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 Post subject: Melt Swell Size
PostPosted: August 16th, 2006, 3:50 pm 
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When you refer to the melt swell size, the amount of molten material prior to the fusion process that develops on the heater face, where do you actually measure this from? Is it from the face of the heater to the outside edge of the molten material that is visible or is it from the crest of the rollback to the heater face? Even though you are not applying force during the butt fusion heating time, material tends to rollback in a wave type shape.


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PostPosted: August 16th, 2006, 3:50 pm 
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The melt swell is measured from the outer surface of the pipe up the face of the heater plate to the higest point of the pipe melt. As PE heats up it expands and softens, this is what you are seeing against the heater plate. As the material swells upward from the pipe ends, the material against the plate is hotter that the back side of the melt swell, this cooling effect on the back side of the melt swell causes this surface to contract, thus the roll back or wave effect you mentioned.
You are very correct in mentioning that you do not apply force during the heating portion of the butt fusion process


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PostPosted: August 16th, 2006, 3:51 pm 
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This is one of the most fundamental questions ever asked about the fusion process, yet our industry has always relied on old doctrine for the answer. Unfortunately the visual indicators commonly used to describe proper heating has been misunderstood even from many of the experts.
The purpose of heating the pipe is to produce a penetrating mass of molten material into the pipe wall. It is this melted mass that is needed to produce the fusion weld. Over the decades our industry has developed a standard of visual indicators to identify the correct amount of melted mass. The term for these measurements is called “Melt swell bead width guidelines”. The definition of “width” describes a distance from side to side not top to bottom. The visual observance of the molten mass is a swelling that rises in height along with width. This is where confusion has entered into our topic and industry.

It is commonly taught that the height of the swelling should be visually observed for the correct melt swell bead width. I believe that the doctrine has eroded to the use of the height visual indicator because of the degree of difficulty to describe the true visual indicator of width and because of the perpetuation of eroded doctrine.

I may be too precise in my definition of the melt swell for the common user because both methods of visual indicators work together. Along with height you also get width in the melt swell; however this is only true if there is no force applied during the procedure of heating. If force was applied during heating you would only achieve height in the melt and no width resulting in a substandard fusion weld. Because of this potential error in using the height as the visual indicator a more correct method would be to use the width.

I would like to also touch on the roll back or wave effect. Temperature is a factor of cause, but there is also another more influential factor. In the process of making HDPE pipe, the outer surface of the pipe is shock cooled to start forming the pipe shape. This shock cooling produces a thin layer of stresses in the skin of the pipe wall. During the heating procedure in the fusion process, these stresses are able to show themselves by the roll back or wave appearance. To illustrate, I would like to point out the how the melt appearance on fittings such as flange adaptors which have been machined to size after their injection molding. The machining process cuts away the stressed skin of the fitting. These fittings when heated don’t produce the same appearance in their melt swell as regular piping regardless of temperature changes.


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