When one thinks of manufacturing, one may think of assembly or light fabrication. Or one may think of plastic injection molding and other processes that are sometimes called “polymer processing”. Let’s focus on them in this article.
Such a process transforms raw resins into the high-performance components that power industries from consumer electronics to medical devices to aerospace.
But here’s the challenge: not all polymers behave the same under heat, pressure, or forming conditions. The processing method you choose doesn’t just affect cost or shape; it directly impacts the mechanical strength, surface finish, dimensional accuracy, chemical resistance, and long-term reliability of your product.
This guide breaks down the six main methods, explaining:
- How each method works
- How it influences product performance
- Which are the best polymers for manufacturing in these ways (and which aren’t)
- Common real-world applications
At Agilian, I help our customers grapple with choices like these every day, because the right process and material match is the foundation of a successful product. So, with that in mind, let’s look at the different polymer processing methods you may use for your product, and why you might use them.
1. Injection Molding
How it works: Molten polymer is injected under high pressure into a closed mold. Once cooled, the part is ejected. Best for high-volume, precise, and complex geometries. (Get answers to common plastic injection molding FAQs here).
Impact on performance:
- High strength and smooth surface finish from pressure packing
- Directional properties (anisotropy) caused by flow orientation
- Gate placement and cooling influence warpage and stress
Best-suited injection molding polymers: ABS, PC, PP, PE, Nylon (PA6/PA66), POM, PBT, PET. Some thermosets via reaction injection molding (RIM).
Unsuitable polymers: Very viscous or unstable polymers (e.g., certain PEEK grades, UHMWPE) and most elastomers (except specialized processes like LSR molding).
Typical products: Automotive dashboards, LEGO bricks, syringes, phone cases, appliance housings.
2. Extrusion
How it works: Melted polymer is forced through a die to create continuous profiles; think pipes, films, or sheets.
Impact on performance:
- Delivers uniform cross-sections
- Molecular orientation along length can increase tensile strength
- Cooling rate affects crystallinity in semi-crystalline polymers
Best-suited extrusion process plastics: LDPE, HDPE, PP, PVC, PS, ABS, PET (sheet), TPU, and fluoropolymers like PTFE.
Unsuitable polymers: Thermosets, polymers with poor melt strength (PLA without additives), or highly filled compounds without specialized screws.
Typical products: Plastic bags, PVC pipes, window profiles, 3D printing filament, synthetic fibers.
3. Blow Molding
How it works: A molten tube (parison) is inflated with air inside a mold to create hollow shapes.
Impact on performance:
- Thin spots reduce durability
- Stretch blow molding (e.g., PET bottles) improves strength and gas barrier
- Residual stress from internal pressure must be managed
Best-suited polymers: HDPE, LDPE, PP, PET, PVC, TPEs.
Unsuitable polymers: Brittle or rigid polymers like PS, PMMA (unless modified), and thermosets.
Typical products: Water bottles, detergent containers, fuel tanks, traffic cones, stadium seats.
4. Thermoforming
How it works: Heated plastic sheets are stretched and shaped over a mold using vacuum, pressure, or mechanical force.
Impact on performance:
- Thinning in corners reduces local strength
- Warpage or stress possible from heating/cooling
- Surface quality depends heavily on mold finish
Best-suited polymers: HIPS, ABS, PVC, PET, PETG, PP, PC, PMMA.
Unsuitable polymers: Narrow-window crystalline polymers (like HDPE) or highly filled sheets.
Typical products: Food packaging, bathtubs, aircraft interior panels, refrigerator liners.
5. Rotational Molding (Rotomolding)
How it works: Polymer powder inside a mold is heated while the mold rotates biaxially. The polymer coats the interior, cools, and solidifies.
Impact on performance:
- Uniform wall thickness possible
- Low residual stress (no pressure involved)
- Surface finish usually matte or textured
Best-suited polymers: PE (LDPE, LLDPE, HDPE), PVC plastisols, nylon (PA11, PA12), PP with stabilizers.
Unsuitable polymers: Polymers that degrade before melting (e.g., PET, PC), materials not available in powder form, most thermosets.
Typical products: Large storage tanks, playground equipment, kayaks, buoys.
6. Additive Manufacturing (3D Printing)
How it works: Layer-by-layer fabrication using FDM, SLS, SLA, or other techniques. Perfect for prototyping, custom parts, or low-volume runs.
Impact on performance:
- Layer adhesion weakens Z-axis compared to X/Y
- Porosity and parameters influence strength and thermal properties
- Strong anisotropy unless post-processed
Best-suited polymers:
- FDM: PLA, ABS, PETG, TPU, PC, Nylon, PEEK
- SLS: PA11, PA12, TPU, PP
- SLA/DLP: Photopolymer resins
Unsuitable polymers: Commodity thermoplastics like HDPE/LDPE (poor adhesion), crystalline polymers like unmodified PP, or very high-temperature materials without industrial machines.
Typical products: Prototypes, jigs, custom implants, aerospace brackets, footwear midsoles, dental aligners.
Key Takeaway
Choosing a polymer processing method is a key strategy for product success.
The wrong polymer processing methods and/or material combination leads to warpage, brittleness, or costly rework. The right choice improves reliability, manufacturability, and lifecycle performance.
At Agilian, we help innovators and manufacturers make calls related to all aspects of plastic part design and production with confidence. No matter what type of product you’re making, our team ensures that your polymer, process, and performance goals align. Learn more about how Agilian supports polymer part design and production here.
