How to Calculate the Required Pulling Force for Plastic Parts: A Real Engineering Case Study

Introduction

Selecting the correct hydraulic cylinder for a core pulling application is not simply a matter of choosing a cylinder with sufficient bore diameter. Every successful injection mould begins with an accurate engineering analysis that considers the geometry of the plastic part, the material characteristics, the mould design and the extraction conditions.

One of the most common mistakes made during mould design is underestimating the extraction force required to remove a plastic component from the cavity. If the required force is underestimated, the consequences can be severe:

deformation of the plastic part;
damage to slides or cores;
bent cylinder rods;
premature wear of guide components;
longer machine downtime;
expensive mould modifications.

This article presents a real engineering case handled by the Vega Technical Department. A customer requested assistance in selecting the correct hydraulic cylinders for extracting an elbow-shaped plastic component. Instead of simply recommending a cylinder, the engineering team analysed the 3D model, estimated the extraction force and proposed several suitable hydraulic solutions based on different operating pressures.

Although the application itself is specific, the engineering method can be applied to thousands of mould designs.

The Customer Request

The customer was designing an injection mould incorporating a mechanically locked slide. Since the mechanical lock would withstand the injection pressure during mould filling, the hydraulic cylinders were required only for the extraction movement.

The requirements were straightforward:

two hydraulic cylinders;
170 mm stroke;
magnetic position switches;
female metric thread on the rod end.

However, before selecting the cylinder, the engineering department needed to answer a much more important question:

How much extraction force is actually required?

Many designers start from the cylinder catalogue.

Experienced engineers start from the physics.

Step 1 – Analysing the 3D Model

The first task consisted of analysing the customer’s CAD model.

Unlike many simplified sizing methods based only on part weight, the Vega engineering team evaluated the actual contact area between the plastic component and the mould surface.

The analysis determined:

lateral contact surface: 177.66 cm²
draft angle: 1°

These two parameters are fundamental because extraction force mainly depends on:

contact surface;
adhesion coefficient;
draft angle;
temperature of the plastic part;
polymer characteristics.

Many designers underestimate the importance of contact area.

A small part with a large contact surface may require a considerably higher pulling force than a heavier component with minimal contact.

Step 2 – Understanding Plastic Adhesion

When a mould opens, the plastic component does not simply “slide” off the core.

Several physical phenomena oppose extraction:

friction;
vacuum effects;
material shrinkage;
elastic deformation;
adhesion between polymer and steel.

Depending on the plastic material, mould finish and processing conditions, adhesion can become the dominant factor.

In this engineering case, Vega adopted an adhesion coefficient of:

20 kg/cm²

This value represents a realistic engineering assumption for:

approximately 1° draft angle;
relatively cold plastic part;
extraction temperature around 30–40°C.

Choosing the correct adhesion coefficient is often more important than selecting the cylinder itself.

An optimistic assumption can produce an undersized cylinder.

An excessively conservative assumption may increase mould cost unnecessarily.

Engineering is always a balance.

Step 3 – Calculating the Pulling Force

Once the contact area and adhesion coefficient were established, the required extraction force could be estimated.

The engineering calculation was based on:

Pulling Force = Contact Area × Adhesion Coefficient

Substituting the values:

177.66 cm² × 20 kg/cm²

Result:

≈ 3553 kgf

This became the target extraction force for the application.

Notice that this value already represents the useful traction force.

It is not simply the theoretical cylinder force obtained from hydraulic pressure.

Instead, it is the force required to safely extract the moulded component under the assumed operating conditions.

Why This Calculation Matters

Many mould designers attempt to size cylinders using only hydraulic pressure charts.

For example:

“A 63 mm cylinder produces more than three tons at 150 bar, therefore it must be sufficient.”

Unfortunately, engineering is rarely that simple.

The required force depends on the application—not on the cylinder catalogue.

Before selecting any actuator, engineers should understand:

why the force is required;
where resistance originates;
whether the resistance changes during extraction;
how the mould behaves throughout the opening sequence.

Only after answering these questions should the cylinder selection begin.

This approach dramatically reduces the risk of oversizing or undersizing hydraulic components while improving mould reliability over its entire service life.

Step 4 – Selecting the Most Suitable Hydraulic Cylinder

After calculating the required extraction force of approximately 3,553 kgf, the next engineering task was to identify the most appropriate hydraulic cylinder.

