Silicone Plug Design Guide: Interference Fit, Pull-Out Force and Sealing Tests - Kunshan Fuhongying Electronic Materials Co., Ltd.

Silicone plugs are used to close ports, holes, tubes and equipment openings in medical devices, consumer electronics, industrial systems, household products and fluid-handling assemblies. Depending on the application, a plug may provide temporary dust protection, high-temperature masking, a removable waterproof closure or a pressure-resistant seal.

Although a silicone plug appears simple, its performance depends on the relationship between plug geometry, material hardness, dimensional tolerances, insertion depth and the mating hole. Excessive interference can make installation difficult or damage the silicone. Insufficient interference can reduce retention and create a leakage path.

This guide explains how to select plug geometry, calculate interference, specify pull-out force and establish sealing tests for custom silicone plugs.

Common Types of Silicone Plugs

Tapered Push-In Plugs

A tapered plug gradually increases in diameter along its insertion length. The operator can adjust insertion depth until the required contact pressure is achieved.

Advantages include:

Fits a range of hole diameters
Easy manual installation
Suitable for masking and temporary protection
Allows gradual engagement
Can work with threaded and smooth holes

The main limitation is that sealing and retention depend on insertion depth. If different operators install the plug to different depths, performance may vary.

Straight Cylindrical Plugs

A straight plug uses a controlled interference fit over a defined engagement length.

Advantages include:

Consistent insertion position
Predictable contact area
Suitable for automated assembly
Easier dimensional inspection
Stable appearance after installation

However, a straight plug requires closer control of the hole diameter and plug outside diameter.

Ribbed or Barbed Plugs

Circumferential ribs or flexible barbs increase retention by creating multiple local contact zones.

They are useful when:

Higher pull-out force is required
The mating hole has a wider tolerance
The plug must resist vibration
A straight friction fit is insufficient
The plug is not removed frequently

Sharp or excessively large barbs may increase installation force, damage the mating surface or tear during removal.

Flanged Plugs

A flange provides a positive stop and prevents the plug from being pushed completely through the opening.

The flange may also:

Cover the edge of the hole
Improve appearance
Provide a secondary face seal
Protect the opening from splash
Support a pull tab or tether

Caplugs offers tapered plugs with wide flanges specifically intended to prevent push-through, illustrating the functional role of this feature. (Caplugs flanged plug example)

Pull-Tab Plugs

A pull tab makes removal easier where the plug is recessed or regularly opened.

The tab must be designed so that:

Normal vibration does not pull the plug loose
The user can grip it with gloves, if required
Pulling does not tear the plug body
The tab does not interfere with surrounding parts
The pull direction is aligned with the plug axis

Hollow Compression Plugs

A hollow core allows the plug to deform inward during insertion. This can reduce insertion force while maintaining contact with the hole.

Hollow designs are useful for:

Large-diameter plugs
Low-hardness silicone
Mating holes with dimensional variation
Applications requiring repeated installation
Assemblies where solid plugs would be too stiff

The hollow cavity must provide enough space for displaced silicone without causing the plug wall to buckle.

Tethered Plugs

A molded tether keeps the plug attached to the product when removed.

Tethers should be evaluated for:

Flex fatigue
Устойчивость к разрыву
Minimum bend radius
Interference with plug installation
Accidental pull-out loading
User access and ergonomics

Understanding Interference Fit

An interference fit exists when the silicone plug is larger than the mating hole before assembly.

For a straight circular plug:

[
\text{Diametral Interference}=D_p-D_h
]

Where:

(D_p) = free outside diameter of the silicone plug
(D_h) = inside diameter of the mating hole

The percentage interference can be expressed as:

[
\text{Interference Percentage}=
\frac{D_p-D_h}{D_h}\times100%
]

For example, if a plug diameter is 10.40 mm and the mating hole is 10.00 mm:

[
\frac{10.40-10.00}{10.00}\times100%=4%
]

This calculation only describes the dimensional relationship. It does not directly predict insertion force, pull-out force or leakage.

Is There a Standard Interference Percentage?

There is no universal interference value for every silicone plug.

Published sizing guidance for silicone masking plugs may recommend a major plug diameter approximately 2–4% larger than the hole for spray masking and 3–7% larger for immersion processes. (Viadon silicone pull-plug guidance)

These figures can be useful as prototype starting points for similar masking applications, but they should not automatically be applied to pressure seals, medical components or long-term waterproof closures.

The correct interference depends on:

Твёрдость силикона
Plug wall thickness
Solid or hollow construction
Hole material
Hole surface finish
Глубина вставки
Plug taper
Температура
Lubrication
Fluid exposure
Required pull-out force
Internal pressure
Number of installation cycles
Dimensional tolerances

Tolerance Stack-Up

A plug should be evaluated under both minimum and maximum interference conditions.

