Injection molds are typically more expensive than rotomolding molds for several reasons:

1. Complexity and Precision: Injection molds are designed to produce parts with high precision and intricate details. The process involves injecting molten plastic into the mold cavity at high pressure, resulting in highly accurate and detailed finished products. This requires the mold to be manufactured with high precision and often includes multiple moving components. In contrast, rotomolding molds are simpler in design since the process relies on rotational movement to evenly coat the mold's interior with melted plastic, producing parts with less precision.
   

2. Material and Construction: Injection molds are usually made from hardened steel or aluminum alloys to withstand the high pressures and temperatures of the injection molding process. These materials are more expensive and require specialized machining techniques. Rotomolding molds can be made from materials like aluminum or less expensive steel, and they don't need to endure the same extreme conditions, resulting in lower material and construction costs.

3. Cavities and Complexity: Injection molds can have multiple cavities, allowing for the simultaneous production of multiple parts in each cycle. This increases production efficiency but also adds complexity to the mold design and manufacturing process. Rotomolding molds typically produce one part at a time, simplifying the mold design and reducing costs.

4. Tooling and Design: The design and engineering of injection molds involve intricate details such as gates, runners, and ejector pins to facilitate the injection molding process and part removal. These features require careful design and precision machining. Rotomolding molds have fewer design complexities, as the rotational process relies more on the shape of the mold and the material's flow properties.
   

5. Production Volume: Injection molding is more suitable for high-volume production runs due to its speed and efficiency. The higher cost of injection molds can be amortized over a large number of parts, making the cost per part more reasonable. Rotomolding, on the other hand, is better suited for lower to medium production volumes, making the cost of mold creation relatively higher on a per-part basis.

6. Lead Time and Labor: The manufacturing process for injection molds involves intricate machining, polishing, and fitting of various components. This labor-intensive process contributes to the higher cost of injection molds compared to the simpler construction of rotomolding molds.

Overall, while injection molds may have a higher initial cost, they offer advantages in terms of precision, complexity, and production efficiency, which makes them a preferred choice for high-quality, high-volume production of plastic parts. Rotomolding molds are better suited for simpler designs and smaller production volumes, offering a more cost-effective solution for certain applications.