The “mismatch” that causes medical tube extrusion problems

Occasionally, I hear from tubing extruders who are having trouble getting the level of precision and quality they want. Some ask if it might be a problem with Conair downstream equipment – like our MedVac® cooling/sizing tanks – or how they are using them. Usually, however, their troubles relate to a mismatch between the materials they are extruding and the sizing/tooling approach that they’ve selected.

Before I explain further, let’s take a quick look at the sizing process. Vacuum sizing begins at the point where the hot material passes from the extruder into the cooling water held in the vacuum tank. At this interface, the extrudate passes through the sizing tooling, which manages the controlled sizing of the tube. Sizing tooling combines external pressure (contact or non-contact) with the natural contraction of the extrudate as water cooling removes heat. While external vacuum sizing takes place, the internal contours of the tube are maintained by a slight positive air pressure within the tube, created by air that was vented into the tube profile during the extrusion process.

When you’ve got the right tooling working, optimizing the extrusion process is a straightforward, incremental process that uses the precision controls offered in the MedVac® tank and MedLine® puller/cutter and, if necessary, coordinates them with upstream extrusion process controls. But if you’ve got a tooling/material mismatch, that “dial in” the process is going to be a struggle, resulting in problems like:

• Slower-than-expected production rates, an inability to get line speed “right.”
• Inconsistencies in surface finish, including “chatter” marks
• Unacceptable dimensional variability, resulting in the constant need to “tweak” settings.

The good news is that, if you’re going to have these problems, you’ll see them early on and they’re fixable. The bad news is that they are usually caused by that mismatch I talked about. With your material in mind, let’s review available options for sizing:

• Contact sizing works best with stiffer materials, such as polyethylene and polypropylene. This process involves placing one or more stainless-steel sizing disks in the tool, then drawing the hot extrusion through them and into the cooling water. Drawdown of the tube is achieved through surface contact with the beveled edge of the sizing disk and material contraction due to water cooling.
• Non-contact sizing was developed to handle softer, stickier materials including flexible PVC, polyurethanes, and TPEs. Non-contact tooling replaces the metal sizing disk, where sticky materials like these tend to catch or hang up, with a bore that is slightly larger than the extrudate. Cooling water is delivered into this bore, then drawn back into the tank by vacuum. The water forms a thin hydraulic layer around the tubing whose turbulence and cooling action help in sizing it.
• Hybrid sizing is a third, much less common approach, which utilizes a specially built tool that combines elements of contact and non-contact sizing for materials and applications that don’t fit neatly with either approach. An example would be an extrusion involving a semi-rigid PVC that sticks to the sizing tooling, yet needs a higher vacuum level than non-contact sizing can provide. In this case, hybrid tooling would utilize a quench chamber to set a skin on the product that would then enable it to be sized using the contact sizing approach.

Determining the right amount of drawdown—the ratio between the extruded size/finished size of a tube—is an important factor when you’re using a contact sizing approach. The diameter you select for the contact sizing disk is especially important, since it plays a dual role: it must be small enough to properly size the tube (and prevent leakage from the vacuum tank), yet large enough to prevent excessive pulling force and an excessive degree of drawdown on the tube.

Providing the right amount of drawdown ensures dimensional stability while retaining the physical properties of the material that are essential to medical tube performance. Too much drawdown can result in longitudinal shrinkage and loss of essential physical properties due to stress on the molecular structure of the material.

MedVac flood-cooling/vacuum-sizing extrusion sizing tanks are part of Conair’s growing MedLine series of cleanroom-ready production equipment. MedVac tanks give extruders a new level of control over all of the downstream factors involved in extrusion of high-quality, high-precision medical tubing. Conair MedLine equipment is pre-evaluated, pre-configured, pre-calibrated, fully-documented and ready for installation in your cleanroom.

If you’ve got questions—about extrusion applications or sizing/tooling problems—Conair has answers. Just send us a message or give us a call.