The road map from a niche product to being a market staple

Introduction
The development of HDPE structured wall pipes for non-pressure and low-pressure applications illustrates a significant transformation of a niche product in the 1960s to an essential component in the infrastructure industry today. This paper mainly focuses on HDPE structured wall pipes that are produced using helical or radial extrusion techniques which enabled the creation of large-sized pipes. The paper is excluding structured wall pipes produced by corrugators. Furthermore, the paper delves into the evolution and refinement of HDPE structured wall pipes, emphasizing the technological advancements and design improvements that have driven their widespread adoption. With a focus on Western Europe, renowned for its leadership in material science and engineering, this study examines the key factors contributing to the success and growth of these innovative piping solutions. By tracing their journey from inception to their current status as essential components of modern infrastructure, this paper provides an overview of the progress and future potential of HDPE structured wall pipes.
An HDPE structured wall pipe is a type of plastic pipe designed to maximize structural integrity while minimizing material usage. The basic idea behind its structure is to leverage the moment of inertia of the pipe wall, creating a stiff yet flexible system. This design results in a robust pipe that uses less material compared to solid wall pipes, in respect to external loads, enabling it to be a suitable material for drainage and sewage systems. HDPE structured wall pipes have a profiled or corrugated exterior to withstand external loads and a smooth inner surface to maintain efficient flow. The winding process of producing these pipes eliminates the sagging problem as the diameter and wall thickness increases which is the opposite case for solid wall pipes. Standards like DIN 16961, ASTM F894 and EN 13476 govern their specifications, ensuring they meet rigorous quality and performance criteria.

Early Development
The journey of HDPE pipes began in the 1960s and 1970s, marked by the increasing use of thermoplastics in various industrial applications. Initially, HDPE pipes were solid-wall constructions, effective but not fully optimized for performance. The concept of structured wall pipes emerged to improve the strength-to-weight ratio by incorporating profiles and corrugations. The DIN 16961 stood out as a pivotal document, ensuring consistency and reliability in the production of thermoplastic pipes with profiled walls and smooth inner surfaces.

Standardization and Technological Advancements
Introduced in 1977, DIN 16961 provided comprehensive guidelines for classifying, sizing, and specifying structured wall pipes as a complete piping system with fittings and jointing methods; which specifies dimensions using inside diameter to compete with the conventional non-pressure concrete material for drainage and sewer. It laid the foundation for their widespread use in various non-pressure to low-pressure applications, ensuring stringent quality and performance criteria suitable even for public tenders. The majority of standards governing structured wall pipes specify the nominal diameter as the inside (hydraulic) diameter which is marked as DN/ID. This approach aligns more closely with concrete and other sewage pipe systems, ensuring compatibility and ease of integration with existing infrastructure. By using the inside diameter as the nominal diameter, the standard ensures that pipes will have the same hydraulic capacity, regardless of their structured wall characteristics.

During the 1980s and 1990s, advancements in extrusion technology allowed for the production of more complex and efficient structured wall pipes. Structured wall pipes provided superior stiffness and strength, ideal for demanding underground applications which allowed for the production of larger diameter pipes up to DN/ID 3000 mm. As HDPE pipe technology advanced, DIN 16961 was periodically updated to incorporate new manufacturing techniques and materials, maintaining its relevance and reflecting the latest technological advancements.

Various production of spirally wound HDPE structured wall pipes
High-Density-PolyEthylene (HDPE) structured wall pipes are known for their versatility, durability, and adaptability across various applications. The following is a detailed comparison of five main different types of structured wall HDPE pipes and their respective production processes; Following named as Type “B” pipe, Type “W” pipe, Type “H” pipe, Type “DWC” pipe and Type “K” pipe, highlighting their production methods and characteristics. In the sketches below, the symbol (1) represents the structured wall profile, which refers to the engineered shape or design of the pipe's profile that provides strength and flexibility. The symbol (2) is the waterway, which is the smooth inner surface of the pipe that facilitates efficient fluid flow. The symbol (3) indicates the overlapping section, a key feature in spirally wound pipes where the profile overlaps during production to ensure a tight, secure and homogeneous structure. And lastly, the symbol (4) marks the welding part, where the pipe profiles are welded together to achieve a homogeneous and leak-proof joint.

