Used to produce high-quality precision parts at large volumes and low costs, plastic injection molding offers versatile solutions for a range of applications.
While this process offers several unique benefits over other production processes, the success of an injection molded part depends on its design; with the right design, long-lasting, quality plastic parts can be made consistently and efficiently. Poor design can lead to costly and time-consuming processing mistakes.
In order to optimize the effectiveness of high-volume injection molding and maximize the precision and quality of your parts, several key design elements should be taken into account before moving on to production.
You can reduce — and even eliminate — most injection molding part defects by taking the time to lay out a smart wall-thickness design. The key is to ensure that the thicknesses of all walls are as uniform as possible, as molten plastic will seek out the path of least resistance (in this scenario, larger wall areas), leaving smaller wall areas potentially unfilled.
Ribs are used to reinforce the strength of a high-volume injection molded part. Ribs should not exceed 70% of your parts wall thickness, however, nor should they fall under 50% of wall thickness; both scenarios can cause sinking in the surface of your part. Also, be sure to pay close attention to the height of the ribs, their location, and their degree of draft for ease of ejection.
Bosses are part features serving as one component of a larger product that requires assembly. During assembly, bosses can serve as anchor locations for screws, pins, or other fasteners. These components have width and height recommendations similar to those of ribs. Pairing bosses and ribs, especially in corners, can strengthen your part and significantly reduce chances of sinking.
In high-volume injection molding, parts with extreme or abrupt geometric features can be prone to defects — molten plastic flows in the path of least resistance, and harsh angles can impede that movement. Whenever possible, all corners and wall seams should be curved, with matching interior and exterior radiuses. Smooth corner transitions allow for better plastic flow.
Weld lines — also known as knit or meld lines — occur when two plastic flows, or two sections of a single flow, meet. They occur most commonly around holes or other obstructions, with the plastic flow separating to pass them and then coming back together afterward. Every plastic injection molded part has weld lines: the goal is to design your part so that weld lines occur in locations that do not compromise your part’s strength or integrity.
Gate and Vent Placement
The gate of a large-volume injection mold is the point at which molten plastic exits the mold’s runner and enters the part cavity. Both the type of gate you choose and where you choose to place it can substantially impact your part’s quality. Vents, which allow air to escape from the mold as plastic rushes in, are similarly important; when positioned properly, vents can help minimize weld lines.
To learn more about these aspects of part design, as well as tool design and build, material selection, and part manufacturing — other factors that are very important to the success of large-volume injection molding production — download our free eBook, How to Manufacture a Perfect Plastic Part.