Automotive Plastics Guide: Types, Uses & CNC Material Selection in the Automotive Industry

Engineering plastics for automotive applications are the ideal choice for achieving lightweight design, cost reduction, and enhanced design flexibility. Dozens of high-performance engineering plastics on the market (such as POM, PA66, PEEK, PPS, etc.) possess unique performance characteristics while sharing common advantages over metals: they are lighter to improve fuel efficiency, more cost-effective and recyclable to reduce production costs, and can be modified with chemical additives to enhance UV resistance or structural strength. 

 

Overview

 

• This article delves into common types of automotive plastics, their key properties, and suitable components. 
• We also illustrate how to select the right material for a coolant distribution block among several viable options based on specific project conditions. 
  
• Furthermore, we share a case study: how our engineering team recommended the optimal material for a thermostat manifold connector based on operating environment demands, ensuring the plastic threads avoid "pull-out" under 2 bar thermal cycling pressure, while meeting strict Ra 1.6 sealing and dimensional stability requirements after CNC machining. 

 

I. Types of Plastics Used in the Automotive Industry and Their Applications

 

The automotive industry extensively uses various plastic materials to meet the requirements of different components.

 

 

Polypropylene (PP)

 

• Characteristics: Polypropylene is an excellent thermoplastic synthetic resin with chemical resistance, heat resistance, electrical insulation, high strength mechanical properties, and good wear-resistant processing performance.
• Applications: Widely used in automotive interior and exterior components and functional parts such as dashboards, door panels, bumpers, etc.

 

Polyurethane (PUR)

 

• Characteristics: Polyurethane foam has high usage temperature and compression performance and is easy to construct. Reactive polyurethane hot melt adhesives have good initial adhesion, excellent curing performance, and durability.
• Applications: Widely used in the bonding and sealing of automotive interior parts, headlights, speakers, filters, and windshields.

 

Polyvinyl Chloride (PVC or Vinyl)

 

• Characteristics: Excellent corrosion resistance, heat resistance, flame retardancy, mechanical properties, insulation properties, and plasticity.
• Applications: Used in underbody coatings, sealants, floor modules, wire harnesses, passenger compartment components, and exterior parts.

 

Acrylonitrile Butadiene Styrene (ABS)

 

• Characteristics: ABS has high strength and hardness, good impact resistance, chemical stability, and heat resistance.
• Applications: Commonly used in the automotive industry for body shells, instrument housings, and light covers.

 

 

 

Polyamide (PA/Nylon 6/6, Nylon 6)

 

• Characteristics: Polyamide has high mechanical properties, heat resistance, and wear resistance.
• Applications: Often used in engine peripheral parts, pipelines, etc.

 

 

 

Polystyrene (PS)

 

• Characteristics: High transparency, good fluidity.
• Applications: Used in the automotive industry to manufacture instrument housings, light covers, and other components.

 

 

Polyethylene (PE/HDPE)

 

• Characteristics: Low cost, corrosion resistance.
• Applications: Used in the automotive industry for fuel tanks, air ducts, and interior parts.

 

 

 

 

Polyoxymethylene (POM)

 

• Characteristics: High strength, wear resistance.
• Applications: Commonly used in the automotive industry for valves, impellers, and other components.

 

 

 

 

Polycarbonate (PC)

 

• Characteristics: Dimensional stability, UV resistance, toughness, high strength.
• Applications: Used in the automotive industry for light covers, windscreen frames, and car windows.

 

Acrylic (PMMA)

 

• Characteristics: Good light transmittance.
• Applications: Used in the automotive industry for lighting signboards, car door windows, etc.

 

 

 

 

Polybutylene Terephthalate (PBT)

 

• Characteristics: Excellent heat resistance, chemical resistance.
• Applications: Used in the automotive industry for electrical connectors, heat exchangers, and other parts.

 

Polyethylene Terephthalate (PET)

 

• Characteristics: Corrosion resistance, impact resistance.
• Applications: Used in the automotive industry for fuel tanks, coolant tanks, and other components.

 

Acrylonitrile Styrene Acrylate (ASA)

 

• Characteristics: Weather resistance, UV resistance, impact resistance.
• Applications: Used in the automotive industry for exterior components, bumpers, etc.

