Vents

Cavity is the hollow region inside the mold that shapes the exterior of the molded part. Together with the core, it defines the final geometry: whatever the melt fills is exactly what becomes the part after cooling. ## Cavity vs. core - Cavity (female side): usually the fixed half of the mold, defines the cosmetic / outer surface. - Core (male side): usually the moving half, defines the inside and houses the ejector pins. The line where the two halves meet is the parting line. ## Single- vs. multi-cavity molds - 1 cavity: prototypes, large parts, low-volume technical work - 2, 4, 8, 16 cavities: medium production (containers, caps) - 32, 64, 96, 128 cavities: high production (PET closures, preforms) - Family molds: different cavities in the same tool to produce a set of parts ## Critical design considerations Balanced runners so all cavities fill simultaneously, symmetrical cooling, draft angle on every vertical wall, surface finish (texture, VDI or SPI polish), and replaceable inserts in high-wear areas (gates, cores). ## Typical cavity defects Imbalance in multi-cavity molds (some parts with flash, others short), scratches from misalignment, ejector marks from poor pin placement, and localized wear at gates of cavities with weaker cooling.

Parting Line is the line or surface where the two halves of the mold (cavity and core) meet when closed. It defines how the mold opens to eject the part and leaves a linear mark on the plastic that is usually visible. ## Importance in design - Determines the mold's opening plane - Defines which surfaces are cosmetic and which can accept the mark - Conditions the location of gates, vents and ejectors - Affects mold complexity and cost ## Parting-line types - Flat: simplest, single horizontal or vertical plane - Stepped: with offsets to accommodate geometries - Curved / 3D: follows the part contour, requires 5-axis machining - Multiple: when there are slides for undercuts ## Design rules - Place the parting line on the part's "natural" edges (corners, flanges) - Avoid crossing cosmetic surfaces - Guarantee accessibility for machining and polishing - Maintain minimum draft of 0.5° on both sides - Plan venting through the parting line ## Associated defects - Flash: the most common, material exits through a poorly sealed line - Visible mark: noticeable on cosmetic parts; mitigated with texture or hidden location - Asymmetric wear: if pressure is poorly distributed across cavities - No venting: if the line is polished perfectly with no air paths ## Maintenance - Visual inspection every 100,000 cycles - Re-grind plates if the line shows recurring flash - Vent cleaning: 0.02 – 0.05 mm standard depth - Plate flatness check: <0.01 mm in precision molds

Short shot is the injection-molding defect in which the cavity is not completely filled and the part comes out incomplete — typically missing material in the zones furthest from the gate, or in bosses, ribs or thin walls. ## Common causes - Insufficient shot size (low dosing) - Injection speed too low: the flow front freezes before filling - Melt or mold temperature out of range (resin too viscous) - Blocked vents: trapped air prevents the melt from advancing - Injection pressure saturating due to upstream restriction (gate, runner, worn check valve) ## Parameters to check Compare actual shot vs. nominal size, multi-stage velocity profile, transfer position, back pressure, barrel temperatures by zone, and vent cleanliness. Cushion must be stable; a cushion at zero means missing material or pressure. ## Systematic fix Increase shot size, ramp velocity in stages, raise melt temperature 5 – 10 °C, open vents, inspect the check valve and verify no restriction in hot runner or gates.

Mold (Tool) is the mechanical assembly of plates, cavities, injection system and cooling that shapes the molded part. It is the single most expensive asset of the operation (10,000 – 500,000 USD) and its design dictates everything: cycle, quality, productivity and unit cost. ## Main components - Cavity plate (fixed half): nozzle side, usually houses the cavity - Core plate (moving half): ejector side, holds the core and ejector pins - Injection system: sprue, runners, gates (cold or hot) - Cooling system: water/glycol channels, conformal in premium molds - Ejection system: pins, sleeves, stripper plates, slides for undercuts - Standard components: leader pins, bushings, retainers, sensors - Replaceable inserts in wear areas ## Mold types - Single-cavity: prototypes, large parts, low production - Multi-cavity (2/4/8/16/32+): mass production - Family mold: different cavities for parts of the same assembly - Cold runner: with cold runners separated each cycle - Hot runner: no runner scrap, shorter cycles - Stack mold: two levels of cavities to double capacity - Two-shot / multi-material: two resins in the same part ## Mold materials - P20 (pre-hardened steel): standard for medium production, easy to machine - H13: hardened inserts, high thermal-wear resistance - S136 (stainless): polished cavities, corrosion resistance (PVC, PET) - Aluminum (7075): prototype or low-volume molds - NAK80: mirror polish without distortion ## Typical service life - Aluminum: 5,000 – 50,000 cycles - P20: 100,000 – 1,000,000 cycles - Hardened H13: 1 – 10 million cycles - Carbide / TZM in hot-runner gates: up to 50 million ## Critical maintenance Cleaning after every production run, vent inspection, lubrication of pins and guides, monitoring of cooling channels (scaling), and repair of cavity damage before it spreads.