Cover Caps for Drill Holes – Professional Closure Solutions for Machined Openings

Cover caps for drill holes are essential components for professionally sealing openings in metal, wood, plastics, and other materials. These blind plugs provide reliable protection against injuries, corrosion, and contamination – while also ensuring an attractive appearance. Our drill hole closure caps are available in four standard colours and in sizes from Ø 4 mm to Ø 30 mm, covering nearly all common bore diameters used in industrial and craft applications.

What are cover caps for drill holes?

Cover caps for drill holes – also referred to as blind plugs, drill hole plugs or cover caps – are closure elements used to seal through-holes and blind holes. They are pressed directly into the bore and held in place by a sophisticated design featuring precision cores and transverse ribs that generate uniform contact pressure against the bore wall. Unlike screw-in closures or bonded solutions, installation is tool-free and reversible.

The functional principle is based on elastic clamping: the cover caps feature a profiled shank with multiple retaining ribs that compress during insertion and then expand to press evenly against the bore wall. The result is a secure, vibration-resistant fit without adhesives, screws, or additional fixing elements.

Functionality and design

Our blind plugs for drill holes are technically refined and optimised for maximum holding force. Their construction includes several carefully designed elements:

Precision core: The central core forms the structural backbone of the cap and is precisely matched to the bore diameter. It ensures accurate centring during insertion and prevents tilting.

Transverse ribs: Several circumferential transverse ribs along the shank generate the holding force. These slightly tapered ribs compress during insertion and then expand back out in the bore. The number and positioning of the ribs are optimised to create uniform contact pressure over the entire insertion depth.

Sealing lip: A surrounding sealing lip at the upper edge provides a flush finish with the material surface. It prevents moisture, dust, and dirt from entering and contributes to a professional appearance.

Tolerance compensation: The flexible PE design allows for tolerance compensation. For example, a cap for Ø 10 mm fits bores from 9.8 mm to 10.2 mm, which simplifies application and reduces stock levels.

Grip recess: Many of our cover caps feature a slight recess on the top surface, making it easier to remove them with a screwdriver or fingernail – important for maintenance and inspection.

Material and technical properties

All cover caps for drill holes are manufactured from high-quality polyethylene (PE). This material has been deliberately selected because it offers optimum properties for closure solutions:

Mechanical durability: PE is characterised by high impact resistance and flexibility. The caps remain elastic even at temperatures down to -40°C and do not crack under mechanical loads. The transverse ribs retain their restoring force even after years of use.

UV stabilisation: All drill hole caps are UV-stabilised and maintain their mechanical properties for many years – even in continuous outdoor use. The colour does not fade and the material does not become brittle.

Chemical resistance: PE is resistant to moisture, saltwater, oils, alkalis, and most acids. In industrial environments, the blind plugs withstand coolants, cleaning agents, and hydraulic oils.

Temperature range: A service range from -40°C to +80°C covers virtually all practical applications – from outdoor constructions in northern climates to machine components in production facilities.

Surface: All caps feature a matte, scratch-resistant surface with slightly rounded edges. The finish integrates harmoniously into metal, wood, and plastic constructions and looks professional.

Available colours

We supply cover caps for drill holes in four standard colours from stock:

• Black (RAL 9005): The most commonly used colour, ideal for industrial applications, mechanical engineering, and all areas where a discreet, technical look is desired.
• Grey (RAL 7040): Matches galvanised or anodised surfaces, popular in aluminium processing.
• Anthracite (RAL 7016): Modern, premium appearance for visible applications in furniture manufacturing, shopfitting, and exhibition construction.
• White (RAL 9010): For light surfaces, cleanroom environments, and medical technology.

All colours are UV-stable and remain colourfast throughout their service life.

Protective functions of drill hole caps

Personal safety and workplace protection

Open drill holes with sharp edges represent a significant risk of injury. Especially in areas with frequent foot traffic – such as stairs, handrails, playground equipment, or workshop furnishings – unsealed bores can cause cuts and abrasions. Cover caps for drill holes seal these hazardous areas and help fulfil occupational safety and accident-prevention requirements.

Corrosion protection for metal structures

Moisture entering blind or through-holes leads to internal rust formation. Particularly on galvanised or painted surfaces, this can cause subsurface corrosion that remains invisible from the outside for a long time. Blind plugs prevent water ingress and significantly extend the service life of steel structures – especially important for outdoor applications such as façade fixings, railings, or machine foundations.

In stainless steel constructions, the caps protect against crevice corrosion that can occur in narrow drill holes when aggressive media accumulate.

