It’s a wrap: Extrusion services for medtech

Dwain Tarmey, Vistamed discusses the jacketing of hypotubes and coils for the medical device industry.

VistaMed provides a range of technical extrusion solutions to deliver the optimum jacketed component for minimally invasive delivery or access device.

Metal materials such as coils, hypotubes, wire and even polymer materials can be coated with a range of thermoplastic materials. VistaMed can jacket discrete lengths, as well as provide continuous jacketing. The discrete lengths can have a tapered OD or have a constant OD whereas the continuous jacketing process will have a constant OD. Over jacketing extrusion allows for the application of an outer layer of plastic onto an existing hypotube, coil, wire, or plastic material.

The process involves a system that will pick and feed the metal parts through the extrusion die head. Prior to entering the die head the metal parts are exposed to heat to promote adhesion to the substrate. Preheating of the metal parts prevents stresses that may occur in the jacket due to premature chilling of hot plastic from the relatively cold metal part. In the case of small metal parts, this can be accomplished by using a low-voltage resistance applied between two properly insulated metallic rolls placed just before the metal part goes into the crosshead. When using larger diameter metal parts and for secondary jacketing operations, the preheating can be done with either a gas flame or an IR preheating tunnel.

Over the years several methods were taken to straighten wire prior to entering the die head. The two widely used methods are the ‘roll’ style straightening method and the ‘rotary’ style straightening method. The rotary method is the best way to straighten wire but it is very expensive to implement and it will be the slowest method. In the case of the roll straightening method, the wire is forced to navigate a course around roll bearings. The wire is bent past its elastic point and then redirected to a straightened state. With the rotary style the straightening rolls are rotated around the wire as it is pulled through the device. The rotational speed and direction of the rolls as well as the feed speed and tension value applied to the wire must all be carefully controlled to obtain good results. Roll straighteners, on the other hand, can be configured with multiple planes that can produce the same results of a rotary straightener but are less expensive, easy to set up and, depending on bearing selection, can be operated at speeds into the thousands of feet per minute. Some general considerations to be taken:

• The wire should travel in as straight a line as possible when being ‘paid off’ from the spool, coil, or reel and though the straightening device.
• As the wire tensile value approaches 300 ksi a ‘roll’ style straightening device is less
• The wire should always be ‘pulled’ through the straightening device with as even a back tension allowed.
• The pulling force applied should be as consistent as possible.

The most critical part of the basic extruder is the screw and its design. The standard metering screw was for many years the standard screw for jacketing extrusion. This screw has a very rapid transition which takes place within ¼ flight of the screw. It has a deep feed section which conveys the pellets and partial melt to the metering zone. Standard design calls for feed section to be ¾ of the total screw length, with the remaining ¼ serving as the metering section. The metering section is the most critical area of the screw since it acts as a pump and is also the region where shear is applied to the material to prepare a homogenous melt. It is recommended that the screw have a long, shallow metering section to improve the homogenizing action.

The crosshead serves as the transition piece between the extruder and the die and supports the guider tip and die. It provides a flow path for the melt as it leaves the extruder and turns in the direction of wire travel. The conventional crosshead for wire jacketing is set at 90 degrees to the extruder. Some newer designs set the crosshead at 30 degrees. The claims for this later design are more streamlined flow and less chance for the material to hang up and degrade. Whichever head is used, the passage should be streamlined and polished.

There are two different types of die tooling used for coating metal parts, (1) Jacket coating and (2) Pressure coating. In jacket coating tooling, the polymer melt does not touch the inner metal part until immediately before the die lips. In pressure coating, the melt contacts the inner metal part long before it reaches the die lips; this is done at high pressures to ensure good adhesion between the two materials. If intimate contact or adhesion is required between the new layer and existing wire, pressure tooling is used. If adhesion is not desired/necessary, jacketing tooling is used instead.

All thermoplastic covered metal parts are cooled after exiting the die head by either passing through a water tank or through an air wipe. Sufficient cooling time is required to allow cooling of the coated product without distortion of the jacketing.