ABS & TPU Overmolding Mold Solution for Plastic Keycaps

• Product: Plastic Button Keycap
• Material Combination: ABS (1st Shot - Rigid Substrate) + TPU (2nd Shot - Soft Tactile Surface)
• Cavitation: 8 + 8 Cavities (1st shot and 2nd shot operate simultaneously)
• Machine Compatibility: Right-angle two-shot injection machine with a 180-degree rotary table.

To accommodate the distinct rheological properties of ABS and TPU, the mold utilizes a customized, two-stage gating system.

The first shot forms the rigid ABS substrate across 8 cavities. We implemented a hot runner system with valve gates, assigning one individual valve pin per cavity.

Technical Advantage: Valve gates provide absolute control over the melt flow and shut-off. For the keycap's internal structure, this ensures zero gate vestige (gate blushing or protrusions), which is critical since the ABS surface must perfectly mate with the TPU overmold in the subsequent step without any dimensional interference.

The second shot overmolds the TPU layer. This sequence utilizes a hot runner transitioning into a cold runner, feeding the 8 cavities through banana gates (cashew gates) fed by 4 hot runner drop points.

Technical Advantage: TPU is a shear-sensitive elastomer. A banana gate allows the material to enter the cavity from the bottom or side, hiding the gate mark on a non-cosmetic surface. Furthermore, the curved profile of the banana gate automatically shears the runner from the molded part during ejection, enabling fully automated, operator-free production.

This tool is specifically engineered for a right-angle injection molding machine featuring a rotary table. A single mold base handles both injection phases simultaneously, doubling the output efficiency.

The Core/Cavity Dynamic: The stationary half (fixed mold) contains two distinct cavity profiles—one for the bare ABS substrate and one for the final ABS+TPU shape. Conversely, the moving half (dynamic mold) rotates 180 degrees after every cycle.

Design Constraint Solved: Because the moving half rotates, the core design must remain 100% identical on both halves of the platen. The core must perfectly seal against the ABS cavity to prevent flash during the first shot, while simultaneously acting as the precise locational substrate for the TPU overmold during the second shot. This requires ultra-high precision CNC machining and spotting to guarantee uniform shut-off across rotating cycles.

One of the most significant engineering challenges in this project was the keycap's mounting mechanism. At the base of the keycap, there are two secure mounting clips (undercuts). Because of these inward-facing clips, the part cannot be ejected straight out of the core.

Traditionally, mold designers use angled guide pillars (horn pins) attached to the cavity side to pull sliders during mold opening. However, in a tightly spaced 16-cavity rotary mold, traditional angle pins consume too much space and limit layout options.

We engineered a specialized, compact slider system where the sliders are driven entirely by the ejection system, rather than the mold opening sequence.

• Each cavity is equipped with two miniature sliders to form the undercuts.
• The sliders are machined with internal angled slots.
• Straight ejector pins with specially machined spherical/rounded heads are inserted into these angled slots.

As the machine's ejector plates push the straight pins forward, the rounded heads travel along the angled slots inside the sliders. This converts the vertical ejection force into a horizontal pulling force, retracting the sliders and releasing the undercuts simultaneously as the part is pushed off the core.

Because this is a multi-component rotary mold, the first shot (ABS) must remain firmly seated on the core as it rotates 180 degrees to receive the second shot (TPU).

To achieve this, the mold's ejection system is designed as two identically configured but independently actuated units. Even though both sides of the moving half have the exact same ejector pin layout, the injection machine only actuates the ejector plates on the side holding the completed second-shot product. This ensures the first-shot substrate is securely retained for rotation while the finished two-color keycaps are seamlessly demolded.

This ABS/TPU keycap project highlights the intersection of advanced mold kinematics and multi-material rheology. By combining valve-gated hot runners, automatic-degating banana runners, and an innovative space-saving ejector-driven slider mechanism, this rotary platen mold delivers high-volume, precision two-shot components with zero secondary processing required.