Corrugated Pipe Talk

I have been wondering lately about the choices our society make in energy consumption and the resulting CO2 emmissions that result from the misuse of energy. Take concrete pipe for instance, from their own statistics the portland cement industry are a large consumer of energy, and in the manufacture of cement produce large emmissions of CO2. Concrete pipe seems to consume intense amounts of energy in all aspects of its construction, delivery and installation.

Plastic pipe, especially light weight profile walled pipes are made from energy, but are very efficient in their use of energy for production. One might argue that they are made from hydro carbon sources of energy but this use of carbon in their molecular structure is not released to the atmosphere. The dellivery of the materials to produce plastic is clean, natural gas being the major building block for most commercial plastics in North America.

When you consider the energy used in manufacturing, transportation and installation it seems intuitive that plastic pipe is a lower contributor of CO2 emmisions, thus a more energy efficient piping material to use. Not to mention it can be reclaimed as an energy source after its useful life.

I am amazed at the lack of interest from the regulatory bodies in North America to look at the real efficiency between the materials they specify for use. It is a complicated comparison to make, and one that to date only opinions form the debate.

Is it not time that our specification of products, especially those that society sponsor, come under a much more rigourous review of their energy efficiency and carbon footprint?

In the end the free market will prefer the products based on the lowest energy cost of manufacture, and the benefits they bring. While this happens their will be intense lobying by those products at a disadvantage to maintain their historic place in the market, so regulators should be much more questioning in this area.

Since the mid 1980’s I have been actively involved in the polyethylene corrugated pipe industry, more specifically in the development of the double-wall (twin wall) product line. In the beginning we were happy to make pipe with a smooth liner, the improved hydraulics on the corrugated pipe with a smooth inner wall presented great opportunity in new applications such as culverts, storm sewers and even sanitary sewers. Our early coupling systems were simple, a coupling band, first made by roto-molding and then blow-molded as volumes increased.

Today, through factors both technical, competitive and market driven, the corrugated pipe industry have developed a plethora of joint designs; band couplings, double bell coupling, spin welded bells, in line bells (both single and double layer), traditional expansion socketing, extrusion welding, hotplate fusion welding, and even wire electro-fusion. I apologize if I’ve omitted any.

Coupling systems have become very complicated to discuss, largely due to the investment by machine producers and manufacturers into their preferred technology. When discussing joint design you will hear many claims, some substantiated but more often theorized about the relative performance of the many systems.

So if you are looking at buying a pipe corrugating line for the first time; and plan to make a pipe that will be used in an application requiring the water to stay in the pipe - or more often is the case out of the pipe, carefully consider the joint design you choose, it has significant long term implications on your business’ competitive position, and even more importantly the satisfaction of your customers.

To select the proper technology you should carefully answer the following questions:

1. What are the competitive pipe products you will compete with, concrete, vitrified and clay, solid wall PVC, solid wall PE, FRP, corrugated PVC PP or PE. Review carefully each of the current joint systems they offer.
2. What are these competitors offering for joint performance: a pressure tested system or a non pressure tested system?
3. Are there local specifications you must meet. One of the common North American standards for joint performance is ASTM D3212.
4. What wall thickness do the specifications require, is your proposed joint system capable of meeting this minimum thickness?
5. Will you need to do testing on the product prior to selling in the market, if so don’t underestimate this step it can take considerable effort and time.
6. Is there local test equipment available or will you need to buy it.
7. Do you have a supply of gaskets, and what are the specifications you must meet for these. Will your gasket supplier do joint testing for you?
8. Does my joint design allow the lowest cost for the gaskets?
With answers to the above questions you should now be in a good position to talk to the pipe machinery suppliers about the requirements of the joint design and the joint design they would recommend to meet your market need.

Here’s a quick review of the most common pipe joining technologies for corrugated pipe

Band couplings / Split couplings– the simplest of all joining systems the band coupling typically requires the purchase of a mold for each part that will be made. Roto-molding or blow molding are both acceptable production methods with roto-molding having a lower capital investment but higher part cost when compared to blow molding. Almost every manufacturer I know offers a band coupler for certain applications such as culverts or repairs. Often they will be used with a geotextile to mitigate soil intrusion through the joint. Specifications covering this joint type often includes the minimum thickness, minimum overlap with the pipe, maximum allowable gap between the pipe and coupler and sometime pull apart, which I have yet to figure a purpose for. Before buying molds to make a band coupler evaluate your proposed profiles relative to already available parts. Both in North America and Europe this coupling type is already widely available, and with the proper profile selection you can save considerable money by using readily available parts.

