Plastic injection molding, as a process, has many parameters but none so important as the right design. Design is particularly important for rapid plastic injection molding, (RIM) where speed is of the essence, as is quality. This post is a top-level examination of many of the vital design considerations that must be taken into account during rapid injection molding.
In a typical RIM process, CAD models are sent directly to the factory floor where mold manufacturing starts. In most instances, molds are made from aluminum, not steel. Aluminum allows for quicker and cheaper tooling compared to conventional steel molds.
RIM can handle side-action and hand-load inserts, in addition to simple over-molding and insert molding. Judicious application of electrical discharge machining (EDM) can optimize mold features such as corners and edges. Several surface finish options are also available with RIM. All of the foregoing means RIM can make parts in a few weeks as opposed to the months — sometimes more than 12 months — needed for traditional injection molding.
From left to right, the components of a RIM press include: ram (1), screw (2), hopper (3), barrel (4), heaters (5), materials (6), nozzle (7), mold (8), and part (9).
Here are some typical RIM applications:
- Get thousands of parts within days
- Iterate quickly with rapidly-built prototypes
- Manage demand variability
- Test functions during product development with production-grade parts
- Test several different plastics
- Test several different CAD designs
- Utilize bridge tooling
- Implement low-volume production for high-demand parts
- Manage demand variability
Critical Part Features For RIM
- Wall Thickness: Perhaps the most crucial design element for optimal molded parts is to constant wall thickness. Optimal wall thickness minimizes the potential for warped or distorted parts.
- Core Geometry: You should core out parts to avoid thick walls at all costs. With minimal wall thickness, you get the same functions as a well-molded part with thicker walls. Unnecessary thickness can warp part dimensions, reduce strength, and increase the need for post-process machining.
- Ramps: Molders should eliminate sharp transitions that cause molded-in stress.
- Fillets: You should design features that support themselves.
There are several other part features which impact RIM but those will have to wait for a future blog post.