Overview
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Theory of Heat Fusion
The principle of heat fusion is to heat two surfaces to a
designated temperature, and then fuse them together by
application of force. This pressure causes flow of the melted
materials, which causes mixing and thus fusion. When
the polyethylene pipe is heated, the molecular structure is
transformed from a crystalline state into an amorphous condition.
When fusion pressure is applied, the molecules from each
pipe end mix. As the joint cools, the molecules return to their
crystalline form, the original interfaces are gone, and the two
pipes have become one homogeneous pipe. The joint area
becomes stronger than the pipe itself in both tensile and pressure
conditions.
The principle operations include:
Clamping The pipe pieces held axially to allow all subsequent
operations to take place.
Facing The pipe ends must be faced to establish clean,
parallel mating surfaces perpendicular to the
centerline of the pipes.
Aligning The pipe ends must be aligned with each other to
minimize mismatch or high-low of the pipe walls.
Heating A melt pattern that penetrates into the pipe must be
formed around both pipe ends.
Joining The melt patterns must be joined with a specified
force. The force must be constant around the interface
area.
Holding The molten joint must be held immobile with a
specified force until adequately cooled.
Inspecting Visually examine the entire circumference of the joint
for compliance with standards established by your
company, customer, industry, federal, state, or local
regulations.
Each pipe manufacturer has a slightly different approach for
fulfilling the heating, joining, and holding phases, but the end
result is the same -- a fusion joint that is as strong or stronger than
the pipe itself.
PH00363B-1-4-96
TX02476-3-30-05
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