Oil Flow, Port Diameter and the Risk of Pressure Spikes in Injection Mold Applications
In injection mold engineering, hydraulic cylinder speed can have a direct impact on cycle time and machine productivity.
When a hydraulic cylinder operates a slide, core, pin or other moving component, the designer may want to increase its speed to reduce the duration of the movement.
At first sight, the solution may appear simple:
increase the oil flow.
But increasing hydraulic flow is not simply a matter of enlarging a hole.
A real engineering case handled by the Vega Team demonstrates why increasing the speed of an existing hydraulic cylinder requires careful consideration of the complete hydraulic circuit and, in particular, of the dynamic pressure conditions inside the cylinder.
The Customer was using a CE050 cylinder that was operating correctly, but the stroke was considered too slow. The hydraulic system was operating at 160 bar, with an 8 mm line that was reduced to 5 mm for the final 20 mm before reaching the cylinder ports. The Customer therefore asked whether the cylinder ports could be enlarged and whether this modification would increase stroke speed.
The Vega Team’s response highlighted a critical engineering consideration: enlarging the internal diameter of the oil ports could increase flow, but could also generate pressure spikes inside the cylinder, potentially leading to fatigue cracks in the cylinder body or piston. For this reason, Vega could not provide a warranty for such a modification.
This case illustrates an important principle:
Increasing hydraulic speed is not simply a question of increasing flow. The entire dynamic behaviour of the hydraulic system must also be considered.
1. The Customer’s Problem
The cylinder itself was working correctly.
There was no reported failure of the cylinder, leakage or inability to complete the stroke.
The problem was simply that:
the stroke was too slow for the Customer’s application.
The Customer therefore investigated whether the hydraulic connection itself could be modified to allow more oil to enter the cylinder.
The operating conditions communicated to the Vega Team were:
- hydraulic pressure: 160 bar;
- supply line: 8 mm;
- final section of the line: 5 mm;
- reduced section length: approximately 20 mm;
- cylinder operating correctly;
- stroke speed considered too slow.
The technical question was very specific:
Could the ports on the actual cylinder be enlarged, and would this increase the stroke speed?
This is a perfectly logical question from an engineering perspective.
However, the answer requires more than simply looking at the diameter of the oil port.
2. Hydraulic Cylinder Speed Depends on Flow
For a hydraulic cylinder, speed is fundamentally related to volumetric flow and piston area.
The simplified relationship is:
v = Q / A
where:
- v = cylinder speed;
- Q = oil flow rate;
- A = effective piston area.
For a given cylinder, increasing the available oil flow can therefore increase the speed of the piston.
This is why restrictions in the hydraulic circuit can become important.
If the circuit cannot deliver sufficient flow to the cylinder, the cylinder may move more slowly than expected even though the hydraulic pressure is adequate.
However, the opposite is also true:
removing a flow restriction can change the dynamic behaviour of the system.
That is exactly where the Customer case becomes interesting.
3. The 8 mm to 5 mm Reduction
The Customer reported that the hydraulic line was 8 mm and then reduced to 5 mm during the final 20 mm before reaching the cylinder ports.
This naturally raises the question of whether the 5 mm section represents a significant flow restriction.
A hydraulic circuit should never be evaluated only according to the nominal diameter of its main hose.
The complete flow path must be considered:
pump → valve → hose → fittings → reducers → cylinder ports → cylinder chamber
Every element can contribute to pressure loss.
Possible restrictions include:
- small internal passages;
- narrow fittings;
- sharp changes in direction;
- valves with insufficient flow capacity;
- reducers;
- long hoses;
- small cylinder ports.
The smallest effective passage can have a significant influence on the available flow.
4. Why Enlarging the Cylinder Ports Appeared to Be a Logical Solution
The Customer’s reasoning was understandable.
If the hydraulic system was already using a relatively large line and then encountered a smaller passage close to the cylinder, increasing the internal diameter of the cylinder ports could potentially reduce flow restriction.
A larger passage can allow greater flow for a given pressure-loss condition.
Therefore, from a purely static flow perspective, enlarging the ports could appear to be an attractive solution.
But a hydraulic cylinder is not simply a pipe.
The cylinder contains moving components, pressurized chambers and structural components that must withstand repeated hydraulic loading.
Changing the flow characteristics can therefore change the pressure behaviour inside the cylinder.
5. The Vega Team’s Technical Concern
The Vega Team identified the main risk very clearly.
