There are two different problems here. In one, every part is off by about the same amount. In the other, parts vary from shot to shot or from hour to hour. Sort out which one you have before you change anything, and start with how and when the parts are measured. After that, part weight is the fastest check you have.
Also described as: out of spec dimensions, size drift, parts too big or too small, inconsistent shrinkage.
Quick diagnosis
| What you see | Likely cause | Check first |
|---|---|---|
| Dimensions wander from shot to shot, and part weight wanders with them. | Packing is not repeating. The cushion is drifting, the check ring is leaking or screw recovery is uneven. | Weight of 20 parts in a row; the cushion on each shot |
| Parts drift for the first hour of a run or after a stop, then settle. | The mold steel has not reached a steady temperature. Barrel and hydraulic oil temperatures move as well. | Steel temperature over time; water temperature and flow |
| Every part is small, or every part is large, by a similar amount. | Hold pressure, hold time or mold temperature is set wrong for this resin, or the steel was cut for a different shrinkage. | Hold time against gate freeze; steel temperature; the steel dimension against the part |
| Parts pass at the press and fail the next day or at the customer. | They were measured too soon. Shrinkage continues after molding, most of all in polypropylene, polyethylene, acetal and nylon. | Measure again at 24 hours (48 for polyolefins) |
| Nylon parts measure differently from week to week, or grow in storage. | The parts are taking up moisture from the air and swelling. | How long the parts sat, and in what humidity, before they were measured |
| One cavity runs out of tolerance and the others are good. | The cavities do not fill, pack or cool alike, or the steel differs. | Measure and weigh by cavity number; a fill-only shot |
| The shift arrived with a new lot, a new color or a new resin. | Melt flow or filler content moved, the pigment changed how the resin crystallizes, or the new resin shrinks by a different amount. | COA of the good lot against the bad lot; shrinkage of the old and new grade in both directions |
Checks in order, cheapest first
- Fix the measurement first. Measure parts at the same age, at the same temperature, on the same fixture, with the same gage and the same method. A part measured warm at the press and a part measured the next day are not comparable. If two people get different readings on the same part, the gage or the method is the first problem.
- Weigh the parts and watch the cushion. Part weight follows packing. If the weight moves with the dimension, look at the cushion, the check ring and screw recovery. If the weight is steady and the dimension still moves, look at temperature, moisture and measurement.
- Let the process settle, and measure the steel. Do not adjust on the first shots after a stop. Measure the mold surface on both halves once the tool is at a steady state, and check that water temperature and flow have not changed since the last good run.
- Confirm hold time and hold pressure. Raise hold time in steps and weigh the parts. When the weight stops rising, the gate is frozen. A hold time set close to gate freeze gives parts that vary with every small change in temperature.
- Hold drying and regrind constant. Wet resin and regrind both make the melt thinner, so the same settings pack the part differently. Keep the moisture and the regrind ratio the same on every run.
- Compare lots and colors. If the shift arrived with a new lot or a new color, compare melt flow and filler content on the COAs before you touch the process.
- Look at the tool. Check gates for wear, vents for buildup and water lines for scale. A blocked water circuit or a worn gate changes one cavity and leaves the others alone.
- Change the resin last. Drift from shot to shot is usually not the resin. A steady offset that began with a resin change usually is, because the tool was cut for the old shrinkage.
The numbers
Typical mold shrinkage, in percent. Where two figures are given, the first is along the flow and the second is across it. A resin with high shrinkage, or a large gap between the two directions, gives the process more to hold steady.
| Resin | Unfilled | Filled |
|---|---|---|
| ABS | 0.4–0.7 | n/a |
| Polycarbonate | 0.5–0.7 (one producer gives 0.70 / 0.75) | 0.1–0.4 along the flow (10–30% glass) |
| PC/ABS | 0.5–0.9, nearly the same in both directions | 0.15–0.35 / 0.30–0.50 (30% glass) |
| Nylon 6 | 0.90 / 0.90 | 0.30 / 0.75 (30% glass) |
| Nylon 66 | 1.50 / 1.80 | 0.50 / 1.10 (30–33% glass) |
| PBT | 1.6–2.0 | 0.30 / 1.10 (30% glass, one producer) |
| Acetal copolymer | 1.2–2.4 | See the grade sheet |
| Polypropylene homopolymer | 0.8–2.5 | 1.10–1.50 with 20% talc; 0.70–1.00 with 40% talc; 0.24 / 1.15 (30% glass) |
| HDPE | 1.70–2.90 | n/a |
Typical values from published processing guides. They are for comparing materials and are not tool design figures. Shrinkage depends on the grade, wall thickness, gate and packing, so confirm against the data sheet for your grade. The full list is in the melt temp, mold temp and shrinkage chart.
