Who is the gatekeeper for your designs? We get many calls from companies and entrepreneurs about new ideas. Some have a sketch on a napkin, others a photo or drawing, most have a design of some sort or another. We talk to them about getting a design in a CAD file so that we can import it and assess the design in 3D. When we are told, “oh it’s ok, I have a person that designs and has created a file,” we approach the file with the same level of scrutiny as a sketch.
One reason is that a major aspect of the design revolves around how the mold will be gated, i.e. where the plastic will enter the mold. Some designers create the part they feel answers the design need without concern for how the gate(s) will allow the plastic to fill the cavity. In some cases, they think about where they want the gate to be, but then put features in locations that will cause difficulties. Knowing and thinking about how, and more importantly where and when, the plastic will flow in a mold is crucial to predicting how your end part will perform.
As the leading edge of plastic flows through a mold it is cooling (by design). When plastic flows around features like bosses (for mounting screws) and openings (e.g. windows and ventilation slots), the plastic divides and then meets up again on the other side and creates a knit line. When the two leading edges meet up, they do not flow together like water. Because of the cooling taking place during flow, there is a definitive line between the two edges. The process can pack them tightly together, but the knit line will be a weak point compared to the plastic around it. If that knit line appears at a structurally stressed location, the part will be weaker and possibly not up to the design task. There are other mold filling issues similar to this that make gate location(s) crucial to the part fulfilling its design purpose.
The message here is that the part designer must work with the mold designer as they are designing the part, not when they have completed it. They need to consider part geometry in relation to what will happen when shooting plastic into the mold. In helping deliver the part that the designer has envisioned, the mold gating is one key to fulfilling that vision.