STRATEGIES ELIMINATE TUBING REJECTS, SCRAP

For companies that extrude polymer tubing for medical use, stability is the Holy Grail. Establish and maintain extrusion process stability and process-related quality problems will disappear. When it comes to optimizing performance within the die, there are tools for optimizing extrusion process control that are simple to apply, and virtually foolproof. Two of the most important calculations are area draw-down ratio (ADDR), and draw ratio balance (DRB).

ADDR is the ratio of the cross-sectional area of the extruded plastic melt to the cross-sectional area of the plastic in its final product form (tube, hose or insulation on a core such as a wire or cable). ADDR is the extent to which the plastic has been reduced in size to form the part; it is a measure of the amount of stretching that occurs between the exit of the die and the takeup.

A larger ADDR enables faster production, while a lower ADDR facilitates closer control of product dimensions.

A low ADDR process tends to be more stable than a higher one; the key is to find the sweet spot that optimizes the balance between the two. Figure 1 illustrates different values of the ADDR.

Various spindle, workholding and tooling options are available on the 5100-S.

DRB is the diameter ratio of the die and tip divided by the diameter ratio of the tubing. When the DRB equals one, the annular shape of the tubing is the same as the annular shape of the tooling. When the DRB is greater than one, the ID of the tubing relative to the OD will be greater than the ID of the tooling (tip diameter) relative to the OD (bushing diameter). For a stable tubing extrusion process, the DRB should be equal to or greater than one.

Smiths Medical Tijuana, Mexico, location operates multiple extrusion lines, extruding PVC, nylon, polypropylene and polyethylene for disposable anesthesia berating circuits, catheters for epidural and tracheal use, and patient monitoring systems. Tolerances must be held to ±0.003".

Manuel Castillo, extrusion engineer, joined Smiths Medical in April 2008 and took responsibility for process control and improvement of extrusion performance and stability. "When I joined the Smiths team," Castillo explains, "there was a big opportunity to tighten control over the extrusion process. Seven out of ten production orders were being rejected because they did not meet specifications. And it was all related to issues with extrusion instability." Castillo used a four-point process to address those issues. "We began by revalidating our portfolio of extruded tubes for uniformity. This involved approximately 20 codes, with each code representing one product. Using tools provided by B&H Tool Co., we changed our ADDR ratios from 4:1 to 2:1 and immediately gained greater dimensional stability."

"We also optimized the land length to control the melt in the die," Castillo adds. "Since PVC is one of our higher runner materials, there is always the opportunity to improve land lengths. I will say, however, that instead of having a ratio of 10:1 land length individually calculated for every PVC tube, it is preferable to have a standard land length for the majority of the tooling.

This allows processors to use existing pins and dies for any new tube coming into production.

"This is possible - and highly beneficial - if the ADDR and DRB rules are met," Castillo states. "It is another reason why the B&H calculators are so vital to stabilizing - and optimizing - the extrusion process." Optimizing the extrusion process has produced immediate and dramatic effects. "After the improvements were made, no production order has been rejected," Castillo says. "Scrap has also been reduced dramatically.

"The use of the B&H internet tooling calculator was invaluable in getting us to where we are today," Castillo explains. "In previous times, I had to resolve these issues in other ways.

Clearly, the ability to use the ADDR and DRB for the different materials and tube dimensions we work with is a huge advantage." The ADDR and DRB calculator tools discussed are located at bhtool.com/calculator.htm#ADDR.

B&H Tool Co.
San Marcos, CA
bhtool.com