Should I Thermoform or Injection Mold My Part?

Deciding whether to thermoform or injection mold your plastic part¹ hinges on understanding your project’s needs—part size, complexity, production volume, and budget all play a role. Thermoforming² heats a plastic sheet and shapes it over a mold, making it great for large, simple parts with low to medium runs. Injection molding, on the other hand, injects molten plastic into a mold under high pressure, excelling at small, intricate parts with high production volumes.

Thermoforming is ideal for large, simple parts with lower volumes, while injection molding³ suits small, complex parts with high precision and scalability.

This guide breaks down both processes, exploring their applications, pros and cons, workflows, and design considerations to help you choose the best method for your part.

What is Thermoforming and Injection Molding?

These two processes transform plastic into functional parts, but their methods and outcomes differ significantly.

Thermoforming shapes heated plastic sheets over a mold, while injection molding fills a mold with molten plastic under pressure.

Thermoforming

Thermoforming heats a thermoplastic sheet until it’s pliable, then uses vacuum or pressure to form it over a single-sided mold. Once cooled, the part is trimmed to its final shape. It’s perfect for large, shallow items like packaging trays, automotive panels, and signage.

Injection Molding

Injection molding melts plastic pellets and injects them into a two-sided mold under high pressure. After cooling, the part is ejected, revealing intricate details and tight tolerances. It’s the go-to for small, complex parts like electronic housings and medical components.

How Do Thermoforming and Injection Molding Differ in Applications?

The right process depends on what you’re making and where it’s headed.

Thermoforming shines in large-scale, simpler applications like trays and panels, while injection molding dominates precision-driven fields like electronics and medical devices.

Thermoforming Applications

• Automotive: Dashboards, interior trim.

• Packaging: Blister packs, food trays.

• Signage: Large displays.

• Medical: Trays, non-precision housings.

Injection Molding Applications

• Consumer Electronics: Phone cases, appliance parts.

• Automotive: Clips, connectors.

• Medical Devices: Syringes, surgical tools.

• Toys: Detailed figures, game pieces.

What are the Pros and Cons of Thermoforming vs. Injection Molding?

Each process has strengths and trade-offs that impact your decision.

Thermoforming offers low startup costs and quick turnarounds, while injection molding delivers precision and cost efficiency at scale.

Factor	Thermoforming	Injection Molding
Volume	Best for 250–5,000 parts	Ideal for 5,000+ parts
Tooling Cost	Low (aluminum molds)	High (steel molds)
Lead Time	6–12 weeks total	12–24 weeks total
Per-Part Cost	Higher at scale	Lower at scale
Complexity	Simple to moderate shapes	Complex, precise shapes
Material Waste	Trimmable, reusable excess	Runner/gate waste, recyclable
Material Variety	Sheet plastics (PP, PET)	Wide range (ABS, PC, nylon)

Thermoforming Pros

• Affordable tooling.

• Fast setup and production.

• Great for large, simple parts.

• Minimal waste with reusable trimmings.

Thermoforming Cons

• Costly per part in high volumes.

• Limited to sheet plastics.

• Less precision for intricate designs.

Injection Molding Pros

• High accuracy and repeatability.

• Broad material options.

• Economical for large runs.

• Handles complex geometries.

Injection Molding Cons

• Expensive initial tooling.

• Longer lead times.

• Higher waste from runners/gates.

What are the Key Steps in Thermoforming and Injection Molding?

Knowing how each process works helps you anticipate challenges and optimize results.

Thermoforming heats, forms, cools, and trims, while injection molding melts, injects, cools, and ejects.

Thermoforming Steps

1. Heat Sheet: Warm a plastic sheet (e.g., PET) until pliable.

2. Form: Use vacuum/pressure to shape it over a mold.

3. Cool: Let it solidify.

4. Trim: Cut away excess material.

Injection Molding Steps

1. Melt Plastic: Heat pellets (e.g., ABS) in a barrel.

2. Inject: Force molten plastic into a mold.

3. Cool: Allow it to harden.

4. Eject: Remove the finished part.

How Do Materials Affect Thermoforming and Injection Molding?

Material choice⁴ shapes the process and part performance.

Thermoforming uses sheet plastics like PP and PET, while injection molding handles a wider range, including ABS and PC.

Thermoforming Materials

• Polypropylene (PP): Flexible, used in packaging.

• Polyethylene Terephthalate (PET): Clear, for trays.

• Polyvinyl Chloride (PVC): Rigid, for signage.

Uniform sheet thickness is vital to avoid weak spots.

Injection Molding Materials

• ABS: Tough, for electronics.

• Polycarbonate (PC): Strong, for lenses.

• Nylon: Durable, for gears.

Flow and shrinkage properties must align with mold design.

What Design Considerations Should I Keep in Mind?

Good design prevents defects and ensures functionality.

Thermoforming needs draft angles and uniform thickness; injection molding requires consistent walls and ejection-friendly angles.

Thermoforming Checklist

• Draft Angles⁵: 1–5° for demolding.

• Thickness: Even to prevent thinning.

• Radii: ≥0.125 inches to avoid tears.

Injection Molding Checklist

• Wall Thickness: 0.080–0.120 inches, uniform.

• Draft Angles: 1–2° for ejection.

• Ribs: Depth ≤ 3x wall thickness.

How Do I Decide Between Thermoforming and Injection Molding?

A clear framework guides your choice.

Weigh part size, volume, complexity, and cost to pick the right process.

Decision Guide

• Size: Thermoforming for parts >1m².

• Volume: Thermoforming <5,000; injection molding >5,000.

• Complexity: Injection molding for tight tolerances (<0.005 inches).

• Budget: Thermoforming for low startup; injection molding for scale.

What Related Technologies Should I Know?

Context matters—other processes and steps influence your choice.

Upstream: material production and mold-making; downstream: assembly and finishing.

Upstream

• Material Prep: Sheets for thermoforming, pellets for injection molding.

• Mold Creation: CNC or 3D printing.

Downstream

• Assembly: Joining parts.

• Finishing: Painting, polishing.

Alternatives

• Blow Molding⁶: Hollow items like bottles.

• Rotational Molding: Large, seamless parts.

Conclusion

Thermoforming suits large, simple parts with lower volumes and quick setup, while injection molding excels at small, complex parts with high precision and cost efficiency at scale. Assess your part’s size, complexity, volume, and budget to choose wisely.