How Draft Angle Determines the Real Force Required for Core Pulling
Designing a hydraulic cylinder for an injection mold is often considered a simple calculation.
Many engineers determine the required cylinder force by multiplying the hydraulic pressure by the piston area, assuming that this value alone guarantees reliable operation.
In reality, this is only part of the engineering calculation.
One of the most underestimated factors is the adhesion force between the molded part and the core.
This force can easily become greater than the force required to move the slide itself.
As a result, a perfectly sized hydraulic cylinder may still fail to retract the core if the adhesion between the plastic part and the mold has been underestimated.
A real engineering case handled by the Vega Technical Department illustrates this perfectly.
During the development of a special bi-material injection mold, a customer asked which adhesion coefficient should be used to calculate the force required for the hydraulic cylinder to pull the slide back after molding.
Rather than providing a single value, the Vega Technical Department explained that the adhesion coefficient changes significantly according to the draft angle and the material involved.
This simple observation has major engineering consequences.
Why Core Pulling Force Is Often Underestimated
When molten plastic cools inside the mold cavity, it shrinks around the core.
This shrinkage generates friction and adhesion between the molded component and the steel surface.
Before the slide can move, the hydraulic cylinder must first overcome this resistance.
Many designers calculate only:
- slide weight;
- mechanical friction;
- hydraulic safety factor.
However, these values often represent only a fraction of the real force required.
The adhesion between the molded part and the core frequently becomes the dominant load acting on the hydraulic cylinder.
Ignoring this effect can lead to:
- undersized cylinders;
- slow slide movement;
- incomplete core retraction;
- increased wear;
- unexpected machine stoppages.
The Role of Draft Angle
The draft angle is one of the most effective methods for reducing adhesion forces.
As the draft angle increases, the contact pressure between the molded part and the core decreases, making ejection significantly easier.
For this reason, even a small increase in draft angle can dramatically reduce the force required from the hydraulic cylinder.
This relationship is well known in mold design but is often overlooked when selecting hydraulic cylinders.
Engineering Data from a Real Case
The Vega Technical Department provided the following adhesion coefficients used during cylinder force calculations.
Plastic Components
| Draft Angle | Adhesion Coefficient |
|---|---|
| 0° | 30 kg/cm² |
| 1° | 25 kg/cm² |
| 2° | 20 kg/cm² |
| 3° | 15 kg/cm² |
Aluminium Components
| Draft Angle | Adhesion Coefficient |
|---|---|
| 0° | 80 kg/cm² |
| 1° | 50 kg/cm² |
| 2° | 40 kg/cm² |
| 3° | 30 kg/cm² |
These engineering values clearly demonstrate that the required extraction force decreases rapidly as the draft angle increases.
Why Aluminium Requires Much Higher Pulling Forces
One particularly interesting observation from these values is the significant difference between plastic and aluminium applications.
At zero draft, aluminium requires almost three times the adhesion force used for plastic components.
This explains why hydraulic cylinders used in die-casting molds often need substantially larger safety margins than those used in plastic injection molds.
Designers who simply apply the same sizing criteria to both applications risk selecting cylinders that are unable to overcome the real extraction forces.
The Mistake of Using a Single Coefficient
Many engineering calculations still rely on a single “standard” adhesion coefficient.
Although this simplifies calculations, it ignores several critical variables:
- draft angle;
- material type;
- surface finish;
- shrinkage characteristics;
- mold temperature;
- lubrication conditions.
In practice, the required pulling force is never constant.
It varies from one application to another and should always be evaluated according to the actual molding conditions.
A Special Mold Design
The engineering case also revealed another important aspect.
The customer explained that the hydraulic cylinder did not move the molded part directly.
Instead, it moved a first slide, which in turn drove a second slide positioned at a right angle.
When the mold closed, mechanical clamping forces were transmitted through the slide mechanism, potentially loading the hydraulic cylinder even before it started moving.
The customer was concerned that these forces might have damaged the self-locking cylinder during the first production trials.
This example illustrates why hydraulic cylinder sizing cannot be separated from the overall kinematics of the mold.
The geometry of the mechanism often determines the real loads acting on the cylinder.
The real challenge is often the adhesion between the molded part and the core.
If this force is underestimated, the cylinder may be perfectly manufactured and correctly installed, yet still fail to retract the slide reliably.
The next question is therefore obvious:
How should engineers calculate the real force required for core pulling?
The answer involves much more than selecting a cylinder from a catalogue.
Step 1 – Calculate the Contact Area
The adhesion force always depends on the surface that remains in contact with the molded component.
For a simple cylindrical core, the contact area can be estimated using:
Contact Area = π × Diameter × Contact Length
For rectangular cores, slides or inserts, the contact area corresponds to the total projected surface that remains in contact with the molded material.
Even relatively small increases in contact area can significantly increase the required extraction force.
This explains why two apparently similar molds may require completely different hydraulic cylinders.
Step 2 – Select the Correct Adhesion Coefficient
Once the contact area has been determined, the next step is selecting the appropriate adhesion coefficient.
The engineering data supplied by the Vega Technical Department shows that the coefficient depends primarily on:
- material;
- draft angle.
