Coloured barrel plugs for fast identification and hole protection
Barrel plugs are widely used across industrial and manufacturing applications to protect openings. As hole plugs, they create an interference fit within a hole.
Most plastic plugs are produced with a nominal diameter above the hole size they're intended for. On insertion, the plug is forced into the hole and held by contact at the wall. No additional fixing is required.
Coloured barrel plugs support fast identification where multiple sizes of hole plugs are used, improving efficiency in high-volume environments. In this guide, we’ll look at:
What are barrel plugs?
Key benefits of barrel plugs
Colour and identification in use
Applications across industries
Material and design features
Design and selection considerations
Conclusion & Expert Resources
What are barrel plugs?
Barrel plugs are cylindrical plastic plugs designed to function as hole plugs in materials such as metal, plastic, or composite components. These are solid, push-fit parts that rely on an interference fit between the plug and the hole diameter.
During installation, barrel plugs compress slightly as they’re inserted. This creates radial pressure against the internal surface of the hole. That pressure allows the hole plugs to remain in place during handling and processing.
Because of this behaviour, barrel plugs are suitable as both temporary hole coverage and longer-term protection. Learn more in our experts’ Guide to Hole Plugs.
Key benefits of barrel plugs
The performance of barrel plugs as hole plugs is based on predictable mechanical behaviour and ease of use in production environments.
| Feature | Benefit |
| Fast identification | Coloured barrel plugs allow quick selection where multiple hole plugs are used |
| Retention performance | Interference fit helps barrel plugs remain in place |
| Surface protection | Covers exposed openings during processing & handling |
| Ease of use | Push-fit design enables quick installation & removal |
| Cost control | Reduces cleaning, damage & rework |
Our experts explain all of your options in The Ultimate Guide to Caps and Plugs.
Colour and identification in use
Colour is used to separate sizes where multiple hole plugs are handled together. In production environments, this reduces selection errors and speeds up identification during installation, particularly where similar diameters are in use.
In production environments, different colours can be assigned to specific stages. Plugs used during machining, finishing or transport are often separated by colour to avoid mix-up between operations.
Colour is also used during inspection. Plugs that contrast with the component surface are easier to see, which helps identify whether openings have been covered or cleared. This is relevant during painting and inspection processes, where missed holes or retained plugs can affect the finished component.
Colour may also be used to separate different specifications within the same part family. This is typically defined by internal practice rather than any external standard.
Colour has no effect on the mechanical performance of hole plugs, but it plays a role in how they’re used and managed.
Applications across industries
Barrel plugs are used in industries where it’s important to effectively plug holes and protect internal areas from contamination.
| Industry | Application |
| Manufacturing | Protecting machined or drilled holes |
| Automotive | Covering openings during production and transport |
| Engineering | Temporary or long-term hole protection |
| HVAC | Used on fabricated sheet metal & holes during transport and installation |
| Indoor & outdoor enclosures | Protecting punched & drilled holes during fabrication & handling |
To learn more, see our guide, How to choose caps and plugs.
Material and design features
Our barrel plugs are made of LDPE-PIR (post-industrial recycled low-density polyethylene). Note, using PIR material can support your own internal sustainability targets.
The material combines flexibility with recovery characteristics suited to interference fit applications.
| Feature | Function |
| LDPE-PIR construction | Provides flexibility for a tight for interference fit applications |
| Cylindrical shape | Aligns with standard drilled & machined hole geometries |
| Convexed sides | Limits contact area at the interface, reducing insertion pressure & aiding removal |
| Material resistance | Allows repeated use with minimal permanent deformation |
| Environmental resistance | Supports use of barrel plugs in general industrial conditions |
Material behavior
LDPE-PIR is characterised by low modulus and high elongation, allowing deformation during fitting without fracture. Typical values are as follows:
| Property | Typical range | Test method |
| Melt flow* | 3–7 g/10 min | ASTM D1238 |
| Density | 0.91–0.925 g/cm³ | ISO 1183 |
| Flexural modulus** | 255.5 N/mm² | ISO 178 |
| Flexural strength** | 10.49 N/mm² | ISO 178 |
| Tensile strength** | 12.98 N/mm² | ISO/R527 |
| Elongation at break** | 211.1% | ISO/R527 |
| Elongation at yield** | 193.9% | ISO/R527 |
| HDT A | 27.3°C at 1820 kPa, 120°C/h | ISO 75-1 |
| HDT B | 38.8°C at 455 kPa, 120°C/h | ISO 75-1 |
| Vicat softening point | 84°C | ISO 306 |
| Colour | Transparent |
*2.16 kg at 190°C ** 23°C at 10 mm/min
Information relates to average values, which may vary. It is the customer’s responsibility to assure suitability by optimising for use in their own products. It is advised that the material pre dries prior to use.
