A part sticks when the force holding it on the steel is more than the ejection system can apply without damaging it. Most sticking comes from overpacking or from ejecting the part at the wrong temperature. Start with hold pressure and cooling time, then look at draft and polish. Sticking is usually not the resin.
Also described as: parts stuck in the cavity, hard ejection, ejector pin marks, sprue sticking.
Quick diagnosis
| What you see | Likely cause | Check first |
|---|---|---|
| The part stays in the cavity half instead of pulling out with the core. | Overpacking, too little draft or a nick in the cavity, or a temperature difference between the halves that holds the part on the wrong side. | Hold pressure and hold time; steel temperature on both halves; the cavity surface under a light |
| The part hangs on the core. Ejector pins dent it or push through it. | The part is too soft at ejection, or it is gripping the core hard: overpacked, too little draft, or cooled so long it has shrunk tight. | Cooling time; core temperature; hold pressure |
| White marks on the part at the ejector pins, ribs or bosses. | The ejection force is more than the plastic can take. Deep ribs and bosses with little draft are the usual place. | Hold pressure; ejection speed; draft and polish in the ribs |
| Drag marks or scuffing down the side walls, in the direction the part is pulled. | Too little draft for the depth of the texture, polish marks that run across the direction of pull, or overpacking. | Draft against texture depth; polish direction; hold pressure |
| A deep, smooth part such as a cup releases with a pop, or collapses inward. | A vacuum forms between the part and a highly polished core as the part is pushed off. | Air poppet or vent in the core; how highly the core is polished |
| The sprue stays in the bushing. | The sprue is not frozen, the bushing is rough or scored, or the nozzle orifice is larger than the bushing opening and leaves an undercut. | Nozzle orifice against sprue bushing opening; cooling time; the sprue puller |
| Sticking started with a new lot or a new resin. | The new material shrinks by a different amount, flows more easily and packs harder, or has a different lubricant package. | Shrinkage of the old and new grade; melt flow on the COAs; whether the grade names a mold release |
Checks in order, cheapest first
- Note where the part sticks and on which half. Run a few cycles by hand and watch. Mark the spot on the part. The location tells you whether to look at packing, the core, a rib or the sprue.
- Lower hold pressure and hold time in steps. Overpacking is the most common cause. Weigh the parts as you go and watch for sink marks. Also check that the cavity is not being filled hard on the first stage before transfer.
- Change the cooling time, and measure the steel. A part that is too hot is soft and the pins push into it. A part cooled too long shrinks tighter onto the core. Try a few seconds either way. Measure both halves, since a hot core is a common cause.
- Slow the ejection. A slower, shorter first stroke gives air time to get behind the part and lowers the load at each pin. Check that every pin moves freely and that the ejector plate is not cocked.
- Clean the mold, and use mold release only as a test. Deposits on the steel can grip the part. A light spray of release that cures the sticking tells you the surface is the issue. It is not a fix: release transfers to the part, spoils paint and print adhesion, and can cause delamination.
- Compare lots. If sticking arrived with a new lot, compare melt flow on the COAs and ask whether the lubricant or mold release package changed.
- Look at the tool. Check for burrs, nicks, rolled edges at the parting line and undercuts that were never meant to be there. Polish in the direction of pull. Add draft where ribs and bosses drag. More or larger ejector pins, sleeves or an air poppet spread the load.
- Change the resin last. If packing, temperature and the steel are right and the part still sticks, a grade with a mold release or one that sets up faster is the next place to look.
Safety. Do not reach into a mold to free a stuck part with the press in cycle. Follow your lockout procedure, and use brass or other soft tools so the steel is not damaged.
The numbers
Typical melt temperature, mold temperature and mold shrinkage. Low-shrink resins such as ABS, polycarbonate, polystyrene and acrylic hardly pull away from the cavity, so they are the easiest to overpack and need the most draft. High-shrink resins pull away from the cavity and grip the core. Where two shrinkage figures are given, the first is along the flow and the second is across it.
| Resin | Melt temperature | Mold temperature | Shrinkage, unfilled % |
|---|---|---|---|
| ABS | 425–525°F / 218–274°C | 80–180°F / 27–82°C | 0.4–0.7 |
| Polycarbonate | 536–653°F / 280–345°C | 158–248°F / 70–120°C | 0.5–0.7 (one producer gives 0.70 / 0.75) |
| General-purpose polystyrene | 356–500°F / 180–260°C | 50–150°F / 10–66°C | 0.3–0.8 |
| Acrylic (PMMA) | 340–520°F / 171–271°C | 100–205°F / 38–96°C | About 0.4 typical |
| Nylon 6 | 482–536°F / 250–280°C | 104–212°F / 40–100°C | 0.90 / 0.90 |
| Nylon 66 | 527–580°F / 275–305°C | 140–212°F / 60–100°C | 1.50 / 1.80 |
| Acetal copolymer | 370–446°F / 188–230°C | 140–250°F / 60–121°C | 1.2–2.4 |
| Polypropylene homopolymer | 374–525°F / 190–274°C | 50–203°F / 10–95°C | 0.8–2.5 |
| HDPE | 380–525°F / 193–275°C | 50–125°F / 10–50°C | 1.70–2.90 |
Typical values from published processing guides. The right setting depends on the grade, the wall thickness and the machine, so confirm against the data sheet for your grade. The full list is in the melt temp, mold temp and shrinkage chart.
