Nanomolding meets the metal catheter tip

Surgical and medical treatment procedures are using smaller and smaller devices, many of which are delivered through tubes called catheters. Typical catheter materials are nylon, TPE, and PE, with or without wire braiding. Soft tips are added to the catheter tubing and there must be a smooth transition between the tubing and the tip. Tips usually are made in softer materials—typically medical grade TPEs, such as Thermedics' Tecoflex—and have an outer-edge radius. Tip joining is usually done in a labor-intensive, multiple-step process. But Murray Inc. (Buffalo Grove, IL) has a better idea. 

Murray has developed a system capable of insert molding catheter tips directly onto catheters. These tips can be .1- to 10-cu-mm parts, with 50- to 500-µm-thick walls shot through 100-µm-diameter gates. Since such diminutive dimensions are involved, the company calls its molding system a "nanomolding" system (see the sidebar for a definition of nanomolding). 

High injection pressure and fast injection speed are required to fill the cavities. Also, high melt temperature is required to reduce viscosity, so the material must move very quickly through the press to avoid degradation. And, because of the small size of the tips, shot size accuracy is vital. Understandably, a special type of molding system is required. Murray suggests the use of its recently patented Sesame molding system. 

Its Sesame press is a fully automatic pneumatic/electric hybrid. Its features include a one-pellet-at-a-time feeder, a plasticating plunger, and injection via an injection pin—a needle driven by a high-speed linear servomotor. The Sesame can deliver injection pressures up to 50,000 psi, achieve injection times as short as .02 second, and inject shots as small as 2.5 cu mm to within ±.012 cu mm accuracy (see "Tiebar-free Hybrid Micromolding Machine," January 1999 IMM, p. 138 for an initial report). 

In fact, the Sesame takes its name from the miniscule seeds found atop many hamburger buns. The system reportedly can mold 20 sesame-seed-sized parts, which measure 2 cu mm and weigh .0022g, from a single plastic pellet. 

Insert Nanomolding 
For insert molding the tips, the catheter shaft (Figure 1) is inserted over a shaped mandrel and advanced into a preheated mold. Murray has found that mold preheating improves bond strength and helps prevent the plastic flow front from freezing in 50-µm gates. Initial mold temperatures as high as 160C have been used. After injection, the mold is cooled to about 40C for part removal. Electrical heating and oil cooling is one approach. 

Several experiments have been conducted by Murray. One involved a mold that used a longitudinal parting line with a tab gate into the side of the tip area on the catheter. Another involved direct injection into the wall of the tip area. Good bonds were achieved, but there was some flashing and gate marks, and the catheter was slightly damaged from the compression in the longitudinal-split cavity. 

Murray's latest electrically heated/oil-cooled mold design for insert molding the tips has a bored cavity that extends perpendicular to the injection pin (Figure 2). The cavity's ID fits the catheter's OD and the catheter's mandrel is shaped to help create the rounded end surface of the tip's leading edge. For an industry-standard 9 French catheter, a 150-µm-diameter gate is used. Moving the mandrel forward shears the gate and pushes the catheter tip from the mold. 

Flash and gate marks are eliminated and catheter damage is minimal. However, the method of part ejection still poses a degree of complexity that must be addressed prior to the full commercialization of the process.