Twin Sheet Forming

Twin sheet forming thermoforms and bonds two heated plastic sheets into a sealed, hollow, double-wall part with high stiffness and integrated features.

Overview

Twin sheet forming heats and forms two separate plastic sheets in matched molds, then fuses them together while still hot to create a sealed, hollow, double-wall structure. The process enables internal ribs, channels, and localized bonding points, so you can build stiffness, routing, and mounting features into the plastic walls without secondary assembly.

This process fits medium to high production volumes where you need lightweight, structural plastic parts larger than typical injection molding envelopes or at lower tooling cost. It excels for panels, pallets, ducts, tanks, doors, and covers that benefit from internal cavities or sound/thermal performance. Expect moderate dimensional accuracy and good repeatability, but not tight-machined tolerances. Tradeoffs include higher tooling cost and complexity than single-sheet thermoforming, more design constraints around bond lines and draw depths, and some geometric limitations versus fully 3D processes like blow molding or injection molding.

Common Materials

  • ABS
  • HDPE
  • HIPS
  • Polycarbonate
  • PETG

Tolerances

±0.015–0.030"

Applications

  • Hollow structural panels and doors
  • Material handling pallets and trays
  • Automotive and HVAC air ducts
  • Enclosed equipment covers and shrouds
  • Plastic fuel or fluid tanks
  • Sound- and thermal-insulated door or wall modules

When to Choose Twin Sheet Forming

Choose twin sheet forming for hollow, double-wall plastic parts where you want higher stiffness, integrated internal features, or internal cavities without secondary assembly. It suits medium to high volumes, moderate tolerances, and part sizes that push beyond economical injection molding. It’s strong for applications needing lightweight structures, ducts, or panels with embedded routing or insulation space.

vs Vacuum Forming

Pick twin sheet forming over vacuum forming when you need a sealed, hollow structure or significantly higher stiffness than a single sheet can provide. It also reduces assembly by fusing two halves in-mold instead of bonding or fastening two vacuum-formed shells later.

vs Pressure Forming

Choose twin sheet forming instead of pressure forming when internal cavities, ducts, or double-wall construction matter more than sharp detail and cosmetic class-A surfaces. Pressure forming is better for one-sided, high-detail skins; twin sheet forming is better for structural hollow parts and integrated internal features.

vs Injection Molding

Use twin sheet forming instead of injection molding for large, hollow, structural parts where injection tooling would be cost-prohibitive or impossible to gate and pack uniformly. Twin sheet forming offers lower tooling cost and easier scaling for big panels, pallets, or ducts, at the expense of looser tolerances and less complex fine features.

vs Blow Molding

Choose twin sheet forming over blow molding when you need better control of external geometry, integrated flat mounting faces, or local internal bonds and ribs. Blow molding is efficient for simple bottles and tanks; twin sheet forming handles more prismatic shapes and structural panels with defined surfaces on both sides.

vs Rotational Molding

Select twin sheet forming instead of rotational molding when cycle time and surface definition matter, and wall thickness can be thinner and more uniform. Twin sheet forming offers shorter cycles and more precise external geometry, while rotational molding suits very thick-walled, heavy-duty hollow parts.

Design Considerations

  • Model both walls with sufficient draft on all formed surfaces, typically 3–5°, to allow clean release from both molds
  • Plan consistent wall thickness and avoid deep, narrow draws to reduce thinning and risk of weak bond areas
  • Define pinch-off and weld/bond lines in low-visibility, low-load regions and call them out clearly in the drawing
  • Use generous radii at corners and transitions (at least material thickness, preferably 2–3×) to maintain material flow and strength
  • Include accessible flat areas for trimming, holes, and inserts, and tolerance these realistically based on thermoforming capability
  • Communicate expected loads, stiffness targets, and any pressure/containment requirements so the shop can design internal ribs and bond patterns appropriately