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Material thickness within a corrugation is well known to be very poor.  If you are not familiar with this, please review the quality tab for supporting information.  This poor distribution results in a significant amount of extra material being pumped into the pipe to maintain minimum required thicknesses.  In addition to this, there are large material deposits at the base of the sidewalls that serve no purpose whatsoever, that naturally occurs due to consequences of the process.

Injection molding on the other hand, is very well known for tight tolerances and high quality.  The RPT process utilizes injection molding to form the pipe.  As a result rib thickness, liner thickness and every other attribute will be well controlled.  Standard deviations of around 0.001" can be expected.  This means, if a rib has a thickness of 0.150" the variation will only be 1% to 2% and not 20% to 30% as is common with corrugation side wall.

Having the ability to maintain tight tolerances creates opportunities for material reduction.  When manufacturing corrugated pipe a minimum thickness is required in several key areas of the pipe to maintain industry standards for stiffness and other performance attributes.  Tighter tolerances allow a minimum thicknesses to be maintained with a lower average thickness. This is probably the most obvious way in which the RPT process will significantly reduce material usage (cost), when compared to a corrugator.

The primary purpose of the corrugation is load bearing.  The soil surrounding the pipe exerts forces on the pipe that force it to ovalize.  The corrugations provide structure to resist these forces and thereby reduce ovality.  These forces act normal/perpendicular to the OD of the pipe but are not uniform.  The forces are higher on top and less on the side.  This is why unpressurized plastic pipe ovalizes in the ground.
A well formed rib on a cylindrical object like pipe is the most efficient structure that can be used to absorb external ground forces and minimize deflection.   Until now, the only practical way to produce a rib like structure has been with a corrugator.  Corrugators however, produce a product with wildly varying thickness.  This level of variation is inherent to the process and cannot be remedied.  In addition, corrugations have what is commonly referred to as a crown (top of corrugation).  This part of the structure is less efficient at load bearing than the sidewalls.  The RPT process eliminates the crown and produces an extremely well formed rib.  As a result, a higher quality, higher performing product can be produced with significantly less material.
Because the sidewalls of a corrugation are not in line with the direction of the applied load the sidewalls start to deflect (bend) at lower loading conditions than if the side wall were directly in line.  As the force is applied the inner surface of the side wall goes into compression and the outer surface goes into tension and thus a bending force is immediately created.  If the sidewall were directly in line with the force it would go into pure compression and a greater load could be absorbed before any significant amount of deflection would occur.
There are good reasons however why corrugated pipe does not have straight side walls.   They would not form well.  Straight side walls would cause the difference between upstream and downstream sidewall thicknesses to be even worse than it already is.  Secondly, the crown would be extremely thin.  It is well proven that a well formed corrugation, with a more uniform material distribution, produces a stronger structure. This is why corrugations are designed with slanted side walls.


It is clear that with regards to material usage a tapered rib is more efficient than a corrugation. 



RPT recently contracted Pi Engineering of Calgary, AB, Canada to perform FEA analysis of one ribbed structure and one corrugated structure.  Pi has 50 years experience in stress analysis of piping systems and has tremendous expertise in this field.  The ribbed structure analyzed by Pi has 15% less material than the corrugated structure.  The corrugated structure is a high quality idealization, that was painstakingly developed by disecting and measuring 30" pipe, manufactured by a US producer.


The results speak for themselves.  Even with 15% less material the ribbed structure is stiffer than the corrugated structure.  The idealized corrugation structure however, was created using average thickness results.  It is highly likely an actual corrugated pipe would not test as well.  Areas of the pipe that measured much less than average would obviously deflect more than what the simulation indicated.  When this is taken into account, it makes these results even more impressive.

A copy of the report provided by Pi Engineering is given below.  Simply click the PDF button to view the report.  Here you will find detailed info on how the evaluation was conducted and the results obtained.  Based on this evaluation, a very solid case can be made that a ribbed structure with 20% less material should perform equal to or better than the corrugated.












Material selection offers another very significant opportunity for cost reduction.  Corrugators require high melt strength materials to properly produce pipe.  Because ribbed pipe is injection molded, melt strength is not an issue.  High melt strength is gained in HDPE and PP by way way of long polymer chains with low MFI.  These attributes require additional processing by the resin manufacture which significantly increases their cost.  The RPT process does not require melt strength, so lower cost materials with higher MFI can readily be utilized.


Post consumer regrind, even in high amounts can easily be processed.  Again, high melt strength is not required, so higher MFI materials like regrind can readily be used.

An injection molding machine is simpler and far less artsy to operate compared to a corrugator.  This will reduce scrap and increase uptime.  Additionally the skill level and therefore cost for operators can also be positively impacted.

20% less material usage, lower cost materials, greater efficiencies and potentially lower labor costs.  When all of this is combined it is easy to see that the RPT process will significantly reduce manufacturing costs.


For many organizations material costs alone represent 70% to 80% of the total manufacturing costs.  With this being the case, the RPT process can easily save such organization 15% or more in total manufacturing costs.

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