1. Type “B” pipes are made using a single extruder that produces an Omega shape which includes the profile and the waterway (1,2). This profile is spirally wound around a mandrel, with the waterway (3) overlapping between each profile. This approach achieves an excellent balance of efficiency and cost-effectiveness while focusing on practical production methods. Type “B” pipes are produced by batch-production and with the possibility of integrated socket and spigot ends.

2. Type “W” pipes are created using a continuous process where a rectangular profile (1) is produced from a main extruder (profile extrusion) and will be spirally wound around a roller system. Welding (4) is done both inside and outside of the pipe, ensuring homogeneity and durability. This process leaves the inner surface not as smooth as the other pipe types due to internal welding on every seam. Integration of socket and spigot is difficult, but pipe length is more flexible.

3. Type “H” pipes follow a two-step process: first, a rectangular profile (1) is produced from the main extruder, same as the Type “W” pipe. Another extruder will produce a smooth waterway (2) on a rotating mandrel. The extruded rectangular profile will be spirally wound on the waterway. Accordingly, the seams outside of the pipe will be subjected to welding (4) as the profile spirally wound around the mandrel. These combined processes will result in a structured profile with a smooth inner surface. Socket and spigot can be produced with mandrel technology.

4. Type “DWC” pipes consist of a smooth inner wall (waterway (2) and a corrugated outer wall or profile (1). Two extruders are used: one forms the corrugated profile by molds, and the other creates the smooth waterway. These layers are fused during production.

5. Type “K” pipes stand out for their advanced co-extrusion technology. Three extruders are used to form the profile (1), waterway (2), and inner color layer. These components are helically wound on a mandrel, melting together to form a homogeneous and seamless structure. The waterway overlaps beneath each profile for added strength, and the pipe is cooled at ambient room temperature to minimize internal stress. The modular extruder setup allows for high flexibility to customize the pipe profile from changing the waterway thickness to creating different profiles with varying moments of inertia as needed. The Type “H” and Type “W” methods incorporate socket and spigot joints, enabling them to handle low-pressure ratings.These designs enhance the versatility and application range of structured wall pipes, making them suitable for various infrastructure needs.

Modern Innovations and Widespread Use
The most recent major update of DIN 16961, in August 2018, introduced modern testing methods for ring stiffness and pressure resistance. It expanded the scope to include helically wounded pipes, reflecting the growing market demand for larger diameter pipes. Internationally, many derivations of the German standard DIN 16961 have been created to facilitate the use of the pipe system globally. Standards such as NBR7373 in Brazil, ASTM F894 in the United States, and the Japanese Standard, JIS K 6780, have been developed based on DIN 16961. These derivations ensure that the high-quality and performance criteria of structured wall pipes are maintained across different regions, promoting consistent and reliable use in various non-pressure and low-pressure applications worldwide. The most significant technological advancement occurred when Krah GmbH and Bauku GmbH eventually parted ways in the early 90s in which Krah focused on technology and machine manufacturing. Following this separation, Krah GmbH successfully formed a new partnership with the German Frank GmbH. This new collaboration allowed Krah to continue advancing its technological innovations and machinery while leveraging on Frank's market presence and expertise to implement the new developments in the pipe market. Early in this partnership, the integrated electrofusion technology was developed and introduced to the market. This innovation combined the ease of socket/spigot jointing with the major benefits of HDPE weldability. Using the same production technology, the socket and spigot together with the fittings (bends, reducers, and branches) are produced as solid walls which ensures proper segmenting during fabrication. Additionally, the production technology was enhanced by incorporating an additional co-extruder, enabling the production of a "bright" inspection-friendly inside surface. Moreso, the production process became more automated and the production flow was optimized, enabling batch production to manufacture pipes with significantly increased cost efficiency. The change over time was drastic, efficiently shortening the time needed to switch the production of a DN/ID 1000mm SN4 pipe to a DN/ID 2000mm SN8 pipe to only 20 minutes with nearly low to zero starting scrap. Following this, the technology allows the production of tailor-made pipes with customized stiffness levels according to customer specifications and order. This process is similar to 3D printing which exemplifies the cutting-edge capabilities of modern manufacturing, offering unparalleled flexibility and efficiency.

