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Caps and Plugs: How do HDPE and PP compare?

Caps and Plugs: How do HDPE and PP compare?

Issue Time:2019-09-11

Caps and plugs can be used in a variety of applications, but how do you go about selecting the right material for your project? How does polypropylene (PP) fare – and is High-Density Polyethylene (HDPE) more efficient? To help you decide, we outline the characteristics of each.

With a wide variety of applications, including automotive components, packaging and even textiles, PP is a thermoplastic polymer manufactured from a monomer known as propylene.

Meanwhile, HDPE can be more rigid that PP, owing to its lower density. Let’s take a look at both materials’ characteristics:




Tensile Strength

0.95 - 1.30 N/mm²

0.20 - 0.40 N/mm²

Thermal Coefficient of expansion

3.0 – 30.0 Kj/m²

no break Kj/m²

Max Cont Use Temp

80˚C / 176˚F

65˚C / 149˚F



0.944 - 0.965 g/cm3

Resistance to chemicals



Dilute Acid

Very good


Dilute Alkalis

Very good


Oil and Greases

Moderate (Variable)

Moderate (Variable)

Aliphatic Hydrocarbons



Aromatic Hydrocarbons



Halogenated Hydrocarbons




Very good


What’s the difference between HDPE and PP?

If you’re wondering if there are any distinct differences between HDPE and PP, you’d be right to wonder. The answer is ‘yes’, with a host of differences in terms of density, temperature and UV and chemical resistance.

Density is a key factor that differentiates HDPE from PP. Because HDPE has a lower density, it can be more rigid. Thanks to its lower density, though, PP, can be used when moulding parts with lower weight.

Like HDPE, Polypropylene offers good chemical resistance. UV resistance, though, is poor; if it is stabilised with additives, it can be improved. Resistant to a wealth of solvents, HDPE and PP have a wide variety of applications.

Why use HDPE for Caps and Plugs

HDPE makes for excellent push fit caps and plugs. Offering smooth and simple assembly, they can protect critical internal and external profiles from damage and there are a wealth of types available. These include taperedeasy-removalflexible, specialist, quick-fit, pull-tab, and tube caps and plugs. Alongside HDPE, they can also be manufactured from LDPE, PE, PVC, Silicone, TPR or EVA.

Why use PP for Caps and Plugs

A relatively inexpensive material, Polypropylene is a good choice when it comes to caps and plugs. It’s versatile, doesn’t suffer damage from the sun or elements in the same way as other plastics, and can withstand high temperatures.

On top of that, it does not absorb water like other plastics, making it ideal for outdoor use. It’s unlikely to shatter, too, though it is not as sturdy as, say, polyethylene. Ensuring your project – and its parts – remain functional and safe, caps and plugs may need to withstand certain chemicals.

In electrical projects specifically, Polypropylene is ideal. The reason for this is because it has a low level of electrical conductivity, meaning it can perform smoothly in electronic products and applications.

How to Select the Right Caps and Plugs

Before choosing the right caps and plugs for your particular project, there are several things you should consider. These include your environment, the application itself, the material, the production process and the removal process.


In the case of your environment, ask yourself the following questions

  • Is there need for UV resistance?
  • Are corrosive substances present?
  • What about the atmosphere; are there high levels of moisture or humidity?
  • Is there a need to conduct or dissipate electricity?
  • Are there Pounds per Square inch (PSI) considerations?
  • Does your application need to operate in cold or hot conditions?

You must also understand your application – and what it requires in order to perform correctly. There are a wealth of caps and plugs available, from O-rings to threaded plugs, with each offering different features.

By identifying the right material for your caps and plugs, you’ll be setting up your project for further success. Consider heat – and the heat tolerance of your chosen material. It’s also advisable to think about the environment your caps and plugs will be used in.

    caps in production

    The production process is vital, too – and by considering how caps and plugs can be applied, you’ll be ensuring a more seamless project. Also critical is the removal process. For example: some caps and plugs can be reused following removal, which allows for some savings within your budgeted project costs. Other caps and plugs, meanwhile, won’t be quite as easy to remove – or you won’t be able to remove them at all.

