Thermoplastics
Thermoplastics are materials that can be repeatedly softened by heating and hardened by cooling within a specific temperature range. Polymers are composed of long molecular chains. Molecular chains of thermoplastic polymers can have linear or branched structures. Relative average molecular mass is used to characterize and measure length of polymer chains. The larger molecular weight, the greater mechanical strength of solid polymer and the higher viscosity of polymer in viscous flow state.
Polymer Aggregate Structure
Table 2-2 shows structures of some carbon chain polymers and heterochain polymers.
Table 2-2 Polymer Structures

 

Aggregation state between molecular chains within a polymer, known as aggregate structure, is also a key structural parameter of polymer. Based on arrangement of molecules, aggregate state of a solid polymer can be divided into crystalline and amorphous (i.e., non-crystalline). Crystalline state refers to linear and branched macromolecules that can be regularly arranged in three dimensions to form a crystal structure. Polymers that have a crystalline structure or are capable of forming a crystalline structure are called crystalline polymers.
In contrast, amorphous polymers are defined as polymers that cannot crystallize under any conditions. During crystal formation process, some macromolecules or macromolecular segments may not have opportunity to crystallize, forming amorphous portion of polymer. Proportion of crystalline portion in polymer is called crystallinity. Even within same polymer, structural differences affect crystallinity. For example, low-density polyethylene (LDPE) has a lower crystallinity than linear high-density polyethylene (HDPE) due to its high number of branched chains, which disrupts chain regularity.

 

Table 2-4 Common Crystalline and Amorphous Polymers

Crystallinity and amorphism are two different states of aggregation, and therefore inevitably lead to significant differences in performance.
Because molecular chains tend to curl freely at high temperatures, they stretch when subjected to external forces. These stretched chains align in an orderly manner along direction of applied force, forming an oriented state. By freezing this oriented polymer at reduced temperature, oriented structure is retained in final product.
Both oriented and crystalline states are characterized by orderly arrangement of polymer molecules. Difference is that crystalline state is three-dimensional and spontaneously forms under suitable external conditions, while oriented state is only one- or two-dimensional. If applied force comes from a single direction, molecular chains are unidirectionally oriented.
Physical States of Plastics
Polymers can exist in three physical states under different temperature conditions: glassy state, elastic state, and viscous flow state. Most plastics are characterized by glassy state at room temperature. Glassy state refers to rigidity of plastic in this state. Even though it is as hard as glass, deformation under external forces is minimal and reversible. It is logical for plastics to be used as rigid materials in this state.
When polymers are subjected to thermal processing, increase in temperature triggers two changes: first, molecular motion continuously gains energy, increasing kinetic energy. Consequently, intermolecular forces weaken, amplitude and frequency of molecular motion increase significantly. Second, volume expands, providing sufficient free space for molecular movement. Temperature at which this transition occurs from glassy state to highly elastic state is called glass transition temperature (GTT).
Molecular motion in highly elastic state undergoes a significant change. Chain segments composed of several or dozens of links in molecular chain begin to move as a moving unit. Under action of a small external force, they can have a large deformation, but deformation is still reversible. Mechanical characteristics of state are like rubber at room temperature.
Temperature when state enters viscous flow state is called viscous flow temperature. Characteristics of viscous flow state molecular motion are that the entire molecular chain can move, and there will be relative movement between chains. Flowing melt cannot maintain its own shape.

 

 

Plastic Processing Performance Evaluation
Among various factors that reflect processing performance of polymers, the most important are processing stability and processing fluidity.
(1) Processing stability. Processing stability can be understood from two aspects: { = 1 \* GB3 |①} Raw material will not undergo chemical changes that are not conducive to processing and change raw material structure during molding process; { = 2 \* GB3 |②} Flow of material is in a relatively stable state.
(2) Processing fluidity. Processing fluidity characterizes flow properties of melt under certain temperature and pressure. Shear stress, shear rate, and viscosity are the most widely used terms to describe melt flow properties.
Melt flow rate (MFR) refers to weight of a thermoplastic melt flowing through a standard die per 10 minutes under a specific temperature and load.
Spiral flow length indicates maximum flow length of a melt of a given wall thickness under a specific injection pressure. It can be used as a design parameter for minimum wall thickness and is usually expressed as FLR. The larger flow ratio of a material, the better melt flow under given process conditions and the greater possibility of designing thinner part walls. Table 2-6 shows flow ratio (FLR) of some thermoplastics.
Table 2-6 Flow Path Ratio (FLR) of Some Thermoplastics

Practical Properties of Crystalline Materials
Crystallization of polymers significantly affects performance of plastic materials. Crystallization results in a denser arrangement of molecular chains and strengthens interactions between macromolecules, thereby improving polymer's density, tensile strength, stiffness, hardness, heat resistance, solvent resistance, and permeation resistance. However, properties that rely on molecular chain motion, such as high elasticity, elongation, and impact toughness, are reduced. In addition, crystallization also has a certain impact on optical properties of plastics. Except for a few varieties, most unmodified crystalline polymers are opaque.