Increasing the internal diameter of the oil ports in order to increase movement speed could generate pressure spikes inside the cylinder.
According to the technical response, these pressure spikes could generate fatigue cracks inside the cylinder body or piston.
This is the key engineering lesson from the case.
A modification intended to improve performance can introduce a different failure mechanism.
The Customer was trying to solve a speed problem.
The proposed modification could potentially create a structural fatigue problem.
6. Why Pressure Spikes Matter
Hydraulic pressure is not necessarily constant during a cylinder’s movement.
The pressure can change rapidly when:
- a valve opens or closes;
- the cylinder accelerates;
- the cylinder decelerates;
- a moving mass reaches the end of its stroke;
- the flow path changes suddenly;
- a restriction is removed;
- the oil flow is interrupted;
- mechanical resistance changes.
These rapid pressure variations can create transient loads that are significantly different from the nominal operating pressure.
This distinction is important.
A system operating at 160 bar does not necessarily experience only 160 bar under every dynamic condition.
The nominal operating pressure and transient pressure peaks are different engineering phenomena.
7. Nominal Pressure Is Not the Whole Story
The Customer reported a working pressure of 160 bar.
At first glance, this value might appear to be the most important pressure parameter.
But when analysing a modification intended to increase cylinder speed, the engineer must also consider transient pressure behaviour.
A hydraulic component can be perfectly capable of operating continuously at a particular nominal pressure while being subjected to damaging repeated pressure peaks.
This is especially relevant in injection molds because hydraulic cylinders can operate through very large numbers of cycles.
A small transient event repeated thousands or millions of times can become a fatigue problem.
8. Fatigue Cracks: Why Repeated Cycles Matter
The Vega Team specifically referred to the possibility of fatigue cracks in the cylinder body or piston.
Fatigue is different from an immediate overload failure.
A component does not necessarily need to exceed its ultimate strength to develop fatigue damage.
Repeated cyclic stress can gradually initiate and propagate a crack.
The process can be influenced by:
- magnitude of cyclic stress;
- number of cycles;
- geometry;
- stress concentrations;
- surface condition;
- pressure fluctuations;
- manufacturing characteristics.
This is why a modification that appears safe during a short test may still require careful evaluation before being used in long-term production.
9. Why Vega Could Not Provide a Warranty for the Modification
The Vega Team stated that it could not provide a warranty if the cylinder was modified in this way.
This is an important engineering distinction.
A cylinder supplied in its original configuration is designed around a specific combination of:
- internal passages;
- hydraulic chambers;
- structural dimensions;
- materials;
- seals;
- pressure conditions;
- flow characteristics.
Changing an internal passage changes part of that design.
The resulting component can no longer automatically be considered equivalent to the original product.
Therefore:
modifying a standard hydraulic cylinder is not the same as selecting a cylinder designed for a higher-speed application.
10. The Modification Proposed by the Vega Team
The Vega Team did not simply reject the Customer’s request.
The technical response explained what would have to be done if the Customer decided to proceed with the modification.
The cylinder would have to be disassembled, and the holes could be enlarged up to a maximum of 3 mm.
The documented limit is therefore:
Maximum hole diameter: 3 mm
This value should be interpreted strictly within the context of this particular technical case.
It should not be interpreted as a general recommendation that every Vega hydraulic cylinder can safely have its ports enlarged to 3 mm.
The documentation does not support such a general conclusion.
11. Why the Cylinder Had to Be Disassembled
The instruction to disassemble the cylinder before carrying out the modification is significant.
The procedure did not recommend simply drilling or machining the cylinder while assembled.
Instead, the Vega Team specified that the cylinder should be disassembled before the modification.
This reflects the fact that the modification concerns internal components and passages.
A controlled engineering modification requires access to the relevant components and proper inspection after machining.
12. Speed Versus Reliability
This case illustrates a common engineering trade-off.
The Customer wanted:
higher speed.
But the hydraulic cylinder also needed:
long-term reliability.
These two objectives cannot be evaluated independently.
Increasing speed can increase:
- flow demand;
- acceleration;
- deceleration energy;
- dynamic pressure variations;
- mechanical loads;
- hydraulic losses elsewhere in the system.
Therefore, the correct engineering question is not:
“How can we make the cylinder faster?”
It is:
“How can we achieve the required speed while keeping the hydraulic and mechanical system within safe operating conditions?”
That is a much more complete engineering question.
