Injection Pressure

Fill (First Stage) is the phase of the cycle in which the screw advances under velocity control, filling the mold cavity approximately to 95 – 99 % of its volume. It ends at the transfer point, where control switches to pressure. ## Key characteristics - Control: velocity (mm/s or cm³/s), not pressure - Purpose: fast and reproducible dynamic filling - Duration: 0.3 – 5 s typically - Volume filled: 95 – 99 % of cavity ## Why separated from hold First stage prioritizes velocity for a uniform flow front; second stage (hold) prioritizes constant pressure to compensate shrinkage. Mixing both in a single-stage process reduces quality and increases variability. ## Multi-stage profile Modern machines allow 5 – 10 velocity steps along the screw stroke: 1. Slow at gate entry (avoids jetting) 2. Fast in wide cavities 3. Slow near critical vents 4. Slow at end for smooth transition ## Typical parameters - Velocity: 30 – 200 mm/s depending on part and resin - Actual pressure (not control): may hit saturation if geometry is restrictive - Time: 0.5 – 3 s on technical parts - Residual volume: 5 – 10 % cushion as margin for hold stage ## Indicators of a good first stage - Uniform flow front (visible in short-shot studies) - Repeatable fill time (±2 % shot-to-shot) - Repeatable pressure peak - Stable final cushion ## Common mistakes - Velocity too high: jetting, splay, burn marks - Velocity too low: cold parts, visible weld lines, short shot - Late transfer: flash, over-pack - Early transfer: sinks, low dimensions

Hold pressure (Packing pressure) is the pressure applied to the material in the cavity after the transfer point, during the hold phase. Its purpose is to compensate volumetric shrinkage as the part cools and solidifies. ## Why it is needed As plastic cools, its volume decreases. Without hold, sinks, internal voids and under-tolerance dimensions appear. Hold pressure pushes additional material to fill this "volumetric deficit" until the gate freezes. ## Typical values - 40 – 80 % of peak injection pressure as starting point - Commodity resins (PE, PP): 300 – 700 bar (plastic) - Engineering resins (ABS, PC, PA): 500 – 1000 bar - Multi-stage: decreasing pressure in 2 – 4 steps as the gate freezes - Time: typically until gate freeze-off (measured by gate-seal study) ## How to set — gate seal study 1. Mold parts with increasing hold times (0.5, 1, 2, 4, 6, 8 s…) 2. Weigh each part 3. Weight rises until it plateaus when the gate freezes 4. The optimal hold time is the first one at which weight no longer grows ## Hold pressure vs. injection pressure These are the two stages of the injection sequence and are easy to confuse: - Injection pressure (first stage) is velocity-controlled and fills the cavity fast — it peaks at the highest value of the shot. - Hold pressure (second stage) is pressure-controlled and only tops up shrinkage after filling — typically 40–80 % of that peak. The instant the machine switches from the first to the second is the transfer (changeover) point. Getting that point right is what separates a stable process from one with flash or short shots. ## Common issues Hold too low: sinks, voids, low dimensions. Hold too high: flash, over-pack, residual stress, ejection difficulty. Hold too long (after gate seal): only wastes cycle time without affecting the part. ## What is hold pressure in injection molding? It is the second-stage pressure that keeps pushing melt into the cavity after filling, compensating for the shrinkage that occurs while the part cools, until the gate freezes. ## How high should hold pressure be? Start at 40–80 % of the peak injection pressure, then fine-tune with a gate-seal study. Commodity resins typically run 300–700 bar plastic pressure; engineering resins 500–1000 bar. ## How do you determine hold time? With a gate-seal (cushion/weight) study: mold parts at increasing hold times and weigh them. Once part weight stops increasing, the gate has frozen and any extra hold time is wasted.

