Packing Pressure and Hold Time: How to Set Them From the Part

What packing actually does

When the screw stops, the cavity is full but the part is not finished. The polymer is cooling and shrinking, and that shrinkage has to be made up with more material — otherwise the part pulls away from the cavity walls, sinks, and ends up underweight and undersized.

Packing pressure does that make-up. It pushes additional melt through the still-open gate to compensate for shrinkage. Hold time is how long that pressure is maintained. Both are set by the part, not by preference, and both have an exact correct answer that can be measured.

The three phases of packing

Table 1 — The phases of a cycle after filling. Everything after gate freeze is cooling, and extending hold beyond gate freeze does nothing except waste time.

Gate freeze is the pivot. Before it, hold pressure controls part weight and dimensions. After it, hold pressure has no effect at all, because there is no path for material to enter. Running hold time past gate freeze is the most common wasted second in injection moulding.

Finding gate freeze

There are two ways: with a cavity pressure sensor, and without one. The sensor method is faster and more accurate; the weight method works on any machine.

With a sensor. Plot cavity pressure against time. The pressure holds flat during packing and then drops when the gate freezes and the part starts to shrink away from the sensor. The time at which the drop begins is gate freeze.

Without a sensor. Run a series of parts at increasing hold times, weighing each one. Part weight rises with hold time and then plateaus. The point where the curve flattens is gate freeze. It takes six to eight parts and about twenty minutes, and it needs no instrumentation.

Table 2 — The weight-plateau method. Gate freeze is where the weight stops changing, which here is about 5 seconds. Hold time should be set at 5 seconds, not 8.

The plateau method has one subtlety: it must be run at the intended pack pressure. At a low pack pressure the gate may freeze earlier because less hot melt is flowing through it; at a high pack pressure the gate stays open longer. Set pack pressure first, then find gate freeze.

Setting pack pressure

Pack pressure is expressed as a percentage of injection pressure on most machines, which makes it machine-specific and not portable between presses. The useful number is cavity pressure, and the useful target is a part that is dimensionally correct and visually acceptable.

Table 3 — Pack pressure effects. Both too low and too high cause warpage, which is why warpage alone does not tell you which way to go.

The practical procedure is to start at a pressure that clearly produces a short shot or visible sink, then raise it in steps until the part is dimensionally correct and cosmetically acceptable, then check for flash. The correct setting is the lowest pressure that produces an acceptable part — not the highest that does not flash.

Running at the lowest acceptable pressure matters because higher pack pressure means higher residual stress, which means more warpage and more stress cracking later. It also means more clamp force required and more wear on the tool.

Packing profiles

Single-stage packing is fine for simple parts. For parts with thick sections, ribs or a long flow path, a two-stage or three-stage profile gives better results.

Table 4 — Packing profiles. The two-stage profile is the most common upgrade: it packs the thick section early while the gate is open, then reduces pressure to avoid over-packing the thin sections.

The reason for reducing pressure later is that thin sections freeze first. Once they are solid, additional pressure cannot pack them, but it still acts on the gate area and on the still-molten core of thick sections. A high pressure held too long over-packs the gate region, which shows as a stress halo around the gate and sometimes as a small crack.

How the resin changes the numbers

Table 5 — Typical packing values by resin. The gate freeze times assume a 1.5–2 mm gate; a bigger gate stays open longer.

Gate size is the strongest influence on freeze time, and it is fixed in the tool. A 1 mm gate on a 3 mm wall freezes in about a second, which leaves almost no packing window and produces sink no matter what the machine does. That is a tooling problem, and no process setting fixes it.

Diagnosing from the part

Table 6 — Reading the part for packing problems. Weight variation is the most diagnostic: a stable process produces parts within 0.2% of each other.

Part weight is the single best production indicator of packing stability. If the weight varies by more than about 0.3% between shots on a stable process, something is changing — switchover position, melt viscosity, or check-ring leakage. Weighing ten parts once an hour catches most packing problems before they become a customer complaint.