Minimum Interference

Minimum interference occurs when the plug is at its smallest allowed diameter and the hole is at its largest allowed diameter:

[
I_{\min}=D_{p,\min}-D_{h,\max}
]

This condition normally produces:

Lowest contact pressure
Lowest insertion force
Lowest pull-out force
Highest risk of leakage

Maximum Interference

Maximum interference occurs when the plug is at its largest allowed diameter and the hole is at its smallest allowed diameter:

[
I_{\max}=D_{p,\max}-D_{h,\min}
]

This condition normally produces:

Highest insertion force
Highest silicone strain
Greater assembly difficulty
Higher risk of tearing or plug damage
Greater stress relaxation over time

A nominal interference value is not sufficient. The drawing and validation plan should account for the complete plug and hole tolerance range.

Why Excessive Interference Can Be a Problem

Silicone rubber can deform substantially, but it does not simply disappear when compressed. The displaced material must move into another area.

Excessive interference can cause:

Very high insertion force
Plug buckling
Flange distortion
Silicone tearing
Damage to thin mating walls
Permanent deformation
Increased compression set
Difficulty removing the plug
Inconsistent insertion depth
Extrusion under pressure

Parker notes that elastomer seals are subject to compression set and that over-compression can reduce long-term sealing effectiveness. (Parker elastomer seal design guidance)

Relief grooves, hollow sections or controlled ribs can provide space for silicone displacement and reduce unnecessary assembly force.

Selecting Silicone Hardness

Silicone plug hardness is commonly specified using Shore A.

Softer Silicone

Softer silicone generally provides:

Easier deformation
Lower insertion force
Better conformance to irregular surfaces
Improved sealing against minor scratches
Greater risk of buckling or accidental pull-out

Harder Silicone

Harder silicone generally provides:

Higher insertion and pull-out force
Better dimensional stability
Reduced tendency to buckle
Less conformance to surface defects
Greater assembly stress on the mating component

Hardness should be selected together with geometry. A thin hollow plug made from relatively firm silicone may install more easily than a thick solid plug made from softer silicone.

Designing the Plug Entry

The leading end should guide the plug into the hole without cutting or folding the silicone.

Common entry features include:

Rounded nose
Conical lead-in
Chamfer
Reduced pilot diameter
Thin flexible first rib

The mating hole should also be reviewed for:

Sharp edges
Burrs
Thread starts
Mold parting lines
Surface damage
Coating buildup

A sharp metal or plastic edge may cut the plug during insertion even when the dimensional interference is correct.

Designing the Sealing Zone

The sealing zone is the area where the plug applies pressure to the mating surface.

A reliable sealing zone should have:

Continuous contact around the circumference
Sufficient engagement length
Controlled surface finish
No mold flash crossing the seal
No deep engraving or texture
No interruption from a pull tab or tether
Adequate material behind the contact surface

Multiple ribs can create several sealing lines. However, additional ribs do not automatically improve performance if the first rib prevents the remaining ribs from fully engaging.

For critical seals, the primary retention feature and primary sealing feature may be separated. For example, a barb can provide retention while a smooth cylindrical band provides sealing.

Blind Holes and Trapped Air

Installing a solid plug into a blind hole can compress trapped air or fluid.

This may cause:

Plug spring-back
Increased installation force
Incomplete insertion
Slow plug movement after assembly
Pressure buildup behind the plug
Unexpected ejection

Possible design solutions include:

Controlled internal venting during assembly
A hollow plug
A vent path that closes after full insertion
Reduced insertion speed
A defined trapped volume
A pressure-relief feature

A vent should not create a permanent leakage path through the final assembly.

Insertion Force

Insertion force affects assembly equipment, operator ergonomics and the risk of plug damage.

It is influenced by:

Interference
Твёрдость
Отделка поверхности
Friction
Taper angle
Engagement length
Insertion speed
Температура
Lubrication
Hole edge condition
Plug geometry

Insertion force should be measured using the actual mating component. A smooth metal test ring may not reproduce the behavior of a threaded, molded or coated production hole.

The specification should define:

Insertion speed
Plug orientation
Maximum insertion force
Required insertion depth
Lubrication condition
Test temperature
Number of insertion cycles

Pull-Out Force

Pull-out force is the axial force required to remove the installed plug.

For a simple friction-fit plug, retention depends on contact pressure, friction coefficient and contact area. In practice, silicone deformation, surface roughness and stress relaxation make pull-out force difficult to predict with a simple equation.

