific timing must be considered in detailed costing.

Injection molding is appropriate for high-volume production of complex parts with repeatable dimensions. It requires substantial tooling investment, but the cost per part can be low at large quantities. Design-for-manufacture principles include uniform wall thickness, draft angles, rounded corners, ribs rather than excessively thick sections, appropriate gate location, and avoidance of enclosed undercuts unless slides or collapsible cores are justified.

2.3 Blow molding

Blow molding produces hollow articles. A heated parison or preform is placed in a mold and inflated against the mold wall using compressed air. The three main variants are extrusion blow molding, injection blow molding, and stretch blow molding.

Extrusion blow molding is suitable for bottles, tanks, ducts, and containers with relatively complex shapes. Injection blow molding produces accurate preforms and good neck finish control. Stretch blow molding biaxially orients materials such as PET, improving strength and barrier performance in bottles.

Critical variables include parison or preform temperature, wall-thickness programming, inflation pressure, mold temperature, and cooling. Defects include uneven wall thickness, pinch-off weakness, wrinkles, holes, excessive flash, and inadequate detail reproduction.

2.4 Thermoforming

Thermoforming heats a thermoplastic sheet until it becomes flexible and forms it over or into a mold. Vacuum forming uses pressure below the sheet to draw it against the mold; pressure forming adds compressed air above the sheet; twin-sheet forming joins two shaped sheets to create a hollow component.

Thermoforming is economical for large, thin-walled parts and relatively low to medium production quantities. Examples include packaging trays, appliance liners, vehicle interior panels, and sanitary products. Compared with injection molding, tooling is generally cheaper, but thickness distribution and dimensional accuracy may be less uniform.

Sheet temperature, heating uniformity, plug assistance, vacuum level, mold geometry, and cooling influence quality. Deep draws can produce severe thinning at corners and sidewalls. Proper radii, pre-stretching, and controlled heating improve thickness distribution.

2.5 Rotational molding

Rotational molding forms hollow parts by placing a measured polymer charge inside a mold, heating the mold while it rotates about two axes, and then cooling it while rotation continues. Powdered thermoplastic, commonly polyethylene, coats the internal surface as it melts and flows.

The process requires relatively low mold pressure, allowing inexpensive molds and large parts. It is useful for tanks, playground equipment, containers, and ducts. Inserts and multiple wall layers can be incorporated. However, cycle times are long, wall thickness control is less precise than in injection molding, and sharp corners or fine details are difficult.

Important variables are powder particle size, charge mass, oven temperature, heating time, mold rotation ratio, cooling rate, and mold venting. Porosity may result from inadequate fusion or trapped gases. Warpage and uneven thickness can result from improper heating or rotation.

2.6 Compression molding

Compression molding places a measured charge of polymer or composite material into an open heated mold. The mold closes and applies pressure until the material flows, fills the cavity, and cures or cools. It is widely used for thermosets, rubber, and fiber-reinforced sheet molding compounds.

Advantages include relatively simple tooling, low waste, and suitability for large flat or moderately complex components. Limitations include flash formation, less freedom for very deep geometries, and potentially longer cycle times than injection molding. Charge placement, mold temperature, pressure, cure time, and venting are central controls.

For thermosets, cure kinetics are particularly important. Insufficient cure causes low strength and poor heat resistance; excessive temperature or time can degrade the resin or reinforcement.

3. Comparative Process Selection

Process selection should consider annual volume, part size, geometry, tolerances, material, surface finish, tooling budget, mechanical requirements, recyclability, and labor. No process is universally superior.

