Injection molding and rotational molding are two common methods used in the manufacture of plastic parts and products. They each have their unique advantages and are suited to different applications. Let's take a closer look at the comparison of these two methods in terms of materials, production time, and some other factors.

Injection Molding 

Injection molding is a manufacturing process for producing parts by injecting molten material into a mold. This process is often used in mass-production and to create intricate parts.

 Materials:

Injection molding can use a wide range of materials, including thermoplastics such as polyethylene, polypropylene, ABS, and nylon, as well as thermosetting plastics, elastomers, and metals.

Production Time:

Injection molding is generally faster than rotational molding. Once the mold is designed and made, the injection molding process can produce a large number of parts in a short period. However, the time taken to create the mold can be lengthy and expensive.

Rotational Molding 

Rotational molding (also known as rotomolding or rotocasting) is a production method typically used to create large, hollow, one-piece plastic products.

Materials:

Rotational molding primarily uses powdered or micropellet thermoplastic polymers, such as polyethylene or PVC. It's less versatile in terms of materials compared to injection molding.

Production Time:

While the time to create a mold for rotational molding is generally less than for injection molding, the time to produce an individual part is longer. The rotational molding process involves slow rotation of the mold while heating it to melt the plastic and cooling it to create the part. This process is slower than injection molding.

Comparison:

- Cost: Injection molding has high upfront costs due to the need for robust molds but can be economical for large production runs. Rotational molding has lower mold costs and is more economical for small to medium-sized production runs or larger, hollow parts.
  
- Design Flexibility: Injection molding can produce complex parts with intricate designs, while rotational molding is suited to simpler, hollow designs.
  
- Waste: Injection molding can produce a significant amount of waste material, especially during color changeovers or startup, whereas rotational molding virtually eliminates significant waste because only the required amount of material is used to make the part.
  
- Finish & Appearance**: Injection molding can achieve a high-quality surface finish and dimensional accuracy. Rotational molding may have minor imperfections such as color or thickness variations.

It's important to note that the choice between injection molding and rotational molding will depend on the specific requirements of the project, including the design of the part, the materials used, the production volume, and the budget.