Page 41 - 2010-2022 hose manufactureing ability and guide

Normally, extrusion is the preferred method Wrapped Tubes
for the tubing process on hoses with ID’s up to For the larger diameter rigid mandrel rubber
1-1/2" when built on a flexible mandrel, to 4" hose constructions, the wrapped tube process is
for rigid mandrel. Beyond these dimensions, utilized. Here, the rubber compound is
wrapped is usually employed. For the larger calendered to a specific thickness and width,
diameter non-mandrel extrusions, the tube may then spirally wrapped on the rigid mandrel with
be lubricated inside to prevent compound sufficient overlap to form the tube. With the
tackiness. Also, an air cushion can be used wrapped process, the challenge is to provide
internally to prevent tube collapse during good bonding at the tube overlap area to prevent
extrusion.
tube delamination.
Extruders are often referred to as crosshead
or straighthead. If the tube is formed in the same COVERING OPERATION
direction as the extruder’s screw orientation, it The covering techniques used for rubber
is a straighthead design, whereas if there is an and thermoplastics are synonymous with the
angle between the tube flow and the screw, it is tubing techniques described previously. In most
a crosshead design. Common crosshead designs instances the same equipment is used.
are 45° or 90° orientation. Crosshead designs Frequently a hose may have an extruded tube
offer more challenges for the process engineer and a wrapped cover. If extruded, covers must
or rubber chemists since the abrupt change in be applied with a crosshead design to allow the
rubber flow direction can induce temperature reinforced uncured tube to be fed properly into
and pressure anomalies, especially with the extruder covering. The wrapped cover
sensitive compounds. concept is shown in Figure 2-7.

Hot feed and cold feed extruder
terminology is common. In the Hot feed process
the rubber is preheated before it is fed into the
extruder, usually on a two-roll mill. This
technique makes the extrusion easier for some
compounds since there is less rapid temperature
increase in the rubber. However with high
equipment and labor cost, it is almost obsolete
in favor of the cold feed process.

Figure 2-7

REINFORCEMENT
The strength component of the hose,
designed to handle the entire pressure load with
appropriate safety factors is the reinforcement.
In most cases it is located between the tube and
cover. Occasionally there are hose applications
not requiring a cover, in which case the
reinforcement also acts as the outer protective
Figure 2-6 layer.

RMA/IP-2/2003 20