4. Mold and Tool Design

A mold must provide accurate geometry, controlled filling, heat transfer, venting, release, and safe ejection. The following principles are broadly applicable:

• Maintain reasonably uniform wall thickness to reduce sink marks and differential cooling.
• Use draft angles so parts can be removed without scuffing or deformation.
• Provide generous radii at internal corners to reduce stress concentration and improve flow.
• Locate gates to promote balanced filling and minimize weld-line weakness.
• Place vents at last-fill regions and potential air traps.
• Design cooling channels to produce uniform mold temperature rather than merely maximum cooling.
• Use ribs and bosses carefully; thick intersections are prone to sink marks and voids.
• Account for material shrinkage, fiber orientation, and thermal expansion in cavity dimensions.

Flow simulation can estimate fill time, pressure, weld-line location, air traps, fiber orientation, and cooling behavior. Simulation is valuable, but it depends on accurate rheological data, thermal properties, boundary conditions, and realistic representation of the machine and mold.

5. Defects, Diagnosis, and Corrective Action

Defect correction should follow a controlled troubleshooting method. First confirm the defect and its location; then examine material condition, machine settings, mold condition, and measurement data. Changing several variables simultaneously may hide the actual cause. Statistical process control, short-shot studies, pressure traces, and dimensional inspection can support diagnosis.

6. Recycling and Sustainability

Mechanical recycling involves sorting, shredding, washing, melting, filtering, and pelletizing thermoplastics. It is most effective when the polymer is clean, well identified, and separated from incompatible materials. Repeated heating and contamination may reduce molecular weight and performance.

Chemical or feedstock recycling converts polymers into monomers, oils, or other chemical intermediates through processes such as depolymerization, solvolysis, pyrolysis, or gasification. These routes can handle some mixed or contaminated streams but generally require more energy and complex facilities.

Thermoset composites are difficult to recycle mechanically because the cross-linked matrix cannot be remelted. Possible routes include mechanical size reduction for filler, thermal treatment to recover fibers, and chemical matrix removal. Recovered fibers may have reduced length, surface quality, or strength.

Design for circularity includes minimizing material variety, using compatible additives, providing identification, enabling disassembly, reducing contamination, and specifying recycled content where performance allows. Environmental assessment should consider raw-material production, processing energy, transport, product life, and end-of-life treatment rather than focusing on one stage alone.

7. Composite Materials and Manufacturing

7.1 Constituents and architecture

A composite consists of a matrix and reinforcement. Common reinforcements include glass, carbon, and aramid fibers. Forms include continuous unidirectional tape, woven fabric, chopped strand mat, short fibers, and particles. The matrix may be thermosetting epoxy or polyester, or a thermoplastic such as PEEK, PPS, or PA.

Continuous fibers provide high directional stiffness and strength. Woven fabrics offer balanced in-plane properties and improved handling, but crimp can reduce axial performance. Short-fiber composites are suitable for injection molding and complex shapes, though their properties are more orientation-dependent and generally lower than those of continuous-fiber laminates.

For a unidirectional composite under ideal longitudinal loading, a simple rule of mixtures is:

where E_L is longitudinal elastic modulus, E_f and E_m are fiber and matrix moduli, and V_f and V_m are their volume fractions, with V_f + V_m = 1. This equation assumes continuous, aligned fibers, effective bonding, elastic constituents, and loading parallel to the fibers. It should not be used directly for transverse properties, discontinuous fibers, or damaged interfaces.

The approximate fiber mass fraction can be related to volume fraction by:

where w_f and w_m are mass fractions and ρ_f and ρ_m are constituent densities. All densities must use consistent units.

7.2 Composite lay-up

In hand lay-up, dry reinforcement or prepreg layers are placed manually over a mold according to a specified ply schedule. The orientation sequence controls stiffness, strength, balance, and resistance to twisting. A laminate may contain 0° plies for axial stiffness, ±45° plies for shear and torsion, and 90° plies for transverse stability.

Vacuum bagging places a flexible airtight membrane over the laminate. Vacuum pressure consolidates the layers, removes air, and assists resin flow. A typical stack may include mold release, reinforcement or prepreg, peel ply, perforated release film, breather, and vacuum bag. Each layer has a distinct function and must not be omitted or incorrectly substituted.