 

These types of plastics cover a wide range of applications in the automotive industry, from interiors to exteriors, functional parts to decorative parts, meeting the requirements of performance, cost, and environmental considerations.

 

 

 

 

 

II. Why Use Plastics in the Automotive Industry?

 

The primary reasons for choosing plastic materials in the automotive industry are their unique performance advantages. The lightweight nature of plastics helps reduce vehicle weight and improve fuel efficiency. Additionally, the corrosion resistance of plastics ensures stable performance in harsh environments. The ease of machining plastic materials allows automotive manufacturers to design more complex structures and shapes, providing more possibilities for innovation.

 

 

 

 

 

Safety and Comfort: Plastics have excellent impact resistance and shock absorption, improving ride comfort and ensuring safety.

Weight Reduction: Plastics are lighter than traditional metal materials, helping to reduce vehicle weight and improve fuel economy.

Minimal Corrosion Damage: Unlike metals, engineering plastics will not rust or corrode, ensuring stable performance and long-term durability even in harsh environments.

Reduced Energy Consumption: Lightweight plastic materials help reduce energy consumption.

Further Creativity and Innovation: Plastics are easy to process and shape, allowing for the design of complex shapes and structures.

Flexibility in Component Integration: Plastics can be combined with other materials (such as metal, rubber) for integrated design. Compared to metals, engineering plastics are softer and easier to CNC machine into complex geometries with high precision. Furthermore, injection molding makes it effortless to manufacture a wide variety of highly complex plastic automotive components.

Cost-Effectiveness: Plastics are highly cost-effective and recyclable, which helps offset raw material costs during production and enhances the overall economic efficiency of automotive manufacturing.

 

 

 

 

III. Choosing the Right Plastic to Manufacture the Best Automotive Parts

 

Selecting the right plastic material is crucial for manufacturing high-quality automotive parts. It is necessary to consider factors such as the specific function of the parts, performance requirements, cost, environment, and processing technology. For example, components that need to withstand high impact should use plastic materials with high strength and toughness; parts that need to be exposed to sunlight for a long time should use UV-resistant plastic materials.

 

 

 

 

 

 

 

 

IV With Several Plastics For the Same Part, How Do You Decide?

 

For a CNC machined automotive part, you often have several material options to choose from. So, how do you decide? This usually requires you to weigh factors like the project phase, vehicle positioning, thermal environment, and cost targets. To demonstrate how this selection logic works in practice, let us examine an engine bay component " a Coolant Distribution Block", and see how the choice of material shifts across four distinct project scenarios.

 

 

 

 

 

Scenario A: Prototype R&D and Testing Stage

 

• Requirements:Rapid turnaround, tight tolerance testing, frequent design iterations.
• Selected Plastic:POM (Acetal/Delrin)
• Selection Rationale:POM is renowned as the most dimensionally stable plastic during precision CNC machining. It allows extremely tight machining tolerances, preventing threaded sensor ports from stripping during repeated assembly and disassembly. With a temperature resistance capable of handling short-term 90°C coolant testing, it offers low material and machining costs, meaning design changes and scrapped prototypes won't derail your project budget.

 

 

Scenario B: Standard Mass Production Vehicles

 

• Requirements:Long service life, continuous high heat resistance, and minimized unit costs.
• Selected Plastic:PA66 + 30% GF (30% Glass Fiber Reinforced Nylon)
• Selection Rationale:Adding 30% glass fiber provides superior structural rigidity, exceptional creep resistance, and the ability to withstand continuous exposure to 120°C hot coolant under constant engine vibration. While initial low-volume batches are CNC machined from modified stock bars, high-volume production transitions smoothly to injection molding, delivering the best long-term cost balance. (Note: Glass fiber increases CNC cutter wear, requiring specialized tooling strategies).

 

 

Scenario C: F1 Racing and High-Performance Supercars

 

• Requirements:Uncompromised performance, extreme weight reduction, unrestricted budget.
• Selected Plastic:PEEK (Polyether Ether Ketone)
• Selection Rationale:PEEK handles extreme continuous operating temperatures exceeding 240°C, remaining unaffected by severe engine bay radiant heat. Its high strength-to-weight ratio allows wall thickness to be minimized for extreme weight savings without compromising structural rigidity. It machines to exceptional tolerances with zero thermal distortion, though material and processing costs are premium.