Why dimensions move
- Shrinkage is a result of the process as well as the resin. Published figures come from a standard test plaque. One producer lists 1.50 / 1.80% for unfilled nylon 66 on the plaque and 0.81% on a constrained real part. Your part will have its own number, and it holds only while the process holds.
- Packing sets how much the part shrinks. More hold pressure, held until the gate freezes, puts more material in the cavity and gives a larger part. Anything that changes the pressure reaching the cavity changes the dimension: cushion, check ring, melt temperature and melt flow.
- Mold temperature works in two directions. A hotter mold gives more shrinkage in the mold and less afterward. A cold mold gives semi-crystalline parts that measure well at the press and then keep moving, sometimes in service.
- Parts keep shrinking after ejection. One polyolefin producer puts a part at about 90% of its total shrinkage after 48 hours, and longer if parts are packed hot. Cooling parts in water or a refrigerator to measure them sooner measures thermal contraction and not shrinkage.
- Nylon and acrylic grow as they absorb moisture. A nylon part measured dry at the press is at its smallest. For acrylic, one producer publishes growth of 0.1% at 40% relative humidity, 0.2% at 65% and 0.3% at 80%. See nylon and moisture.
- Lots are not identical. A lot at the high end of the melt flow range packs differently from one at the low end. In filled grades, a small change in filler content changes shrinkage. Both figures are on the COA. See melt flow index explained.
- Glass fiber makes the gate part of the dimension. Glass-filled grades shrink much less along the flow than across it. The dimension then depends on the direction of fill, and a gate change can move it more than any process setting.
Material changes, and what each one costs you
| Change | Why it helps | What it costs you |
|---|---|---|
| A melt flow range and a filler range on the specification | Dimensions that move by lot often trace to melt flow or filler content moving between lots. | Nothing in the part. It takes an incoming check, and it may narrow your choice of sources. |
| A replacement grade matched for shrinkage in both directions | When you add a second source or replace a discontinued grade, a match on shrinkage keeps the parts in tolerance from the same steel. | A narrower choice of grades, a trial run, and a new grade for your customer to approve. |
| An amorphous resin in place of a semi-crystalline one | Lower and more even shrinkage, and less change after molding. | Weaker chemical resistance and a higher risk of stress cracking near oils and solvents. The part will come out larger from the same steel, so the tool may need work. |
| A glass-filled or mineral-filled grade | Lower shrinkage and less growth with heat and moisture. Mineral fillers shrink more evenly than glass fiber. | Lower impact strength in most cases, a different surface, more tool wear with glass, and with glass fiber uneven shrinkage that can cause warp. |
| For nylon parts that grow, a resin that absorbs less water | Nylon 11 and nylon 12 absorb far less water than nylon 6 and 66. Acetal and PBT absorb very little. | A different price, different strength and heat resistance, different shrinkage, and a full part approval. |
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
- Warp. Warp changes the shape of the part: it bows, twists or cups. A dimensional problem changes the size and leaves the shape alone. The two often come together. See warped parts.
- Nylon growing with moisture. A nylon part that was in tolerance when molded and is larger weeks later has absorbed water. No press setting will change that. See nylon and moisture.
- Flash. A thin fin at the parting line makes a part read large across that line. Look at the edge before you chase shrinkage. See flash.
- Change in service. A part that is in tolerance when shipped and out of tolerance after time under load or heat is creeping or softening. See parts softening under heat.
Quick answers
Why are my parts out of tolerance?
There are two different problems: every part is off by about the same amount, or parts vary from shot to shot. Sort out which it is first, starting with how and when parts are measured. A steady offset usually points to hold pressure, hold time, mold temperature, or steel cut for a different shrinkage. Part weight is the fastest check.
Why do part dimensions change from shot to shot?
Shot-to-shot variation usually means packing is not repeating: the cushion is drifting, the check ring is leaking or screw recovery is uneven. Weigh 20 parts in a row and watch the cushion on each shot. If the weight is steady, look at temperature, moisture and measurement. Drift from shot to shot is usually not the resin.
Why are my parts bigger after changing resin?
The tool was cut for the shrinkage of the old resin, and a resin that shrinks less makes a bigger part from the same steel. A steady offset that began with a resin change usually is the resin. Compare the shrinkage of the old and new grade in both directions, and check critical dimensions before approving a swap.
When should molded parts be measured?
Measure parts at the same age, at the same temperature, on the same fixture, with the same gage and method. Shrinkage continues after molding, most of all in polypropylene, polyethylene, acetal and nylon. Parts that pass at the press and fail the next day were measured too soon, so measure again at 24 hours, or 48 for polyolefins.
Why do nylon parts grow after molding?
Nylon parts take up moisture from the air and swell. A nylon part measured dry at the press is at its smallest, and one that was in tolerance when molded can be larger weeks later. No press setting will change that. Note how long the parts sat, and in what humidity, before they were measured.
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