Automated fiber placement and automated tape laying improve deposition speed, repeatability, and geometric accuracy for large structures. Their limitations include high capital cost, steering defects, gaps, overlaps, and the need for sophisticated path planning.

7.3 Molding methods

Wet lay-up. Resin is applied during placement of dry reinforcement. It is inexpensive and flexible but depends strongly on operator skill and may produce variable fiber volume and void content.

Prepreg molding. Reinforcement is pre-impregnated with a controlled resin content. Lay-up is followed by vacuum-bag or autoclave curing. Prepregs provide excellent consistency but require refrigerated storage or controlled out-time and involve higher cost.

Resin transfer molding (RTM). Dry reinforcement is placed in a closed mold, the mold is sealed, and resin is injected. RTM provides good surface finish on both sides and better dimensional control than open molding. Resin viscosity, injection pressure, permeability, and vent location govern complete impregnation.

Vacuum-assisted resin infusion (VARI or VARTM). Vacuum draws resin through dry reinforcement under a flexible bag. It is suitable for large structures, including marine panels and wind-turbine components. Race-tracking, dry spots, leaks, and uneven flow are important risks.

Compression molding of sheet molding compound (SMC). A resin-and-fiber charge is compressed in a heated mold. It supports relatively high production rates and complex automotive panels, but fiber length and orientation differ from those in continuous-fiber laminates.

Autoclave molding. A vacuum-bagged prepreg is cured using controlled temperature and external pressure in an autoclave. The process gives high consolidation and low void content but requires expensive equipment and substantial energy.

7.4 Curing and quality

Thermoset curing involves chemical reaction, heat generation, viscosity change, gelation, and cross-linking. A cure cycle specifies temperature, heating rate, dwell temperature, dwell time, pressure or vacuum, and cooling rate. Thick laminates may experience exotherm: heat generated by reaction raises the internal temperature above the programmed temperature. Excessive exotherm can damage the resin or create residual stresses.

Quality assurance includes incoming-material inspection, resin and prepreg storage control, fiber orientation checks, vacuum-leak testing, cure monitoring, dimensional inspection, and nondestructive testing. Ultrasonic inspection can detect delamination, voids, and disbonds; thermography and radiography may provide complementary information. Destructive tests such as tensile, compression, interlaminar shear, and short-beam tests are used for material qualification.

8. Illustrative Case: Selecting a Process for an Automotive Battery Enclosure

Background

An electric-vehicle supplier needs a corrosion-resistant polymer-composite enclosure. The enclosure is approximately 1.2 m long, must contain a sealed cavity, requires moderate stiffness, and will be produced initially at 8,000 units per year, with an expected increase to 60,000 units per year.

Problem

The enclosure must satisfy dimensional, sealing, impact, thermal, and fire-performance requirements. Candidate processes include rotational molding, thermoforming, compression molding of a glass-fiber composite, and injection molding of a filled thermoplastic.

Analysis

Rotational molding offers low tooling cost and large hollow forms, but cycle time and dimensional control may be inadequate for the sealing flange. Thermoforming is attractive for large shells but may require secondary joining and can produce nonuniform thickness. Injection molding gives excellent repeatability but would require a very large machine and expensive tooling. Compression molding of a glass-fiber SMC can provide stiffness, integrated ribs, and good production economics.

The design team therefore compares tooling cost, cycle time, projected volume, part consolidation, fire performance, repairability, and end-of-life treatment. A prototype tool is used to evaluate charge placement, flow, weld regions, flange flatness, and insert retention.

Approach and solution

The selected initial approach is compression molding of a fire-retardant glass-fiber SMC, with molded-in ribs and machined sealing surfaces. The mold includes balanced charge zones, controlled vents, replaceable inserts, and temperature sensors. A later transition to a thermoplastic compression process is retained as a development option to improve recyclability.

