VAST QUALITY GAINS
CORRUGATOR vs INJECTION MOLDING MACHINE
OUT OF CONTROL
The typical corrugator produces pipe with thickness variations so high, that many manufacturers do not even attempt to apply standard SQC or SPC control methods. Corrugation side wall thickness is a critical attribute for pipe performance. Yet this attribute, and many others vary greatly from side to side and from top to bottom in every single corrugation.
The image below shows a cross section from a single corrugation. As you can see, thickness varies wildly from the left sidewall to the right sidewall. Multiple areas are miss-formed, bowed out and warped. There is also a large deposit of wasted material near the bottom of the left side wall. These variations and defects are all inherent to the process and will be present at some level on all pipe produced on a corrugator.
A section of 30 inch pipe was cut into quadrants and a thin cross section of each quadrant was then prepared for analysis. A photo of these cross sections is given below. As you can see, not only is there the variation previously mentioned, but there is also huge variation from quadrant to quadrant and then again from corrugation to corrugation. The pipe depicted here is not scrap or defective pipe. This pipe was sold commercially and randomly selected. In my opinion, this is relatively typical pipe and I have 15 years experience developing products in this specific industry.
Measurement of side wall thickness at 1.75 inches from the pipe ID yielded an average thickness of 0.141 "with a standard deviation of .0126". If we apply the accepted limits of plus or minus 3 standard deviations, the expected range is then 0.103" to 0.179". This is the range derived from just one section of pipe, four corrugations long. Imagine what this range would be if similar data were collected from an entire run, where thousands of feet of pipe were produced.
Even with this much variation in the critical attribute of sidewall thickness, this product will perform extremely well. Corrugated HDPE pipe is a proven product that has stood the test of time. Performance is not at issue. What is at issue is the efficient use of material. Any manufacturing process with this much variation is not efficiently utilizing material to produce the product. The obvious take away here, is if corrugated pipe could be produced with tight tolerances a much lighter product could be produced that performs just as well.
A WELL CONTROLLED PROCESS
Injection molding 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 nearly every other attribute will be well controlled. Standard deviations of only 0.001" can be expected. This means, if a rib has an average thickness of 0.141" the variation will only be 1% to 2% and not 20% to 30% (or more) like a corrugation side wall.
Having the ability to maintain tight tolerances creates opportunities for material reduction. To maintain industry standards for stiffness and other attributes a minimum thickness is required in numerous areas. Tighter tolerances allow a minimum thicknesses to be maintained with a lower average thickness.
Corrugated plastic pipe is notorious for having a rough inner liner and there is a good reason for this. In a corrugator the inner liner is formed while being dragged across the surface of a cooling mandrel. This action pulls the liner causing thickness variation and uneven cooling. Secondly the liner is cooled on only one side which creates more uneven cooling. As a result the liner warps and bends creating what is known as inner liner roughness. Again, this is a defect that is inherent to the process and will always be present at some level in pipe produced on a corrugator.
The RPT process is vastly different. The inner liner is formed between a stationary mandrel and a stationary mold. This creates a liner with uniform thickness that has been uniformly cooled from both sides. The result will be an inner liner that is well formed before existing the mold with very little roughness.