How dumbbells are made

One dumbbell patent presses rubber sheet around each iron head at about 170 °C, one head at a time.

stations: 8
things upstream: 52
raw sources: 20
steps from the ground: 8

The short version

Fixed rubber dumbbells begin as cast iron heads with a hole through the center and a separate handle knurled for grip. Heads and handle are joined into one piece, then a head goes into a mold packed with rubber sheet and is pressed hot under a hydraulic ram. The bar is flipped so the other head gets the same treatment, and a blade trims the flash.

Cast the hex heads

Iron is cast into hexagons, each with a hole straight through it for the handle. One patent complains that weight and size are hard to control during casting, and that rejects run high.

Goes in
Pig iron
Knurl the handle

The handle is machined separately and its grip is knurled into a non-slip texture. One maker contours the bar to the palm.

Goes in
Steel
Chrome the handle

The handle takes a chrome finish on rubber hex models such as Body-Solid's.

Goes in
Chromium
Weld heads to bar

In one Chinese patent the handle and both heads are welded into one solid body. Another patent threads the handle through the plate instead, after its coating has gone on.
Pack rubber around iron

The bar is clamped upright in a slot on the machine table with one head on top. A sleeve goes around that head and sheet rubber is stuffed into the gap until it overflows. Molded letters and numbers for the weight marking sit in a top cap.

Goes in
Vulcanized rubber
Hot-press the cover

A hydraulic ram pushes the top cap down, and the rubber flows tight around the iron while the marking fuses into its end. Pressure is released in steps to let air out, up to 22 times in one patent.

one patent: 170 °C, 100 MPa, 15 minutes
Flip and repeat

The ram retracts, the bar slides out and the mold parts come off in order. The bar is turned upside down and the second head goes through the same packing and pressing.
Trim the flash

A blade slices off the thin fin of rubber squeezed out at the mold joints, leaving the finished dumbbell.

End of the lineDumbbell

Worth repeating

The ancestor of the dumbbell was the halteres, weights that ancient Greek athletes held to jump farther.
The name comes from bell-ringing. In the 17th century, silent weighted devices copied the motion of a church bell so people could exercise at home.
Cheap fixed dumbbells may be a plastic shell filled with concrete instead of iron.