Rather than recommending a single solution, the Vega Technical Department evaluated multiple alternatives, allowing the customer to select the configuration that best suited the hydraulic system already installed on the injection moulding machine.

The proposed solutions were:

2 × CM063 cylinders with reducer, operating at 150 bar
2 × CR063 cylinders, operating at 150 bar
2 × CM080 cylinders with reducer, operating at 100 bar
2 × CR080 cylinders, operating at 100 bar

This approach demonstrates an important engineering principle.

Hydraulic cylinder selection is rarely limited to a single “correct” answer.

Different hydraulic pressures, bore diameters and cylinder designs may all satisfy the same application while offering different advantages in terms of machine compatibility, installation space and operating efficiency.

Why Two Different Bore Sizes?

At first glance, selecting either a Ø63 mm or Ø80 mm cylinder may seem inconsistent.

However, the hydraulic force generated by a cylinder depends on two variables:

piston area;
hydraulic pressure.

A smaller cylinder operating at a higher pressure can produce a similar output force to a larger cylinder operating at a lower pressure.

This flexibility is extremely valuable during mould design.

For example:

some moulders operate permanently at 150 bar;
others intentionally limit pressure to around 100 bar to reduce stress on hydraulic components;
certain moulding machines have pressure limitations on auxiliary hydraulic circuits.

Providing alternative cylinder sizes allows the customer to optimise the mould according to the available hydraulic system rather than modifying the machine itself.

This engineering flexibility is one of the reasons experienced technical departments usually provide several acceptable solutions instead of only one catalogue reference.

The Importance of Hydraulic Pressure

One of the most overlooked aspects of cylinder selection is the available hydraulic pressure.

Many designers calculate the cylinder force assuming the maximum pressure indicated in the machine manual.

In reality, the effective pressure available at the cylinder can be significantly lower because of:

pressure losses through valves;
long hydraulic hoses;
flow restrictions;
manifold design;
simultaneous movements of multiple hydraulic actuators.

For this reason, selecting a cylinder that only just satisfies the theoretical force requirement may leave little safety margin during actual production.

A properly engineered solution always considers the real operating conditions rather than ideal laboratory values.

The Influence of Draft Angle

The engineering calculation in this case assumed a draft angle of approximately 1°.

Although this value may appear insignificant, even small variations in draft angle can have a major influence on extraction force.

Increasing the draft angle generally:

reduces contact pressure;
lowers friction;
decreases adhesion;
improves part release.

Conversely, reducing the draft angle increases the extraction force and raises the likelihood of damaging both the moulded component and the mechanical system.

Design engineers often focus on cosmetic requirements or dimensional accuracy without fully appreciating how strongly draft angle influences mould reliability.

Finding the right compromise between product design and manufacturability is one of the most important responsibilities of mould engineers.

Temperature Changes Everything

One of the most valuable observations included in the original engineering response concerns the temperature of the plastic part.

The calculation assumed that the component would be extracted at approximately 30–40°C. However, the engineer clearly explained that if the part temperature during extraction were higher, the required pulling force could be lower, although it could not be calculated accurately without additional information.

This statement reflects real engineering experience.

Plastic behaviour changes continuously with temperature.

A hotter component generally exhibits:

reduced stiffness;
lower friction;
decreased adhesion to polished steel surfaces;
easier release from the mould.

However, every polymer behaves differently.

Materials such as polypropylene, ABS, polycarbonate or reinforced engineering plastics each have unique shrinkage characteristics and surface interactions.

Because of these variables, no universal adhesion coefficient exists.

Engineering calculations therefore provide an informed estimate rather than an absolute value.

This is why experienced engineers often combine theoretical calculations with practical validation during mould commissioning.

Why Engineering Calculations Are Never Absolute

One of the strongest aspects of this engineering case is that the Vega Technical Department did not present the calculated force as an unquestionable truth.

Instead, the engineer explicitly recommended that the customer verify the assumptions under actual operating conditions.

This is not uncertainty.

It is good engineering practice.

Real mould performance depends on many variables that cannot always be predicted perfectly during the design stage, including:

polymer formulation;
mould cooling efficiency;
surface finish;
processing parameters;
lubrication;
production tolerances;
wear after thousands of cycles.

Theoretical calculations establish a solid engineering starting point, but practical validation remains an essential part of every successful mould development project.

The best engineering decisions are based on both calculation and experience.