The ultimate guide to caps and plugs

General Protection

Not every general protection product is suited for every application. Materials play a critical role. For instance, you wouldn’t use a stud bolt protection cap as a high-temperature masking solution. For starters, they’re made of Low Density Polyethylene, which can’t stand up to high temperatures, but provides excellent performance for a basic application, such as for storage and transportation.

To help you select the right caps and plugs for your application, we’ve put together this guide. We strongly recommend that you try before you buy – request free samples to ensure that the Essentra product selected matches your requirements.

What do you want to know?

• Types of caps and plugs
• Materials overview
• When materials are similar
• Examples of applications by industries
• Material and properties

Types of caps and plugs

Before you can understand the materials, you need to know the types of caps and plugs available. Caps and plugs are designed for every use in a variety of materials. You can even have them custom made. Decide your priorities and then you can choose the most suitable caps and plugs.

• Manufacturing – To keep production moving efficiently, always consider the time it takes to apply and remove caps and plugs without damaging threads or profiles; an easy pull plug or tear tab caps work well, for example. If you can adapt your protection to your production methods, you’ll work far more productively.
• Masking – Areas you especially need to protect include electrical contact points, threaded holes or studs and welding areas. Materials react differently to processes, so before you actually start masking, request a free sample of the masking product you’d like to use and test it. Keep in mind that caps and plugs aren’t your only option. Masking tape might be best for your needs.
• Storage and transportation – Components are especially vulnerable to damage and contamination during transportation and storage, which is why caps and plugs are often used to protect critical profiles in those situations. Tapered round plugs with flange, for example, are often used for storage and transportation, providing protection against dirt, debris, moisture and corrosion.
• End use – Caps and plugs are often used to finish off a product, such as furniture or panels. Decorative hex bolt caps or a round ferrule are good examples, while also providing protection.

Specialty caps and plugs

Materials overview

How do you know which material you should use for your application? To give you a head start, here’s a quick look at the characteristics of materials for caps and plugs:

Plastics

Rubber

Other materials

When materials are similar

This often makes choosing a material for your caps and plugs difficult. Here are the differences in the materials that many find confusing.

LDPE vs. HDPE

They’re both made of polyethylene (PE), one having low and the other, high density, In fact, LDPE has a density in the range between 0.910 and 0.925 gm/cm3. HDPE, within the range of 0.941 to 0.965 gm/cm3.

As to characteristics, LPDE has a high ductile nature and mediocre tensile strength. HPDE has high tensile strength. Both LDPE and HDPE are flexible. LDPE stretches and conforms better to surfaces than HDPE, which is tougher and more rigid. 

HDPE vs Polypropylene

Polypropylene is a thermoplast polymer. Like HDPE, it’s rugged and can resist chemical solvents and acids. However, they differ in density, with polypropylene between 0.895 and 0.92 g/cm², making it more flexible than HDPE. Another difference is that polypropylene has poor UV resistance.

EPDM vs Silicone

Both are rubbers. EPDM is popular in the automotive and construction industries. Your car’s door and window seals are most likely EPDM, for example. Silicone has a longer life than EPDM, but the main difference in these two materials comes down to their ability to withstand extreme temperatures. Silicone is the runaway winner. Here’s a closer look at EDPM vs. Silicone.

TPE vs TPR

Both materials belong to the family of thermoplastic elastomers. TPR has qualities that combine the properties of rubber with the moulding capabilities of thermoplastics. Yet these two materials are so much alike in their characteristics. Both have high-flexural fatigue resistance, high-impact strength – even their temperature ranges are the same. The difference comes down to their base materials.

TPE is modified from Styrene Ethylene Butylene Styrene Block Copolymer (SEBS) and has a lower density than TPR. TPR is often modified from styrene-butadiene-styrene block copolymer (SBS). The result is that TPE is essentially a softer version of TPR. 

Industries: which solutions?

Not every material works for every cap and plug, of course. Here’s a glance at examples of what material works best in specific applications.

• Equipment Manufacturing
• Masking
• Vehicles
• Hoses and Hydraulics
• Pipe Protection

Equipment Manufacturing

Blanking plugs, for example, are often used to finish off panel holes on a piece of equipment, giving it a nice, clean finish and providing safety against sharp edges. They’re available in different materials, but which should you choose and why?

Nylon, PVC or copolyester?

Your choice will depend on the equipment being manufactured. If you’re dealing with irregular shapes, or wires and cables that will need to be accessed, you want a flexible material for your blanking plug which can easily be removed when needed. This is why the automotive industry uses flexible blanking plugs. If it’s a straightforward job, and the blanking holes are merely there for appearance, then nylon will fulfil your need. It’s a hard, long-lasting material. If you want a clear plug, opt for copolyester.

