Method for extruding silicone tubing

A method for extruding silicone tubing which maintains consistently precise dimensions in the final tubing product by continuously monitoring the tubing profile after cooling and adjusting the height the loop of uncured tubing as necessary to neck the tubing down to the desired size.

BACKGROUND OF THE INVENTION 
This invention relates generally to methods for extruding elastomeric 
tubing and, more particularly, to improved methods for extruding silicone 
tubing. 
In the course of manufacturing silicone tubing by extrusion, operational 
parameters such as temperature, material viscosity and speed typically 
drift, resulting in undesirable variations in the tubing dimensions. It is 
therefore an object of the present invention to provide an improved method 
for making silicone tubing which automatically compensates for variations 
in system parameters, in order to maintain consistently precise dimensions 
in the final tubing product. 
SUMMARY OF THE INVENTION 
The present invention is therefore directed to a method for extruding 
tubing which maintains consistently precise dimensions in the final tubing 
product in which silicone rubber is extruded through a pin and die to 
produce uncured tubing which may be of circular, oblong and other 
cross-sectional profiles. A loop of predetermined height is formed from 
the uncured tubing in order to neck the dimensions of the tubing down to a 
predetermined value, and it is passed through an oven to vulcanize or cure 
the tubing. The tubing is then cooled, an electrical signal indicative of 
the outside diameter of the tubing is generated, and the height of the 
loop is adjusted to compensate for variations in the outside diameter of 
the tubing from the predetermined value.

DESCRIPTION OF THE PREFERRED EMBODIMENTS 
The practice of the method of this invention is described below in 
connection with the apparatus of FIG. 1, using silicone rubber as the 
extrudate, although other thermosetting materials may be used. 
Thus, turning to FIG. 1, a raw silicone rubber blend (not shown) is worked 
in a conventional rubber extruder 10 which is water-chilled in order to 
prevent heating which would bring on premature curing of the silicone 
rubber as it moves down the barrel 12 of the extruder. The silicone tubing 
emerges through a conventional pin 13 and die 15 (FIG. 1A), identified 
collectively as 14, in the form of uncured tubing 18 which forms a loop 16 
before it passes through a conventional hot-air vulcanizer 20, over 
take-up reel 22, past idler pulley 24 and through cooling tower 26. 
The outer diameter of the tubing may be varied by varying the height of 
loop 16. A longer loop will neck down the tubing to a greater degree than 
a shorter loop, since the tubing tends to neck down as long as it is 
uncured and subject to the force of gravity. This necking down process 
slows as curing begins in the hot-air vulcanizer 20. 
Once the tubing emerges from the hot-air vulcanizer, the curing process is 
nearly complete. Since the tubing is hot, however, the diameter of the 
tubing at this point will be somewhat greater than that of the final 
tubing, at room temperature. Thus, since it is important to monitor the 
final outside diameter of the tubing, it is passed over a take-up reel 22 
and idler pulley 24 through cooling tube 26 which is continuously injected 
with cool, anti-static air to cool and prevent the silicone tubing from 
adhering to the sides of the cooling tube. 
Monitoring means is provided at 28, to continuously measure the outside 
diameter of the tubing and to produce an electrical signal indicative 
thereof. A height sensor 30, connected to monitoring means 28, is 
positioned below the loop to determine the height A of tubing loop 16 and 
establish a correspondence between a particular loop height and a 
particular final tubing outside diameter at room temperature. 
Thus, when monitoring means 28 indicates that the outside diameter of the 
tubing is beginning to waiver from the predetermined value, it signals 
height sensor 30 which causes take-up reel 22 to either speed up or slow 
down the movement of the tubing, thereby adjusting the loop to a height 
which restores the outside diameter of the tubing to the desired value. 
The process of this invention can be used not only with single-extruded 
tubing, but also with coextruded tubing. In such applications, a second 
extruder 40 is connected to a cross head which combines the extrudate 
before it enters pin and die 14, to produce a coextruded product. One 
coextruded product which may be produced in this fashion is a 
half-clear/half-radiopaque tubing made from clear silicone and silicone 
containing a radiopaque agent. Such coextruded tubing finds particular 
application in the medical field. 
While particular embodiments of the invention have been shown and 
described, it will be obvious to those skilled in the art that various 
changes and modifications may be made therein without departing from the 
spirit and the scope of the invention and, therefore, it is intended in 
the appended claims to cover all such changes and modifications which fall 
within the true spirit and scope of the invention.