Why parts stick
- Overpacking leaves pressure in the cavity. If the plastic is still pressed against the steel when the mold opens, friction holds it there. Lowering hold pressure lets the part shrink away from the cavity walls.
- Plastic shrinks onto cores and away from cavities. That is why the part normally stays on the core half, where the ejectors are. Anything that upsets it (an overpacked cavity, a hot core, a rough cavity wall) can leave the part on the wrong side.
- Draft is what lets the part go. With enough taper, the part is free of the wall as soon as it moves. With too little, it drags the whole way. Textured walls need more draft than polished ones, and deeper textures need more again.
- Polish matters in both directions. A rough surface, or polish lines across the direction of pull, grips the part like a file. A mirror finish on a deep core can seal so well that a vacuum holds the part, most often with soft and flexible resins.
- Temperature at ejection sets how much force the part can take. A part ejected hot is soft, so pins sink in. Brittle resins such as general-purpose polystyrene and acrylic tend to crack at the pins. Tougher resins such as polypropylene, ABS and impact polystyrene turn white where they are over-stressed.
- Many grades carry a lubricant package. An internal lubricant or mold release in the grade lowers the friction against the steel. Two grades with the same data sheet properties can release very differently if one has it and the other does not.
Material changes, and what each one costs you
| Change | Why it helps | What it costs you |
|---|---|---|
| A grade with an internal mold release or lubricant | Lower friction against the steel, so less ejection force. | It can interfere with painting, printing, plating, adhesive bonding and welding, and it can leave deposits on the tool. A new grade for your customer to approve. |
| A nucleated or fast-cycling grade (polypropylene, nylon, PBT) | The part crystallizes sooner and is stiffer when the pins hit it. | Different shrinkage, so dimensions and warp can change. A new grade for your customer to approve. |
| A stiffer grade, or a filled grade | A stiffer part takes more ejection force before the pins mark it. | Lower impact strength in most cases, different shrinkage, a different surface, and more tool wear with glass. |
| A higher melt flow grade | It fills and packs at lower pressure, which leaves less pressure in the cavity at mold open. | Lower molecular weight, which usually means lower impact strength and stress-crack resistance, and more tendency to flash. |
| The lubricant package written into the specification | Sticking that arrives with a new lot or a second source often traces to a missing or different release additive. | Nothing in the part. It may narrow your choice of sources. |
Any resin change moves your dimensions. The tool was cut for the shrinkage of the resin you run now. A resin that shrinks less makes a bigger part from the same steel, so check critical dimensions before you approve a swap.
Often confused with
- Ejector pin marks. Every part shows a faint outline where each pin sits, and that is normal. A pin mark that is pushed in, raised, glossy or white means the part resisted ejection or was too soft. A pin that is simply too long or too short leaves a clean step with no whitening.
- Stress whitening. White marks at pins, ribs and bosses straight out of the mold are ejection damage. Whitening that appears later, where the part was bent, snapped together or hit, is the material being over-stressed in use. See brittle or cracking parts.
- Warp. A part bent by the ejectors shows pin marks or whitening where it was pushed. Warp from uneven shrinkage does not. See warped parts.
- Flash holding the part. A thin fin at the parting line, around a pin or in a vent can lock the part in place. Fix the flash and the sticking goes with it. See flash.
Quick answers
Why are my parts sticking in the mold?
Most sticking comes from overpacking or from ejecting the part at the wrong temperature. Overpacking leaves the plastic pressed against the steel when the mold opens, and friction holds it there. Too little draft, a rough or nicked surface and a hot core are the other usual causes. Sticking is usually not the resin.
How do I stop parts sticking to the cavity?
Start by lowering hold pressure and hold time in steps, since overpacking is the most common cause. Then measure steel temperature on both halves and look at the cavity surface under a light for nicks and too little draft. Do not reach into a mold with the press in cycle. Follow your lockout procedure.
What causes ejector pin marks?
Pin marks that are pushed in, raised, glossy or white mean the part resisted ejection or was too soft when the pins hit it. Check hold pressure, cooling time, ejection speed, and draft and polish in the ribs. A faint outline at each pin is normal, and a pin that is too long or too short leaves a clean step.
Should I use mold release spray to stop sticking?
Use it only as a test. A light spray that cures the sticking tells you the steel surface is the issue. It is not a fix, because release transfers to the part, spoils paint and print adhesion, and can cause delamination. Clean the mold first, since deposits on the steel can grip the part.
Why did sticking start with a new lot of resin?
The new material may shrink by a different amount, flow more easily and pack harder, or carry a different lubricant package. Compare melt flow on the COAs and ask whether the lubricant or mold release package changed. Two grades with the same data sheet properties can release very differently.
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