Masking

For masking to be successful, your material selection is critical. It’s got to behave as required during wet painting, powder coating, e-coating, anodizing, plating, blasting and drying. Your material has to stand up to the operating temperature that your application requires. Before you actually start masking, request free samples of the masking product you’d like to use and test it.

Masking tape can also give you a very economical, easy-to-use solution. For instance, if you’re masking sheet metal, tape is easy to apply and remove.

Which material for your masking application?

Masking solutions to consider, depending on the process you’re using:

We do not assume any liability for the materials, specifications or information provided. For complete temperature and other physical properties of our products’ material, please refer to the actual product material technical data sheet, available upon request.

Vehicles

The concern here is with precision. Key vehicle parts to protect are:

• Diesel/petrol pumps
• Engines
• Specialist vehicle fittings
• Powertrains
• Brake systems

What materials will help you protect fuel lines with integrity? The aim should be to prevent contamination while protecting threads from damage during shipment and masking.

LDPE is usually used to protect products during transit and storage. It’s relatively rigid and offers a secure fit for maximum protection, with good impact resistance. Its protection parts are sometimes threaded so that it can be screwed onto threaded studs and removed without too much fuss. An example of an unthreaded LPDE solution is a grease nipple cap, which protects grease nipples from dirt ingress when not engaged with its mating part.

Vinyl offers a snug fit, even for small dimensions or irregular shapes. Despite its softness, vinyl resists tearing, cracking, splitting and shredding and is easy to remove. Vinyl caps provide an airtight seal while also insulating and cushioning. If your first priority is shock resistance, vinyl offers excellent protection during transport and handling.

Hoses and Hydraulics

You’ll need specialist protection to ensure the safe operation of any fluid hydraulic component. A cap or a plug will protect the coupling whilst minimising the impact of fluid spills and maintaining hose integrity. Common materials for caps and plugs for this application include:

• LDPE
• HDPE
• TPE
• PVC

LDPE material is both cost effective and efficient. For these applications, caps are flexible and easy to fit and do an excellent job of sealing hydraulic applications from damage during transit, heat, dirt, moisture and corrosion. HDPE is more rigid and resists solvents, which can be an important factor in hydraulics. TPE’s broad temperature range means it can withstand the outdoors well. Its ability to be soft enables it to shape to fit and retain its position.

Pipe Protection

Materials aren’t quite as varied when it comes to industrial pipe protection, but you still have some decisions to make: LPDE, PE and vinyl.

You have to check these materials off against the three areas that matter most to your pipes, and prioritise them in order of importance to you:

• Durability against impact
• Ease of use
• Quality of your materials

LDPE is a low-cost, weatherproof material, and resists chemicals. Durable yet lightweight, these caps are easy to remove and 100% recyclable. Vinyl is attractive as pipe protection due to its flexibility. It stretches over threads to provide a tight seal that keeps out contaminants. PE is a good choice too. It’s a cost-effective material while protecting from damage during storage and transportation.

Materials Properties

Below is a list of materials, and their properties, commonly associated with caps and plugs. Note, these are not the properties of the caps and plugs themselves, but of the material. Materials can be formulated to enhance or take on specific qualities. Still, it’s a good starting point for understanding certain materials.

Plastics

Low Density Polyethylene (LDPE)

The most popular choice for caps and plugs for using as protection during transport, and not just because of its low cost relative to other materials. LDPE is rigid, but compared to Nylon or HDPE, it’s soft. It also has high impact strength and a reputation as a cross between elastomers and thermoplastics. LDPE can withstand the cold, specifically temperatures down to 0˚C before it becomes brittle. When placed in high temperatures, it quickly becomes soft.

Generally, LDPE offers:

Resistance to chemicals:

High density Polyethylene (HDPE)

HDPE is a stiff, strong, high-density material and similar to LDPE in opacity and impact resistance. With good chemical resistance and low permeability to gases and moisture, it’s a good – although not the most common – choice for protecting NPT thread ports from dirt and other contaminants.

Generally, HDPE material has:

Resistance to chemicals:

Nylon

Nylon is a tough material with good thermal and chemical resistance. It’s incredibly strong and can be used in place of low-strength metals. It’s a popular material, used to make everything from fasteners to cable ties. Nylon 6.6, a variant, is stronger and absorbs less moisture than Nylon. It’s the ideal material when high level of torque is required.