Below we list several of the most prominent advantages of plastics to illustrate that using plastics as preferred material for industrial design should be a logical choice.
(1) Low density. Density of plastics is generally between 0.9 and 1.4 g/cm3. Its weight can be 20% and 50% lighter than aluminum and steel respectively;
(2) Transparency and impact resistance. Many plastics have very good transparency. Organic glass with good transparency can have a light transmittance of up to 92%, and its impact strength is 250 times that of inorganic glass.
(3) Excellent molding processability, specifically manifested in: { = 1 \* GB3 |①} Multiple molding methods; { = 2 \* GB3 |②} From raw materials to finished products, complex-shaped parts can also be molded from raw materials to finished products in one go, while metal parts may require dozens of processes to process a complex-shaped part; { = 3 \* GB3 |③} Greater design flexibility;
(4) Strong material designability. There are more than 300 synthetic resins that can be used for plastics, and more than 40 are commonly used;
(5) Ideal feel, touch and visual effects;
1. Polyethylene (PE) It is a crystalline plastic polymerized from ethylene. Melt has good flow properties. Low-density polyethylene (LDPE) is produced by high-pressure method, with a low crystallinity of 45%-65%. It has good softness, fracture growth rate, impact strength and transparency. High-density polyethylene (HDPE) is produced using a low-pressure process and has a high crystallinity of 85%-95%. It possesses high mechanical strength and a high operating temperature, making it suitable for blow molding, injection molding, and extrusion of various bottles, basins, barrels, sheets, pipes, and special-shaped materials.
Design Notes:
{ = 1 \* GB3 | ①} It is not resistant to high concentrations of oxidizing acids and other strong oxidants, but is soluble in some organic solvents above 60℃.
{ = 2 \* GB3 | ②} It is best not to embed metal directly into PE plastic. Plastic surrounding metal can break and detach due to excessive load stress.
{ = 3 \* GB3 | ③} Animal and vegetable oils and mineral oils can cause PE to swell, which can cause stress cracking around mechanically stressed areas of product. This is known as environmental stress cracking (ESC) of polyethylene.
{ = 4 \* GB3 | ④} Due to its non-polar nature and low surface energy, printing and bonding are difficult.
{ = 5 \* GB3 |⑤} Shrinkage is high and directional, making injection molded products prone to warping and deformation.
2. Polypropylene (PP): This is a crystalline polymer with low density and good heat resistance. Its performance is similar to that of PE, with high molding shrinkage, good melt flowability, and outstanding fatigue resistance. Products exhibit excellent mechanical properties, high rigidity and surface hardness, and particularly exceptional flex fatigue resistance, capable of withstanding hundreds of thousands of folding and bending cycles without failure, making them ideal for hinges. Long-term operating temperatures can reach 120℃, and up to 150℃ when not subjected to external forces. PP has low water absorption and excellent chemical resistance, being resistant to acids, alkalis, salts, and many polar organic solvents below 80℃. However, PP has low low-temperature impact strength, and its glass transition temperature is around -20℃, at which it becomes brittle. PP-made structural parts, such as housings, should be considered if subjected to freezing temperatures below 0℃. Therefore, compounding or blending modification methods are necessary to improve their properties.
Design Notes:
{ = 1 \* GB3 |①} Poor sunlight resistance and prone to thermal oxidative aging. For outdoor use, antioxidants and light stabilizers must be added.
{ = 2 \* GB3 |②} Poor low-temperature impact resistance, creep resistance, and wear resistance are also poor.
{ = 3 \* GB3 |③} Avoid contact with copper. Copper salt solutions have a particularly destructive effect on PP, so copper inserts are not recommended in products.
{ = 4 \* GB3 |④} PP has a high molding shrinkage and a relatively large thermal expansion coefficient.
{ = 5 \* GB3 |⑤} Similar to PE, due to its non-polar nature, surface treatment is required for coating and bonding.
PP accounts for the largest share of plastics used in automobiles, accounting for approximately 42% of the total market. It is used in a large number of interior and exterior parts such as bumpers, instrument panels, steering wheels, battery housings, decorative panels, cooling fans, radiator covers, lamp housings, and fenders. High-impact elastomer-toughened PP is used in automotive bumper materials. In Europe, 95% of automotive bumpers are made of this material. PP's hinged properties make it widely used in boxes, covers, and container lids.
3. Polyvinyl chloride (PVC): Various additives can be added to PVC resin to produce plastic products with various properties. PVC, as an amorphous polymer, has poor melt thermal stability, a narrow molding temperature range, and is corrosive to molds. At low temperatures, PVC tends to harden and become brittle. Soft PVC is often used as a sealing and decorative material in automotive design.
4. Polystyrene (PS): This is a colorless, transparent plastic. PS is an amorphous polymer with good melt flowability and resistance to decomposition, making it easy to mold. However, its mechanical properties are average, and its resistance to heavy impact is poor. It is prone to fracture when dropped or hit. Wall thickness of plastic part should be uniform, and all joints should have rounded corners. Inserts should not be designed. Excessive residual stress in plastic part can cause stress whitening and cracking. PS has a large thermal expansion coefficient. Alternating expansion and contraction forces can cause cracking in connection base of plastic parts.