13. Pressure Generates Force, Flow Generates Speed
One of the most important concepts when designing hydraulic systems is the difference between pressure and flow.
Cylinder force is approximately related to:
F = P × A
where:
- F = hydraulic force;
- P = hydraulic pressure;
- A = piston area.
Cylinder speed is related to:
v = Q / A
where:
- v = speed;
- Q = flow;
- A = piston area.
Therefore:
Pressure is primarily associated with force.
Flow is primarily associated with speed.
This distinction is fundamental when diagnosing a slow hydraulic cylinder.
Increasing pressure does not automatically solve a speed problem.
Likewise, increasing flow does not automatically solve a force problem.
14. A Slow Cylinder Should Be Analysed as a System
When a hydraulic cylinder moves too slowly, the first step should be to identify where the flow is being restricted.
A systematic analysis should consider:
Hydraulic pump
Is the required flow actually available?
Hydraulic valves
Can the valves deliver the required flow without excessive pressure loss?
Hoses
Are the internal diameters adequate?
Fittings
Do the fittings have sufficiently large internal passages?
Reducers
Are there unnecessary reductions in the circuit?
Cylinder ports
Are the cylinder ports appropriate for the required flow?
Cylinder
Is the cylinder itself suitable for the required speed?
Moving load
Is the mechanical load compatible with the desired acceleration and speed?
This approach is more reliable than modifying one component without evaluating the complete circuit.
15. The Importance of the Final Connection
The Customer’s hydraulic line was 8 mm but reduced to 5 mm over the final 20 mm before the cylinder ports.
This illustrates an important practical issue.
A hydraulic circuit can have a large main hose and still be limited by a small internal passage near the actuator.
The effective flow capacity is therefore influenced by the complete connection architecture.
This is one reason why hydraulic cylinder design should consider the cylinder, fittings and hoses as an integrated system.
16. A Better Engineering Approach to Increasing Cylinder Speed
If a Customer needs a faster hydraulic cylinder, a structured analysis can follow several stages.
Step 1 — Define the required speed
Determine the target stroke time.
Step 2 — Determine the cylinder volume
Calculate the oil volume required for the complete stroke.
Step 3 — Calculate the required flow
For a given cylinder:
Q = v × A
Step 4 — Analyse the complete hydraulic circuit
Check whether the pump, valves, hoses and fittings can deliver the required flow.
Step 5 — Identify restrictions
Determine where the pressure losses occur.
Step 6 — Analyse dynamic behaviour
Consider acceleration, deceleration and end-of-stroke conditions.
Step 7 — Verify cylinder suitability
Confirm that the cylinder itself is designed for the required speed and flow.
Step 8 — Select the appropriate solution
If the existing cylinder cannot safely provide the required performance, a cylinder or configuration designed for higher speed may be preferable to modifying a standard cylinder.
17. From Cylinder Modification to High-Speed Cylinder Design
The historical Customer case is particularly interesting when considered alongside Vega’s current product technology.
Today, Vega offers SpeedPorts, a technology specifically developed for high-speed hydraulic-cylinder applications. The official Vega website describes SpeedPorts as a technology designed to increase oil flow while maintaining the overall dimensions of the oil supply arrangement.
The current Vega range also includes the V500CZ, described as a compact high-speed block cylinder with SpeedPorts oil delivery.
This represents a fundamentally different engineering philosophy from modifying an existing standard cylinder.
Instead of asking:
“How can we modify this cylinder to make it faster?”
the designer can ask:
“Which cylinder configuration is designed for the required speed?”
18. SpeedPorts as a Design Solution
The current SpeedPorts technology is particularly relevant to the engineering lesson from this case.
The Vega website presents SpeedPorts as a solution for increasing oil flow without increasing the overall dimensions of the oil supply arrangement.
This addresses one of the fundamental challenges of high-speed hydraulic systems:
how to increase flow while keeping the hydraulic connection compact.
This can be especially valuable in injection molds, where available space around hydraulic cylinders is often restricted.
19. The V250CE and Compact Cylinder Applications
The current Vega range also includes the V250CE, a compact short-stroke block cylinder intended for injection-mold applications such as the movement of slides, punches and pins. The official Vega site lists V250CE configurations with BSP (Gas) threaded oil delivery.
This is relevant because the historical Customer case involved a CE050 cylinder and a requirement for faster stroke movement.
However, the historical case and current product specifications should not be mixed.
