When it comes to chemical storage tanks, the outlet fitting is one of the most critical points in the entire system. It's where stored chemicals meet the outside world, where pressure concentrates, and where tank failure tends to happen first.
For decades, the industry has used metallic inserts as a go-to solution for low sidewall outlets — either molded directly into the plastic tank wall during manufacturing or installed after the fact.
They feel sturdy. They look industrial. They seem like the logical choice.
They're not.
Whether molded into the chemical tank during production or threaded in post-production, a metallic insert introduces structural, chemical, and mechanical vulnerabilities that compound over time. These vulnerabilities can cause massive leaks that endanger equipment, personnel, and the environment — as well as result in significant chemical and financial losses.
To better understand the impact, let’s take a closer look at what metallic inserts actually do to a plastic tank over its service life.
Related: Avoid These 4 Common Chemical Tank Mistakes
That constant push and pull creates stress concentration around the metal insert. Over time, micro-cracking inevitably develops in the plastic, the material begins to creep, and the seal that looked perfect on installation day gradually loosens. The result is a slow leak that worsens with each thermal cycle, often appearing to come and go with the seasons, making diagnosis frustratingly difficult.
This isn't a manufacturing defect or an installation error — it's physics, and no amount of torque or sealant can permanently overcome it.
As the chemical storage tank’s plastic slowly deforms around the hard-metal fitting, gasket compression is lost, and the insert hole begins to ovalize or elongate. What was once a tight, leak-free connection becomes a pathway for chemical migration. This process can take months or even a couple of years, which means a tank can pass every inspection and appear perfectly sound right up until the moment it fails — making creep-related failure one of the most dangerous and deceptive failure modes in chemical storage.
Under normal operating conditions, this stress concentration can initiate cracks that radiate outward from the insert through the tank wall. When aggressive chemicals are involved, this risk escalates dramatically.
Environmental stress cracking (ESC) is a well-documented failure mode in polyethylene tanks, particularly in HDPE tanks exposed to acids, oxidizers, solvents, and surfactants. These chemicals attack the plastic at points of elevated stress — exactly the kind of stress that a metal insert creates. The outcome isn't a slow drip, but catastrophic sidewall cracking.
An HDPE tank can rapidly and without warning release its entire contents.
A tank specified for a particular chemical is only as compatible as its weakest component. Even when a polyethylene tank wall is fully compatible with a stored chemical, the metal insert may not be. Which means, in the end, your entire chemical storage solution is incompatible with the chemical you’re storing.
Chlorides cause pitting in stainless steel. Acids accelerate corrosion in carbon steel. When dissimilar metals are present, galvanic corrosion becomes an additional threat.
As the metal corrodes, rough surfaces develop at the fitting interface, creating new leak paths and introducing corrosion byproducts that can further degrade the surrounding plastic. The tank material was carefully selected by expert engineers — a metal fitting undermines the entire tank design.
Related: How Chemical Stress Can Lead To Polyethylene Tank Cracks
Low sidewall fittings connect to piping systems that, in some cases, carry their own weight, expand and contract with temperature, and transmit vibration from pumps. Metal inserts transfer those mechanical loads directly into the plastic tank wall, which is poorly suited to handle concentrated point loads.
The result can be wall distortion, cracking at the fitting, or complete fitting pull-out — particularly in larger diameter outlets or systems without proper external pipe support.
Every failure mode described above shares a common consequence: it is expensive and difficult to fix. Once a metal insert fails in a plastic tank, repair options are severely limited. Plastic welding is unreliable or impossible when metal is embedded in the wall, and field repairs rarely withstand chemical exposure and hydrostatic pressure.
In most cases, the tank must be replaced entirely, along with the downtime, disposal costs, and the liability that accompany a chemical release.
Poly Processing’s Integrally Molded Flanged Outlet® (IMFO) was engineered specifically to eliminate the critical vulnerabilities of metallic tank inserts. The IMFO takes a fundamentally different approach. Rather than introducing a foreign material into the tank wall, the IMFO is integrally molded as part of the tank itself. The flange and the tank wall are one continuous piece of the same material, eliminating the interface where differential thermal expansion, creep, stress concentration, and chemical incompatibility all originate.
Because the IMFO is the same material as the tank, it moves with the tank. It responds to temperature changes, hydrostatic pressure, and chemical exposure exactly as the tank wall does — because it is the tank wall. There is no metal to corrode, no embedded fitting to loosen, no stress riser to initiate cracking, and no dissimilar material to create a compatibility gap.
For any application involving aggressive chemicals, outdoor exposure, or cyclic filling, the IMFO is the best option available. And it’s the only full-discharge fitting that addresses the full range of risks that metallic inserts, in any form, inevitably carry.
Get more details about the IMFO system and discover the advantages over other discharge systems by downloading our full discharge eBook.