Outcome

The pilot parts meet stiffness and dimensional requirements after optimization of charge mass, mold temperature, pressure, and cure time. Early trials reveal voids near a rib intersection and warpage at the flange. The voids are reduced by changing charge placement and venting; flange warpage is reduced by improving temperature uniformity and adding a controlled cooling stage.

Learning points

The case demonstrates that process selection is a systems decision. The cheapest tool is not necessarily the lowest-cost solution over the product life. Geometry, production volume, quality requirements, joining, repair, safety, recycling, and future scale-up must be considered together.

9. Professional Practice, Safety, and Emerging Technology

Polymer processing involves hot surfaces, pressurized melts, rotating machinery, fumes, solvents, cutting tools, and heavy molds. Risk controls include guarding, interlocks, ventilation, appropriate personal protective equipment, lockout procedures, safe handling of resins and catalysts, and documented emergency response. Safety data sheets and applicable occupational regulations must guide chemical handling.

Manufacturing engineers also have responsibilities concerning emissions, worker exposure, traceability, and truthful quality reporting. A part should not be released by concealing defects or by substituting unapproved materials.

Digital manufacturing increasingly uses sensors, machine-learning models, and digital twins to predict defects, estimate remaining tool life, and optimize cycle conditions. These tools are valuable when supported by reliable data and physical understanding. Automated recommendations should be validated because a model trained on one resin grade, mold, or machine may not generalize to another. Human oversight remains essential for safety-critical components.

Chapter Summary

Polymer manufacturing depends on the interaction of thermal behavior, viscosity, molecular orientation, shrinkage, mold design, and cooling. Extrusion is a continuous process for constant-section products; injection molding produces accurate discrete parts; blow molding creates hollow articles; thermoforming shapes heated sheet; rotational molding produces large hollow parts at low pressure; and compression molding is important for thermosets, elastomers, and composites.

Good mold design requires balanced flow, suitable gates, vents, cooling channels, draft, radii, and allowance for shrinkage. Defects such as short shots, flash, sink marks, warpage, weld lines, burns, and voids should be diagnosed systematically. Thermoplastic recycling is comparatively feasible through mechanical and chemical routes, while cross-linked composite recycling remains more challenging.

Composite performance depends on reinforcement type, orientation, volume fraction, matrix bonding, consolidation, and cure. Hand lay-up, prepreg, vacuum infusion, RTM, compression molding, automated placement, and autoclave processing offer different balances of quality, cost, rate, and flexibility. Process selection must integrate technical performance, production volume, tooling, safety, sustainability, and end-of-life considerations.

Key Terms

Thermoplastic: Polymer that can be softened and reshaped repeatedly by heating.

Thermoset: Polymer that becomes permanently cross-linked during curing.

Glass-transition temperature: Temperature range associated with transition between glassy and rubbery behavior.

Shear thinning: Reduction in apparent viscosity as shear rate increases.

Extrusion: Continuous forcing of a polymer melt through a shaped die.

Injection molding: Discrete-part process in which melt is injected into a closed mold.

Blow molding: Process in which a heated polymer tube or preform is inflated inside a mold.

Thermoforming: Shaping of heated thermoplastic sheet by vacuum, pressure, or mechanical means.

Rotational molding: Heating and rotating a mold containing polymer charge to form a hollow part.

Compression molding: Forming by pressing a material charge in a heated mold.

Gate: Mold passage through which polymer enters the cavity.

Weld line: Line formed where separate flow fronts meet.

Warpage: Unintended distortion caused by nonuniform contraction or residual stress.

Composite: Material formed from two or more distinct constituent phases.

Matrix: Continuous phase that surrounds and supports reinforcement.

Prepreg: Reinforcement pre-impregnated with a controlled amount of resin.

Resin transfer molding: Closed-mold injection of resin into dry reinforcement.

Vacuum infusion: Use of vacuum pressure to draw resin through dry reinforcement.