Double bell coupling - a requirement for almost every manufacturer the double bell coupling is primarily offered as a repair coupling, although in the early stages of a product line development they are often the primary joining system. These products can easily be molded using the three methods of roto, blow and injection molding, however injection molding has become the principal method of manufacture for these parts offering low part cost and high volume production. A simple injection mold making a half coupler which is then welded together to make the double bell is a good way to develop a bell for spin welding, and supply a part for fittings production and double bell field joints. The double bell can be made with locking tabs, and is often used with a gasket. Equipment has been developed by Mecasoft to automatically push on a gasket and coupler in-line for many European manufacturers which sell the smaller diameter pipes with this type of coupler. The double bell coupler is prominent in Europe in pipe diameters below 200mm, but is still found in use in many areas on larger sizes. In North America it has been overcome by the success of the in-line single layer bell systems, and is typically available as a repair joint. Double-bell couplers are available almost everywhere, but must be matched to the diameter of the pipe. If you are in the design phase of your profiles, pay attention to what parts are commercially available in the US and Europe.

Spin welding / Friction welding – a robust well proven joining system, used in Europe and to a lesser extent in North America this joining method offers the flexibility of a secondary belling process, fast cycle times, infinite pipe length variations and low scrap rates. When you consider developing your corrugated pipe product line and require a bell to be molded for each pipe diameter then the ability to use this bell and friction weld it to the pipe becomes a very attractive and flexible option. The bells used for friction welding can also be used for making couplers and fittings allowing a complete product line for one mold investment. The molding methods are less flexible, and any manufacturer of spin welding equipment will demand am injection molded part offering the wall and dimensional consistency demanded by the process. The bell design has a significant impact on the injection mold cost, and if following North American design practice can result in a mold cost less than a blow mold for an equivalent sized bell. Adescor offer spin welders for sizes of 50mm (2”) ID to a size of 1200mm (48”) OD sized pipes in semi automatic to fully automatic machines. The process is most suitable for polyolefin’s such as PP and PE. The spin welding process is ideal for diameters less than 300mm ID, as this is the range of pipe diameters with highest line speeds and other processes result in slowed corrugator operation and increased control complexity. The friction welded bell is not the lowest initial capital investment, but results in a complete product line and excellent production speeds.

In line belling – seemingly the lowest production cost process for making a bell joint on a corrugated pipe the single layer IB process has become the dominant joining system on the North American market for corrugated polyethylene double wall pipe. It is also used on PVC in the manufacture of ribbed pipes and to a lesser extent on PP pipes in Europe. There is a significant debate over the benefits of single layer and double layer in-corrugator belling systems of which I have seen very little documented evidence supporting the benefits of one vs. the other. The market fact is the single layer bell has been widely accepted, albeit some designs are now reinforcing the larger diameter bells specifically where the gasket seats, and the double layer bell has had some adoption, but with the expected line speed slow downs the industry understands is the result of double layer bell production. The selection of an IB system and technology is complicated, there is no doubt it offers a great joint, with a low mold investment, and good throughputs, but make certain you understand the local standards and joint pressure specifications you must compete with. IB systems were first introduced in North America in the late 1980’s and there has been significant work in the area since. The most recent designs allow for a reduced corrugation with a gasket pocket fitting within a bell the same diameter of the pipe, offering many advantages such as decreased gasket cost, and ease of handling. When talking to your corrugator supplier about IB, ensure you ask for references, even better would be some joint test data, and don’t forget how critical the gasket designs are to make this joint design work well. Choose a reputable supplier.

Expansion Socketing – Developed over 40 years ago for PVC smooth wall pipes, traditional bell forming using a secondary heating process and expansion mandrel until recently has been limited to PVC double wall pipes. Work in the past few years has resulted in new technology that can bell successfully bell polyolefin’s more specifically PE corrugated pipes using the more traditional expansion forming technology. This system offers the flexibility to make various lengths without the purchase of additional molds for making a bell, albeit pipe bell tooling for each size must still be purchased and can add up to a significant investment. When reviewing this technology for PE pipes ensure you ask all the questions about production variability, memory and process stability. If the newer processes developed for polyolefin’s have the stability known of PVC belling this technology will be worthy of consideration.

Extrusion Welding – A method of adding a bell to larger pipes typically 450mm and greater is the manufacturing of a bell using roto molding and extrusion welding it to the pipe as it revolves on a rotary table. This can be automated and there are machines available on the market to do this with reasonable production rates. This method I consider an entry level technology and is quickly surpassed in the market by manufacturers who introduce one of the lower cost production methods of making and attaching a bell.

Field Welding – Traditional field welding of polyethylene pipe is widely accepted with tremendous machinery capacity available to welding pipes in the field. In the corrugated industry the lighter weight wall and difficulty in clamping the pipes has made field welding much less popular than in the PE pressure pipe industry. Systems using electro fusion wire and lower pressure field clamps have been developed, but also have resulted in high installation costs and poor acceptance by the contractor. Field welding of corrugated pipes is possible, proven and can offer special industrial applications a solid joint without the use of a gasket.

