The size of a gate is important because it will impact the aesthetics and performance of the parts you design. This is because the gate size determines the shear stress molten plastic will experience as it fills out your part. It also determines how well part dimensions and sink marks can be controlled. This video explores the basics of sizing gates for your injection molded parts, to reduce shear stress and optimize part dimensions.
The gate is a designed restriction in the mold, which will increase the shear stress on the resin as it flows into the mold’s cavity. The amount of stress the material experiences can be influenced by controlling the shear rate of the material moving through the gate. This is achieved by adjusting the dimensions of the gate. This is very important, because the shear rate will influence the molecular alignment, part aesthetics, and the amount of residual stress in the part, which can make the gate area sensitive to impact.
The gate also needs to be sized to allow the part to fill, and to sustain pressure on the part as it cools. Because molten plastics are highly viscous materials, high pressures are required to push them into the mold’s cavity. Restrictions in the flow path, such as gates, cause a high pressure-drop. This means that the smaller the gate is, the more pressure the machine must use to pressurize the part and maintain control over the part dimensions. If sufficient pressure cannot be maintained on the molten polymer as it cools, the result is sink marks, voids, warpage, and reduced mechanical properties. Larger gates will allow for longer packing times, which may allow for more flexibility in the packing pressure profile. However, a larger gate will also be more difficult to remove after ejection and could cause a larger vestige on the part’s surface. Therefore, gate sizing needs to balance your customer’s requirements for both part aesthetics and the ability to develop a robust process.
Gate sizes can be compared with mold-filling simulation software, such as Moldflow. Click here to learn about Moldflow analysis and manufacturing support at The Madison Group.
Want to learn more, check out How Do Gates Affect Part Quality and Introduction to Hot and Cold Runners for Injection Molded Parts.