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<\/p>\nHEE&HATO Engineering Team | Plastics Processing Equipment | Published: August 17, 2026<\/p>\n
Set injection moulding process parameters in a fixed order: barrel and mould temperature, shot size and cushion, injection speed, transfer point, holding pressure and time, cooling, back pressure, clamp force. Each step has a verification test. Skipping the order is what turns a two-hour setup into a two-day one.<\/p>\n
Four conditions have to be true, or every number you set afterwards will be measuring something other than what you think.<\/p>\n
The material is dried to the resin datasheet, not to habit. Hygroscopic resins pull moisture back out of plant air in minutes once the hopper is opened. Splay that appears after a lunch break is usually a drying problem being blamed on injection speed.<\/p>\n
The barrel and mould are at temperature and have held there. Reaching setpoint is not the same as being at temperature. Steel and melt need soak time. On a hydraulic machine, the oil also needs to be at working temperature, because oil viscosity changes injection speed even when the setpoint on the screen has not moved.<\/p>\n
The mould is clean, vented and its cooling circuits are flowing. Blocked vents show up as burn marks that operators then try to fix by dropping injection speed, which loses them fill.<\/p>\n
You have a scale. A balance reading to 0.01 g is the single most useful tool in the setup. Most of the verification steps below are weight comparisons.<\/p>\n
Compared with the DOE and optimisation approach common in the academic literature, where a matrix of runs is designed in advance and analysed afterwards, this sequence is built for a machine that has to make sellable parts by the end of the shift. Both are valid. They answer different questions.<\/p>\n
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Work through these in order. Do not jump ahead because a parameter looks obviously wrong; a wrong-looking value at step 6 is often a symptom of step 3.<\/p>\n
Step 1 \u2014 Barrel profile and mould temperature.<\/strong> Set the barrel zones from the resin datasheet, typically rising from feed throat toward the nozzle. Set mould temperature to the datasheet mid-range. Verify: Purge and take a melt temperature reading with a probe in an air shot. The reading will often differ from the barrel setpoint, sometimes substantially, because shear heating from the screw adds to it. The probe reading is the number that matters. Record it.<\/p>\n Step 2 \u2014 Shot size, cushion and decompression.<\/strong> Set screw stroke to deliver an estimated shot, then set a cushion so the screw does not bottom out at the end of injection. Set decompression to the minimum that stops nozzle drool. Verify: Watch the screw position at the end of hold across five shots. The cushion should be present and repeating. A cushion that reads zero on any shot means the screw is bottoming, and every pressure number after that point is meaningless.<\/p>\n Step 3 \u2014 Injection speed, using a short-shot ladder.<\/strong> Turn holding pressure to zero. Run a series of shots, increasing shot size in small increments, and lay the parts out in order. Verify: You are looking for a fill that progresses smoothly, with no hesitation and no jetting from the gate. If flow lines or jetting appear, adjust speed rather than temperature. Stop when the cavity is filled to roughly 95\u201398% by volume, with the part visibly short at the last-to-fill area.<\/p>\n Step 4 \u2014 Lock the V\/P transfer point.<\/strong> The screw position from step 3 that gives that 95\u201398% fill is your transfer point from velocity control to pressure control. Set it. Verify: Run five shots with holding pressure still at zero. Every part should be short by the same amount. If the short varies, the transfer point is being reached at different screw positions and you have a check ring or a shot size problem to fix before continuing.<\/p>\n Step 5 \u2014 Holding pressure and holding time, by gate seal study.<\/strong> Set a starting holding pressure and a deliberately long holding time. Now run a series, reducing holding time in steps, and weigh one part from each setting. Verify: Plot weight against holding time. The weight climbs, then flattens. The point where it flattens is gate seal, and holding beyond it adds cycle time without adding material. Set holding time just past that point. Then adjust holding pressure to bring part weight to target, re-checking that the part is free of sink and not flashing.<\/p>\n Step 6 \u2014 Cooling time. Start long. Reduce in steps.<\/strong> Verify: Reduce until you see the first sign of trouble \u2014 ejector pin push marks, warpage after cooling on the bench, parts sticking in the cavity \u2014 then go back up by a margin. Measure a dimension on parts from each cooling setting, not just look at them. Parts that look fine at ejection can move for hours afterwards.<\/p>\n Step 7 \u2014 Back pressure and screw rotation speed.<\/strong> Set back pressure to the lowest value that gives consistent shot weight and no visible unmelt or air entrapment. Set screw speed so plasticising finishes before cooling ends. Verify: Two checks. First, weigh ten consecutive shots; back pressure that is too low shows up as weight scatter. Second, watch the recovery time against the cooling time on the cycle display. If recovery finishes right at the end of cooling with no margin, any drop in screw speed will extend your cycle. If it finishes far too early, screw speed is higher than it needs to be and you are adding shear heat for nothing.<\/p>\n Step 8 \u2014 Clamp force.