Packing Is Where Most Injection Molding Quality Is Decided
Ask a molder what stage ruins the most prototype parts and the honest answer is not filling — it is packing. Fill gets the polymer to the far end of the cavity; packing decides whether that part comes out dimensionally correct, sink-free, and repeatable. Yet packing is routinely treated as a single number ("hold at 60% of injection pressure") when it is really a time-pressure curve. Getting the curve right is one of the cheapest quality upgrades available, because it costs nothing but process discipline.
What the Packing Stage Actually Does
As molten plastic cools inside the cavity it shrinks — typically 0.5% for filled PC, up to 1.5–2% for unfilled PP and 0.4–0.7% for ABS. Without compensation, that shrink shows up as sink marks over ribs and bosses, voids in thick sections, and dimensions that land below print. The packing stage pushes a small additional volume of melt into the cavity to offset shrinkage until the gate freezes. Two consequences follow:
- Everything depends on gate seal time. Pressure applied after the gate freezes does nothing. Pressure cut off before seal leaves the part under-packed.
- More packing pressure is not automatically better. Over-packing locks in residual stress near the gate, makes parts stick in the cavity, and can flash a mold that was borderline.
Building a Packing Profile That Works
Step 1: Find the gate seal time by weighing parts
Set a packing time, shoot a part, weigh it. Increase packing time in 0.5–1 s steps and weigh again. Part weight climbs and then plateaus — the plateau start is the gate seal time. A 2 mm wall ABS part through a 1.2 mm edge gate might seal in 4–6 s; a thicker wall with the same gate can seal in 10 s or more. Packing time beyond seal is wasted cycle time, and on prototype molds run at 40–60 s cycles, 5 wasted seconds is 10% of your throughput.
Step 2: Start the pressure where the fill left off
Most modern presses switch from velocity to pressure control at 95–99% fill. The packing pressure at switch-over should be close to the pressure the melt already has — often 50–80% of peak injection pressure for semi-crystalline materials, less for easy-flow ABS. Starting too low lets the melt backflow through the gate briefly, which shows up as gates with dull, drool-heavy vestiges.
Step 3: Profile down, don't hold flat
The cavity pressure decays as the part cools, so the ideal packing curve is usually descending: full compensation pressure right after switchover, stepping down (2–4 segments) toward the end of hold. A flat profile over-packs the region near the gate during the second half of the hold. Profiling matters most for:
- Parts with gate-side cosmetic surfaces (less stress whitening and gloss mismatch)
- Glass-filled grades like PA66-GF30, where over-pack amplifies warpage from differential fiber orientation
- Prototype molds in aluminum, which reach steady state faster and punish aggressive over-pack with flash
Step 4: Validate with the right checkpoints
Don't judge the profile from one shot. We check:
- Part weight stability across 5 consecutive shots — within roughly 0.2–0.5% means the process is stable.
- Sink depth over the thickest rib/boss, by feel or gauge, not just by eye.
- Dimensional check on the feature tied to the thickest wall, since shrinkage lands there first.
- Flash check at the parting line, especially on aluminum prototype tooling.
Material Notes That Change the Numbers
- ABS: forgiving, wide processing window, shrink 0.4–0.7%. A good first material to learn profiling on.
- PC: stiff melt and higher viscosity; needs higher packing pressure and is notch- and gate-stress sensitive. Keep hold pressure honest but watch gate-area stress.
- PP: shrink up to 1.5–2% and strong sink behavior; compensate generously but expect longer effective hold on thick sections.
- PA12 (SLS parts) is a different world: no packing stage at all, which is why printed prototypes with ±0.2–0.3 mm dimensional spread can't match plastic injection parts held to ±0.05–0.1 mm.
Prototype-Mold Specifics
Prototype and soft tooling molds change the packing conversation in three ways. Aluminum transfers heat faster than P20 steel, so cycle times are short and seal times arrive sooner. Aluminum also flashes earlier, which caps how hard you can pack — and that is fine, because the right answer for most prototype parts is moderate pressure plus a correct profile, not brute force. Finally, if the tool is a bridge to production, document the validated packing curve: it becomes the transfer baseline for the production mold, and it surfaces gates or wall sections that will need redesign before hard tooling.
Common Failure Patterns and the Fix
- Sink over boss: boss wall too thick or hold pressure insufficient after seal check — reduce boss cored diameter, then re-run the seal-time test.
- Flash on one side: often unbalanced fill or clamp tonnage margin, not just packing pressure. Check vent closure and fill balance first.
- Part sticks on ejection after heavy pack: over-pack near gate — introduce profile step-down or shorten hold.
- Weight drifts shot to shot: nonuniform regrind/moisture or inconsistent cushion — stabilize the front end before tuning the profile.
Packing profiles reward patience: an hour of weighing parts and stepping the curve buys you dimensionally stable parts for the rest of the program. If you want a second pair of eyes on a molding DFM question before tooling starts, talk to our engineering team — fixing gate or wall-section issues at the drawing stage is dramatically cheaper than re-cutting steel later.
