• Skip to primary navigation
  • Skip to main content
Plastic Expert Group

Materials Testing & Plastic Consultants

  • HOME
  • ABOUT US
  • SERVICES
    • Overview
    • Plastic Failure
    • Pipe Failure Analysis
    • Injection Molding Defects
    • Materials Testing
    • CPVC Fire Sprinkler Piping
  • LEARN
  • CONTACT US

Categories

Why Are My Plastic Parts Failing at Weld Lines?

You are here: Home / Plastic Failure Analysis & Testing / Why Are My Plastic Parts Failing at Weld Lines?

February 16, 2023

If the PEX manufacturer’s installation rules are not followed, failures can result. One of the most common installation defects is sharply bending the pipe to make a directional change. Sharply bending the tubing causes the tubing to kink. The kink in the tubing places localized stress at a spot in the wall of the tubing. Stress in the PEX material facilitates oxidative degradation of the PEX causing it to become brittle at the high-stress point.

A CASE STUDY 

The following case study is the property of Plastic Expert Group and cannot be copied or distributed without prior permission. Our case analysis does not imply that this specific plastic failure will happen systematically. Every case is unique and should be treated accordingly.

 

PROBLEM: Defective PEX pipes with expansion joints leak causing water damage

There are a lot of different types of PEX pipes and fittings. The three types of PEX are PEXa, PEXb, and PEXc. PEXa and PEXb are the most common. There are several different types of fittings used to assemble a PEX piping system. The most common fittings are brass and plastic. Because of the potential for corrosion (dezincification) of brass fittings, plastic fittings are generally preferred. Once the installer decides on whether to use brass or plastic fittings, he or she then has to decide whether to use standard size fittings or use expansion fittings. As shown in the figure below, expansion fittings are larger in diameter than the inside of the pipe. 

 

In order to be able to insert the larger diameter expansion fitting into the end of the PEX pipe, the end of the pipe is expanded using an expansion tool like shown in the figure below.

 

Once the fitting is inserted into the expanded tubing, the tubing shrinks onto the fitting.  There are potential problems with the use of expansion fittings including 1) the teeth on the tool damages the inside of the tubing by placing indents into the inside surface, and 2) the stress/strain on the expanded tubing are high as revealed in the finite element stress analysis below.

PEX Pipe tubing failure spots

 

The photograph below shows an image of an example of a typical PEX failure at a connection to an expansion fitting.

 

SOLUTION:

Degradation and embrittlement of PEX piping are accelerated by stress. The pipe is designed to
handle the stress of normal water pressure. However, stressors are additive. The use of expansion fittings places significant additional stress on the wall of the pipe making the pipe more prone to chemical attack and degradation by chlorinated water. The image below shows an image of the inside surface of the pipe installed using an expansion fitting. Notice the damage to the inside surface caused by the teeth of the expansion tool and the cracks that formed on the inside surface resulting from embrittlement of the PEX material accelerated by the additional stress of the expansion. The use of fittings that are the same diameter as the inside of the PEX pipe and crimp fitting results in a more reliable joint because of the lower stress.

Inside-Surface-of-the-Pipe-Installer

With decades of experience, Plastic Expert Group are experts at why various plastics fail at weld lines.

FEEL FREE TO SHARE YOUR PROBLEM OR QUESTIONS WITH US AND LET’S DISCUSS!

CONTACT US

Plastic Expert Group & Failure Labs Copyright © 2025 Toll Free USA: (877) 668-4345   •   International: +1 (989) 281-4465

www.plasticpipefailure.com   •   www.plasticfailure.com

Plastic Failure Services:  Molding Defects   •   Pipe Failure Analysis   •   Materials Testing Consultants   •   Plastic Failure   •   PEX Failure   •   Polymer Consulting

Manage Consent
To provide the best experiences, we use technologies like cookies to store and/or access device information. Consenting to these technologies will allow us to process data such as browsing behavior or unique IDs on this site. Not consenting or withdrawing consent, may adversely affect certain features and functions.
Functional Always active
The technical storage or access is strictly necessary for the legitimate purpose of enabling the use of a specific service explicitly requested by the subscriber or user, or for the sole purpose of carrying out the transmission of a communication over an electronic communications network.
Preferences
The technical storage or access is necessary for the legitimate purpose of storing preferences that are not requested by the subscriber or user.
Statistics
The technical storage or access that is used exclusively for statistical purposes. The technical storage or access that is used exclusively for anonymous statistical purposes. Without a subpoena, voluntary compliance on the part of your Internet Service Provider, or additional records from a third party, information stored or retrieved for this purpose alone cannot usually be used to identify you.
Marketing
The technical storage or access is required to create user profiles to send advertising, or to track the user on a website or across several websites for similar marketing purposes.
Manage options Manage services Manage {vendor_count} vendors Read more about these purposes
View preferences
{title} {title} {title}