How to Set the Injection to Packing Switchover: Time, Pressure or Position

Same machine, same mould, same material, and the part weight still scatters from one shift to the next. More often than not the cause sits at the instant injection changes to packing, because that is where the amount of material in the cavity is settled.

What triggers the handover decides whether that point can repeat. Time, pressure and position look at three different quantities, and the difference is not an accuracy figure.

Why There Is a Switchover at All

Injection is velocity controlled: the screw is pushed forward at a set speed and drives melt into the cavity. Packing is pressure controlled: the screw barely moves while pressure is held to compact the material and compensate shrinkage.

The two stages aim at different things, so a moment has to exist where control passes from velocity to pressure. Where that moment sits is effectively a statement about how full the cavity is.

Switch too early and no amount of packing recovers the shortfall. Switch too late and the cavity is already over-pressurised, bringing flash and internal stress. The switchover point is a fill criterion, not a time stamp or a pressure number.

Three Triggers, One Thing They Differ In

There are three ways a controller decides that the moment has come: the injection timer expires, the injection pressure reaches a threshold, or the screw arrives at a position.

All three work. What separates them is how they behave when the process moves underneath them. Only one question matters: under the same conditions, does it switch at the same fill level every time?

The third row is steady for a plain reason: position is a geometric quantity. It has nothing to do with oil temperature, viscosity or cavity resistance. The screw is at that coordinate or it is not.

Time Triggering: Easiest, and First to Drift

Set a time and switch when it expires. No position feedback needed, which is why the method survives on older machines.

The trouble is that how much material goes in during a fixed time depends on melt viscosity and cavity resistance. Warmer oil lowers viscosity and fills faster within the same window; poor venting or a worn gate fills less.

On the floor it appears as weight running high for the first couple of hours and then falling off as the machine warms, or as the whole curve shifting when a new material batch arrives. Time triggering does not create process variation, it translates it directly into part variation.

Pressure Triggering: Fast, but It Reads an Instant

Switch when injection pressure crosses a threshold. This sits closer to the fill level than a timer does, since rising pressure means the cavity is filling.

But it reads an instantaneous value, and instantaneous pressure carries noise. Pressure fluctuates during injection, and valve shift, high shear at the gate and the folding front of the flow all make the reading twitch. Set the threshold close and it triggers early; set it far and the switch happens after the cavity is already full.

There is also unavoidable lag. The pressure element has its own response time, and a further interval passes before the controller acts on it. On large machines with long runners that interval grows.

Position Triggering: It Waits for the Screw

Screw position is delivered continuously by a magnetostrictive displacement sensor on the injection unit, and the controller switches the moment the reading passes the set point.

Its advantage is a single criterion: whatever the oil temperature, viscosity or cavity resistance, the switch happens at that coordinate. Process drift cannot move the switching moment.

Two things need attention. First, repeatability rather than resolution — the consistency of the point rests on repeatability, and more decimal places do not help if the unit does not return to the same place. Second, refresh rate against the injection stage: a controller deciding on the previous cycle position adds a delay of random length to the switchover.

Injection carries the tightest accuracy demand of the measuring points on an injection moulding machine; the grade comparison is set out in clamping force and shot size closed loop.

How the Switchover Point Is Decided

No universal number exists to copy. The point depends on cavity volume, gate size, material shrinkage and the target part weight.

In practice a first value is tried near the point where the cavity is close to full, the part is inspected, and the point is trimmed by weighing. The direction is unambiguous: light parts with sink marks mean the switch came early and the point moves later; flash and high internal stress mean it came late and the point moves earlier.

A change of material, of mould, or even of batch within one grade calls for the point to be confirmed again.

What Early and Late Switching Produce

• Too early: the cavity is handed to packing while still short of full, and the hold phase moves the screw so little that the shortfall cannot be made up. The signs are short shots, sink marks, undersized parts and low weight, with large differences between locations in the same cavity.
• Too late: the cavity is already over-pressurised and more material is forced in. The signs are flash, difficult ejection, internal stress that shows up later as warping or cracking, and damage to the parting line.
• A drifting point: neither early nor late, but different each cycle. Weight scatter is large while the average looks normal, and checking the trigger method pays off faster than adjusting process parameters.

Verifying It: Weighing Beats Watching the Curve

Watching the injection curve shows a trend but not how much material is involved.

Weighing is more direct. Hold the other parameters, step the switchover point forward and back in fixed increments, mould a few parts at each step and record the weight. The sensitivity of weight to the switchover point draws a slope, and the point belongs where that slope is flat and weight is stable.

The same curve also answers whether repeatability is adequate: if consecutive parts within one step already scatter, the problem is not the switchover point but the feedback or the mechanics.

Beyond the feedback, mechanical effects at the end of a stroke are worth a look, as covered in cylinder cushioning and the end of stroke.