Design and selection considerations
Correct selection of barrel plugs ensures consistent performance when used as hole plugs.
Hole size and interference fit
Hole size sets the fit. Barrel plugs are specified with an outer diameter above the hole so that interference is present on installation. Contact at the wall provides the holding force. If the fit is too loose, the hole plugs will not remain securely in place. If too tight, insertion forces increase and may affect both the plug and the surrounding material.
Fit specification
Barrel plugs are not sized to a single hole value. They’re used across a hole size range. At the large end of the hole, interference is reduced. At the small end, insertion force increases. Both conditions sit within the working range of the plastic plugs.
Where hole size variation is wide, a single size of hole plugs may not provide consistent fit. In these cases, multiple plug sizes are used to cover the full tolerance range.
Retention versus removal
The level of interference between the barrel plugs and the hole determines both retention and removal force. Increased interference results in higher contact pressure at the interface, which raises both retention and extraction forces.
Material performance
LDPE-PIR plastic plugs remain under compression once fitted. Deformation occurs during insertion, followed by partial recovery after unloading. Under sustained load, contact pressure at the interface reduces.
Environmental conditions
Temperature changes how the material behaves. It softens as temperature rises. Chemical exposure comes down to what the plastic plugs are in contact with. Moisture isn’t a factor for fit.
Failure modes and limitations
Contact pressure drops under load. The plug doesn’t hold as tightly once it’s been in place for a while.
With tighter fits, insertion becomes more demanding. It’s not unusual to see deformation at the surface of the barrel plugs, or damage at the hole edge where force is concentrated.
Where hole size is below the intended range, insertion forces rise sharply. Surface marking or damage to the plastic plugs can occur during fitting.
Material stiffness changes with temperature. At higher temperatures, resistance to deformation is lower and the plug is more easily displaced under load.
Example:
A hole specified at 10.0 mm with a tolerance of ±0.1 mm gives a range of 9.9 to 10.1 mm. A barrel plug with a nominal diameter of 10.2 mm will sit with higher interference at the lower limit and reduced interference at the upper limit.
Conclusion & expert resources
Barrel plugs provide a consistent method of covering and protecting holes in industrial applications. Their performance is based on controlled interference fit, material flexibility, and predictable mechanical behaviour.
Hole size, tolerance, and operating conditions set the fit. These define how the plastic plugs sit and hold in place.
Coloured barrel plugs further improve operational efficiency by enabling fast identification, particularly in environments where multiple hole plugs are used.
We also recommend you use these resources:
Plastic standards, ASTM
How plastics are tested for mechanical, thermal, and chemical performance.
A discrete event simulation model of compressive creep for LDPE
LDPE creep behaviour shows multiple deformation rates and time-dependent variability under compression.
MIT OpenCourseWare – mechanical design & tolerances
Lecture notes that cover interference fits, tolerances and manufacturing variation.
Download free CADs
Free CADs are available for most solutions, which you can download. You can also request free samples to make sure you’ve chosen exactly what you need. If you’re not quite sure which plastic plugs will work best for your application, our experts are always happy to advise you.
Whatever your requirements, you can depend on fast dispatch.
Questions?
Email us at sales@essentracomponents.co.uk or speak to one of our experts for further information on the ideal solution for your application at 0345 528 0474.