Molded Part

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.

Former holes (hole-forming features, core pins) are the parts of the mold whose job is to create the holes, bores and openings called for by the part design. Where the cavity forms the outer shape, a former hole is the positive steel — a pin or core — that the plastic flows around, leaving a hole in the finished molded part. ## How they are built - Integral (same formation): machined directly into the core or cavity block when the hole runs in the mold's opening direction, so it clears as the mold opens. - Sliders / side actions: when a hole runs across the opening direction (a side hole or undercut), the hole-forming pin is mounted on a slider (side-action core) that retracts sideways before ejection, then returns for the next shot. ## Why they matter - Function & assembly: molded-in holes for screws, snaps, shafts and ports avoid secondary drilling and support clean design for assembly. - Flow & defects: melt splitting around a pin rejoins on the far side, forming a weld line; pins also need support, or they deflect under melt pressure and shift the hole. Worn or mismatched pins let plastic seep and create flash around the hole. - Cooling & wear: thin pins run hot and wear fastest, so they are a common maintenance and dimensional-control point. ## Design notes Holes are placed and sized with draft and steel strength in mind; through-holes are often formed by a pin meeting the opposite face ("shut-off"), while blind holes use a single supported pin. Their location relative to the parting line decides whether a simple integral pin or a slider is needed. ## Related terms - See also: cavity, molded part, parting line, design for assembly, flash ## What are former holes in a mold? Mold features — pins or cores — that form the holes and openings in a molded part; the plastic flows around the steel pin, leaving a hole when the part is ejected. ## How are holes made in injection-molded parts? By hole-forming pins or cores in the mold: integral pins for holes in the opening direction, and sliders (side-action cores) that retract sideways for holes or undercuts running across the opening direction. ## Why do molded holes sometimes have weld lines or flash? The melt splits around the pin and rejoins on the far side, leaving a weld line; if the pin is worn, unsupported or mismatched to its mating face, plastic seeps past it and forms flash around the hole.

Molding Cycle is the complete sequence of phases that produces one injection-molded part, from mold close to the next mold open. Each phase contributes its time and together they determine press productivity and part cost. ## Phases of the cycle 1. Clamp close and full tonnage applied 2. Injection: the screw forces molten material into the cavity along a velocity profile 3. Hold (packing): constant pressure to compensate for shrinkage during initial cooling 4. Cooling + plasticizing: the screw rotates to prepare the next shot while the part cools 5. Clamp open 6. Ejection and robot / EOAT motion ## How to calculate cycle time Cycle time is the sum of every phase: Cycle time = clamp close + injection + hold + cooling + mold open + ejection Plasticizing (screw recovery) runs in parallel with cooling, so it only counts when it is longer than the cooling phase. Cooling dominates and scales with the square of the thickest wall: doubling wall thickness roughly quadruples cooling time. This wall-thickness rule is the single biggest lever on overall cycle time. ## Cycle time example A representative thin-wall part on a 150-ton press: | Phase | Time | |-------|------| | Clamp close | 1.0 s | | Injection | 1.5 s | | Hold | 4.0 s | | Cooling | 8.0 s | | Mold open + eject | 2.0 s | | Total cycle | 16.5 s | ## How does material affect the cycle? Semi-crystalline resins (PP, PA, POM, HDPE) release more latent heat as they crystallize and usually need longer cooling than amorphous resins (ABS, PC, PS) at the same wall thickness. Melt temperature, mold temperature and the resin's ejection (heat-deflection) temperature all set the minimum cooling time. ## Optimization Conformal cooling channels that follow part geometry, a multi-stage injection profile, plasticizing in parallel with opening, valve gates on hot runners for clean closure, and elimination of dead time on the robot side. ## How long is an injection molding cycle? Thin-wall packaging parts cycle in 3–15 s, while thick technical or engineering parts can take 30–60 s or more. Cooling sets the floor: the thicker the wall, the longer the minimum achievable cycle. ## Which phase takes the longest? Cooling, almost always. It typically accounts for 50–70 % of the total cycle, which is why conformal cooling and wall-thickness reduction give the biggest gains. ## What is the difference between molding cycle and cycle time? "Molding cycle" describes the sequence of phases; "cycle time" is the numerical total in seconds reported on the cell's OEE. The cycle is the what, the cycle time is the how long.

Part ejection is the final stage of the molding cycle, where the cooled molded part is pushed out of the open mold so the next shot can run. It happens after the clamp opens and the part has solidified enough to hold its shape. ## How parts are ejected The ejector system pushes the part off the cores: - Ejector pins: the most common — round pins behind the part. - Ejector sleeves / blades: for bosses and ribs. - Stripper plate / ring: pushes on a large rim to avoid pin marks on cosmetic parts. - Air ejection: a puff of air breaks the vacuum on thin, deep parts. ## How the part leaves the cell - Free-fall: the part drops onto a conveyor or bin — typical in an automatic cycle. - Robot / eoat end of arm tool: picks and places the part for handling, gating or inspection. - Manual: an operator removes it (semi-automatic). ## Why it matters Eject too early (before enough cooling time) and the warm part distorts, sticks or shows ejector-pin push marks; too late and you waste cycle time. Proper draft, polish and ejector layout let parts release cleanly without drag marks, white stress or warpage. ## Related terms - See also: molding cycle, molded part, cooling time, eoat end of arm tool, automatic cycle ## What is part ejection in injection molding? It is the final cycle stage where the cooled part is pushed out of the open mold by ejector pins, sleeves, a stripper plate or air, then removed by free-fall, robot or by hand. ## What causes ejector pin marks? Ejecting before the part is cool enough, too few or too small ejector pins, or insufficient draft — the pins push into a still-soft surface and leave witness marks. ## How is part ejection automated? Either by free-fall onto a conveyor in a fully automatic cycle, or by a robot with end-of-arm tooling that picks the part for downstream handling.

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.