This video documents a maker's ambitious project to 3D print and assemble a full-scale replica of the $3,500 Terje Ekstrøm (Extramm) chair from 1972. The process involved complex techniques: 3D printing dozens of single-use PLA molds, casting expanding foam components, and integrating a rigid internal skeleton constructed from aluminum tubing and 3D printed PET CF joints, bypassing traditional welding.
The creator faced multiple engineering and design challenges, including issues with mold removal, errors from using old casting foam, and initial structural weakness due to failing to glue the skeleton components. The biggest hurdle was the discovery that the chair’s closed-loop design prevented the use of the intended tubular knit "sock" upholstery method. This crisis prompted a creative pivot, resulting in a unique, yarn-wrapped finish. After final structural reinforcement using 3D printed triangles, the chair was confirmed to be highly comfortable and structurally sound, ultimately earning a place in the creator's home.
The DIY Extramm Chair: 3D Printing, Foam Casting, and Upholstery Challenges
0:03 The Goal: Replicate the $3,500 Terje Ekstrøm (Extramm) chair (designed 1972) using 3D printing and custom fabrication.
1:15 Modeling: A perfect 3D model of the chair was sourced online (requiring file format conversion) and imported into Fusion 360.
1:34 Foam Casting Method: The plan involved 3D printing molds to cast the chair in pieces using expanding foam rubber, a technique derived from previous experiments replicating the Panton chair.
3:45 Skeleton Design: The internal skeleton was modeled by tracing the centerline of the chair. To avoid welding/tube bending, the creator used straight aluminum tubing segments connected by custom 3D printed joints (a method inspired by go-kart building).
4:46 Materials: 3D printed joints were made from strong PET CF (carbon fiber reinforced nylon). Molds were printed using PLA.
5:07 Initial Casting Preparation: Molds were pre-treated with mold release agent. A back leg section was cast first, revealing that the complex sliding dovetails used for mold connections made demolding impossible without destroying the plastic.
8:06 Mistakes and Corrections: The first mold contained several design errors, including an unnecessary alignment cavity. The creator used the ongoing project to debug and refine future mold designs.
10:07 Mold Destruction and Waste: Demolding subsequent pieces required breaking the molds, highlighting the uncomfortable reality of generating 10 kg of single-use plastic waste. The creator plans to recycle this plastic into a new dining table (16:16).
11:36 Upholstery Plan: The creator sourced tubular knit fabric, hoping to pull it over the foam sections like a sock to achieve a seamless upholstery finish similar to the original design.
14:04 Structural Flaw 1 (Weak Skeleton): After demolding the first half, the skeleton was "flopping like a wet noodle." The critical mistake was realizing the aluminum tubing was not secured/glued to the 3D printed joints.
15:35 Structural Fix 1: For the second half, the tubing was secured to the PET CF joints using polyurethane construction adhesive, significantly stiffening the components.
18:44 Assembly: The two halves aligned perfectly, but the chair remained flexible/bouncy even with the glued joints, likely due to the inherent flexibility of the PET CF material.
20:44 Upholstery Crisis: A major design flaw was discovered: the center segment of the chair forms a closed loop, making it impossible to pull the tubular knit fabric over the arms.
22:33 Creative Pivot: The creator abandoned the tubular knit plan and chose a third option: wrapping the entire chair in plush, bulky yarn, resulting in a unique, creature-like aesthetic.
24:14 Structural Flaw 2 (Final Failure): The official first sit test caused the chair’s legs to splay outwards, proving the frame was still functionally too weak.
25:18 Final Structural Fix: Hidden triangular reinforcements (a principle of structural engineering) were 3D printed and installed to lock the frame and prevent outward swaying. Cork circles were added to the feet for grip.
26:56 Outcome: The completed, reinforced chair was tested by the creator and his wife (Eden). It was found to be "surprisingly comfortable" and extremely stable (passing a joint weight test at 27:47).