In recent years, the proportion of plastics applied in automobiles has increased year by year. At present, the average plastic consumption per vehicle reaches 110 kg, accounting for 10% of the vehicle’s total weight. A wide variety of plastics are adopted in different positions of automobiles, including interior parts, exterior trim parts and functional components.

According to different service characteristics, automotive plastics are generally divided into general-purpose plastics and engineering plastics.

1. General-purpose Plastics

General-purpose plastics feature high output, good moldability and low cost, making them the most widely used plastic category. All general-purpose plastics in this section are thermoplastics, which can be processed upon heating. This category includes polypropylene (PP), polyethylene (PE), polyvinyl chloride (PVC), polystyrene (PS) and acrylonitrile-butadiene-styrene copolymer (ABS).

1.1 PP - Polypropylene

Overview: PP is commonly known as polypropene, a white wax-like material with transparency and low density. It flows well during injection molding. Its water absorption is less than 0.02%, shrinkage rate ranges from 1.0% to 3.5%, and the recommended wall thickness for finished products is 1.5–2.5 mm.
Modification methods:
a. PP is flammable with dripping burning debris and becomes brittle under low-temperature conditions. To tackle these drawbacks, appropriate additives are blended for PP modification in industrial production.
b. Flame retardants are added to improve flame resistance. To enhance low-temperature toughness, toughening materials, rigid particles or elastomers such as EPDM rubber, talc powder (TD) and glass fiber (GF) are incorporated.
Applications: Thanks to modification solutions, PP has extensive automotive applications, including instrument panels, door trims, pillar trims, bumpers, water tanks and decorative panels. The usage of PP in automobiles keeps growing.

1.2 PE - Polyethylene

Overview: PE is one of the lightest plastics. It is a translucent white waxy material with excellent water resistance, acid & alkali resistance, organic solvent resistance and great electrical insulation. Its shrinkage rate is 1.5～4.0%, and the recommended wall thickness is 1.0–2.5 mm.
Modification methods: The biggest weakness of PE for automotive use is flammability. Adding suitable flame retardants is the most common and simple method to improve its combustion performance.
Applications: Automotive PE components mainly include fuel tanks, air ducts, front seat decorative covers, air duct assemblies (HDPE), left & right vent pipe assemblies, upper/lower air vane connecting bars, and left & right foot air outlet assemblies.

1.3 PVC - Polyvinyl Chloride

Overview: Conventional PVC is a multi-component plastic with economical price and balanced mechanical properties. Different ratios of ingredients produce PVC pellets of varying hardness, classified into soft PVC and rigid PVC. Shrinkage rate: 1–1.5% for rigid PVC, 2–2.3% for soft PVC. Recommended wall thickness: 2–3.5 mm.
Applications: PVC is among the most common raw materials for automotive plastic parts. Typical products include instrument panel skins, plastic steering wheels, plastic lining panels and automotive floor materials.

1.4 PS - Polystyrene

Overview: PS, also called hard glue, is a transparent glass-like material with good fluidity and nearly no water absorption.
Modification methods: PS is inherently brittle. It is often graft-copolymerized with other substances. For example, graft copolymerization with butadiene forms HIPS; the added rubber phase greatly boosts impact strength and surface hardness. Shrinkage rate: 0.4–0.7%. Recommended wall thickness: 2–3 mm.
Applications: PS is used for instrument housings, lamp covers and lighting components in vehicles. Foamed PS can serve as packaging and thermal insulation materials for automobile roof liners. PS can also be modified and alloyed with other materials, such as SMA applied for instrument panel skeletons.

1.5 ABS - Acrylonitrile-Butadiene-Styrene Copolymer

Overview: ABS is nicknamed shatter-resistant plastic, naturally white. Shrinkage rate is 0.4–0.7%, recommended wall thickness 1.8–3 mm. It has multiple grades, excellent surface treatment performance and competitive pricing.
Modification methods: ABS suffers from poor weather resistance. Weather-resistant alternatives such as AES and ASA are widely adopted in commercial vehicles to supplement ABS.
Applications: ABS is selected for parts requiring surface finishing (such as water transfer printing and painting). ASA / AES are preferred if no surface treatment is needed. Typical automotive parts: decorative bright strips, gear shift knobs, interior opening handles, license decorative plates, rear spoiler assemblies, rearview mirror bodies, radiator grille bodies and armrests.

2. Engineering Plastics

Engineering plastics deliver superior comprehensive mechanical properties and are widely used in automotive manufacturing. This section introduces polyamide (PA), polymethyl methacrylate (PMMA), polyoxymethylene (POM), polyurethane (PU) and polycarbonate (PC).

2.1 PA - Polyamide (Nylon)

Overview: PA, commonly known as nylon, has many variants; PA6, PA66 and PA610 are widely used. Affected by thermal expansion and water absorption, PA parts have poor dimensional stability after molding. Its shrinkage rate is 1～2%, and dimensional change caused by moisture absorption after molding must be considered. Its saturated water absorption can reach up to 8%. Recommended wall thickness: 2–3.5 mm.
Applications: PA is mainly used to produce hoses (brake hoses, fuel hoses), fuel filters, air filters, oil filters, water pump housings, water pump impellers, fans, brake fluid reservoirs, power steering fluid tanks, louvers, headlamp housings and safety belt components.

2.2 PMMA - Polymethyl Methacrylate

Overview: PMMA, or acrylic / organic glass, features outstanding outdoor aging resistance and excellent light transmittance. It is suitable for transparent structural parts with certain strength requirements.
Applications: Relying on its high light transmission, PMMA is widely used for automotive lighting signs, door window components and lamp lenses.

