Thermoforming

RIM vs Thermoforming: Engineering the Right Solution for Large, Complex Parts

When your design requires large plastic parts, thermoforming and reaction injection molding often emerge as the two most viable manufacturing options. Both processes can produce sizable components at reasonable volumes, but the similarities end there. The fundamental differences between heating and forming sheet plastic versus injecting reactive liquid components lead to distinct capabilities that can make or break your design.

Process Fundamentals

Thermoforming starts with a heated plastic sheet draped or vacuumed over a mold surface. The process excels at creating simple, shell-like structures where uniform wall thickness suits the application. RIM injects two liquid chemical components that react and cure inside the mold cavity, allowing the material to flow into complex geometries and build varying wall thicknesses as it solidifies.

This chemical reaction happens at temperatures around 90-105°F under pressures of only 50-150 psi. Thermoforming requires heating plastic sheets to their forming temperature and uses vacuum or pressure forming to shape the part. The low-pressure, low-temperature nature of RIM fundamentally changes what's possible in part design.

Wall Thickness Capabilities

Thermoformed parts maintain the thickness of the source sheet material, typically ranging from thin-gauge applications under 0.125 inches to heavy-gauge parts up to 0.5 inches. Any variation requires bonding additional material sections together, adding assembly time and creating potential failure points.

RIM produces parts with wall thickness variations from 0.125 to 1.125 inches within a single molded component. This range comes from the Covestro RIM Part & Mold Design Guide, which documents RIM's ability to handle these thickness transitions without secondary operations. When you need structural reinforcement in specific areas while keeping other sections lighter, RIM delivers this capability as part of the molding process rather than through post-mold assembly.

Design Complexity and Features

The sheet-forming nature of thermoforming limits what designers can achieve. Inside features, mounting bosses, and structural ribs require bonding separate components after forming. Each bonded joint introduces assembly labor, potential quality issues, and stress concentrations that can compromise part integrity.

RIM molds these features directly into the part. Stiffening ribs, mounting bosses, recessed areas, and complex surface details emerge from the mold as integral components of the finished part. This design freedom extends to undercuts and side-action features that RIM's slower reaction cycle can accommodate without the time penalties that make these features prohibitively expensive in high-speed injection molding.

Dimensional Control and Repeatability

Thermoformed parts often struggle with dimensional consistency. The forming process can produce what industry professionals call "oil canning"—localized deformation or waviness in large, flat surfaces caused by process variation and the limitations of sheet forming. Tolerances tend to be looser, and part-to-part consistency becomes harder to maintain as part size increases.

RIM holds dimensional tolerances of 0.1% or less according to the Covestro design guide. The thermoset nature of polyurethane eliminates sink marks that plague other molding processes. Parts emerge from RIM molds with consistent dimensions because the material cures against the mold surface rather than being formed through mechanical deformation of heated plastic.

RIM Structural Performance

A thermoformed part derives its strength solely from the formed shell. Without molded-in ribs or reinforcement, designers must rely on the base material thickness and geometry to provide structural integrity. This limitation often forces compromises between weight, material cost, and required strength.

RIM parts can incorporate internal structures during the molding process. Ribs, gussets, and reinforcing features become part of the component rather than add-ons. The process can also accommodate glass reinforcement in the material system itself, achieving strength-to-weight ratios that thermoformed shells cannot match without extensive secondary bonding operations.

RIM Encapsulation Capabilities

Thermoforming cannot encapsulate components. Any embedded hardware, electronics, or reinforcing inserts must be added through secondary assembly operations with adhesives or mechanical fasteners.

RIM's low processing temperature protects sensitive components during encapsulation. Electronics, metal hardware, antennas, and other inserts can be positioned in the mold and completely surrounded by the polyurethane as it cures. This creates weatherproof, impact-resistant assemblies that would require complex post-mold assembly with thermoformed parts.

Tooling Considerations

Both processes offer lower tooling costs compared to injection molding, but the comparison reveals important differences. Thermoforming tools are generally less expensive initially, particularly for simple geometries. The vacuum forming process requires minimal clamping pressure, allowing simpler tool construction.

RIM tooling operates at molding pressures between 50-100 psi—more than an order of magnitude lower than thermoplastic injection molding. According to the Covestro mold design data, this enables aluminum tooling that costs 50% of equivalent steel tooling and machines more easily than the steel tools required for high-pressure processes.

The critical advantage emerges when design changes become necessary. RIM molds machined from aluminum accommodate modifications far more economically than thermoforming tools. When market feedback or engineering considerations require feature changes, RIM tooling adapts without the complete tool replacement sometimes needed for thermoforming.

Surface Finish and Aesthetics

Thermoformed parts replicate the finish of the source sheet material. This can yield acceptable cosmetics for many applications but limits the range of achievable surface textures and leaves visible texture inconsistencies at forming transition areas.

RIM parts accept paint, silk screening, and texturing with excellent adhesion. The molded surface can be finished to match specific aesthetic requirements, from high-gloss to textured surfaces that hide minor cosmetic imperfections. This finishing flexibility often proves essential for consumer-facing products or applications where specific surface characteristics matter.

Production Economics

The volume sweet spot differs between these processes. Thermoforming can be economical at very low volumes where simple geometries suit the application. RIM becomes cost-effective when part complexity increases and production volumes fall between 100 and 5,000 units annually.

For complex parts requiring assembled features, RIM's lower unit cost often offsets higher initial tooling expense. The elimination of secondary bonding operations, improved dimensional consistency, and reduced scrap rates shift the total cost equation in RIM's favor as part complexity increases.

When Each Process Makes Sense

Choose thermoforming when your design meets specific criteria: simple shell geometries, uniform wall thickness throughout, minimal internal features, very thin walls below 0.125 inches, or ultra-low volumes where tooling cost dominates all other considerations.

Select RIM when the design requires variable wall thickness, molded-in features, encapsulation of components, tight dimensional tolerances, improved structural performance, or when design changes during development seem likely. RIM particularly suits applications where part consolidation reduces assembly operations and improves reliability.

Technical Decision Framework

Engineers evaluating these processes should examine their designs against several key questions: Does the part require varying wall thickness? Do internal features need integration into the molded component? Will encapsulation protect sensitive components? What dimensional tolerances must the manufacturing process hold consistently? How likely are design changes during development and early production?

If your answers point toward complexity, integration, or tight tolerances, RIM provides capabilities that thermoforming cannot match. If the design truly needs only a simple formed shell with uniform thickness, thermoforming might serve the application at lower cost.

The manufacturing landscape continues evolving, but the fundamental physics of these processes remain constant. Thermoforming shapes heated sheets. RIM cures reactive liquids in complex molds. Understanding which capabilities your design requires leads directly to the right manufacturing decision.