Stack Mold Case Study: High-Efficiency Dual-Product Ice Cream Lid Production

This mold is uniquely engineered to produce two distinct products within the same cycle. Both products are thin-wall packaging components requiring high-precision filling to prevent warpage and short shots.

• Product 1: Square Ice Cream Lid (92 x 92 x 6.3 mm)
• Product 2: Rectangular Ice Cream Lid (78 x 117 x 6.3 mm)
• Wall Thickness: 0.8 mm (Both products)
• Cavitation: 14 Cavities Total (8-Cavity Square + 6-Cavity Rectangular)

Industry Insight: A wall thickness of 0.8mm classifies this as a thin-wall injection molding application. It requires high injection speeds and pressures, making the structural integrity of the mold and the balance of the hot runner system absolutely critical to ensure dimensional stability and uniform cooling.

The gating system utilizes a sophisticated Hot Runner System configured for 1 Drop per Cavity. Because this is a stack mold, it features two distinct parting lines requiring two separate hot runner subsystems to work in perfect unison.

The most complex engineering challenge in this mold is the material delivery. The plastic melt must be transferred from the machine nozzle across the first parting line to reach the center of the mold, ensuring zero defects or pressure drops across all 14 cavities.

To achieve this, the mold utilizes a specialized Stack Mold Melt Transfer System (often referred to as a valve-gated sprue bar).

• Mold Open State: When the mold opens for ejection, the hot runner pathway must physically disconnect across the first parting line. To prevent plastic drooling, a precision pneumatic or hydraulic valve pin instantly shuts the transfer nozzle.
• Mold Closed State: When the mold clamps shut, the primary transfer nozzle and the receiving nozzle in the center manifold align and compress against each other. The valve gates open simultaneously, recreating a continuous, leak-proof melt channel to supply both the first and second parting line hot runner plates.

Adding to the complexity, the first parting line utilizes a reverse mold configuration (倒装结构). The A Plate (Cavity) is positioned behind the B Plate (Core). This is strategically designed to accommodate the melt transfer mechanism and the custom pneumatic ejection system on the machine's fixed platen side.

In a stack mold, ensuring both parting lines open simultaneously and equally is essential for part clearance and cycle time optimization. The driving force for opening is provided by the injection molding machine's standard opening stroke, but the synchronization is managed by a robust Rack and Pinion Gear System mounted on the exterior sides of the mold.

• The Upper Rack: Fixed to the side of the Manifold Plate (just behind the Top Clamping Plate), extending all the way to the A Plate of the second parting line. This rack remains stationary relative to the fixed platen.
• The Lower Rack: Fixed to the side of the Bottom Clamping Plate, extending to the B Plate of the first parting line.
• The Pinion (Gear): Mounted centrally between the two hot runner plates in the middle block of the mold.

As the machine pulls the mold open, the Bottom Clamping Plate moves, pulling the lower rack with it. The lower rack forces the center gear to rotate against the stationary upper rack. This mechanical gearing ensures that the center block (containing the 1st Parting Line A Plate, both Hot Runner Plates, and the 2nd Parting Line A Plate) moves at exactly half the speed and half the distance of the main moving platen. This guarantees that both Parting Line 1 and Parting Line 2 open simultaneously, allowing clearance for the parts to drop.

As the machine pulls the mold open, the Bottom Clamping Plate moves, pulling the lower rack with it. The lower rack forces the center gear to rotate against the stationary upper rack. This mechanical gearing ensures that the center block (containing the 1st Parting Line A Plate, both Hot Runner Plates, and the 2nd Parting Line A Plate) moves at exactly half the speed and half the distance of the main moving platen. This guarantees that both Parting Line 1 and Parting Line 2 open simultaneously, allowing clearance for the parts to drop.

Because the mold features a reverse layout on one side and a center block, traditional machine ejector rods cannot be used for the first parting line. Instead, the mold features a fully independent, automated Pneumatic Ejection System.

First Parting Line Ejection: Pneumatic cylinders are mounted on the top and bottom sides of the Top Clamping Plate. Their actuation rods bypass the fixed plates and connect directly to the 1st Parting Line B Plate (Core). During the ejection phase, these cylinders fire, pushing the B Plate forward to strip the 8 square lids, allowing them to free-fall.

Second Parting Line Ejection: A mirrored pneumatic setup is configured on the Bottom Clamping Plate. Cylinders push the 2nd Parting Line B Plate (Core) to eject the 6 rectangular lids, facilitating a fully automated, gravity-drop ejection cycle.

Designing a stack mold for thin-wall packaging requires a masterful balance of polymer rheology, mechanical gearing, and thermal management. By integrating a dynamic melt transfer valve system, a precise rack-and-pinion synchronization unit, and customized pneumatic ejection, this mold successfully achieves continuous, high-yield production of two different products within a single compact footprint. This case study exemplifies the pinnacle of modern injection molding efficiency, delivering reduced part costs and maximized ROI for high-volume packaging manufacturers.