Short Shot in Injection Molding: Ten Causes Ranked by How Often They Are the Answer

What a short shot tells you

A short shot is a cavity that did not fill. The part is incomplete, with a rounded or torn edge where the flow stopped, and the location of that edge is the most useful piece of information in the whole diagnosis.

A short shot at the end of flow, far from the gate, is a pressure or a temperature problem. A short shot close to the gate, or in one cavity of a multi-cavity tool, is a restriction or a balance problem. A short shot that moves from shot to shot is a feed or a vent problem.

That single observation eliminates half the list before any setting is touched, and it is the step most often skipped in favour of trying things.

The pressure chain

Filling a cavity is a pressure budget. The machine supplies a pressure at the nozzle, and the melt spends it on the runner, the gate, and the flow through the cavity. If the budget runs out before the cavity is full, the part is short.

• Machine: is the injection pressure limit reached? If the pressure peak sits on the limit, the machine is the constraint and no setting will help.
• Shot size: is the cushion at zero? A shot that is too small runs out of material before it runs out of pressure, and the two look identical on the part.
• Runner and gate: a gate that is too small consumes a disproportionate share of the budget, and it does so at the highest shear.
• Cavity: a thin wall, a long flow path or a tight tolerance all raise the pressure needed.
• Vent: trapped gas resists fill and produces a short shot at the last point to fill, which is where the vent should have been.

The pressure needed to fill a wall scales roughly with the inverse square of the thickness. Halving the wall from 1.2 to 0.6 mm takes the fill pressure from 31 MPa to 180, which is beyond most machines and most tools.

The ten causes, ranked

The ranking below comes from the pattern of what actually turns out to be the answer, not from a list of possibilities. It is worth reading in order, because the first three account for more than half.

Table 1 - The ten causes in the order they turn out to be the answer, with the single observation that confirms each one.

The first three account for about three quarters of cases in a shop that is running an established tool. A tool that is new accounts for a different distribution, where gate size and venting dominate, and a tool that has run for years accounts for a third, where wear and blocked vents dominate.

The diagnosis sequence

The sequence is designed so that each step eliminates a group of causes rather than testing one hypothesis at a time.

1. Look at where the short shot is. Far from the gate means pressure, temperature or speed. Near the gate or in one cavity means restriction or balance.
2. Look at the pressure peak. On the limit means the machine or the tool is the constraint. Well below the limit means the shot ended for another reason.
3. Look at the cushion. Zero means the shot size is the problem, and no amount of pressure will fix it.
4. Look at the trend over five shots. Getting better means a thermal problem. Constant means a restriction. Random means feed or venting.
5. Only then change one setting, and change it in the direction the first four steps indicated.

The most common wasted hour in a moulding shop is raising the injection pressure on a machine that is already at its limit, or on a shot that has run out of material. Both look like a pressure problem on the part and neither is.

Thin wall: where short shots are structural

Below about 1 mm the fill pressure requirement rises so steeply that the process window closes. A 1 mm wall needs about 45 MPa, a 0.8 mm wall 70, and a 0.5 mm wall 180. At that point the machine, the tool steel and the material are all at their limits at once.

Table 2 - Fill pressure and practical flow length ratio against wall thickness. The flow length ratio is the longest flow divided by the wall, and it is the design number that predicts a short shot.

Thin-wall parts need high injection speed, a hot melt, a hot tool and a gate that is large relative to the wall. They also need a material with a low viscosity at high shear, which is why thin-wall grades exist and why substituting a standard grade produces a short shot that no setting will fix.

The flow length to thickness ratio is the design number: a wall of 1 mm with a flow length of 200 mm is a ratio of 200, which is achievable in polypropylene and marginal in polycarbonate. Above about 250, the part needs a different gate position or a thicker wall.

Fixes that make it worse

• Raising the packing pressure. Packing happens after the cavity is full, so it cannot fill a cavity that did not fill. It only adds flash and stress.
• Increasing the injection speed without checking the gate. A higher speed through a small gate raises the shear and the melt temperature locally, which can degrade the material at the gate while the cavity is still short.
• Raising the melt temperature to the top of the window to compensate for a cold tool. It works, and it produces a part with degraded properties and a longer cycle.
• Adding a release agent or a lubricant to the compound. It improves flow and it changes the material, which is not acceptable on a qualified part.
• Opening the vent by machining the parting line. It fixes the vent and creates a flash problem, and the flash is harder to remove than the vent was to clean.

Prevention

• Simulate the fill before the tool is cut, and look at the pressure at the end of fill as a fraction of the machine capacity. Above 80 percent, the process has no margin.
• Size the gate from the wall and the flow length, not from what fits. A gate of 60 to 80 percent of the wall thickness is the starting point for most materials.
• Vent at the last point to fill, and vent the runner as well as the cavity.
• Record the process window at the first trial: the melt temperature range, the speed range and the pressure range in which the part is good. A tool with a narrow window will produce short shots on a hot day.
• Clean the vents on a schedule, not when a part goes short. A blocked vent is a slow failure that looks sudden.

The best predictor of a short shot in production is a first trial that needed more than 80 percent of the available pressure to fill. That tool will run, and it will run short on the day the ambient temperature rises or the material lot changes.