By Rachel Golan
No design challenge is too difficult for Root3 Labs to tackle. When one of our clients requested that our team complete the design for manufacturing (DFM) of a medical device with complex curvature spanning over various planes, Root3 Labs could not utilize the most common manufacturing technique of injection molding to produce production-grade prototypes. Instead, we turned to a lesser known method called dip molding. Let’s get a glimpse into what this process entails!
Dip molding is a manufacturing technique that involves dipping heated metal molds into a liquid polymer to form a plastic part. It is commonly used for medical balloons, endoscope components, cannulas and surgical gloves.
The first step to prepare a product for dip molding is to design and model the metal molds, commonly referred to as mandrels. It is important to incorporate sufficiently large radii into the design to facilitate the de-molding process. Also, extra space should be left at the top of the mandrel, so the manufacturer has room to grip the mandrel while dipping. After the design is complete, the mandrels are manufactured on a CNC machine from either aluminum or steel. Standard finish can be utilized, but electropolishing is preferable since it facilitates the de-molding process and results in a smoother internal part cavity. When the mandrels are finished, they are sent to the dip molding facility.
In the dip molding plant, the mandrels are heated and subsequently dipped into a liquid polymer. The most commonly used polymer is liquid PVC, or plastisol. Additional resin choices include latex, polyurethanes, silicone, leneoprene and epoxy. The mandrels may be dipped by hand or with a programmable, automated machine. Automated dipping enables precise control of the orientation of the dipped mandrel and speed of dipping, resulting in consistent and repeatable samples. Once the mandrel is immersed in the resin, the liquid polymer attaches to the mandrel. The thickness of the product is determined by the initial mold temperature, temperature of the resin, speed of dipping and the length of time the mandrel stays in the liquid polymer (also known as the “dwell time”). After a set dwell time, the mandrels are removed from the resin tank and placed in an oven to dry. The heat from the oven cross-links the polymer, curing it into a solid. Finally, the mandrels are cooled, and the finished parts are stripped from the mandrels. The parts are now ready to use!
Dip molding has several advantages over other manufacturing techniques. It has low tooling and production costs, especially compared to injection molding. Prototypes can be produced quickly, and order sizes can range from a few dipped parts to high production orders. There are no parting lines on dip molded parts and a variety of different materials and durometers are available to choose from. Lastly, dip molding can accommodate parts with complex curvatures that could otherwise not be manufactured with injection molding.
The disadvantages of dip molding are that some samples may contain drip marks and bubbles, and the wall thickness of the parts can be difficult to control. However, the orientation and dipping speed of the mandrels can be optimized to control these factors in production. Despite this, manufacturers may not be able to accommodate a request for two or more wall thicknesses in a given part. A final factor to be aware of is that mandrels should be designed so that smaller cross-sectional areas that will pulled over larger cross-sectional areas are at least half the size of the larger cross-sectional area. If this is not possible, demolding the part may be difficult and the mandrel may need to be manufactured in separate parts.
Even though dip molding has some limitations, it may be the preferred manufacturing solution in some cases. Overall, dip molding is a valuable manufacturing technique that can be utilized to transform ideas into finish products in a cost-effective, quick, and high-quality manner.