2.3 POM - Polyoxymethylene

Overview: POM is also known as acetal plastic. Shrinkage rate: 2–3.5%. Recommended wall thickness:1.5–2.5 mm.
Applications: POM is used for glove box accessories on instrument panels, various valves (drain valves, air conditioner valves), impellers (water pump impellers, heater impellers, oil pump wheels), small gears for electrical switches & meters, handles and door latches.

2.4 PC - Polycarbonate

Overview: PC has outstanding impact toughness and creep resistance, together with good heat resistance and low-temperature performance. Shrinkage rate:0.5–0.7%. Recommended wall thickness:2–3.5 mm. Adding glass fiber improves shrinkage, mechanical strength and temperature resistance. Its rigidity increases under long-term service at about 100℃, and annealing can release internal stress. PC-ABS alloy is a common blend; it features high surface hardness, rigidity, toughness and resistance to stress cracking, with mechanical performance between PC and ABS.
Applications: PC is applied for lamp housings, wheel arch liners, instrument panel skeletons (PC+ABS), left & right air frame covers, central air frame covers (PC+ABS) and rear bumper energy-absorbing pads.

2.5 PU - Polyurethane

Overview: PU is categorized into rigid polyurethane and flexible polyurethane according to polymerization products.
Applications: Polyurethane foams are widely used in automotive interior and vibration-damping components, including coating materials, rigid PU sheets, PU elastomers, soft foam for seats, decorative parts, synthetic leather, and roof ornaments.
Flexible & rigid PU foams are the most common grades, providing vibration isolation, sound insulation, noise reduction and thermal insulation. PU can also be made into automotive coatings, adhesives and sealants.
Typical automotive PU parts: instrument panels, rearview mirrors, bumpers, seat cushions, headrests, steering wheels, instrument panel anti-vibration pads, pillar trims, front headliners, window frames, roof & side roof decorations, door liners, sun visors and rear roof trims.

3. Common Additives

Pure general or engineering plastics often fail to meet the performance requirements of engineering design. Therefore, additives are blended into plastic pellets to optimize material performance. This article introduces two widely used additives: glass fiber and talc powder.

3.1 Glass Fiber

Overview: Glass fiber reinforced plastics are produced by mixing glass fiber and other additives into neat resin to expand application scope. Most glass fiber reinforced grades are structural engineering materials for structural parts, such as modified PP, ABS, PA66, PA6, PC and POM.

Advantages

a. Glass fiber is heat-resistant, so reinforced plastics have much higher heat resistance than unfilled resins, especially nylon materials.
b. Glass fiber restricts movement of polymer molecular chains, greatly reducing shrinkage and boosting rigidity.
c. Reinforced plastics resist stress cracking and deliver improved impact performance.
d. High-strength glass fiber greatly increases tensile, compressive and flexural strength of plastics.
e. Glass fiber and additives reduce flammability; most reinforced grades become non-flammable.

Disadvantages

a. Transparent neat resin will turn opaque after glass fiber filling.
b. The material becomes less tough and more brittle.
c. Melt viscosity rises and fluidity declines, requiring higher injection pressure during molding.
d. Higher molding temperature (10℃–30℃ higher) is needed for normal injection molding.
e. Moisture absorption increases significantly. Even originally non-hygroscopic plastics absorb water after fiber filling, so drying is mandatory before injection molding.
f. Glass fiber may migrate to the part surface and create rough speckled texture. Mold temperature controllers can help polymer flow to the surface for better appearance, though it cannot reach the surface quality of neat plastic.
g. Glass fiber has high hardness. Volatile gases of additives at high temperature are corrosive, causing wear and corrosion on injection screws and molds. Anti-corrosion and surface hardening treatment are required for molds and machines.

Applications

- Body parts: Body shells, hardtops, sunroofs, doors, radiator grilles, headlight reflectors, front & rear bumpers and interior trims. This is the main application field for glass fiber composites, meeting streamlined styling and high appearance requirements, with huge development potential. The materials are mainly glass fiber reinforced thermosetting plastics, processed via SMC/BMC, RTM, hand lay-up and spray-up.
- Structural parts: Front end brackets, bumper frames, seat frames and floor panels. They offer greater design freedom, multi-functionality and component integration. High-strength SMC, GMT and LFT are commonly adopted.
- Functional parts: Require high temperature resistance and oil corrosion resistance, mainly for engine and peripheral components, such as valve covers, intake manifolds, oil pans, air filter covers, gear chamber covers, air shrouds, intake guards, fan blades, fan shrouds, heater covers, water tank parts, water outlet housings, water pump turbines and engine acoustic shields. Main materials & processes: SMC/BMC, RTM, GMT and glass fiber reinforced nylon.

3.2 Talc Powder

Overview: Talc has a unique lamellar structure. As a filler, it reduces resin consumption and greatly improves physical properties, serving as an important functional filler in plastics.

Advantages

a. Functional benefits: Talc powder optimizes multiple properties including molding shrinkage, surface hardness, flexural modulus, tensile strength, impact strength, heat deflection temperature, processability and dimensional stability.
Adding talc to ABS & PA improves heat resistance and reduces molding shrinkage. In PE, talc enhances rigidity, heat resistance and lowers shrinkage. In PP, talc increases overall strength, heat resistance, shrinkage control and rigidity.
b. Economic benefits: Talc filler significantly cuts raw material cost and improves economic efficiency.

Applications

a. When added to automotive plastics and pipe-grade engineering plastics, talc improves impact strength, high-temperature creep resistance, reduces shrinkage, and delivers superior surface quality, better visual effect and comfortable hand feel.
b. In injection molded products, talc enhances mechanical performance and reduces sink marks at gates and edges caused by shrinkage, optimizing appearance and touch.