Twin Sheet Forming

Twin sheet forming thermoforms two heated plastic sheets and fuses them into a hollow, stiff part with internal cavities and integrated features.

Overview

Twin sheet forming is a thermoforming process where two heated plastic sheets are formed in matched molds, then pressed together so the molten interfaces bond into a closed, hollow structure. The result is a lightweight part with much higher stiffness than a single-sheet form, plus the ability to create internal air gaps, channels, and double-wall sections.

Choose it for medium-to-large plastic parts that need structural rigidity, enclosed volumes, or clean “two-sided” geometry without secondary bonding. Tooling cost is higher than single-sheet forming because you’re building two tools and controlling alignment and weld areas. Detail resolution is limited compared to injection molding; deep draws, sharp corners, and tight features can thin out and print through. Expect secondary ops (trim, CNC cut, inserts, fittings) for precision holes and interfaces.

Common Materials

  • HDPE
  • ABS
  • HIPS
  • Polycarbonate
  • PETG
  • PP

Tolerances

±0.030"

Applications

  • Returnable dunnage trays and totes
  • Hollow doors and access panels
  • Equipment housings and enclosures
  • Medical device shells and carts
  • Recreational vehicle and marine storage compartments
  • Air ducts and plenums

When to Choose Twin Sheet Forming

Pick twin sheet forming for parts that need a closed, hollow structure for stiffness, insulation, flotation, or integrated air/flow cavities. It fits low-to-medium production where injection tooling is hard to justify, especially for large footprints. Plan on trimming and machining for critical interfaces and hole locations.

vs Vacuum Forming

Choose twin sheet forming when you need a rigid, double-wall or hollow part rather than a single-shell form. It eliminates many secondary bonding steps by welding the two halves in the press and enables enclosed channels and thicker structural sections. Expect higher tooling cost and more process control than vacuum forming.

vs Pressure Forming

Choose twin sheet forming when part stiffness comes from a closed section or you need two finished sides with internal volume. Pressure forming is better for sharper cosmetic detail on one side, but it still produces a single wall unless you add assembly steps. Twin sheet trades surface detail for structure and integration.

vs Injection Molding

Choose twin sheet forming for large, lightweight hollow parts where injection molds and press tonnage would be expensive or impractical. It delivers structural geometry at lower tooling investment and faster lead times for bigger parts. Injection molding wins when you need tight tolerances, high feature detail, and high volumes.

vs Rotational Molding

Choose twin sheet forming when you need faster cycles, more consistent wall control in formed areas, and defined parting/weld locations. Rotomolding handles fully enclosed 3D shapes well but often has thicker, less controlled walls and fewer crisp features. Twin sheet is a better fit for panel-like geometries with internal cavities and flat datum areas.

Design Considerations

  • Define and dimension the intended weld/bond flange areas; avoid critical features that cross the bond line
  • Use generous radii and gradual transitions to reduce thinning in deep draws and around ribs
  • Keep critical holes, slots, and gasket surfaces as post-trim/CNC features and add trim datums
  • Add alignment features and robust parting geometry to control sheet-to-sheet registration
  • Design for uniform draw depth where possible; large depth changes drive wall thickness variation
  • Specify texture and cosmetic side requirements clearly; show A-side/B-side and acceptable witness lines