Measuring Tubing

Because Grayline’s tubing is flexible and easily deformed, ID and OD specifications do not apply to tubing in the free state. ID and OD specifications are only applied to tubing when it is constrained to a perfectly round condition.

Grayline measures the ID using fixed diameter cylindrical pin gages. A series of pins having gradually increasing diameters is inserted into the gently rounded tubing to find the largest pin that fits without stretching the tubing. Water is often required as a lubricant on softer materials to prevent the pin from sticking to the tacky polymer surface.

Grayline measures the OD with this largest ID pin in place. Grayline uses a special thickness gage (Mitutoyo 7304) to measure the OD in multiple places around the tube. A caliper or micrometer can also be used if care is taken to avoid compressing the soft tubing. The OD is defined as the average of the minimum and maximum values measured using this method.

Grayline measures wall thickness with a custom made wall thickness gage. The gage uses an indicator with an 0.062" or larger pin for the anvil. The indicator allows for a light and consistent pressure on the tube wall in order to minimize compression. The tube is rotated in order to measure several places around the tube to find the minimum and maximum wall thickness. Tubing with IDs less than 0.062" are measured by finding half of the difference between the ID and OD

For parts under 3", the part cut length is measured using a caliper. Calipers require a very light touch to avoid compressing the tubing.

Parts over 3" are measured using a rule or tape. Grayline uses steel rules with 1/50th inch graduations where possible. Longer parts will use a 1/32" steel tape. These rules and tapes are mounted on a base that includes an end stop. One end of the tubing is gently aligned with the end stop. Care is taken to avoid compressing or stretching the tubing and it should be kept as straight as possible. Lengths are read to the nearest half graduation.

Even if our tubing is perfectly round as it leaves the extrusion process, Grayline cannot guarantee that our flexible tubing will be round when received by a customer. There are several mechanisms that can cause tubing to become flattened or out of round after it is manufactured and packaged.

For Reeled Tubing:

• As a round tube is bent around the core of the spool, there is a natural tendency for the tubing to flatten. This flattening will be more severe for smaller bend radii, smaller wall-to-ID ratios, and softer materials.
• All extruded tubing has some degree of longitudinal shrinkage that occurs after the extrusion process, as internal stresses caused by drawing down the material as it exits the die are naturally relieved. Most of this shrinkage occurs within the first hour after extrusion, with virtually all of the shrinkage occurring within the first twelve hours after extrusion. This longitudinal shrinkage results in the tubing tightening on the core of the reel, which flattens the inner wraps of tubing.
• Small deformations can occur at pressure points at which one wrap of tubing crosses over a lower wrap, or if the tubing is pressed against the side of the reel flange.

For Tubing Cut to Length:

• Tubes at the bottom of cartons can become oval due to the weight of the parts above.
• Small deformations can occur at pressure points at which tubes lay over each other in the carton, or if tubes are pressed against the carton flaps.
• The ends of the tubes can be deformed if they are pushed up against the side of the carton or against other tubes.

The factors listed above can have a greater effect on the roundness of flexible tubing if exposure to elevated temperatures during transportation and subsequent storage takes place. The higher the temperature, and the greater the amount of time that the tubing is exposed to that temperature, the greater the effects.

Since the factors that cause out of round tubing are beyond our control, Grayline cannot accept rejections for out of round tubing unless the condition is severe and clearly worse than normal.

If customers have special roundness requirements, they should be discussed with Grayline prior to quoting.