The 2024 Polestar 5 won’t be the first vehicle to use adhesive bonding to join most of its aluminum unibody structure, but most of the others are low-volume models from niche brands like Lotus and Aston Martin. The upcoming Polestar 5 hatchback’s first completely in-house-designed architecture’s significance will be employing the construction technique at a higher volume than any car before. The payoff is a five-seat unitized body-chassis structure that boasts the torsional rigidity of a two-seat sportscar or supercar, yet is comparable in weight to a smaller car’s frame.
What Are We Talking About?
When developing an aluminum unibody structure, the various castings, extrusions, and stampings can be joined by spot-welding, seam-welding, riveting, epoxy-bonding, or more likely some combination of several of the above. Sorting out which to choose is a headache for any automaker that’s working with aluminum for its weight-savings and other properties.
There are structural, cost, and weight tradeoffs to each technique that must be balanced, and Polestar’s balance sheet has tipped heavily in favor of one method in particular: epoxy bonding.
Polestar 5 Advantages
The epoxy-based adhesives such as the one Polestar is using can be more “forgiving” of slight mismatches in part shapes, as the bonding material expands to fill small voids and improve surface-to-surface contact. Adhesive joins the parts uniformly along the bead, and not locally like spot welds. It’s similar to the costly laser seam weld method.
Because welding melts a small area of each part being joined and could warp the material out of its intended design, some parts must be made thicker than necessary to tolerate the high temperature, adding weight that isn’t necessary with the epxoy-bonding method Polestar is developing.
There’s also a great advantage to assembly with epoxy-bonding, since the parts don’t need to be designed to provide clearance for welding tips to access both sides of a flange, essentially reducing the complexity of the structure that’s only required because of the assembly method.
So, considerable mass can be taken out of the parts being joined by using smaller flanges in the construction, and by holistically designing the structure to leverage the strength of bonded joints and geometric sections that might not otherwise be possible if protecting for welding-tip access.
What Makes Adhesive Bonding More Labor Intense?
Most other volume-produced aluminum vehicles—like the Jaguar I-Pace—use some blend of adhesive bonding and mechanical fastening, with considerably more riveting and welding employed along with the adhesive. Polestar’s approach is similar but hopefully more streamlined.
At lower production volumes, it tends to be more cost effective and expeditious to both apply the adhesive and clamp the joined parts manually until the adhesive gets thermally cured. At the Polestar 5’s higher anticipated (five-digit) production volume, it pays to find manufacturing efficiency in the adhesive application with robotized assembly, and tool more elaborate jigs to hold certain parts through the curing process.
Certain parts are fastened with both adhesive and a mechanical fastener like self-piercing rivets, either because their structural requirements call for it, including crumple-zone crash structures, or simply to hold the parts together as they pass through the curing oven.
How Does Baking The Polestar 5 Work?
The bonded structure and its many jigs get a pass through a curing oven ahead of the painting process. This solidifies the entire unibody structure and ensures all the epoxy resins are fully set. Note that a fully bonded structure requires much less sealing than one held together largely with spot welds (to prevent dust and water intrusion), but the Polestar 5 does still require some external body sealing.
Polestar promises to quantify things like the torsional rigidity and mass, comparing each to the competition in the months to come. The Polestar 5 is expected to arrive in 2024. If you’re waiting to buy one, you can already start bragging to your friends that it’ll be built like like exotics, just like Aston Martin Rapide.
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