Pull-out testing is therefore normally required.

Factors That Increase Pull-Out Force

Greater interference
Longer engagement length
Higher-friction surface
Multiple retention ribs
Undercut or barb
Larger plug diameter
Mechanical locking feature
Higher material stiffness

Factors That Reduce Pull-Out Force

Lubrication
Smooth or polished mating surface
Fluid contamination
Reduced interference
Short engagement
Low hardness
Thermal aging
Остаточная деформация при сжатии
Repeated installation
Material swelling or shrinkage

A plug that passes a pull-out test immediately after installation may not provide the same retention after aging or repeated removal.

Internal Pressure and Plug Ejection

Internal pressure generates an axial force that can push the plug out of the opening.

The approximate pressure-generated force is:

[
F_p=\Delta P\times A
]

For a circular opening:

[
A=\frac{\pi D_h^2}{4}
]

Where:

(F_p) = pressure-generated ejection force
(\Delta P) = pressure difference across the plug
(A) = projected area of the opening
(D_h) = hole diameter

The minimum validated pull-out force should exceed the maximum expected pressure force plus vibration, handling and other service loads, using an appropriate project-specific safety margin.

A friction fit alone may not be suitable where internal pressure is high or failure could be hazardous. A flange, barb, retaining ring, thread or other positive mechanical feature may be required.

How to Perform a Pull-Out Test

A typical pull-out test includes:

Condition the plug and mating component.
Install the plug using the specified method.
Allow the required dwell or relaxation time.
Secure the mating component in a rigid fixture.
Grip the plug head or defined pull feature.
Pull axially at a controlled speed.
Record the peak force.
Record the failure mode.
Inspect the plug and mating component.

Possible failure modes include:

Normal sliding removal
Rib collapse
Barb release
Silicone tearing
Pull-tab separation
Flange inversion
Mating-hole damage
Fixture slip

The test report should record both force and failure mode.

Sealing Test Methods

Pressure Decay Testing

The sealed assembly is pressurized to a defined level. After filling and stabilization, the pressure source is isolated and pressure loss is measured over a fixed period.

Cincinnati Test Systems describes pressure decay testing as filling a component to a target pressure, isolating it, stabilizing it and measuring pressure loss during a defined test time. (CTS pressure decay explanation)

Pressure decay is suitable for:

Closed ports
Waterproof housings
Fluid connectors
Electronic enclosures
Silicone-plastic assemblies
Production leak testing

The test fixture must seal all other possible leakage paths.

Vacuum Decay Testing

The assembly is evacuated and monitored for pressure rise.

Vacuum decay may be useful when:

The product operates under vacuum
Positive pressure could eject the plug
A thin housing may expand during pressurization
The seal behaves differently under inward pressure

Bubble Leak Testing

The plugged component is pressurized and submerged in water or covered with a leak-detection solution. Escaping bubbles indicate a leakage path.

Bubble testing is useful for:

Engineering trials
Locating leaks
Comparing prototype geometries
Visual production checks

However, it is operator-dependent and does not always provide an accurate quantitative leak rate.

Water Immersion Testing

A plug intended for splash or immersion protection can be tested under defined depth, time and orientation conditions.

The specification should distinguish between:

Water resistance of the individual plugged port
Water resistance of the complete product enclosure
Temporary immersion
Continuous immersion
Static water exposure
Water jets or spray

IEC 60529 classifies the ingress protection provided by complete electrical enclosures. A silicone plug alone should not be described as “IP67” or “IP68” unless the final assembled enclosure has been tested under the relevant requirements. (IEC explanation of IP ratings)

Tracer-Gas Testing

Helium or another tracer gas can be used when very small leaks must be detected.

This method is suitable for:

High-value assemblies
Medical or analytical equipment
Vacuum systems
Critical electronics
Engineering validation

Tracer-gas testing requires specialized equipment and carefully designed fixtures.

Proof-Pressure and Ejection Testing

The assembly is exposed to a specified pressure above normal operating pressure to verify that:

The plug remains installed
No permanent deformation occurs
The housing is not damaged
Leakage remains acceptable
The plug does not move beyond the allowed position

Proof testing is not the same as burst testing. A proof test verifies survival at a specified pressure, while a burst test determines the pressure at which the assembly fails.

Test Conditions That Must Be Defined

A sealing test should specify:

Test medium
Test pressure or vacuum
Fill time
Stabilization time
Measurement time
Maximum allowable leak rate
Температура
Plug insertion depth
Plug conditioning
Wet or dry condition
Test direction
Number of installation cycles
Fixture sealing location
Accept/reject criteria

Flexible silicone may continue to deform after pressure is applied. Adequate stabilization time is needed to prevent normal material movement from being interpreted as leakage.