For example, reducing the draft angle from 3° to 0° doubles the adhesion coefficient for plastic components.
This means that a designer who removes draft angle to improve aesthetics may unknowingly double the force required from the hydraulic cylinder.
Step 3 – Calculate the Adhesion Force
The extraction force generated by adhesion can be estimated using:
Adhesion Force = Contact Area × Adhesion Coefficient
Although this equation appears simple, it represents one of the most frequently overlooked calculations in hydraulic cylinder sizing.
Many designers calculate only the hydraulic force available from the cylinder while ignoring the force needed to separate the molded component from the core.
Step 4 – Add Mechanical Loads
The adhesion force is only one part of the total load.
The hydraulic cylinder must also overcome:
- slide friction;
- guide friction;
- mechanical transmissions;
- wedges;
- angle pins;
- return springs;
- inertia of moving masses;
- contamination;
- lubrication losses.
Every additional resistance increases the total cylinder force requirement.
Step 5 – Consider the Mold Kinematics
The engineering case analysed by the Vega Technical Department involved an unusual mold design.
The hydraulic cylinder moved a primary slide which then actuated a second slide positioned at 90 degrees.
During mold closing, the clamping force generated a mechanical load on the slide mechanism, potentially transmitting additional stress directly to the self-locking hydraulic cylinder.
This type of mechanism demonstrates that hydraulic cylinder calculations cannot be based solely on linear motion.
Engineers must analyse the complete force path through the mold.
Depending on the geometry, mechanical leverage may either increase or reduce the actual force acting on the cylinder.
Why Safety Factors Are Essential
Real production conditions rarely match theoretical calculations.
Several variables continuously change during production:
- plastic shrinkage;
- mold temperature;
- lubrication conditions;
- wear;
- production speed;
- material batches;
- cooling efficiency.
For this reason, experienced mold designers always include an appropriate engineering safety factor.
An oversized cylinder generally offers greater reliability than a cylinder operating continuously near its maximum theoretical capacity.
Common Engineering Mistakes
Many hydraulic cylinder problems originate long before production begins.
Some of the most frequent design errors include:
- calculating only hydraulic pressure;
- ignoring adhesion forces;
- assuming the draft angle has little influence;
- neglecting force multiplication inside complex slide mechanisms;
- selecting cylinders without considering long-term wear;
- using generic adhesion coefficients for every application.
These mistakes often result in cylinders that appear correctly sized on paper but fail during production.
Engineering Lessons from This Case
This real engineering case provides several valuable design principles:
- Core pulling force depends largely on adhesion between the molded component and the core.
- Draft angle has a direct influence on adhesion and therefore on cylinder sizing.
- Aluminium molds require significantly higher adhesion coefficients than plastic applications.
- Complex slide mechanisms can transmit unexpected loads to hydraulic cylinders.
- Hydraulic cylinder sizing should always be based on the complete mold mechanism rather than hydraulic pressure alone.
Conclusion
Selecting the correct hydraulic cylinder requires much more than calculating piston force.
Engineers must understand how adhesion, draft angle, mold geometry and mechanical force transmission interact throughout the molding cycle.
The engineering data provided by the Vega Technical Department clearly demonstrates that adhesion coefficients vary significantly with both material type and draft angle, making these parameters essential for accurate cylinder sizing.
Likewise, the special bi-material mold discussed in this case illustrates how complex slide arrangements can introduce unexpected mechanical loads that should always be considered during the design phase.
By combining accurate adhesion calculations with a complete analysis of mold kinematics, engineers can select hydraulic cylinders that operate reliably throughout the entire service life of the mold.
Further Technical Reading
To deepen your understanding of hydraulic cylinder sizing, draft angle and core pulling calculations, explore these related engineering articles from the Vega Technical Blog:
- How to Calculate the Correct Hydraulic Cylinder Size for Injection Molds
https://www.icvega.com/support/how-to-calculate-the-correct-hydraulic-cylinder-size-for-injection-molds
This article explains how to calculate both pushing and pulling forces, why plastic adhesion is often underestimated, and how hydraulic pressure influences cylinder selection. - How to Calculate the Pulling Force of Hydraulic Cylinders in Injection Molds with Mechanical Locks
https://www.icvega.com/support/how-to-calculate-the-pulling-force-of-hydraulic-cylinders-in-injection-molds-with-mechanical-locks
Learn how to size self-locking hydraulic cylinders by correctly calculating extraction forces throughout the molding cycle. - Why Doesn’t the Slide Move? A Guide to Hydraulic Cylinder Sizing
https://www.icvega.com/support/why-doesnt-the-slide-move-a-guide-to-hydraulic-cylinder-sizing
Discover why theoretical calculations sometimes fail during mold trials and which real-world factors influence slide movement. - How to Size Two Hydraulic Cylinders Working Simultaneously in an Injection Mold
https://www.icvega.com/support/how-to-size-two-hydraulic-cylinders-working-simultaneously-in-an-injection-mold
A practical engineering guide for applications where multiple hydraulic cylinders operate together and extraction forces must be shared correctly. - The Force Nobody Calculated
https://www.icvega.com/support/the-force-nobody-calculated
A real engineering case showing why the most critical force in an injection mold is often the one that designers forget to include in their calculations.