In Summary

Joining systems are a complicated and competitive area of the pipe industry. There are machinery and technology companies promoting their specific technologies; pipe companies competing on specifications designed around their proprietary design, and customer preference on the system to use locally.

The joining system you pick should be a big and long term investments, make sure you only have to do it once, and if in doubt get good third party advice.

Tony Kime

Corrugated Pipe Machinery and Technology

I’ve often wondered why the use of a continuous caterpillar style take off / puller, or modified pipe corrugator has never been adapted to perform continuous extrusion injection molding or sophisticated profile extrusion.

I can think of many products that would benefit from a continuous extrusion process: architectural trim, corner molding, foam pool noodles (imagine a snake design for your kids), possibly even specially profiled gasket designs with indents and detents not suitable for traditional profile extrusion.

Pipe corrugators when fitted with molds can many make shapes suited for a straight open close mold mechanics, it is even possible to fit molds with special inserts that can be retracted prior to opening.

Pipe corrugators by their name pigeon hole themselves into a very narrow technology niche, rename them and a pipe corrugator is a very sophisticated continuous extrusion takeoff system capable of moving molds past a fixed injection point, at high speed, great precision while supplying vacuum to the mold cavity for even filling and having very controllable cooling or heating systems.

Over the past 40 years pipe corrugators have evolved in the light weight profile walled pipe extrusion industry with tremendous mechanical advances over the traditional Reifenhauser / Corma vertical chain carrier air cooled style machines. Today very high speed liquid cooled or heated molding systems are available and could easily be adapted into a multitude of possible continuous extrusion or continuous filling applications.

Low pressure injection molding on a continuous extrusion system has already been proven with the development of the Ultra Rib design of pipe first invented by Uponor. While this process is relatively slow in line speeds, it resulted in a new design of pipe not previously possible with traditional profile vacuum tank extrusion s in tremendous pounds throughput and was a significant advance on pipe forming technology at the time.

With rapid development of the composite extrusion industry, more conversion of traditional building materials, the penetration of foaming technology into the extrusion industry it seems imminent that development groups will be looking for more advanced systems to perform high speed continuous mold filling of infinite length products.

So, what would one look for if evaluating a technology supplier for a continuous injection molding extrusion process.

- How does the material flow, does it pour or extrude? Material with low viscosity at exit of the injection nozzle are likely best run in a vertical process using gravity to their favour. Materials with high viscosity are likely to handle the more typical horizontal extrusion filling orientation.
- What is the mold cooling or curing temperature needed? Molds typically are cooled, and water cooling systems are well proven and advanced. Heating systems, which are really higher temperature operating curing systems are also available, but more expensive due to the oil heat transfer system used.
- What is the tolerance in the product design for flashing? As a general rule, the less tolerance for flashing, the more expensive the machinery. The function of the mold mechanics requires mold halves in repeating increments typically called the mold length. This characteristic of the extrusion filling process requires thorough evaluation of not only the product design, but the capabilities of the machinery, and the ongoing maintenance to maintain the required flashing specification.
- How does the technology manage thermal expansion and contraction of the mold set? This is a critical component of setup and efficiency. When a mold train running in an endless loop changes in length form thermal expansion or contraction it affects the drive gear train, it’s important to review how this is accommodated in the machine design.
- Can the continuous extrusion device be easily changed in mold train length? In many applications a discreet part will be made and later cut, when dealing with a continuous mold train of a fixed length you must design the discrete part length to result in an integer quotient when divided into mold train length. This relationship should always result in a review of the finest gear pitch increment that the mold train is driven by. Resulting in the possible mold length variations for part design.
- What is the line speed you can get to. In the corrugated pipe industry line speeds approaching 200’/min have been talked about. In the 4” corrugated drainage tubing technology line speeds of 120’/min are known. IN the end the line speed is governed by the contact time the product needs to cool or heat prior to be released from the mold. Continuous extrusion technology for pipe corrugating has been made with a mold – product contact distance from 12” top as long as 20’, it very quickly becomes a ROI calculation.
- Are changeovers important? If it’s a one product line likely not, but with the outputs of this technology that’s unlikely. A complicated area to discuss, but intuitively the faster you can change a line the more money you can make. Make sure you understand the complete changeover process: die, molds alignment, height adjustment.

When looking for break through technology your mind has to stretch, and a thorough evaluation is always smart. Advanced takeoff system allowing continuous filling of molds at high speeds under tight temperature control is highly developed in the pipe industry, we need to find technology cross over opportunities to share our technical developments and expand complicated profile extrusion into non-traditional extrusion applications.