In plastic injection molding, one of the most important aspects of the mold design is how and where it is gated. The “gate” is the opening in a mold through which the molten plastic is injected into the final part. It is the boundary between part and scrap. The location, size, and shape of the gate can have a significant effect on everything from the structural integrity to the visual appearance of a finished piece.
Two Types of Tending: Manually and Automatically Trimmed Gates
There are two types of gates that are used in plastic injection molding: manually trimmed and automatically trimmed gates. With manually trimmed gates, an operator must separate the parts from the runners after every cycle. Manually trimmed gates are used when the material used for the part must not be exposed to the forces of automatic shearing or when the gate is too thick to be separated automatically.
With automatically trimmed gates, no human intervention is needed to separate parts from runners. When the tool opens to eject the part, it is automatically sheared from the runner. Robots and other automation equipment used with automatic trimming can also reduce scratching and labor content. This also eliminates the need for manual separating as an additional operation and ensures that cycle times remain constant.
Common Plastic Gate Designs
While gates come in a wide range of designs, four of the most commonly used are:
Edge Gate. Probably the most common gate design, it is best suited for flat parts and is located (not surprisingly) on the edge of the part. Edge gates work well with medium and thick sections, and can be used on multicavity two-plate tools. They leave a scar at the parting line.
Hot Tip Gate. Typically located at the top of a part as opposed to the parting line, this type of gate is great for conical or round shapes that require uniform flow. This type of gate requires a heated element to deliver hot material directly to the part, but it can improve flow and reduce cycle times. It leaves a small raised spot on the surface of the part.
Sub Gate. This design is the only automatically trimmed gate on the list, and it requires ejector pins. A very common gate, it has a number of variations, some with memorable names like the smiley gate and the banana gate, as well as the tunnel gate. Sub gates can be used away from the parting line, giving you great flexibility in placement. They leave a very small scar on parts.
Direct or Sprue Gate. This manually trimmed gate is used to produce symmetrical filling of single cavity molds for large cylindrical parts. Easy to design and with low maintenance costs, it produces parts that are typically lower stressed and high strength. This gate leaves a significant scar at the point of contact.
We Get Gates
In our more than two decades in plastic injection molding, we’ve seen gate designs that produced outstanding parts and others that required a trip back to the drawing board. As a result, we have significant expertise in this area and can provide tremendous insight on your mold design. Give us a call or stop by and let’s talk about your next project.