Unit
- Injection molding cycle: The process repeats four phases - clamp, inject, cool, eject. Cycle time directly affects productivity; reducing cooling time by 20 percent can increase output by 15 percent in high-cavitation molds.
- Two-plate mold: Simplest configuration with one parting line. The sprue, runner, and gate are on the stationary half; ejection occurs on the moving half. Used for parts with simple geometry and moderate quality requirements.
- Three-plate mold: Contains an intermediate stripper plate that separates the runner from the part automatically. Suitable for center-gated parts and multiple-drop scenarios where gate location flexibility is needed.
- Hot runner mold: Uses heated manifolds to keep plastic molten in the runner system, eliminating regrind and reducing cycle time. Requires precise temperature control to avoid degradation of temperature-sensitive materials.
- Family mold: Produces multiple different parts in a single cycle. Design must balance cavity pressures and fill times to ensure all parts fill simultaneously; otherwise, short shots or flashing occur.
- Unscrewing mold: Used for threaded parts such as bottle caps. The core rotates via a hydraulic motor or rack-and-pinion mechanism while the mold opens, unscrewing the part without damage.
- Stripper plate mold: Ejection is performed by a plate that pushes against the entire part perimeter, ideal for thin-walled containers that would distort under pin ejection.
- Insert mold: A preformed component (metal, ceramic, or plastic) is placed in the cavity before injection. The plastic encapsulates the insert. Proper insert holding and sealing prevent flash around the insert.
- Side-action mold: Used for parts with external undercuts, holes, or recesses perpendicular to the mold opening direction. Slides are driven by angled pins, hydraulic cylinders, or cam mechanisms. The slide must fully lock before injection to prevent movement under cavity pressure. Proper wear plates, gibs, and lubrication are critical to maintain alignment over long production runs.
- Stack mold: Uses two or more parting planes to double or triple the number of cavities without increasing clamp force proportionally. Applications include high-volume packaging parts like thin-wall containers. Requires a hot runner system in the center section; melt distribution must be carefully balanced across all levels to ensure uniform filling and consistent part quality.
- Gas-assisted injection molding: Nitrogen gas is injected into the melt to hollow out thick sections, reducing sink marks, warpage, and material usage. Common for handles, automotive ducts, and structural ribs. Gas channels must be designed to direct the gas path; timing and pressure control are critical to avoid gas penetration into thin walls and ensure consistent hollow core formation.
- Multi-shot mold: Produces parts with two or more materials or colors in a single cycle by sequential injection into the same cavity or different cavities on a rotating core. Applications include soft-touch grips and two-color automotive lenses. Requires precise alignment between the first and second shots; mold design must accommodate separate injection units and careful isolation of melt streams.
Process Flow
- Stage 1 - Mold Concept: Analyze part geometry, material, and production volume to determine mold type (two-plate, three-plate, hot runner).
- Stage 2 - Preliminary Layout: Sketch parting line, cavity count, and ejection direction. Check for undercuts and side-core requirements.
- Stage 3 - Design Review: Evaluate cooling channel placement, gate location, and venting. Simulate fill and packing.
- Stage 4 - Manufacturing Planning: Select machining methods (CNC, EDM, grinding) and order specialized steel blocks.
- Stage 5 - Assembly and Tryout: Fit all components, mount on press, and run test shots. Adjust process parameters until part meets specifications.
Quick Revision
- Injection molding cycle has four phases: clamp, inject, cool, eject.
- Two-plate molds are simplest; three-plate and hot runner systems offer gating flexibility.
- Family molds produce multiple parts per cycle; unscrewing molds handle threads.
- Insert molds encapsulate preformed components; stripper plate molds eject thin-walled parts.