Contamination protection in production environments

In production halls, workshops, and industrial environments, metal chips, grinding dust, coolants, or welding spatter can enter open bores. These contaminants not only impair appearance, but can also damage tools or make subsequent assembly steps more difficult. Sealing drill holes with suitable caps is therefore standard in industrial manufacturing.

Outdoors, insects may nest in open bores and dirt, leaves, and organic material can accumulate. Cover caps keep these contaminants out and significantly reduce maintenance effort.

Aesthetics and professional appearance

Sealed drill holes create a cleaner and more professional look – a key factor in visible constructions in furniture manufacturing, shopfitting, or exhibition construction. The flush finish without visible fasteners contributes to a high-quality overall impression. In representative areas such as reception halls, showrooms, or public buildings, cover caps for drill holes are indispensable for a flawless finish.

Installation and handling

Installation of cover caps for drill holes is tool-free and takes only a few seconds:

• Preparation: Ensure the bore is clean and free of burrs. Remove burrs with a countersink, deburring tool, or sandpaper, as they can damage the sealing lip and ribs. For oily surfaces, degrease the bore with a suitable cleaner.
• Insertion: Position the blind plug straight and press it in evenly. The transverse ribs compress during insertion and then lock securely in place. For larger caps (from approx. Ø 20 mm), a rubber or plastic mallet may be used. Never use a metal hammer directly – this can damage the cap or permanently deform the ribs.
• Inspection: Check the fit. The cap should sit flush with the surface and not protrude. The sealing lip should rest fully on the material surface. A light pull test confirms the secure fit.

Removal and reuse: Cover caps can be removed when required. Carefully lift the cap at the sealing lip with a flat screwdriver or extraction hook and lever it out. If the transverse ribs and sealing lip are undamaged, the cap can be reused – important for maintenance, inspection, or component replacement.

Application areas by industry

• Metal construction and steel structures: In metal construction, numerous bores are created for fastening screws, welding points, or temporary fixings. Not all of these bores are ultimately used – surplus or modified bores must be sealed. Cover caps for drill holes offer the ideal solution here: quick installation, professional appearance, and reliable corrosion protection. Typical bore diameters: Ø 6–16 mm for fastening holes, Ø 20–30 mm for cable routing.
• Mechanical engineering and plant construction: Machine frames, guide plates, and mounting rails often have threaded or through-holes that are not permanently used. Blind plugs protect against coolants, oils, chips, and dirt. They prevent threads from clogging or hydraulic connections from becoming contaminated. In the food and pharmaceutical industries, PE caps also help to meet hygiene requirements.
• Furniture manufacturing and interior design: In furniture manufacturing, bores are created for handles, connectors, hinges, or shelving systems. When designs change or drilling errors occur, these holes must be covered. Cover caps in matching colours make mis-drilled holes almost invisible. Common diameters: Ø 5 mm (hinges), Ø 8 mm (shelf supports), Ø 10 mm (connector fittings), Ø 35 mm (cup hinge bores).
• Automotive and vehicle body construction: In the automotive and commercial vehicle industries, bodywork, frames, and superstructures are drilled. Surplus bores or holes from previous models must be sealed. Drill hole caps made from UV-stable PE withstand weather, road salt, and temperature fluctuations. They prevent rust and help reduce noise.
• Construction and façade technology: During façade fixings, railing installations, or window mounting, bores are created in structural elements. Unused holes or bores from modifications must be permanently sealed. Cover caps for drill holes provide reliable weather protection and prevent moisture from penetrating the building structure.
• Electrical engineering and control cabinet construction: Control cabinets, distribution boxes, and enclosures contain numerous bores for cable entries, fixings, and ventilation openings. Unused bores must be sealed to maintain the protection rating (IP protection class). Blind plugs prevent dust ingress and contribute to touch protection.
• Playground and sports equipment: Climbing frames, bars, playhouses, and fitness equipment are often made from tubes and profiles with many drill holes. Cover caps for drill holes fulfil important safety functions here: they cover sharp edges and prevent injuries. Our PE caps meet the increased requirements for impact resistance, UV stability, and low emissions.

Size selection and tolerances

The correct cover cap is selected based on the bore diameter. Our blind plugs are available in the following standard sizes:

• Ø 4 mm to Ø 10 mm: For small bores in thin-walled constructions, electronic enclosures, and furniture.
• Ø 12 mm to Ø 16 mm: For standard bores in metal construction, mechanical engineering, and general industrial applications.
• Ø 18 mm to Ø 30 mm: For larger bores in steel structures, cable routing, and pipeline construction.