Injection Speed (Injection Velocity) is the linear speed at which the screw advances during fill, programmed in mm/s (or volumetric cm³/s). It is one of the parameters that controls fill quality, alongside temperature and hold pressure. ## Why it matters Speed determines: - Fill time: 0.3 – 3 s on technical parts - Shear on the material (faster → more shear → lower effective viscosity) - Cosmetic marks: jetting (excessive speed on small gate), flow marks (too slow or interrupted) - Molecular orientation and residual stress ## Multi-stage profile Modern machines allow 5 – 10 velocity steps along the screw stroke: 1. Slow at gate entry (avoids jetting) 2. Fast in wide cavities 3. Slow near critical vent zones (avoid air trap) 4. Slow at end of fill for smooth transition to hold ## Typical values - Commodity resins, standard wall thickness: 50 – 150 mm/s - Technical detailed parts: 30 – 80 mm/s - Very thin wall parts (<0.8 mm): 200 – 500 mm/s (high-dynamic servo machines) - Shear-sensitive resins (PVC, PMMA): moderate speed ## Optimization Moldflow / Moldex3D analysis to define theoretical profile, iterative tuning with short-shot studies, and melt-temperature monitoring at end of fill (must not rise more than 5 – 10 °C from over-shear). ## Common issues Jetting from high speed on point gates, flow marks from insufficient speed, burn marks from trapped air at end of fill, and delamination if the flow front cools partially.

Injection Time (Fill Time) is the time the screw takes to move from its initial position to the transfer point, executing the dynamic filling phase. It results from the programmed velocity profile and the shot volume. ## Typical values - Small parts (<10 g): 0.3 – 0.8 s - Medium parts (10 – 100 g): 0.8 – 2.5 s - Large parts (>100 g) or thin walls: 2 – 5 s - Thick technical parts: 3 – 8 s ## Why it matters A repeatable injection time is a stable-process signal. Shot-to-shot variation indicates: - Worn check valve (poor seal, melt back-flow) - Resin viscosity change (moisture, temperature) - Variable gate restriction (degradation, contamination) - Real velocity not reaching programmed (pressure saturation) ## Programmed vs. actual time The programmed time is ideal per the profile; actual can be larger if injection pressure saturates (velocity drops). Monitoring actual time is key in scientific molding. ## Optimization - Constant volumetric flow rate filling requires adjusting velocity per step - Inject as fast as possible without defects (jetting, splay, burn marks) - Short times shorten cycle but generate more shear and orientation ## Variation diagnostics Record injection-time histogram across 100 shots. Deviation >5 % indicates a problem: - Rising trend: check valve wearing out - Random jumps: moisture variation in resin - Sudden increase: partial blockage at some gate

Screw is the helical component inside the barrel of the injection unit. It rotates on its axis to feed, plasticize (melt) and meter the resin; during injection it acts as a piston pushing the melt toward the mold. ## Screw anatomy Three functional zones along its length: 1. Feed zone: deep, takes pellets from the hopper. 50 – 60 % of length 2. Compression zone: depth tapers down, compacts and starts melting. 20 – 30 % 3. Metering zone: minimum constant depth, homogenizes and meters the shot. 20 % ## Geometric parameters - Diameter (D): 18 – 200 mm in commercial machines - L/D ratio: 18:1 to 24:1 standard; up to 30:1 for high mixing - Compression ratio: 2.0:1 to 3.5:1 depending on resin - Material: nitrided steel (standard), bimetallic (PVC, flame retardants), tungsten-carbide coatings (glass fiber) ## Special screw types - Barrier screw: divides the channel in two for better melting - Mixing screw: with extra shear/mixing elements - For PVC: low compression ratio, no hot zone - For fiber-reinforced: low shear so as not to break fibers ## Maintenance - Visual inspection every 6 months - Diameter and clearance check with three-point micrometer - Typical replacement: 1 – 3 million cycles depending on resin abrasiveness - Wear indicators: shot-weight variation, unstable cushion, uneven color ## Common issues Flight wear from abrasive resins, corrosion from PVC without proper coating, pellet bridging in feed from moisture or irregular size, and worn check valve allowing material backflow during injection.