 

 

Scenario D: EV Battery Cooling System Components

 

• Requirements:Native flame retardancy, zero moisture absorption, aggressive electrolyte resistance.
• Selected Plastic:PPS (Polyphenylene Sulfide)
• Selection Rationale:PPS offers outstanding chemical stability, making it immune to novel automotive coolants and battery electrolytes, paired with native V-0 flame retardancy. With near-zero water absorption, PPS maintains its precise physical dimensions under harsh, humid-thermal cycling, ensuring a long-lasting leakproof seal for critical EV cooling circuits.

 

 

 

 

V. VMT CNC Machining Factory Case Study

 

 

How We Solved Leaks and Thread Stripping in a Thermostat Manifold Connector by Material and CNC Detail Control

 

Minor material oversight or neglect of machining detail control can cause sealing failures in thermostat manifold connector components operating under high-temperature and high-pressure cycling. Let us look at how our CNC machining facility started with failure analysis, helped the client select the right plastic material, and solved the issues of coolant leakage and thread stripping in thermostat manifold connectors through precise CNC machining detail control.

 

 

1. Project Background

 

When the client approached us, the thermostat manifold connectors produced by their previous supplier had experienced thread stripping and coolant leakage caused by sealing failure during actual use.

 

Through communication, we first clarified and confirmed the specific operating requirements of the component with the client:

 

• Operating Environment:Long-term contact with 50% ethylene glycol coolant; operating temperature from -40°C to 120°C (peaking at 135°C); operating pressure from 1.5 to 2.0 bar.
• Dimensions and Mating Precision:Depth tolerance of the O-ring seal groove connecting to the engine block was required to be ±0.05 mm, with a seal surface roughness requirement of Ra 1.6.
• Assembly Requirements:Direct insertion of an M12×1.5 water temperature sensor with an installation torque of 15 Nm; external connection to a rubber hose secured with a clamp.
• Project Positioning:Small-batch (100 units) vehicle assembly and road testing for a lightweight vehicle or high-performance modified car.

 

2. Failure Cause Analysis

 

The previous supplier recommended standard unmodified Nylon 66 (PA66) for this part. We inferred that the previous supplier failed to fully understand the actual operating environment of the client, overlooking the hydrolysis characteristics and moisture absorption swell rate of standard PA66 in high-temperature aqueous environments:

 

• Hydrolysis and Creep:Under a thermal cycling pressure of 2 bar, the pipe wall of PA66 swelled and deformed due to creep after absorbing moisture, leading to loosening at the clamp connection and triggering coolant leakage.
• Thread Stripping:As the nylon absorbed moisture and softened, its thermal strength decreased. Under the combined effect of engine vibration and thermal stress, the engagement force of the sensor thread became insufficient, ultimately causing the thread to strip and pull loose.

 

3. Material Recommendation and Selection

 

Considering that this component operates in a high-temperature steam and water cycling environment:

 

If the client had a limited budget, POM (Delrin/Acetal) could be considered; If they decided to continue using nylon materials, a hydrolysis-resistant grade must be chosen (such as PA612 or specialized long-carbon-chain nylon).

 

After comprehensively comparing performance and cost, the client ultimately selected PA612.

 

 

4. CNC Machining Details and Quality Control

 

After selecting the material, we focused on controlling dimensional stability and sealing surfaces during the CNC machining process:

 

Dimensional Stability Control:

 

PA612 stock bars were pre-annealed prior to machining to relieve internal stress. The machining process was divided into roughing and finishing stages to prevent post-machining deformation caused by concentrated cutting stress, ensuring the O-ring groove depth tolerance remained stable within ±0.05 mm.

 

Sealing Surface and Tool Mark Control:

 

To achieve the Ra 1.6 surface roughness requirement and prevent microscopic leakage, we used a small-radius ball-end mill to finish the O-ring groove. The tool path direction was strictly controlled to run along the circumference of the O-ring, avoiding transverse tool marks that would cut across the sealing surface.

 

Thread Machining:

 

A CNC thread mill was used to machine the M12×1.5 sensor thread, ensuring full profile form without burrs to meet the 15 Nm assembly torque requirement.

