Hot-Plate Welding Design Guidelines

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Weld Joint Design

A well-designed joint is critical to achieving a robust hot-plate weld. When optimized, a hot-plate weld joint should be able to achieve strengths comparable to that of the parent material. Use the following examples to properly design your weld joint for hot-plate welding.

Un-welded components

A properly designed weld joint should include sacrificial material that will be displaced during the melt (red) and seal (gold) phases of the welding cycle.

Welded assembly

When the joint is welded together, the displaced material will extrude out either side of the joint. The extruded material is called flash.


Welded components should be overlapped in computer modeling (CAD). The overlapped material will allow for displacement during welding later.


  • Assembled components should be shown in the data with the weld ribs overlapping one another and with the final weld dimensions, see welded overlap above.
  • For assemblies where large dimensional variations may be possible, more melt may be required to ensure contact is complete prior to beginning the melt phase.
  • Minimize welding angles as much as possible (max recommended 30 degrees); a flat weld plane normal to the direction of pressure is ideal.
  • Always keep the top surfaces of weld ribs parallel with each other, see weld surface design below.
  • Parts should be designed so that hot-plate welding tools can support directly under or adjacent to the weld rib to ensure that proper melt and seal force can be applied.
  • A capable weld joint consists of: two weld ribs which oppose each other; and features for flash control, alignment, and tooling.
  • The measurements shown in the weld joint design above represent typical wall thickness and should be used for reference only.

Weld Surface Design

Best Design

Good Design

Poor Design

Weld Clearance Design


  • Do not locate injection molding parting lines, gates, or ejector pins on weld ribs.
  • Maintain a minimum of 1.52 mm (0.06″) gap between edge of weld rib and adjacent features (side walls, interior components, etc.) for heating tooling clearance.
  • Calculate expected displaced weld rib volume and evaluate the volume of the adjacent area to determine if the gap needs to be larger than the minimum 1.52 mm (0.06″) for displaced plastic (flash).

Flash Trap Design


  • The cross-sectional area of the flash trap must be larger than the cross-sectional area of half of the weld rib.
  • The weld rib must be tall enough so that the opposing flash trap side walls do not come in contact with each other during the seal phase.

Return Flange Design


  • For tall parts and parts with thin or unsupported walls, a return flange may be added to perimeter walls for additional tooling support and location control at the weld joint.

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