Environmental Validation

A silicone plug should be tested after exposure to the conditions expected in service.

Validation may include:

High-temperature aging
Low-temperature exposure
Thermal cycling
Влажность
Water immersion
Чистящие химикаты
Oils or fuels
UV exposure
Sterilization
Repeated insertion and removal
Vibration
Pressure cycling
Long-term installed compression

Material compatibility must be confirmed using the exact silicone formulation and service fluid.

Common Silicone Plug Failures

Failure	Possible Cause	Recommended Review
Plug is difficult to install	Excessive interference, hard material or sharp entry	Reduce interference or improve lead-in geometry
Plug falls out	Insufficient interference or low engagement	Increase contact length or add retention ribs
Plug slowly moves outward	Trapped pressure or stress relaxation	Review blind-hole venting and retention
Plug leaks	Low contact pressure, flash or surface damage	Improve sealing zone and inspect mating surface
Plug tears during removal	Sharp barb, thin wall or weak pull tab	Add radii and increase local thickness
Flange deforms	Excessive insertion depth or poor support	Add a positive stop and review flange thickness
Pull-out force varies	Hole tolerance, lubrication or surface variation	Tighten mating-part controls and test conditions
Seal fails after aging	Compression set or fluid incompatibility	Review material grade and long-term compression
Plug ejects under pressure	Pressure force exceeds retention	Add positive mechanical retention
Water enters around the port	Incomplete circumferential contact	Review sealing-band geometry and assembly alignment

Silicone Plug Inspection Plan

Production inspection may include:

Plug outside diameter
Rib or barb diameter
Engagement length
Flange thickness
Pull-tab dimensions
Твёрдость
Visual surface inspection
Flash and parting-line condition
Insertion force
Pull-out force
Leak rate
Proof-pressure result
Repeated-use testing
Packaging cleanliness

Critical plug dimensions should be measured using methods that do not excessively compress the silicone.

What to Include in a Silicone Plug RFQ

Provide the following information:

2D drawing and 3D model
Mating-hole diameter and tolerance
Hole material
Hole surface finish
Threaded or smooth bore
Through-hole or blind-hole design
Required insertion depth
Твёрдость силикона
Цвет
Required insertion force
Minimum pull-out force
Operating pressure
Proof or burst pressure
Maximum allowable leak rate
Service temperature
Fluid and chemical exposure
Number of removal cycles
Нормативные требования
Annual production volume
Inspection and packaging requirements

If the mating component is available, physical samples can help verify insertion, retention and sealing performance before production tooling is finalized.

Заключение

A reliable silicone plug requires more than selecting a diameter slightly larger than the mating hole.

Interference fit, hardness, wall thickness, taper, engagement length and surface condition work together to determine insertion force, pull-out force and leakage. The design must also consider tolerance extremes, trapped pressure, internal pressure-generated ejection force, repeated use and environmental aging.

Prototype testing should include both minimum- and maximum-interference samples. Pull-out testing verifies retention, while pressure decay, vacuum decay, bubble or tracer-gas testing confirms sealing performance. Where internal pressure or safety risk is significant, mechanical retention should supplement the friction fit.

FHY Silicone provides custom silicone plugs, sealing caps, protective covers and molded silicone components for electronics, medical, industrial and consumer applications. Send us your mating-hole dimensions, material requirements, pull-out force and leak-test conditions for a manufacturability review.

Часто задаваемые вопросы

How much larger should a silicone plug be than the hole?

There is no universal percentage. Some masking applications begin with a few percent diametral interference, but the correct value depends on hardness, geometry, tolerance, sealing pressure and required removal force.

Does more interference always improve sealing?

No. Excessive interference can increase insertion force, damage the silicone, distort the flange and reduce long-term sealing through over-compression.

How can silicone plug pull-out force be increased?

Increase engagement length, add retention ribs or barbs, improve surface contact or use a positive mechanical feature such as a flange, groove or retaining ring.

Can a silicone plug seal a threaded hole?

Yes, but the thread creates an irregular sealing surface. A tapered or compliant plug with sufficient engagement is usually required, and the final design must be leak tested.

Why does a plug move out of a blind hole?

The plug may compress trapped air or liquid during insertion. The resulting pressure can push it outward unless the design provides controlled venting or stronger retention.

Is a waterproof silicone plug automatically IP67?

No. IP ratings apply to the tested complete enclosure, not only the individual silicone component.

Should pull-out force be tested before or after aging?

Both. Initial testing confirms production assembly, while aged testing evaluates changes caused by temperature, fluid exposure, compression set and repeated use.