Last updated: 2026-09-05 · 5 min read
Packing Pressure in Injection Molding — What Does It Do, How Much Is Too Much or Too Little, and How Do You Set It for Each Resin?
Packing pressure holds the melt compressed after cavity fill — it pushes additional material into the cavity to compensate for shrinkage as the part cools. Too low: sink marks, voids and undersized dimensions. Too high: flash, over-packing and ejection problems. Set packing pressure at 50–80% of injection pressure, then adjust by weight — increase until part weight stops rising.
- Packing pressure acts after the cavity is filled (first stage switches to second stage). While the part cools, the melt shrinks — volume decreases by 1–3% depending on resin. Packing pressure keeps pushing material into the cavity through the gate to fill the shrinkage void. Once the gate freezes (solidifies), packing can no longer reach the cavity — anything after gate freeze has no effect. For a typical 2mm gate: gate freeze time is 8–15 seconds. For a 0.8mm pin gate: 4–8 seconds.
- Too-low packing pressure: the melt shrinks without compensation — sink marks appear on thick sections, voids form in the center, and dimensions fall below nominal. For a 100mm ABS part: at 30 bar packing, dimension is 99.6mm (undersized by 0.4mm). At 60 bar: 99.9mm. At 80 bar: 100.0mm (nominal). The relationship is linear — each 10 bar increase adds roughly 0.01–0.02mm to the dimension.
- Too-high packing pressure: the cavity is over-packed — excess material is forced in, creating internal stress. The part is difficult to eject (sticks to the core), shows flash at the parting line, and warps after ejection because residual stress is released. For semi-crystalline resins (PP, nylon, POM): over-packing causes post-mold shrinkage that continues for days — dimensions drift below nominal over 48–72 hours.
How Do You Set Packing Pressure for Different Resins?
| Resin | Injection Pressure | Packing Pressure (50–80%) | Packing Time | Notes |
|---|---|---|---|---|
| ABS | 80–120 bar | 40–90 bar | 8–15s | Amorphous — gate freeze is clear |
| PP | 60–100 bar | 30–80 bar | 10–20s | Semi-crystalline — high shrinkage, needs long packing |
| Nylon (PA66) | 80–140 bar | 40–100 bar | 8–15s | Semi-crystalline — moisture-sensitive |
| PC | 100–160 bar | 50–120 bar | 10–20s | Amorphous — high viscosity, needs high packing |
| POM | 70–120 bar | 35–90 bar | 5–12s | Semi-crystalline — fast freeze, short packing window |
The 50–80% rule is a starting point. The exact value depends on part geometry (thick sections need more packing), gate size (small gates freeze faster, reducing effective packing time), and resin (semi-crystalline resins shrink more and need longer packing). The definitive method is the weight-vs.-packing-pressure curve — see Step 2 below.
Gate freeze time — the time after cavity fill when the gate solidifies and packing pressure can no longer reach the cavity. For a 2mm submarine gate: 10–12 seconds. For a 0.8mm pin gate: 5–7 seconds. After gate freeze, additional packing has no effect on part dimensions. The fiber orientation warpage guide shows how decaying packing reduces fiber alignment in glass-filled resins.
What Three Steps Optimize Packing Pressure for a New Mold?
- Start at 50% of injection pressure and run 5 shots — weigh each part. If the average weight is below the target (calculated from part volume × resin density × 98%), increase packing by 5 bar and run 5 more shots. Continue increasing until the weight stops rising — that point is the gate freeze limit. The optimal packing pressure is 5–10 bar below the gate freeze point (this provides a safety margin against process variation). The brittle parts guide explains how over-packing creates residual stress that causes brittle failure.
- Set packing time based on gate freeze — not on a fixed number. Run a packing time study: hold packing pressure constant, vary packing time from 2 to 30 seconds. Weigh the parts. When weight stops increasing with longer packing time, the gate has frozen. Set the machine packing time to the point where weight plateaus — plus 1–2 seconds safety margin. For a 2mm submarine gate: gate freeze is typically 10–12 seconds. For a 0.8mm pin gate: 5–7 seconds.