The structure of the pipes continuously improved, resulting in lighter pipes and increased production output, even for large diameter pipes. With a tailor-made production, overdesigning the pipes will be avoided. Today, the profile wall of structured wall pipes could be design with a smooth interior, a profile that could be in multiple layers and with a top layer (close profile) to increase the stiffness of pipe as the diameter increases. This led to a significant reduction in costs per meter. One main contributor of decreasing the pipe weight is the continuous development of increasing the pipe profile height which yields to a higher moment of inertia. This allows a pipe to have the same stiffness but with a lighter weight. Hence, the pipe uses lesser raw material. Additionally, the homogeneous waterway wall enabled the pipes to be effectively used even in low-pressure applications. Nowadays, reducing the CO2 footprint is achievable by using the optimal amount of material in production. The approach to reduce the material before reusing and putting recycling as the last resort will not only conserve resources but will also decreases the energy required for manufacturing, transportation, and installation, leading to lower overall greenhouse gas emissions. By optimizing material use, industries can contribute significantly to environmental sustainability and carbon footprint reduction.

HDPE Structured wall pipe as a market staple
From a niche product initially used for specialty applications, HDPE structured wall pipes have become a standard in infrastructure projects. The (real) production output has increased dramatically from around 200 kg/hr in the 1980s to approximately 1500 kg/hr in 2024. This growth is attributed to advancements such as the transition from manual to fully automated production and the integration of co-extrusion technology, allowing efficient production of larger-sized structured wall pipes.

The growing demand for production lines of HDPE structured wall pipes is evident in Krah's output: in the 1980s, Krah produced one machine every two years, but by the 2020s, they were producing 3-5 machines annually. There has also been a significant shift in the diameter of the pipes produced. Today, 60% of all mandrels (production tool) are for pipes greater than DN/ID 1200 mm, whereas in the past, the majority of pipes had diameters less than DN/ID 1200 mm. HDPE structured wall pipes are expected to find new applications, particularly in areas such as renewable energy (e.g., hydropower plants), agriculture (e.g., irrigation systems), and disaster management (e.g., flood control systems). The versatility and adaptability of these pipes make them suitable for a wide range of emerging needs (Lee & Wang, 2023). Today, the largest HDPE structured wall pipes have diameter of up to DN/ID 4000 mm with projects under design phase of up to DN/ID 5000 mm. These pipes are finding attractive applications in marine pipelines, drinking water reservoirs, the headrace of hydropower plants, and large-scale flooding control systems. Krah technology alone has spread to over 30 countries across six continents, including Germany, Norway, Sweden, Denmark, Poland, the UK, Italy, Spain, Russia, the USA, Argentina, Chile, Mexico, Iran, Saudi Arabia, Algeria, Egypt, Turkey, Estonia, Australia, New Zealand, Philippines, Malaysia, and more. Countries like Indonesia, the Czech Republic, and South Africa are expected to follow by the second half of 2024.

Conclusion
The advancement of HDPE structured wall pipes for non-pressure to low-pressure applications is intricately connected to the development and continuous refinement of international standards and acceptance of the pipe system, backed up with successfully realized large pipe projects (worldwide) from the 1960s up to present. With ongoing progress in material science and manufacturing technologies, HDPE structured wall pipes are poised to achieve even greater enhancements in performance and sustainability, solidifying its role as a fundamental component of modern infrastructure—specially with a unique integrated electro-fusion socket in the pipe for jointing.

With the further development of large size production, innovative material-saving applications, and a focus on sustainability, the future of HDPE structured wall pipes is highly favorable. Customized to meet specific client needs, these versatile pipes are expanding their use across sectors making it a significant market staple product with consistent market demand. Technological advancements are making them more cost-effective and optimizing performance. HDPE structured wall pipes present a highly efficient alternative to traditional pipe materials (e.g. concrete and metal) offering a cost-effective, adaptable and eco-friendly solution for infrastructure projects. This efficiency means that with the same government budget, it is possible to lay down longer pipelines compared to using conventional materials like concrete or metal. By opting for HDPE structured wall pipes, governments can maximize their budget, extending the reach of essential services such as water supply and sewage systems to more communities.

Author:
Jeneleen Lansangan