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Home / Troubleshooting Short Shots: Flow and Pressure in the Injection Molding Process

Troubleshooting Short Shots: Flow and Pressure in the Injection Molding Process

2025/09/15 By le zhan

Injection Molding Process 1-2

Short shots, flash, weld lines, and inconsistent part weight—these are symptoms, not the root causes. At the heart of many recurring defects lies the relationship between flow and pressure in the injection molding process. First, we must understand the core facts. First, polymer melts behave differently from water; their viscosity is highly dependent on temperature and shear. Second, injection molding machines are energy-constrained systems. Their screw speed, pump capacity, barrel temperature, and nozzle geometry create a layered hierarchy of constraints that dictate the achievable flow and pressure combinations. Addressing these defects requires proper part design, mold hydraulics, and injection molding machine performance to ensure the melt front fills the cavity before cooling locks in, avoiding excessive pressure at the gate and bottlenecks.

Melt Rheology, Pressure Drop, and Injection Molding Machine Performance in the Injection Molding Process

To address defects related to flow and pressure, it’s important to understand the fundamental principles of melt delivery control in the injection molding process. Polymer viscosity is not constant, and many engineering plastics exhibit shear-thinning properties and are highly temperature-sensitive. This means that flow rate, applied shear, and barrel/nozzle temperature form a feedback loop. Increasing injection speed increases shear heating and reduces apparent viscosity, which can improve filling, but may also raise melt temperature and risk degradation.

During pressure drop, the system needs a pressure head capable of overcoming flow resistance to push the melt through runners, gates, and thin walls. In practice, the required injection pressure increases with increasing flow rate and path difficulty. Therefore, designers often use narrow gates, which reduce the instantaneous flow area and thus require higher nozzle pressure.

Injection molding machines typically deliver energy in two ways: volumetric rate and pressure capacity. For hydraulic presses, peak pressure and available flow are determined by pump displacement, drive power, and relief valve settings. For all electric systems, motor torque and screw design limit the achievable pressure at a given speed. Therefore, it’s important to understand the key practical specifications for each injection molding machine in your facility: maximum injection pressure, maximum injection speed or volumetric flow rate, screw/plunger stroke and diameter, and pressure/speed control response time. This data can help you determine whether a short shot is a machine limitation or a molding process issue.

Melt Rheology, Pressure Drop, and Injection Molding Machine Performance in the Injection Molding Process

The Impact of Mold and Part Design on the Injection Molding Process

Adjusting gate size, runner layout, venting, and wall thickness is often a quicker and more economical way to address deficiencies than replacing the injection molding machine. In injection molding, each of these design elements directly impacts the required injection pressure and achievable flow rate.

Gate size determines the local shear rate and pressure drop as the melt enters the cavity. A small gate can prevent the injection molding machine from achieving the required flow rate, resulting in slow filling or undershot. Conversely, an oversized gate reduces injection pressure requirements but may result in jetting, weakened weld line strength, or cosmetic issues. For multi-cavity molds, ensure that runner resistance is balanced so that all cavities achieve similar pressures and fill times.

Long runners or thin walls significantly increase pressure requirements. The pressure required to fill long, narrow runners is directly proportional to runner length and inversely proportional to thickness. If a part has thick-to-thin transitions or thin ribs, consider carefully before attempting radical injection molding machine modifications. Furthermore, insufficient venting can cause trapped air to obstruct melt flow. Despite increasing pressure, filling stalls often leads to incomplete filling or burning. Effective venting at weld lines, deep ribs, or corners allows air to escape, reducing the required injection pressure. For these issues, subtle but targeted modifications, such as slightly increasing the gate diameter, shortening the flow length with springback beads, or improving the vent size, can often restore a complete part without significant mold downtime.

Injection Molding Mold

Speed-Pressure Curves, Switchover, and Advanced Control

In the injection molding process, determining when to switch from speed control to pressure control and how to adjust the speed and pressure curves is crucial. This decision determines whether the cavity fills, whether holding pressure stabilizes the material, and whether the process minimizes defects. A common practice is to use speed-pressure switching, where operators inject at a specified screw speed until they reach the set injection position or a preset switchover point, and then switch to pressure control for holding pressure.

Of course, many parts benefit from a phased injection process, rather than a single speed. Initially, a strong injection speed is used to fill long runners or gates, followed by a gentler injection speed in the middle to limit shear, and finally, a high pressure in the final stage to compensate for solidification. Similarly, holding pressure can be multi-staged, with peak pressure applied immediately after the switch to the critical holding zone, followed by a gradual reduction in pressure to avoid excessive flash while maintaining dimensional stability.

Furthermore, Topstar’s injection molding machines support adaptive control: real-time feedback from cavity pressure, screw position, and barrel pressure adjusts speed and pressure to keep the process within a defined window. This allows for the conversion of marginally short processes into stable production. In production, the maximum injection speed can be limited to within the acceptable range for the nozzle and gate. Use shot size monitoring and cross-check with cavity pressure to detect short circuits early.

Targeted Fixes for Common Flow and Pressure-Related Defects

In production, defects can occur quickly and require rapid diagnosis. Below are common symptoms you may encounter during injection molding, their causes, and precise corrective measures.

Short shot causes Include Insufficient pressure or flow to fill the mold cavity before the gate freezes, runner blockage, and decreased melt fluidity. Solutions: Increase injection speed and/or injection pressure, moderately increase melt temperature to reduce viscosity, enlarge the gate or smooth the gate entrance, improve venting, and check for nozzle/check valve blockage.

Scorch marks and air pockets: Caused by compression of the melt as it advances through poorly vented areas, resulting in air trapping. Solution: Add or enlarge the vent, reduce injection speed to allow air to escape, and decrease back pressure to promote flow.

Flash: Caused by excessive holding pressure or injection pressure, as well as excessive clamping force loss. Common solutions include reducing holding pressure or holding time, checking clamping force and tie rod alignment, and inspecting the mold seals or ejector bosses for wear.

Jet and flow marks: Excessive melt velocity at the gate causes jetting, and the gate geometry may be inappropriate. Solution: Reduce the initial fill speed or reshape the gate to create a laminar inlet.

Causes of weight variation or cycle instability include inconsistent shot weight, screw slippage, compressibility, or outgassing. Check the shot weight control and rezero the scale, inspect the screw head and rings for wear, and calibrate the injection molding machine pump/servo drive.

Targeted repairs to common injection molding flow and pressure-related defects

Troubleshooting and Preventing Flow and Pressure Insufficiency

To address flow and pressure insufficiency in the injection molding process, it’s necessary to instrument the process, systematically test variables, and implement a control strategy that matches the injection molding machine’s capabilities to the mold design. In production, this means collecting baseline data, conducting controlled experiments, varying one variable at a time, prioritizing cavity pressure-based switching and staged speed/pressure profiles over simple fixed-position control, and validating corrective actions through capability studies. This disciplined approach can shorten the time to root cause, prevent recurrence of defects, and safeguard production capabilities.

 

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