Design Notes:
{ = 1 \* GB3 |①} Low heat resistance, continuous operating temperature 60-75℃. Flammable, producing thick black smoke after ignition.
{ = 2 \* GB3 |②} Poor impact resistance, brittle, and prone to cracking.
{ = 3 \* GB3 |③} Most vegetable oils and aromatic oils can cause swelling or cracking in product.
{ = 4 \* GB3 |④} Poor sunlight resistance, prone to discoloration in sunlight.
{ = 5 \* GB3 |⑤} Similar to PE, due to its non-polar nature, surface treatment is required for coating and bonding.
PS requires material modification for greater application, such as high-impact polystyrene (HIPS), which is polystyrene toughened with styrene-butadiene rubber. Compared to PS, it has higher toughness and impact strength. SMA resin, a new type of engineering plastic produced by random copolymerization of maleic anhydride and styrene monomers, offers high thermal stability, good low-temperature impact toughness, and outstanding chemical stability. It is a new thermoplastic with great potential for development, with a growth rate of 15-20% annually. SMA will be increasingly widely used in automotive industry.
5. ABS: This is an amorphous plastic formed by copolymerization of styrene, butadiene, acrylonitrile. ABS generally has better toughness than HIPS and offers better overall mechanical properties. Its melt flowability is moderate, making it easy to injection mold. It also offers low molding shrinkage, stable product dimensions, and an operating temperature range of -40℃ to 100℃. It also exhibits excellent chemical stability, printability, dyeability, and adhesion, as well as excellent electroplating properties. Its acoustic properties make it suitable for audio equipment housings. ABS is commonly used to manufacture various automotive parts and structures, such as handles, lampshades, dashboards, grilles, and decorative parts with surface treatments such as electroplating. Instruments made with ABS panels, housings, and structural components typically have a design life of 10 years. The earliest areas to crack in instruments are housing threads, adjacent screw holes, and base, which are subject to assembly tightening forces.
Design Notes:
{ = 1 \* GB3 |①} Due to butadiene component in its composition, it has poor weather resistance and is prone to discoloration and strength loss with prolonged exposure to sunlight.
{ = 2 \* GB3 |②} is flammable, with an oxygen index of 19, and produces significant smoke when burned.
{ = 3 \* GB3 |③} has poor low-temperature impact resistance, and its toughness decreases significantly at -20℃.
Glass fiber can significantly improve mechanical properties of ABS, increasing tensile and flexural strength by 2-3 times. ASA replaces easily aged butadiene component in ABS with an acrylic ester, resulting in 6-10 times greater UV radiation resistance than ABS.
6. Polymethyl methacrylate (PMMA): This is a highly transparent amorphous plastic, commonly known as organic glass. Acrylic glass has poor surface hardness and is easily scratched by hard objects. Environmental stress cracking is another weakness of PMMA.
7. Polyamide (PA): This is a multi-variety crystalline polymer, commonly known as nylon. It is tough, wear-resistant, and fatigue-resistant, but it is highly hygroscopic. It has a high melting point and a narrow melting temperature range. Material must be thoroughly dried before injection. Melt viscosity is low, making it prone to flow and overflow during injection. Mold temperature affects crystallinity. Molding shrinkage of plastic parts is large, fluctuating. Molded products are dimensionally unstable due to moisture absorption and other factors. PA6 has excellent elasticity, high impact strength, and high water absorption. PA66 has high strength and good wear resistance.
Nylon has excellent friction and abrasion resistance and is self-lubricating. Friction coefficient for oil-free lubrication is typically 0.1-0.3. Long-term operating temperature for most varieties is generally around 80℃, with some varieties reaching 150℃. Brittle temperature generally reaches -30℃. Nylon is insoluble in most non-polar solvents, particularly gasoline, lubricants, animal and vegetable oils.
Four characteristics of polyamide product failure are observed. First, hydrolysis during high-temperature processing of PA can cause degradation. Therefore, water content in raw materials must be strictly limited. Degradation reduces average molecular weight of PA, resulting in a decrease in strength of product. Second, PA requires ample time for crystallization during curing. The higher crystallinity, the finer grain size, and the better impact toughness. To achieve certain properties, crystallinity must be carefully controlled. Third, during product's service life, water absorption can enhance toughness. Water may or may not be necessary for PA products. Controlling water content in product is crucial for preventing plastic part failure. Fourth, water content in PA products also affects their dimensions. Water absorption and expansion can alter dimensions, causing plastic to exceed tolerances.
Design Notes:
{ = 1 \* GB3 | ①} Most varieties are highly hygroscopic.
{ = 2 \* GB3 | ②} Nylon has poor toughness when used in a dry state or at low temperatures, making it particularly susceptible to stress cracking in products with metal inserts.
