Understanding Low Melt Fiber (LM Fiber) and the Reason for Using the 4D × 51mm Specification

In the nonwoven industry, Low Melt Polyester Fiber (LM fiber) plays a critical role in determining the structural strength and bonding performance of the final product.

In applications such as PET-based acoustic insulation, thermal insulation, industrial filtration media, and geotextiles, the quality and specifications of LM fiber directly influence durability, dimensional stability, and overall product integrity.

But why is LM fiber commonly produced in a 4 denier (4D) fineness and a 51 mm cut length?

In this post, we will explore what LM fiber is and why these standard specifications have become widely used in manufacturing.

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What is LM Fiber?

LM fiber literally stands for Low Melt Fiber, meaning a polyester fiber engineered to melt at a significantly lower temperature than standard polyester fibers.

Whereas conventional polyester melts at approximately 255–265°C, LM fiber typically melts at around 110–150°C.

This fiber is commonly produced as a bicomponent (sheath–core) structure:

• The outer sheath is made of a low-melting polymer.

• The inner core is standard high-melting PET.

During heat treatment, only the sheath softens and melts, allowing it to bond with surrounding fibers. As a result, LM fiber enables strong structural adhesion without the need for chemical adhesives.

In other words, LM fiber functions as a binder fiber in nonwoven materials, providing structural integrity and durability.

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Why 4 Denier (4D) Became the Standard

So why is LM fiber most commonly produced in a 4D specification?

The denier, which represents the thickness of the fiber, directly affects both mechanical strength and the uniformity of thermal bonding during processing.

LM fiber is widely standardized at 4 denier because it offers the best balance between process stability and bonding consistency.

• If the fiber is too fine (e.g., 2D or below):Heat transfer occurs too quickly, causing the low-melt sheath to over-soften. This often leads to agglomeration (clumping), resulting in uneven bonding and reduced structural stability.

• If the fiber is too coarse (e.g., 6D or above):The spacing between fibers becomes larger, leading to fewer and weaker bonding points. This reduces uniformity in the web and creates instability in pore structure.

  
  

By contrast, 4D provides an ideal intermediate fiber thickness:

• Stable bonding during thermal processing

• Sufficient mechanical strength

• Uniform fiber dispersion during carding and web formation

For these reasons, 4D has become the standard and most widely used denier specification for LM fibers in the nonwoven industry.

Category	Advantages
Bonding Uniformity	The 4D thickness provides an optimal balance between contact area and heat transfer rate, resulting in clean and uniform thermal bonding.
Mechanical Strength Retention	Maintains sufficient tensile strength for products such as filters and acoustic insulation materials while also providing a soft hand-feel.
Process Stability	Operates most reliably in carding, cross-lapping, and thermal bonding production lines, ensuring stable web formation.
Cost Efficiency	Offers excellent bonding performance relative to production cost, making it highly cost-effective for large-volume manufacturing.

It is used under these conditions to produce a wide range of products in combination with other fibers.

Product Category	Primary Fiber Composition	Notes
Acoustic Pads	PET 6D + LM 4D	Thermal bonding at approx. 110°C
Thermal Insulation Materials	HCS 7D + LM 4D	Maintains soft touch while retaining shape
Air Filters	PET 3D/6D + LM 4D	Forms a uniform bonding layer
Geotextiles	Solid 6D + LM 4D	Requires high-strength bonding performance

Accordingly, LM fiber is widely used as the standard specification for thermally bonded nonwoven products such as acoustic pads, insulation materials, and filtration media.

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Why is the 51 mm Cut Length the Standard?

LM fiber is also typically cut to a length of 51 mm.

This cut length is important because it directly affects the stability of the carding and web-forming processes.

If the fibers are too short, they disperse easily, but the strength of the finished nonwoven decreases.If the fibers are too long, they tend to entangle, which reduces opening efficiency and disrupts carding performance.

A length of 51 mm (approximately 2 inches) provides the optimal balance between processability and product uniformity.When matched with the standard cut length of PET staple fibers, it produces the most stable and consistent web structure.

Criteria	Technical Advantages of the 51 mm Cut Length	Notes
1) Compatibility with Carding Machines	Most carding and cross-lapping equipment is designed to process fibers in the 38–64 mm range. A 51 mm cut length can be opened and processed smoothly without tangling.	Widely accepted global standard
2) Web Uniformity	51 mm fibers provide the optimal balance between mechanical bonding strength and fiber dispersion, resulting in a uniform web structure.	Fibers shorter than 32 mm cause dusting; fibers longer than 64 mm cause clumping
3) Fiber Orientation Stability in Needle-Punching	If fibers are too short, they slip during punching, and if too long, they create directionality issues. 51 mm maintains stable fiber orientation.	51 mm is the most balanced length
4) Thermal Bonding (Binder Function)	When LM fiber is the same length as PET staple fibers (typically 51 mm), the contact area is maximized, forming a uniform bonding network.	Recommended for PET + LM blend applications
5) Production and Cost Efficiency	Longer fibers require more cutting and opening energy. 51 mm minimizes processing energy while maintaining bonding quality.	Based on international design standards such as ISO 9073

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Exceptional Cases Where Other Cut Lengths Are Used

Application	Typical Cut Length	Reason
High-density filters / Nonwoven mask media	32 mm	Prioritizes fine fiber dispersion
Stuffing / Padding materials	64 mm ~ 76 mm	Improves loft (bulkiness and resilience)
Geotextiles	51 mm or 64 mm	Slightly longer fibers are used to increase tensile strength during production
Acoustic pads / Thermal insulation (PET + LM blends)	51 mm	Maintains standard structure and ensures uniform bonding

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LM fiber is not merely a "secondary additive" but a functional binder fiber that forms the structural foundation of nonwoven materials.

And the standard specification of 4D × 51 mm is an industry-validated optimum, proven through extensive production experience and performance testing.

DK FIBER supplies LM fibers that meet international manufacturing standards.