Generally there are two main types of plastic flower pots/planters, Thermoformed or Injection Moulded. In thermoforming, a flat sheet of plastic is heated to a pliable temperature, then moulded to shape using suction from a vacuum or both suction and pressure. There are other variations of this process, but thermoformed pots are usually lightweight and of a thinner gauge and therefore less expensive than injection moulded pots. However they can still be quite durable, often they are smooth walled and stackable. Although many pot designs include an indented base, protruded collar and rolled lip which all add some rigidity.
In injection moulding, high-quality polypropylene resin is heated to a liquid state and injected into the mould. When the plastic "cools" or "solidifies", the mould is then removed, leaving the plastic in the required size and shape. Injection moulded plastic plant pots are often thicker, firmer and stronger. Generally, Injection moulded are more costly than their thinner thermoformed counterparts. however, they are very durable and can be reused time and again.