<\/strong> Start from projected part area including runners, multiplied by the cavity pressure the resin needs, and add margin. Then work downward. Verify: Reduce clamp force in steps until flash appears at the parting line, then set above that point with margin. Excess clamp force compresses vents and blocks them, and the burn marks that follow get misdiagnosed as an injection speed problem. Re-check that mould height adjustment is correct after any mould change; an uncalibrated mould height makes the tonnage reading on the screen a fiction.<\/p>\n Step 9 \u2014 Ejection and cycle lock-in.<\/strong> Set ejector stroke, speed and any multiple-stroke function. Then run twenty consecutive shots without touching anything. Verify: Weigh all twenty. Calculate the mean and the standard deviation. That standard deviation is your process baseline. Write it on the mould data sheet. Every future argument about whether the process has drifted gets settled against this number.<\/p>\n This is where most setups go wrong, and it has nothing to do with knowing the parameters. This table gives a first move and a second move. It is a triage order, not a complete diagnosis, and it assumes the mould itself is sound.<\/p>\n The “do not start with” column exists because those are the moves operators reach for most often, and they usually hide the defect for a while rather than remove it.<\/p>\n Raising melt temperature as the universal fix. It works. That is the problem. Higher melt temperature improves fill, reduces flow lines, softens weld lines and covers a lot of small sins, and it also lengthens cooling time, increases shrinkage variation and degrades heat-sensitive resins. When temperature is used first for everything, the process ends up running hot, slow and close to the degradation limit, with no headroom left when a real problem arrives. Use it as the second or third move on most defects.<\/p>\n Trusting the shift-start settings on a hydraulic machine. Oil viscosity falls as oil warms, and injection speed follows it. This is the mechanism behind the classic pattern of scrap concentrated in the first shots after every startup and every long stop. It is not an operator error. Fix it with an oil temperature control loop, a defined warm-up before the first accepted shot, and closed-loop injection control on the machine.<\/p>\n The edge case worth planning for: plants running short campaigns with frequent changeovers never let the oil settle, so the drift never gets a chance to disappear. If that is your production pattern, the setup discipline above matters more than it does in a continuous three-shift plant, and a servo drive with closed-loop injection earns its cost in scrap rather than in kilowatt-hours.<\/p>\n One material check that belongs upstream of all of this: if part weight drifts between resin lots with no machine change, the melt flow rate of the incoming material has moved. ISO 1133 defines how melt flow rate is determined for thermoplastics, and asking your supplier for lot-level MFR data measured to that standard turns an unexplained process drift into a purchasing conversation.<\/p>\nMove one parameter, then prove it moved something<\/h2>\n
\nChange one thing. Purge the effect of the previous setting through the machine, which usually takes three to five shots depending on cycle time and shot size. Then take your sample. Parts made during the transition are not data.
\nWhen you evaluate, use a measurement rather than an opinion. Weight for fill and pack changes. A dimension with a caliper or gauge for shrinkage and cooling changes. A visual standard, physically kept at the machine, for cosmetic defects. “It looks better” is not a result that survives the next shift handover.
\nIf a change produces no measurable difference, put the parameter back. Carrying forward adjustments that did nothing is how process sheets accumulate settings nobody can explain two years later.<\/p>\nDefect appears: which parameter to move first<\/h2>\n
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\n Defect<\/b><\/strong><\/td>\n Move this first<\/b><\/strong><\/td>\n If that fails, then<\/b><\/strong><\/td>\n Do not start with<\/b><\/strong><\/td>\n<\/tr>\n \n Short shot<\/td>\n Injection speed, then shot size<\/td>\n Melt and mould temperature<\/td>\n Holding pressure<\/td>\n<\/tr>\n \n Flash<\/td>\n Clamp force and mould height calibration<\/td>\n Injection speed, then melt temperature<\/td>\n Tempo di raffreddamento<\/td>\n<\/tr>\n \n Sink marks<\/td>\n Holding pressure, then holding time<\/td>\n Mould temperature at the thick section<\/td>\n Injection speed<\/td>\n<\/tr>\n \n Voids<\/td>\n Holding pressure and time<\/td>\n Cooling time, then melt temperature<\/td>\n Back pressure<\/td>\n<\/tr>\n \n Weld line weakness<\/td>\n Melt and mould temperature<\/td>\n Injection speed at the meeting point<\/td>\n Holding pressure<\/td>\n<\/tr>\n \n Burn marks<\/td>\n Vent condition and clamp force<\/td>\n Injection speed at end of fill<\/td>\n Melt temperature<\/td>\n<\/tr>\n \n Splay or silver streaks<\/td>\n Drying \u2014 check dryer, dew point, hopper time<\/td>\n Back pressure, then melt temperature<\/td>\n Injection speed<\/td>\n<\/tr>\n \n Warpage<\/td>\n Cooling time and mould temperature balance<\/td>\n Holding pressure profile<\/td>\n Melt temperature<\/td>\n<\/tr>\n \n Jetting<\/td>\n Injection speed at start of fill<\/td>\n Gate condition, then melt temperature<\/td>\n Holding pressure<\/td>\n<\/tr>\n \n Flow lines<\/td>\n Injection speed profile<\/td>\n Melt and mould temperature<\/td>\n Clamp force<\/td>\n<\/tr>\n \n Weight drift over a shift<\/td>\n Barrel and oil temperature stability, check ring<\/td>\n Back pressure<\/td>\n Holding pressure<\/td>\n<\/tr>\n \n Dimensional drift over a shift<\/td>\n Mould temperature control, cooling water flow<\/td>\n Cycle time consistency<\/td>\n Shot size<\/td>\n<\/tr>\n<\/tbody>\n<\/table>\n <\/p>\nTwo habits that cost more than they save<\/h2>\n
What the machine gives you during setup: the IJT-SV330<\/h2>\n
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