{ = 3 \* GB3 | ③} Because nylon's high water absorption affects its dimensional stability and creep resistance, it is not suitable for parts requiring high dimensional accuracy.
{ = 4 \* GB3 |④} It is not suitable for prolonged contact with media containing colorants, such as fruit juice, coffee, tea, and lipstick.
8. Polycarbonate (PC): This amorphous polymer offers outstanding impact strength and creep resistance, is relatively resistant to cold and heat. PC possesses excellent mechanical and electrical insulation properties, as well as good transparency. Its products exhibit minimal molding shrinkage and high dimensional accuracy. It is the second most produced engineering plastic after PA. PC's impact strength is 2-3 times that of polymethyl methacrylate (PMMA), and its creep resistance is approximately 10% of PMMA's. PC can be used for extended periods within temperature range of -100℃ to 300℃, with a thermal decomposition temperature exceeding 310℃. Its flammability is self-extinguishing. It is resistant to dilute acids at room temperature and relatively stable to alcohols, oils, and salts, with exception of formaldehyde.
PC melts have high viscosity and poor flowability. This requires high mold temperatures, making molded parts difficult to demold. High residual stresses are generated by material flow and temperature differences during injection molding. Stress cracking is particularly common around inserts in plastic parts, around molded holes, and at cross-sectional changes. External forces and other environmental factors can easily cause stress cracking. PC products have high notch sensitivity, with a low-temperature brittleness temperature below -15℃. PC's transparency, weather resistance, and impact resistance can be leveraged to create high-strength, impact-resistant transparent sheet materials to replace glass and plexiglass. Some important PC alloys, such as PC/ABS, are primarily used in automotive industry, accounting for over half of all PC applications. Typical applications include interior components such as instrument panels, instrument housings, steering column cladding; exterior components such as fenders, bumpers, wheel covers, taillight housings, body parts, and mirrors. For components requiring high weather resistance, alloys such as PC/ASA and PC/SMA can be used.
Design Notes:
{ = 1 \* GB3 |①} has low fatigue strength and is prone to stress cracking. Minimize use of metal inserts in products. If necessary, refer to relevant design guidelines.
{ = 2 \* GB3 |②} is sensitive to notches; notched parts significantly reduce impact strength.
{ = 3 \* GB3 |③} is not alkali-resistant and soluble in aromatic hydrocarbons and halogenated aliphatic hydrocarbons. Acetone can cause stress cracking in parts.
9. Polyoxymethylene (POM): This highly crystalline polymer exhibits excellent physical and mechanical properties. It is wear-resistant, water-resistant, and corrosion-resistant, with good dimensional stability. It has moderate melt flowability, a narrow melting temperature range, high thermal sensitivity, and is prone to decomposition. Glass fiber reinforcement and modification can reduce molding shrinkage. It ages rapidly after prolonged exposure to sunlight. Long-term operating temperature range: -40-100℃. High rigidity, high hardness, high elastic modulus, and good impact strength are achieved. Its specific strength and rigidity approach those of metals, with low friction coefficients and wear, excellent self-lubrication. High dielectric strength and insulation resistance, low dielectric loss, and excellent arc resistance are also achieved. Humidity has minimal impact on electrical properties. High resistance to organic solvents such as hydrocarbons, alcohols, aldehydes, esters, and ethers, as well as oils such as automotive coolant.
Design Notes:
{ = 1 \* GB3 |①} is sensitive to notches; impact strength will be significantly reduced in presence of notches.
{ = 2 \* GB3 |②} is not resistant to strong acids and oxidants and may experience stress cracking.
{ = 3 \* GB3 |③} will experience a certain degree of damp heat aging under long-term exposure to hot water.
{ = 4 \* GB3 |④} has poor radiation resistance.
In automotive industry, polyoxymethylene is primarily used in automotive engine fuel system components, various transmission components, and wear-resistant parts. Oil-containing polyoxymethylene is particularly useful in manufacture of various components subject to high-load, low-speed swinging motion, particularly ball seats in automotive steering and suspension systems.
10. Polyphenylene ether (PPO): This amorphous polymer ranks fifth in terms of volume among the top five engineering plastics. It exhibits low molding shrinkage and water absorption, and exhibits excellent flame retardancy. It requires high temperatures of 300-330℃ to melt. Various modified PPOs are used in production, with Noryl, a modified PPO blended with polystyrene, being the most widely used.
Design Notes:
{ = 1 \* GB3 |①} is similar to PC and is prone to stress cracking.
{ = 2 \* GB3 |②} is not very solvent-resistant. Under stress, mineral oils, esters, and ketones can cause stress cracking.
In automotive industry, polyphenylene ether is used to manufacture instrument panels, window frames, shock absorbers, timers, battery panels, grilles, wheel arches, connectors, fenders, integral bumpers, damping plates, rear quarter panels, front and rear fenders, tailgates, and fuel tank caps.
Table 1.1 Thermal properties of various plastics Unit: ℃

