Scott Byal

As technology miniaturizes, traditional design engineering aspects of injection molded parts are now causing problems. When looking at the micro mold gate, where polymer enters the mold cavity, only a small percentage of part geometry and surface area is affected. In micro molding however, the gate becomes a part feature. This poses problems and delays OEM production timelines, but there’s a simple fix.

It’s easy to forget gate features on a small injection molded part are often bigger than the overall size of a micro molded part. For the uninitiated, gate features on an injection molded part appear as small nubs or areas where the plastic “bumps out.” It’s acceptable in small parts or larger parts, but in micro molding, it thwarts the entire project.

When considering reduced clearances in assemblies like wearables and thin electronics packages for smart clothing, finalized designs may not even allow space for even the smallest gate remnants. As tech’s rapid decrease in hardware size continues, and micro mold gate placement is increasingly impacted, many designers are scratching their heads, wondering how to design their parts moving forward. After decades of experience, here are some best practices.

Change your thinking of mold design

Simply changing how a designer looks at gate and ejection pin location is important to success of the project. As your company and OEMs go micro, it’s time to consider injection-molded gate details as a part feature in the design phase.

micro mold examples next to coins
Micro mold examples next to coins

Strategic gate relief designs will have to be considered to either hide vestige from the external profile of the part, provide clearance in mating parts, or to limit material dragging away from the gate. This applies specifically in the case of a self-shearing sub/ tunnel gate.

Micro Mold gate-focused part design

Counterbore

The most common best-practice example of this is when micro mold gate relief features are cut into or recessed from the part surface that contains the gate. This can be in the form of a round counterbore sunken into the part surface, but also a straight cut through the part exterior to provide the relieved face. This results in a gate that is hidden below the part’s exterior dimensions. It’s a simple solution but only works when considered before the design phase begins.

Sub-gate

In another example, sub-gates can be used in production applications because their self-shearing action generally results in little gate remnant protruding from the part’s surface. It’s important to note this shearing action drags material along the part surface opposite the direction of ejection causing what some refer to as gate drag. When a part is small enough the gate drag can extend beyond the border of the part causing interference issues.

Stepped gate

Stepped gate reliefs can take away the wall to drag against. This will keep the feature in the gate relief where it belongs. In addition, angled gate reliefs are also effective as they eliminate the rubbing action that creates gate drag.

Get DFM micro mold insight

The final solution is to simply get the right insight as early as possible. Micro mold DFM teams are great at determining the best location and style for a micro mold gate. Because we see many of the same challenges, working with great micro molders lets you benefit from the institutional knowledge and experience they have working with other leading tech companies.

Getting them involved early can often result in a more robust molding process, potentially gating on a less critical part surface or providing relief in a mating component to avoid the risk of interference.

Scott Byal of Accumold

Scott Byal is Senior Project Engineer at Accumold. This is an updated version of a piece that originally ran in “Make Parts Fast.” Read the original version here >

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