Make TPU and PLA hold together: beam interlocking in PrusaSlicer 2.9.1

You can print PLA with TPU, or PLA with PETG, in one part, but the joint tends to peel because the two materials do not fuse. PrusaSlicer 2.9.1 (March 2025) added beam interlocking: small beams of each material, crossed like a woven mesh at the boundary, so the two sides lock mechanically while the outside stays clean. A toy with a soft tyre on a rigid hub, or a soft grip on a rigid core, prints in one go.

You need a printer that handles more than one filament (an automatic changer or multiple heads). If you are not sure what yours can do, read multi-colour printing options first.

Why interlock at all

The two usual approaches:

1. Let it fuse. Works for the same material in different colours. Pairs that do not bond peel apart easily.
2. Model a lock. Dovetails, pins, through-holes for TPU to flow into. Prusa’s announcement notes that this used to be the way. It works, but you design it for every part.

Interlocking automates option 2 at the toolpath level. It works on separate parts in one object and on materials assigned with the painting tools or modifiers.

How the woven beams work

The slicer divides the region around the boundary into a grid of cells. In each cell, material A gets one half and material B the other, as parallel beams. After the set number of layers, the beam direction turns 90°, so A’s beams sit across B’s and then the other way round, like basket weaving. You cannot pull one side out without breaking beams.

No beams are made close to the outer surface (boundary avoidance), so the visible faces keep the colour split from your model.

The settings

In PrusaSlicer 2.9.1 they sit under Print Settings → Multiple Extruders, Advanced section. Switch to Advanced or Expert mode to see them.

Setting	Default	What it does
Use beam interlocking	off	turns the woven beams on
Interlocking beam width	0.8 mm	beam width
Interlocking direction	22.5°	beam angle
Interlocking beam layers	2	beam height in layers; fewer is stronger but more prone to defects
Interlocking depth	2	how far from the boundary beams go, in cells; too few means poor adhesion
Interlocking boundary avoidance	2	how far from the outer surface no beams are made, in cells

All six defaults match the original Ultimaker Cura feature.

Worked example: cell size and how much material it takes

In Cura’s original code, cell width = beam A + beam B and cell height = 2 × beam layers. With the defaults and 0.2 mm layers:

1. Cell width = 0.8 + 0.8 = 1.6 mm.
2. Depth into each material: 2 cells = 2 × 1.6 ≈ 3.2 mm from the boundary.
3. Clearance from the outer surface: 2 cells ≈ 3.2 mm.
4. Height: beams are 2 layers = 0.4 mm, so direction flips every 0.4 mm; a cell is 4 layers = 0.8 mm tall.

Try it on a toy wheel: a 4 mm thick TPU tyre on a PLA hub, 12 mm wide.

• Take 3.2 mm off each edge of the 12 mm contact face: 12 − 6.4 = 5.6 mm of band left for beams, about three and a half cells.
• Beams reach about 3.2 mm into the 4 mm tyre, leaving under 1 mm of solid rubber outside. If the wheel takes heavy load, try depth 1 or a thicker tyre.

A 6 mm wide wheel would leave almost no room for beams. These figures are estimates from the defaults and Cura’s code; PrusaSlicer says it adapted the feature with minor changes. Always check the preview to see where beams actually appear.

The joint face must be wide and thick enough for beams to fit. Give two-material parts a wide contact face from the start, in CAD.

Compared with other ways to join

Method	How it holds	Good for	Limits
Plain fusion	plastics melt together	same material, different colours	dissimilar pairs peel
Beam interlocking	woven beams inside the part	PLA+TPU, PLA+PETG with wide contact faces	needs enough width and thickness
Modelled lock	dovetails, pins, through-holes	thin parts, narrow joints	designed by hand for each part
Print separately and join	glue, screws, press fit	big parts, single-filament printers	extra assembly step

For glue and dovetails, see joining printed parts.

Your first test

1. Set the model up as two parts (or paint materials onto one part) and assign PLA and TPU.
2. Print Settings → Multiple Extruders → turn on Use beam interlocking. Start with the defaults.
3. Pick a bed temperature both materials accept. PrusaSlicer 2.9.1 also added a setting that helps choose a bed temperature from the filaments in a multi-material job.
4. Slice and scrub through the layers in the preview at the joint. You should see two-colour beams crossing, alternating by layer.
5. Print two small test pieces, interlocking on and off, and pull them apart by hand. Then print the real thing.

Prusa’s own examples are a shopping-bag carrier with a partly flexible handle and a cable grommet with a soft centre and a rigid outer frame.

Common mistakes

• Staying in Simple mode. The Use beam interlocking box is hidden.
• Narrow faces or thin parts. The edge clearance eats all the room and beams may not appear at all. Check the preview.
• Too much depth in thin TPU. Beams eat into the rubber and leave a thin, uneven skin.
• Dropping beam layers to 1 before your TPU prints reliably. Stronger in theory, more defects in practice.
• Using it on one material in two colours. Extra filament changes for no gain.

Sources

• PrusaSlicer 2.9.1: smarter sequential printing, stronger multi-material interlocking — Prusa Blog
• PrusaSlicer 2.9.1 PrintConfig.cpp (interlocking defaults) — GitHub
• Cura fdmprinter.def.json (Generate Interlocking Structure) — GitHub
• CuraEngine InterlockingGenerator.cpp (cell size) — GitHub