The documented case describes the technical conditions and modification considered in 2015. The current product range represents Vega’s present-day product offering.
20. Why a Purpose-Designed Solution Is Preferable
When a standard cylinder is modified, the engineer must consider how the modification affects:
- pressure behaviour;
- fatigue;
- internal flow;
- structural integrity;
- seals;
- reliability;
- warranty.
A cylinder designed specifically for higher flow and higher speed can instead be engineered around those requirements from the beginning.
This is one of the most important lessons that can be drawn from the Customer case.
Performance should ideally be designed into the component rather than obtained by modifying a component after production.
21. What the Customer Case Teaches Mold Designers
The case provides several practical lessons.
1. A slow cylinder does not necessarily mean that the cylinder is defective.
The cylinder in this case was operating correctly; the issue was that its stroke speed was considered too slow.
2. The entire hydraulic circuit must be analysed.
The Customer’s circuit included an 8 mm line followed by a 5 mm section before the cylinder.
3. Increasing flow can create new engineering problems.
The Vega Team specifically warned about possible pressure spikes.
4. Pressure spikes can become a fatigue issue.
The documented concern was the possibility of fatigue cracks in the cylinder body or piston.
5. A modification can affect the product warranty.
Vega explicitly stated that it could not provide a warranty for the modified cylinder.
6. A high-speed application may require a purpose-designed solution.
Vega’s current SpeedPorts technology provides an example of a design specifically aimed at increasing hydraulic flow for high-speed applications.
Conclusion
A hydraulic cylinder that moves too slowly may appear to present a simple problem.
The obvious solution is to increase the oil flow.
But as this Customer case demonstrates, increasing flow through a hydraulic cylinder is an engineering problem that must be considered together with pressure dynamics and structural reliability.
The Customer was using a CE050 cylinder at 160 bar, supplied through an 8 mm line that reduced to 5 mm during the final 20 mm before the cylinder ports. Because the stroke was considered too slow, the Customer asked whether the cylinder ports could be enlarged to increase speed.
The Vega Team warned that increasing the internal diameter of the oil ports could generate pressure spikes inside the cylinder and potentially cause fatigue cracks in the cylinder body or piston. For this reason, Vega could not provide a warranty for the modification.
If the Customer nevertheless decided to proceed, the documented technical instruction was to disassemble the cylinder and increase the holes up to a maximum of 3 mm.
The broader engineering lesson is more important than the modification itself:
A faster hydraulic cylinder is not simply a cylinder with larger holes.
The designer must consider:
flow → pressure losses → acceleration → pressure spikes → structural stress → fatigue → service life.
This is why high-speed hydraulic applications should ideally be addressed through a complete system analysis and, where appropriate, through a cylinder configuration specifically designed for higher flow.
Vega’s current SpeedPorts technology is an example of this approach, providing a dedicated solution for increasing oil flow in high-speed hydraulic-cylinder applications.
Ultimately:
More speed requires more than more flow. It requires controlled flow, controlled pressure and a hydraulic cylinder designed to withstand the resulting dynamic conditions.
Useful and Verified URLs
- SpeedPorts – High-Speed Hydraulic Cylinder Technology — Vega’s current technology for high-flow, high-speed hydraulic-cylinder applications. This is the most relevant link for connecting the historical Customer problem with Vega’s current engineering solution.
SpeedPorts – High-Speed Hydraulic Cylinder Technology - V250CE Block Cylinder – Short Stroke — compact hydraulic-cylinder family for injection-mold applications, including slide, punch and pin movement. The current Vega catalogue lists V250CE configurations with BSP (Gas) threaded oil delivery.
V250CE Block Cylinder – Short Stroke - Hydraulic Cylinders for Injection Molds — Vega’s complete hydraulic-cylinder range for plastic injection molding and die casting.
Hydraulic Cylinders for Injection Molds - Vega Hydraulic Cylinder Products — current online product catalogue, including standard and high-speed configurations.
Vega Hydraulic Cylinder Products - Hydraulic Cushioning in Injection Mold Cylinders — useful complementary technical article concerning speed, moving mass, end-of-stroke energy and pressure spikes in high-speed injection-mold applications.
Hydraulic Cushioning in Injection Mold Cylinders - How to Choose the Fastest Hydraulic Cylinder and the Right Oil Ports — relevant technical article specifically addressing cylinder speed, flow rate and oil-port selection.
How to Choose the Fastest Hydraulic Cylinder and the Right Oil Ports