    HDPE Caps and Plugs

    So, where do you begin with HDPE caps and plugs; which is best for your project? Below, we lay out a few examples and some details on how they might be utilised in your application.

    Plug or Cap Type

    Plug or Cap Characteristics

    UNF/Metric Threaded Sealing Caps

    Limiting fluid leakage on UNF threads, the UNF/metric threaded sealing cap has a height of 452 in (11.5 mm) and reduces the potential of shearing

    NPT Threaded Square-headed Plugs

    The NPT Threaded Square-headed Plug has an easy-grip head for efficient removal and application. It’s also used to protect NPT threads against moisture and contaminants.

    Metric Threaded O-Ring Plugs

    Equipped with an O-ring to protect M8 x 1 Metric Threads from leakage, the Yellow HDPE Threaded O-Ring Plug can be applied or removed by spanner, screwdriver or socket. It can even be removed by hand, if needed.

    Overfill Prevention Device Cylinder Valve Plugs

    Compatible with Propane gas to protect valve threads during use and transport, Overfill Prevention Device Cylinder Valve Plugs are easy to apply via a flat strap attachment device.

    UNJ/UNJS Threaded Plugs

    With an easy to grip had for efficient removal and application, the UNJ/UNJS Threaded Plug protects components from dust, moisture and damage during the production, storage or shipment process.

    UNF Standard Threaded Plugs

    A threaded plug with a serrated grip, UNF Standard Threaded Plugs can be used in a variety of applications and can be removed by hand, hex wrench or screwdriver.

    Polypropylene Caps and Plugs

    Looking for Polypropylene caps and plugs? Take a look at some of the options available to you – and their characteristics:

    Plug or Cap Type

    Plug or Cap Characteristics

    Fluid Absorption Plugs

    Providing an attractive finish with a fitting height of 15.0mm, Fluid Absorption Plugs can control excess fluid residue from leaking. Easy installation is possible, too, thanks to its unique design, and the plugs are safer, cleaner and easier to use than their conventional counterparts.

    UNF Standard Threaded Plugs (O-Ring optional)

    Enjoy efficient application and removal with the UNF Standard Threaded Plug (O-ring optional). They can be applied or removed by hand, or with a hex wrench or screwdriver and an optional O-ring gives this component a water-tight seal.

    NS & NF Class 1-2-3 Threaded Plugs (O-Ring Optional)

    With an optional O-ring, which gives this component a water-tight seal, NS & NF Class 1-2-3 Threaded Plugs have an easy to grip head for efficient application and removal, and will fit NS and NF Class 1-2-3 standard threads.

    Flange Protection Discs

    Protecting a DN10 inch pipe size with a 10, 16, 25, 40 Class Pressure Rating flange, these Flange Protection Discs provide simple, cost-effective protection for flanges. Covering a variety of DIN sizes and pressure ratings, the outside diameter of the component is manufactured from Corrugated PP Material.

    HDPE Vs Polypropylene: In Summary

    Ready to choose your preferred caps and plugs? There are lots to think about, with safety, efficiency and cost amongst the key factors. Light and flexible, HDPE offers quick installation, excellent resistance and a long lifespan.

    HDPE products aren’t known to transmit any chemicals, either, which deems them safe for use in everything from food packaging to automotive components.

    Polypropylene, meanwhile, can withstand higher temperatures, making it ideal for hotter environments. Possessing high flexural strength, due to its semi-crystalline nature, it’s a relatively inexpensive material and can be used in a variety of applications.

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    Qingdao Haixin Steel and Plastic Product Co.,Ltd was established in Qingdao,China. We focus on manufacturing high quality plastic injection Products. With over 10 years development, now we have owned an excellent ...