The phrase "are composed of plastic" is correct and usable in written English.
It can be used when describing the material or substance that makes up an object or item.
Example: "The toys are composed of plastic, making them lightweight and durable."
Alternatives: "are made of plastic" or "consist of plastic".
We therefore conclude that age-related alterations are composed of plastic-adaptive alterations in response to modified use and degenerational changes developing with age.
The developed model revealed that the bond stress was not uniform along the entire bonded length when the ultimate state was reached; instead, the bonded length was composed of plastic and elastic zones.
Aesthetically, there's something about them that's reminiscent of U-Ram Choe's cyber creatures but instead of shiny metals they're composed of plastic tubes that use the wind as their sustenance and fuel.
The objects were composed of plastic and were printed using a Dimension Elite 3D Printer (Stratasys, Inc).
The metal-stained samples used in these experiments are geometrically complex: They are composed of part plastic insulator and part heavy-metal-stained tissue with a multitude of densities.
The bodies are composed of nylon plastic shells created using selective laser sintering (SLS), stainless steel threaded rods, and aluminum end plugs.
The sides and top of each arm are composed of clear plastic to allow mice to use visual cues in the room to spatially navigate the maze.
In the case of the EN-Vs, that philosophy translates into bodies that are made from carbon-fibre composites and doors that are composed of polycarbonate plastics.Automated driving, moreover, takes the EN-Vs to a new level of sophistication.
In addition, some food and drink containers are composed of polycarbonate plastics [ 9, 10].
