Some of the more challenging aspects of medical device manufacturing come from the unique opportunities presented by plastic insert molding. Many of today’s new and innovative healthcare products are made of a combination of thermoplastic resin and components like cannula, tubing, wires, cables, stampings and delicate sensors. Great advances have come with the development of specialized insert-molding equipment and mold-tooling designs that allow delicate components to be molded directly into the devices, instead of being incorporated later through machining, gluing, or ultrasonic welding.
However, it is often difficult to hold these components in place during the injection-molding process. The vast majority of equipment on the market is configured so the top half of the mold is attached to the upper platen of the molding machine, with either one bottom mounted directly below, or multiple bottoms that travel on a shuttle or rotary table. When one bottom is mounted below, the top closes down with some force onto the bottom, plastic is injected, and then the top half rises again to give access to the finished molded parts.
The drawback with this method is that not much else can be accomplished while the cycle is being completed—for example, no additional inserts can be loaded in the mold. To solve this problem, multiple mold bottoms are often used so operators can continue to work during the molding cycle, reducing overall cycle times and improving output.
Whichever process is used, the inserts loaded in the bottom mold half must be held securely in place so that, when the top closes, neither the mold nor the often-expensive insert is damaged. Damage to the mold can be substantial, especially when the two halves close under high tonnage around steel or other rigid materials, so it is important to locate and secure components in place carefully during molding. Still, factors such as operator misplacement or shuttle/rotary table vibration can mean damage to the mold or the inserts.
One solution used by a growing number of companies is bookmolds mounted on a rotary table press. Bookmolds, which have been around for many decades, were especially popular in transfer-molding of rubber products and components, but they have become even more prevalent in the production of medical devices that use thermoplastic resins or even silicone rubber. Understanding how these bookmolds function will help illustrate how they can help improve a company’s quality and efficiency.
Bookmolds are mounted to vertical clamp/vertical injection molding machines that have multi-station rotary tables. The molds consist of top and bottom mold halves that are connected with a hinge in the back so the top half can be lifted open to give access to the bottom half. These molds are often made from a prehard stainless steel, with a high grade tool steel hinge for wear and support. Like conventional molds, bookmolds may have taper locks to maintain alignment between both halves and avoid shifting. A tapered sprue bushing is located on the top of the mold so that, when the mold is opened, the sprue and runner will remain in the bottom half. Ejectors are installed in the bottom half to lift out finished parts along with the sprue and runner, so multiple cavities are often well suited for bookmolds. A handle is often mounted on the top half for easy opening, which can be done either manually by the press operator, or with a stationary ramp.
Bookmolds have a major advantage when delicate inserts have to be loaded because it is easy to load them into the bottom half of the mold, and they are kept secure when the top of the mold is closed prior to any table movement during indexing. If an insert is misloaded, the top half will remain slightly open. When the misloaded book mold then passes under a sensor before it reaches the clamp position, the misload will be detected and corrected before any pressure is applied to the top of the mold that could damage the insert. This system virtually eliminates mold or insert damage, not to mention down time, costly repairs and lost production.
Another attractive feature of book molds is the use of multiple molds to reduce cycle times and increase productivity. During the molding cycle, the two mold halves are clamped together and plastic is injected. Molded parts can cure in the bookmold during travel, since both halves remain closed together, resulting in the fastest per mold cycles and the highest possible yields. This advantage is even greater when molding liquid silicone rubber (LSR), whose cure time can often be a minute or more. Multiple bookmolds can reduce the per-mold cycle time by up to 70% in some cases.
Another advantage is that, so long as they use a common molding resin, different bookmolds can be run together on a rotary table at one time, which means complete assemblies or family parts can be manufactured together. In this way, when the rotary table makes a complete turn, it yields a finished assembly containing multiple molded parts. This is possible because some machine brands now offer controls that can be programmed and stored to automatically change and adjust the molding profile at each station. This system can be an important ingredient of the Lean Manufacturing and JIT requirements for many companies.
Bookmolds can also be used for short runs and specialty manufacturing; different bookmolds remain mounted on the press, with recipes in place for each product they represent. Molds can be selected easily, introduced into production quickly and efficiently, and removed just as easily when they’re no longer needed. Molds that aren’t being used stay mounted on the rotary table, ready for the next short run.
Some operators prefer smaller bookmolds that have fewer cavities per mold over one large multi-cavity mold that can require careful runner balancing and a larger press with increased clamp tonnage, and create problems with pressure/flow constraints. For example, four bookmolds with four cavities each may out-produce one sixteen-cavity mold while also permitting better processing control because of their shorter and more direct runners. In addition, if a problem occurs with the sixteen-cavity mold, production shuts down until the problem is resolved, while one smaller bookmold can easily be taken out of service should a problem arise, with the three remaining molds continuing to produce parts. Bookmolds are typically lightweight, easy to handle and store, and typically require less than five minutes to be installed on the press.
Insert molding of medical devices presents a growing and promising opportunity for companies that want to capitalize on the growth potential of this industry. New and innovative methods are necessary to keep pace with the ever-increasing demands that these products require, and bookmolds address many of the challenges medical molders face on the path to success.
Aberdeen Technologies, Inc.
If you’d like to learn more about how bookmolds and other molding technologies can help provide superior production results and quality control for molding of medical devices, contact us at 1- 800-323-8095.