Curing: Chemical reaction that converts a reactive resin into a solid network.

Fiber volume fraction: Fraction of composite volume occupied by reinforcement fibers.

Review Questions

Short Answer Questions

1. Distinguish thermoplastics from thermosets in terms of heating and recycling.
2. Why are polymer melts commonly described as shear-thinning fluids?
3. State two advantages and two limitations of injection molding.
4. What functions do the matrix and reinforcement perform in a composite?
5. Why is venting important in polymer molds?

Long Answer Questions

1. Explain the operating sequence, applications, advantages, and limitations of extrusion, injection molding, and blow molding.
2. Compare thermoforming, rotational molding, and compression molding for large polymer components.
3. Discuss how gate location, cooling, wall thickness, and draft angle influence injection-molded part quality.
4. Explain wet lay-up, prepreg molding, RTM, vacuum infusion, and autoclave processing.
5. Describe mechanical and chemical recycling of polymers and explain why thermoset composites are difficult to recycle.

Analytical / Application Questions

1. A manufacturer must produce a large hollow tank in low annual volume. Compare rotational molding, blow molding, and thermoforming, then recommend a process with justification.
2. An injection-molded housing shows sink marks near thick bosses and warpage near one wall. Diagnose the likely causes and propose changes to geometry, packing, and cooling.
3. Design a preliminary laminate schedule for a flat panel subjected to axial tension, transverse loading, and in-plane shear. Explain the purpose of 0°, 90°, and ±45° plies and identify manufacturing controls needed to limit voids and delamination.

Multiple Choice Questions with Answers

Question 1

Which process is most naturally suited to producing a continuous polymer pipe?

A. Extrusion

B. Blow molding

C. Compression molding

D. Rotational molding

Correct Answer: A. Extrusion

Question 2

Why are vents provided in an injection mold?

A. To increase polymer crystallinity

B. To remove trapped air and gases

C. To prevent all polymer shrinkage

D. To replace the cooling system

Correct Answer: B. To remove trapped air and gases

Question 3

Which process forms a hollow product by inflating a heated parison or preform?

A. Thermoforming

B. Blow molding

C. Extrusion coating

D. Compression molding

Correct Answer: B. Blow molding

Question 4

For a shear-thinning polymer represented by τ = Kγ̇ⁿ, which condition applies?

A. n = 2

B. n = 1 only

C. 0 < n < 1

D. n < 0

Correct Answer: C. 0 < n < 1

Question 5

Which process is especially suitable for large, low-volume hollow thermoplastic parts made at low pressure?

A. Rotational molding

B. Injection molding

C. Autoclave molding

D. Pultrusion

Correct Answer: A. Rotational molding

Question 6

A sink mark is most likely to occur near which feature?

A. A uniform thin wall

B. A thick boss or section

C. A properly vented edge

D. A cooled ejector pin only

Correct Answer: B. A thick boss or section

Question 7

In a fiber-reinforced composite, the matrix primarily

A. provides all longitudinal strength

B. transfers load and protects the reinforcement

C. eliminates directional properties

D. acts only as a temporary mold release

Correct Answer: B. transfers load and protects the reinforcement

Question 8

Which composite process injects resin into dry reinforcement contained in a closed mold?

A. Resin transfer molding

B. Hand trimming

C. Extrusion blow molding

D. Vacuum metallizing

Correct Answer: A. Resin transfer molding

Question 9

In a laminate, ±45° plies are particularly important for carrying

A. in-plane shear and torsional loads

B. only through-thickness compression

C. mold-closing force

D. thermal radiation

Correct Answer: A. in-plane shear and torsional loads

Question 10

Why is mechanical recycling generally easier for thermoplastics than for thermosets?

A. Thermoplastics contain no additives

B. Thermoplastics can be softened and remelted

C. Thermosets always contain fibers

D. Thermosets dissolve in water

Correct Answer: B. Thermoplastics can be softened and remelted