 

 

5. Delivery and Test Results

 

• Sealing Performance:The delivered 100 PA612 parts passed air-tightness and water pressure testing at 3.0 bar with zero leakage, and the O-ring groove dimensions met the ±0.05 mm design specification.
• Assembly and Road Testing:The water temperature sensor was installed to a torque of 15 Nm with tight thread engagement. In subsequent vehicle road trials, the threads showed no loosening, and the clamp connection remained secure. The parts successfully met the client's small-batch assembly and testing requirements.

 

 

 

VI. Conclusion

 

There is a wide variety of plastics used in the automotive industry, each with its unique characteristics and application scenarios. By selecting the appropriate plastic materials, automotive manufacturers can produce lighter, more durable, and more aesthetically pleasing automotive parts, thereby improving the overall performance and competitiveness of vehicles. Furthermore, it is common for a single automotive part to have several plastic material options, which requires you to weigh factors such as the project phase, operating conditions, and costs. In many cases, simply choosing a material with matching properties is not enough to ensure success; analyzing factors like temperature, sealing requirements, and tool mark orientation is what defines an excellent custom automotive parts manufacturer worth considering as a backup supplier.

 

Need consultation on operating conditions and material recommendations for automotive parts, or solutions to resolve issues with sealing and dimensional stability during machining? Welcome to contact us for a free consultation. We are glad to provide you with manufacturing solutions, efficient DFM analysis, and quotations.[We support 2D Drawings (PDF files), 3D Drawings (IGS/STP/STEP files)].

 

 

VII. Frequently Asked Questions

 

 

Is PVC or ABS stronger?

 

Answer:PVC and ABS each have their strengths. ABS generally has higher strength and toughness, making it more suitable for parts that need to withstand greater impact. PVC, on the other hand, emphasizes good processability and corrosion resistance.

 

 

What is the best manufacturing method for automotive plastics?

 

Answer:The best manufacturing method for automotive plastics depends on the part's design, volume, and material. Common methods include:

• Injection Molding:Ideal for mass production of complex, high-volume components like dashboards and interior trims with low unit costs.
• CNC Machining:Best for high-precision, low-volume custom parts, functional prototypes, or complex components requiring tight tolerances without expensive mold costs.
• Blow Molding:Suitable for producing hollow automotive components, such as fuel tanks and air intake ducts.
• Extrusion:Best for continuous profile parts with uniform cross-sections, such as door seals and wiring conduits.

 

 

How to choose the right plastic?

 

Answer:Choosing the right plastic requires consideration of the part’s function, performance requirements, cost, and processing technology. Additionally, key indicators such as the mechanical properties, heat resistance, and corrosion resistance of the plastic should be considered. It is recommended to consult professional engineers or material experts for more accurate advice.

 

 

Which automotive plastic has the best dimensional stability during CNC machining, and what parts is it suitable for?

 

Answer:POM (Acetal/Delrin) offers the best dimensional stability during CNC machining due to its high stiffness, low internal stress, and ultra-low moisture absorption. It is ideal for precision parts like gears, valve bodies, impellers, and threaded sensor housings.

 

 

Which plastics are commonly reinforced with glass fiber, and what automotive parts are they suitable for?

 

Answer:Glass fiber is typically added to PA6, PA66, PBT, PP, and PEEK to boost mechanical strength and heat resistance. These materials are perfect for structural engine brackets, intake manifolds, cooling fan blades, and water pump housings.

 

 

What plastic materials are UV-resistant, and what automotive parts are they suitable for?

 

Answer:ASA (Acrylonitrile Styrene Acrylate), PC/PBT blends, and UV-stabilized PP or PA (with carbon black) offer excellent weatherability. They are widely used for exterior components such as mirror housings, radiator grilles, bumper trims, and cowl vents.

 

 

Which metal automotive parts are being replaced by plastics, and how much weight does this save?

 

Answer:Parts like engine intake manifolds, oil pans, valve covers, water pump housings, and suspension bushings are increasingly made from engineering plastics instead of aluminum or cast iron. This replacement typically achieves a 30% to 50% weight reduction per component.

 

 

 

Disclaimer

 

The technical information and manufacturing advice shared on the VMT website are for general guidance only. While we strive for accuracy, VMT does not guarantee that the processes, tolerances, or material properties mentioned are applicable to every specific project. Any reliance you place on such information is strictly at your own risk. It is the buyer's responsibility to provide definitive engineering specifications for any production orders. Final specifications and service terms shall be subject to the formal contract or quotation confirmed by both parties.