Generally, nylon material has:

Resistance to chemicals:

Ethylene Vinyl Acetate (EVA)

Flexible even in low temperatures, EVA has good chemical and stress-crack resistance and its properties work well for electrical applications. Good cushioning performance add an extra touch of protection, which is why it’s often used for handles. It’s especially an ideal material for plugging hydraulic hoses. Look for ones with ribs – these plugs provide a comfortable, snug fit without stretching the hose.

EVA is also excellent at protecting applications from dust ingress and damage.

Generally, EVA material offers:

Resistance to chemicals:

Polypropylene (PP)

Polypropylene caps are known for good impact strength, cost effectiveness, and pliability. The material is considered tough, though it can act with elasticity. It doesn’t suffer stress-cracking problems. It has excellent fatigue resistance, retaining its shape after a lot of torsion, (which is one reason why it’s a great material for hinges). Polypropylene also offers excellent electrical resistance, so it’s often used in electronics. Not suited for freezing temperatures, where it can become brittle.

Generally, Polypropylene material offers:

Resistance to chemicals:

Polyvinyl Chloride (PVC)

PVC is a very durable and long-lasting material. It’s also flexible, and as a cap or plug, this means they’re easy to fit and provide an excellent seal. PVC won’t crack or shred, as hard plastics can do, leaving residue on the area they’re meant to protect from contamination.

PVC can withstand high temperatures, making these caps and plugs good for all masking applications except blasting. Its chemical resistance makes it especially suitable for plating. It’s also an insulative material, which makes it a good choice for enclosures with electrical applications.

Generally, PVC material offers:

Resistance to chemicals:

Polyolefin

HDPE, LDPE and PP are polyolefins. See those specific materials for guidance.

Thermoplastic elastomers (TPE)

A thermoplastic elastomer, TPE is considered more of a rubber by some manufacturers because of its characteristics. Dense, flexible and soft, TPE offers excellent weather and ozone resistance. With good tear and abrasion resistance, TPE also has excellent flexural fatigue resistance and high impact strength.

Generally, TPE material offers:

Resistance to chemicals:

Rubber

Ethylene Propylene Diene Monomer (EPDM)

Used mainly as a masking product, EPDM is versatile enough to also act as a protection solution. It’s generally a less expensive alternative to silicone. It has excellent weather resistance, which is why it’s popular as a roofing material in construction. The automotive, oil and gas industries like caps and plugs made from EPDM for its resistance to acids and alkalis. EPDM’s flexibility makes it an ideal masking solution for sealing round and irregular shapes.

Generally, EPDM material offers:

*EPDM has poor compatibility with the alcohol Hexyl

Resistance to chemicals:

Silicone

Silicone is excellent at resisting high temperatures, making it the ideal material for masking. An excellent example: silicone tapered plugs. However, silicone’s abrasion resistance is poor, though it can stand up to repeated production runs and conditions without wearing, breaking or degrading. And while it is ill-suited as a vibration insulator, it’s an excellent choice as an electrical insulator. Silicone’s flexibility allows for easy installation in the masking process, while providing an airtight seal.

Generally, Silicone material offers:

When not to use Silicone

For protective purposes. Only use for masking

Resistance to chemicals:

Thermoplastic rubber (TPR)

Lightweight with good tear strength, TPR has qualities that combine the properties of rubber with the moulding capabilities of thermoplastics. It holds its shape and comes in varying levels of flexibility and has excellent electrical properties. Good tear strength, reusable and recyclable, TPR is an ideal solution for masking protection.

Generally, TPR material offers:

Resistance to chemicals:

Paper

Caps and plugs made of crepe paper are a low-cost alternative to silicone. Used for masking in painting and powder-coating applications, it can stand up to high temperatures. Made from a blend of virgin pulp and post-consumer recycled paper, it’s a good option when you need environmentally friendly solutions.

Generally, crepe paper has:

Material vs chemicals

It’s critical to choose a general-protection material capable of standing up to chemicals. Chemicals can include anything from water to acid, so no matter what your application’s environment, you need to carefully consider the material you use. 

Materials vs shock vibrations

During transport, caps and plugs are vulnerable to damage due to bumps and falls. This is why choosing a material with shock absorption is critical.

What exactly is shore hardness?

The shore hardness of materials is important for your application. Get it wrong, and your application could fail. 

Download free CADs and try before you buy

Free CADs are available for most solutions, which you can download for free. You can also request free samples to ensure the solutions you’ve chosen are exactly what you need. If you’re not quite sure which product will work best for your application, our experts are always happy to advise you.

Request your free samples or download free CADs now.

Download The Ultimate Guide to Caps And Plugs