Note: TS refers to thermosetting plastics.
One of the key performance indicators for plastics replacing metals is maximum temperature. Table 1.2 lists maximum temperatures of various plastics in order of thermal performance.
Table 1.2 Maximum Temperature Range of Various Plastics Unit: ℃

Note: TS refers to thermosetting plastics.
Decomposition temperature of a plastic is temperature around which it can be cooled while retaining all its original properties.

Material selection must not only ensure functionality of plastic product but also consider processing, production, cost, and supply. Material properties generally include four aspects:
{ = 1 \* GB3 |①} Processing performance: melt flow rate, melting temperature, processing temperature range, and molding shrinkage.
{ = 2 \* GB3 |②} Mechanical properties include tensile strength, elongation at break, tensile yield stress, tensile modulus, flexural modulus (at 22℃, 92℃, 120℃, and 150℃, respectively), notched cantilever impact strength, and hardness.
{ = 3 \* GB3 |③} Thermal properties include coefficient of linear expansion (CLTE), heat distortion temperature (1.82 MPa and 0.45 MPa), thermal conductivity, and specific heat capacity.
{ = 4 \* GB3 |④} Physical properties include density, water absorption (24 hours), and dielectric strength (short-term voltage rise).
{ = 5 \* GB3 |⑤} At a higher level, modern plastic product design also requires flow curve of plastic melt, creep curves at various temperatures, and fatigue curves.
Regarding material selection, you can first provide a data sheet of raw material properties required for plastic part. Then, select type and even grade of plastic material based on this data. Material selection generally involves four steps.
{ = 1 \* GB3 | ①} Required performance items for plastic parts are proposed. In addition to processing, mechanical, thermal, and physical properties, specialized properties are also considered to achieve various functions.
{ = 2 \* GB3 | ②} A list of minimum numerical values for raw material performance items is proposed. Table 1.5 lists minimum raw material performance requirements for designing a rigid thermoplastic structural component, injection molding of plastic part has been determined.
Table 15 Material Performance Requirements for Rigid Thermoplastic Injection Parts

{ = 3 \* GB3 | ③} A preliminary list of candidate materials is selected, such as eight candidate plastic materials considered in Table 1.6.
Table 1.6 Candidate Material Selection for an Injection Molded Part

{ = 4 \* GB3 | ④} Final material selection is based on specialized performance and material cost, as shown in Table 1.7.
Table 1.7 Second Material Selection for an Injection Molded Part