Case Study of a 2000ml Food-Grade PP Cheese Bucket Injection Mold

For medium-to-thin-walled rigid packaging like this 2000ml pail, a 1-drop valve gate hot runner system was deployed. Direct gating at the center of the bucket base ensures balanced melt distribution and minimizes residual internal stress. Crucially for food packaging:

• Zero Gate Vestige: The valve pin mechanically seals the orifice flush with the component surface, eliminating post-molding trimming and preventing cosmetic defects or stacking interference.
• Minimized Drooling & Stringing: Given the high thermal sensitivity and flowability of food-grade PP, the positive mechanical shut-off prevents material drooling, keeping cycle times highly consistent.

The container's geometry features three critical external undercuts near the lower section of the bucket body: two opposing handle holes and one side-mounted tear-off pull ring.

Because the primary parting line is established at the neck/rim of the bucket, standard horn-pin-driven external sliders cannot be utilized. When the mold splits, these undercut features remain located deep within the cavity matrix, preventing them from being pulled radially outward in a conventional sequence.

To overcome this layout constraint, our engineering team integrated a specialized sub-cavity slider mechanism mounted directly onto an internal ejection tray:

1. Integrated Floating Layout: The three slider blocks are nested on a precision-ground circular ejector tray situated at the bottom of the cavity.
2. Synchronized Ejection Advance: Upon mold opening, the circular tray advances concurrently with the product during the first phase of the ejection stroke. At this stage, the sliders remain physically confined by the internal walls of the cavity steel.
3. Spring-Driven Radial Release: As the tray clears the top edge of the cavity plate, the external mechanical constraints vanish. Heavy-duty internal compression springs built within the slider blocks immediately exert outward radial force. The sliders instantly snap outward, backing out of the 2mm deep handle apertures and the tear-ring geometry without scratching or distorting the fresh PP plastic.

To secure the container's snap-on lid, the mouth/rim of the cheese bucket is engineered with a continuous 2mm deep peripheral retention groove. This geometric configuration creates a significant undercut on the core side, making standard single-stroke ejection high-risk for part deformation.

To execute a flawless, damage-free release, a customized Two-Stage Mechanical Rocker Ejection System (locally referred to as the "Seesaw" Mechanism) was engineered. This mechanism comprises a pair of hook/checkmark-shaped levers positioned between the main ejector plate and the ejector retainer plate.

• Pivot Points & Links: One end of the rocker arm is mechanically linked to the mold's spacer/support block, the central pivot is anchored to the ejector retainer plate, and the driving end acts directly upon the secondary stripper/top plate.
• Stage 1 (Simultaneous Travel): When the machine’s hydraulic ejector rods actuate, the entire ejector package (ejector plate, retainer plate, and secondary plate) moves forward as a single unit for a predetermined distance. This initial stroke breaks the vacuum between the PP container and the core insert.
• Stage 2 (Accelerated Secondary Ejection): Once the mechanism reaches its built-in mechanical stroke limit stop, the stationary link forces the hook-shaped rocker arm to pivot rapidly. Utilizing mechanical leverage, the "seesaw" action instantly accelerates the secondary top plate, driving it forward an additional distance at a higher velocity relative to the main ejector plates.

Food-grade PP exhibits excellent elasticity at demolding temperatures. By utilizing the rapid, concentrated secondary thrust of the stripper ring driven by the seesaw mechanism, the mold utilizes forced stripping (mechanical stripping). The 2mm deep groove expands elastically over the core insert and immediately snaps back to its nominal dimensions without stress-whitening, tearing, or oval distortion.

By pairing a clean-running valve gate hot runner with innovative internal kinematics—the floating sub-cavity sliders and the leverage-driven two-stage ejection—this mold architecture optimizes cycle efficiency and eliminates manual handling risks. For B2B industrial packaging suppliers, this tooling configuration guarantees million-cycle longevity, minimal maintenance intervals, and absolute structural compliance with international food-grade standards.