Recovery

Back Pressure is the hydraulic pressure applied against the screw while it rotates during plasticizing, intentionally slowing its retraction. Its purpose is to improve melt homogeneity, disperse pigments and additives, and remove entrapped air. ## Why it is applied Without back pressure, the screw retracts as fast as it can and the melt may exit with bubbles, color streaks or shot-to-shot viscosity variation. Adequate back pressure accumulates shear work in the melt, improving temperature uniformity and mixing. ## Typical values - Unpigmented commodity resins (PP, PE): 30 – 50 bar (plastic) - Pigmented or masterbatch-loaded compounds: 60 – 120 bar - Engineering grades (PC, PA, ABS): 50 – 100 bar - Fiber-reinforced: 30 – 60 bar (higher degrades the fiber) - Highly abrasive materials (PVDF, flame retardants): as low as possible ## How to tune - Start at minimum and raise until: - Color is shot-to-shot homogeneous - Shot weight is stable (±0.5 %) - Plasticizing time does not exceed cooling time (must not extend the cycle) - Verify melt temperature rises no more than 5 °C as back pressure increases ## Common issues - Low back pressure: color streaks, bubbles, unstable weight, unmelted pellets - High back pressure: thermal degradation, fiber breakage, plasticizing > cooling (lengthens cycle), screw wear - Confusing hydraulic back pressure with plastic back pressure (relates to intensification ratio)

Residence time is the time the plastic spends inside the heated barrel — from the moment pellets melt until that material is injected into the mold. It is one of the most overlooked drivers of melt quality: too long and the polymer thermally degrades; too short and you get inconsistent melt and poor process control. ## How to estimate residence time A practical estimate uses how many shots the barrel holds: - Shots in barrel = barrel shot capacity (g) ÷ shot weight (g) - Residence time = shots in barrel × cycle time Example: a barrel rated at 230 g running a 40 g shot holds 5.75 shots; at a 30 s cycle that is 5.75 × 30 ≈ 172.5 s (about 2.9 min). ## Barrel occupancy — the safe window Shot weight should use roughly 20–65 % of the barrel's rated capacity (the barrel occupancy): - Below ~20 %: the shot is too small for the barrel, residence time stretches out and the resin sits and degrades. - Above ~65 %: too little melt reserve — unmelt, poor melt uniformity and long screw recovery. ## Typical targets and degradation Most thermoplastics tolerate ~2–10 minutes; heat-sensitive resins (PVC, POM, some flame-retardant grades) usually want it under ~5 minutes. Excessive residence shows up as discoloration, brown streaks, black specks, a drop in molecular weight and brittle parts. ## Related terms - See also: barrel, barrel occupancy, cycle time, melt, shot weight ## What is residence time in injection molding? It is how long the polymer stays in the heated barrel before injection. Estimate it as the number of shots the barrel holds (barrel capacity ÷ shot size) multiplied by the cycle time. ## How do you reduce residence time? Move the job to a smaller-barrel machine, bring barrel occupancy into the 20–65 % window, shorten the cycle, or lower melt temperature — so the resin spends less time hot and does not degrade. ## What is a typical residence time? For most resins 2–10 minutes is acceptable; heat-sensitive materials like PVC and POM should usually stay under about 5 minutes to avoid degradation.

Revolutions per minute (RPM) is the rotational speed of the screw during recovery (plastication) — how fast the screw turns to convey, melt and meter the next shot. It is one of the input settings a technician controls, and along with back pressure it governs how the melt is prepared. (RPM also names motor and pump speeds elsewhere, but in molding it usually means screw speed.) ## What screw RPM does - Conveying & metering: higher RPM moves plastic forward faster, shortening recovery time so it fits inside the cooling time. - Shear heating: turning the screw shears the plastic and generates heat — much of the melting energy actually comes from this mechanical shear, not just the barrel heaters. Higher RPM = more shear heat. - Melt quality: enough RPM gives a uniform, well-mixed melt; too much overheats and can degrade shear-sensitive resins, raise melt temperature and add color streaking. ## Setting it well - Match recovery to cooling: set RPM so recovery finishes just before the mold opens — not so slow it extends the cycle, not so fast it spikes shear and wear. - Surface speed matters more than RPM alone: the same RPM is gentler on a small screw and harsher on a large one, because the screw's outer surface moves faster — so target screw surface speed (m/s) when comparing machines. - Pair with back pressure: RPM and back-pressure together set melt uniformity and shot size consistency; watch melt temperature and residence time for degradation. ## Why it matters Screw RPM is a direct lever on recovery time, melt temperature and melt homogeneity — it affects cycle time, part consistency and resin degradation. Shear-sensitive materials (PVC, some flame-retardant grades) need conservative RPM; robust commodity resins tolerate more. ## Related terms - See also: screw, recovery, back pressure, melt, residence time ## What is screw RPM in injection molding? The rotational speed of the screw during recovery — it conveys and melts the next shot. Higher RPM shortens recovery and adds shear heat; it is set together with back pressure to prepare a uniform melt. ## How does screw speed affect the melt? Faster rotation shears the plastic more, generating heat that helps melt it and mixing the melt, but too high an RPM overheats and can degrade shear-sensitive resins and shift melt temperature and color. ## Why is screw surface speed used instead of RPM? Because the same RPM produces different shear on different screw diameters — a large screw's surface moves faster — so surface speed (m/s) compares melting conditions fairly across machines, while RPM alone does not.

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.

Shot size is the volume of melt the screw meters and injects each cycle — set as a screw-position stroke (mm) or a volume (cm³). It is the volumetric twin of shot weight, which is the same material expressed as mass. ## How it is set - During recovery (dosing) the screw rotates and retracts to a set position; that retraction distance defines the shot size. - Set it so the first-stage (velocity-controlled) fill reaches about 95–99 % of the part, then hold packs it out — leaving a stable cushion so the screw never bottoms out. - The shot should use roughly 20–80 % of the barrel's rated capacity (see barrel occupancy) to keep residence time in range. ## Why it matters Shot size drives part-weight repeatability and where the transfer position cut off lands. Too small and you cannot fill the part and still keep a cushion; too large and you waste material, stretch residence time and risk degradation. ## Shot size vs shot weight - Shot size: the volume or screw stroke the machine delivers per cycle. - shot weight: the mass of that same shot (parts + runners + sprue), read on a scale. Shot size × melt density ≈ shot weight. ## Related terms - See also: shot weight, cushion, recovery, barrel occupancy, transfer position cut off ## What is shot size in injection molding? It is the metered volume (or screw stroke) of melt injected per cycle, set during recovery so the part fills on first stage and a cushion remains. ## How do you set shot size? Dose to a screw position that fills about 95–99 % on first stage and leaves a small, stable cushion, keeping the shot within roughly 20–80 % of barrel capacity. ## What is the difference between shot size and shot weight? Shot size is volumetric (screw stroke or cm³); shot weight is the mass of the same shot in grams. Multiplying shot size by melt density gives approximately the shot weight.