- Use a decaying packing profile — high pressure first, then taper off. Most modern presses support multi-stage packing: Stage 1 at 80% for 3 seconds (compensates for initial shrinkage), Stage 2 at 50% for 5 seconds, Stage 3 at 30% for remaining time. This reduces residual stress compared to constant packing — the part warps less and ejects more easily. For glass-filled resins: decaying packing reduces fiber orientation warpage by 10–15%.
Decaying packing profile — a multi-stage packing strategy where pressure starts high and tapers off over time. Stage 1 at 80% for 3 seconds compensates for initial shrinkage; Stage 2 at 50% for 5 seconds; Stage 3 at 30% for remaining time. This reduces residual stress compared to constant packing — the part warps less and ejects more easily. The short shots guide covers how packing interacts with fill phase settings.
Packing pressure: 50–80% of injection pressure. Compensates for shrinkage after fill. Too low = sink, voids, undersize. Too high = flash, over-pack, warp. Optimize by weight curve: increase until weight stops rising, then set 5–10 bar below.
- Packing at 50–80% of injection pressure — starting point; adjust by weight curve.
- Weight curve run before production — increase until weight plateaus, set 5–10 bar below.
- Packing time set to gate freeze — not a fixed number; verify with time study.
- Decaying profile preferred — 80/50/30% stages reduce residual stress and warpage.
- Semi-crystalline resins: measure after 48h — post-mold crystallization adds 0.1–0.5% shrinkage.
Frequently Asked Questions
What is the difference between packing pressure and holding pressure?
They are the same thing — different names for the second-stage pressure that compensates for shrinkage after cavity fill. Some press manufacturers call it “holding pressure,” others call it “packing pressure.” The function is identical: push additional melt into the cavity through the gate while the part cools and shrinks. The key parameter is not just the pressure value but also the packing time — it must be long enough to compensate until the gate freezes.
Why does my part dimension drift over 48 hours after molding?
Semi-crystalline resins (PP, nylon, POM) continue to crystallize after molding — the crystallization causes additional shrinkage over 24–72 hours. For PP: post-mold shrinkage adds 0.1–0.3% over 48 hours. For nylon: 0.2–0.5% (also absorbs moisture, which expands the part). Measure dimensions after 48 hours of conditioning (23°C, 50% RH) for the final value. T1 dimensions at 1 hour are always larger than the final dimension.
Can packing pressure fix sink marks on thick sections?
Yes — if the sink mark is caused by insufficient packing. Increase packing pressure by 10 bar and check if the sink mark depth decreases. If it does, the cause was low packing. If the sink mark remains unchanged, the cause is something else: wall thickness ratio (thick section too thick relative to adjacent wall), or the gate has already frozen before packing reaches the thick section. In that case, relocate the gate closer to the thick section or increase the gate size.
Key Takeaways
- Packing = 50–80% of injection pressure. Compensates shrinkage.
- Too low: sink, voids, undersize. Too high: flash, over-pack, warp.
- Optimize by weight curve — increase until weight plateaus.
- Use decaying profile to reduce residual stress and warpage.
Send the part drawing, resin grade and current process sheet — the review checks whether packing pressure and time are set correctly, and specifies the weight-based optimization needed to eliminate sink marks and hold dimensions within tolerance.
Sources: Plastics Technology — Packing Pressure and Holding Time Optimization · Plastics Technology Online — Second-Stage Pressure Settings for Engineering Resins · Society of Plastics Engineers (SPE) — Packing Pressure and Shrinkage Technical Papers · BASF — Processing Guidelines for Engineering Resins · ENGEL — iQ Weight Control: Adaptive Packing Optimization
— Arthur, Tooling Engineer at aCatMould