Thermoforming vs. Injection Molding: What’s the Difference?

Choosing between injection molding and thermoforming is a key decision for businesses developing custom plastic parts. These two manufacturing processes, while both shaping plastics, differ significantly in their mechanics, applications, and costs. This article compares injection molding vs thermoforming, exploring their processes, key differences, thermoforming vs injection molding cost, and decision-making criteria.

What is Injection Molding?

Injection molding is a molding manufacturing process where molten plastic is injected under high pressure into a complex mold to form detailed, high-strength parts. The process begins with melting thermoplastic or thermoset pellets, injecting the material into a steel or aluminum mold, and cooling it to solidify the part, which is then ejected with minimal post-processing. Known for tight tolerances (±0.005 inches), injection molding excels in high-volume production of intricate components.

What is Thermoforming?

Thermoforming involves heating a thermoplastic sheet until pliable, then forming it over a single-sided mold using vacuum, pressure, or mechanical force. After shaping, the part is cooled, trimmed, and finished as needed. Producing large, simpler parts with looser tolerances (±0.020 inches), thermoforming is ideal for low to medium production volumes.

There are 3 common methods of thermoforming used to shape heated plastic sheets into desired forms:

1. Vacuum Forming

In vacuum forming, a plastic sheet is heated until pliable and then placed over a mold. A vacuum is applied to suck the air out from between the sheet and the mold, causing the sheet to conform to the mold’s shape. This method is cost-effective, fast, and suitable for low to large-volume production, especially when high precision is not critical.

2. Pressure Forming

Pressure forming is similar to vacuum forming but adds positive air pressure on top of the heated plastic sheet, pushing it more firmly against the mold. This results in better detail, uniform thickness, and higher quality surface finishes.

3. Twin Sheet Forming

Twin sheet forming involves heating two plastic sheets simultaneously and forming them over two molds (one male, one female). The sheets are then fused together around the edges with air pressure injected between them to create hollow or double-walled parts. This method is ideal for manufacturing hollow items.

Key Differences Between Injection Molding and Thermoforming

The difference between thermoforming and injection molding spans several critical factors, from tooling to applications. Understanding these distinctions helps manufacturers choose the right process for their project, balancing thermoforming vs injection molding cost and performance.

Tooling and Mold Design

Injection molding relies on complex, dual-sided molds made from steel or thick aluminum alloys, designed to withstand high pressures and repeated use. These molds are costly and take months to design and build, increasing upfront investment. Thermoforming, in contrast, uses simpler, single-sided molds, typically made from affordable aluminum. These molds are faster and cheaper to produce and easier to modify, making thermoforming ideal for rapid prototyping and design iterations.

Production Volume

Injection molding process leverage multi-cavity molds to produce thousands of parts quickly. So, despite high initial tooling costs, it becomes cost-effective for large quantities. Thermoforming suits smaller production runs, typically hundreds to thousands of parts annually, due to lower tooling costs.

Part Size and Complexity

Injection molding excels in producing small, highly detailed parts with tight tolerances (±0.005 inches), supporting complex features like ribs, metal inserts, screw threads, and intricate geometries. Thermoforming is better suited for larger parts with simpler shapes and looser tolerances.

Material Forms and Waste

Injection molding uses plastic pellets melted and injected into molds, minimizing material waste through precise control. Thermoforming, however, relies on plastic sheets, which generate more waste due to trimming and reworking.

Lead Times and Flexibility

Injection molding requires longer lead times, often months, due to complex mold design and fabrication. Mold modifications are costly and time-consuming, limiting flexibility. Thermoforming offers shorter lead times, with simpler molds produced in days to weeks, enabling rapid prototyping and easy design changes.

Cost Considerations

Injection molding involves high upfront tooling costs but lower per-part costs at high volumes, making it economical for mass production. Thermoforming’s lower tooling costs make it more cost-effective for low to medium volumes, especially for larger parts where tooling savings are significant.

Thermoforming vs Injection Molding, Which One is More Cost-Effective?

Injection molding incurs high upfront tooling costs, with complex steel molds ranging from $10,000 to $100,000, but its automated, high-volume production yields lower per-unit costs. Material use is efficient, though high-performance plastics like polycarbonate can be pricey.

Thermoforming’s simpler aluminum molds cost $1,000 to $10,000, reducing setup expenses, but sheet trimming increases material waste, and per-unit costs are higher for large runs.

Injection molding breaks even at 1,000–10,000 units, while thermoforming is economical for 50–1,000 units or large parts. These cost dynamics guide budget-conscious decisions in injection molding vs thermoforming.

Injection Molding vs Thermoforming: How to Choose

Choosing the right plastic molding process depends on your project demands. Injection molding process is cost-effective for high volumes, tight tolerances, and durable materials. Thermoforming is suitable for lower volumes, simpler shapes, and faster, more affordable production. Working with professional manufacturer will help you to choose the right molding process for your projects. Zhongde, an experienced manufacturer in plastic parts, helps bring your design into reality. Ready to start your project? Connect with Zhongde’s dedicated team today to explore injection molding solutions and achieve outstanding results.