Each cap covers a certain tolerance range. Example: A drill hole cap for Ø 10 mm fits bores from 9.8 mm to 10.3 mm. This tolerance takes manufacturing deviations into account and makes application easier.

Our product descriptions contain precise dimensional information with nominal diameter, outer diameter of the sealing lip, and insertion depth. If you are unsure, we can provide free samples for testing.

Differences compared with other closure solutions

• Cover caps vs. threaded plugs: Threaded plugs are screwed into existing threads and offer higher resistance under pressure. However, they require prepared threads and are more expensive. Cover caps for drill holes are universally usable, quicker to install, and more economical.
• Cover caps vs. adhesive plugs: Bonded closures provide maximum sealing, but are not reversible. Our blind plugs can be removed when required – important for maintenance, inspection, or modifications.
• Plastic vs. rubber: Rubber plugs offer higher elasticity and better sealing in irregular bores, but age more quickly due to UV radiation and ozone. Plastic blind plugs made from PE are more durable, UV-stable, dimensionally stable, and sufficiently tight for most applications.
• Plastic vs. metal: Metal plugs provide higher mechanical strength and temperature resistance. Plastic cover caps are corrosion-free, lighter, more economical, electrically insulating, and sufficiently robust for almost all industrial applications.

Quality and warranty

• Precision manufacturing: High quality standards ensure a precise fit in common bore tolerances. The transverse ribs are exactly dimensioned for optimum holding force.
• Quality control: Each batch is tested for dimensional accuracy, material properties, and functionality. Random tests check extraction forces and long-term stability.
• Long-term testing: Regular tests verify UV resistance, resistance to temperature cycling, impact strength, and chemical resistance under real operating conditions.
• 10-year warranty on dimensional stability, UV resistance, and colourfastness under proper use.
• Made in Germany – German quality, reliability, and fast availability.

Delivery and service

• Delivery time: Standard sizes and colours available from stock, shipping within 24 hours after receipt of order.
• Shipping costs: Free shipping within Germany for orders over €100.
• Quantity discounts: Automatic tiered pricing for registered B2B customers – the larger the quantity, the lower the unit price.
• Free samples: Samples available on request for test fitting before placing bulk orders.
• Technical support: Our experienced team supports you in product selection and answers technical questions.

FAQ – Frequently asked questions about cover caps for drill holes

1. What material are the blind plugs made from?

Our cover caps for drill holes are made from high-quality polyethylene (PE) – durable, weather-resistant, impact-resistant, and UV-stabilised. The matte surface is scratch-resistant and remains dimensionally stable for many years.

2. Are the drill hole caps suitable for outdoor use?

Yes, all caps are UV-stabilised and designed for permanent outdoor use. They are resistant to rain, snow, sunlight, frost, and temperature fluctuations (-40°C to +80°C) and have been proven in façade construction, metal construction, and outdoor installations for many years.

3. How do I find the correct size for my bore?

Measure the diameter of your bore with calipers. Each product description specifies the corresponding diameter range (e.g. Ø 9.8–10.3 mm for caps rated Ø 10 mm). If you are unsure, we can provide free samples for testing.

4. How are the cover caps installed?

Installation is tool-free by pressing in by hand or using a rubber mallet for larger sizes. The bore should be clean and free of burrs. The cap sits flush with the surface – no adhesive is required. Installation takes only a few seconds per cap.

5. Are the blind plugs watertight?

They provide reliable protection against splash water, rain, moisture, and dust, but they are not designed as pressure seals for high-pressure applications. For typical industrial uses (metal construction, mechanical engineering, furniture manufacturing), the protection they offer is fully sufficient.

6. Can the cover caps be removed and reused?

Yes, they can be carefully lifted and removed with a flat screwdriver. If the transverse ribs and sealing lip remain intact, the caps can be reused. This is important for maintenance work, inspections, or subsequent modifications.

7. Which colours are available?

Black (RAL 9005), grey (RAL 7040), anthracite (RAL 7016), and white (RAL 9010) are available from stock. All colours are UV-stabilised and remain colourfast throughout their service life.

8. How do cover caps differ from rubber plugs?

Our PE cover caps with transverse-rib design are more dimensionally stable, UV-resistant, and durable than conventional rubber plugs. Rubber plugs offer higher elasticity for irregular bores but age more quickly. For permanent outdoor applications and precise bores, we recommend PE blind plugs.

9. Can the blind plugs be painted or coated?

Yes, the PE surface can be painted with suitable plastic paints or 2-component coatings. For optimum adhesion, we recommend light sanding with fine sandpaper (grit 320) and the use of a plastic primer. This allows the caps to be colour-matched precisely to the surrounding surface.