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Molding: hot runners meet the challenge of molding small medical parts.

The cost-effectiveness of hot-runner systems has become more evident in recent years, during which increasing numbers of companies have specified hot-runner molds because of the promise of reducing cycle times, eliminating scrap material, and saving overall energy costs.

The medical/health care industry, one of the fastest-growing markets for plastics, is particularly demanding and challenging-for injection molders. Healthcare companies require thin-walled products that can be molded in large volume to the tightest tolerances, but at reasonable cost. Medical applications specifically require temperature uniformity, individual cavity control, leakproof seals, elimination of O-rings, reduced control zones, gate control, and guaranteed vestige height. However, a hot-runner system engineered to satisfy all of their specific requirements has, historically, been unavailable to molders of medical products.

Economical hot-runner systems that satisfy even the most difficult medical/ health care applications are now available. But it is for high-cavitation, small medical-part molds that hot-runner molding has real economic value. Many applications demand zero regrind, and when runner weights equal or exceed actual part weights, runner storage and handling can be costly as well as troublesome. Economic Justification Typically, a hot-runner system requires a capital investment of 10% to 20% of the total price of a mold. A return on investment can be quickly realized, however, through the many economic benefits of hot-runner molding.

Energy savings. These are among the biggest and frequently overlooked savings that can be realized through hot-runner molding. In small-part molding, the volume and projected area of the runner can be as large as or larger than those of the parts. By eliminating the excess volume of plastic used in runners and sprues or reducing the projected area, the molder can utilize a smaller machine. A smaller machine requires smaller motors, pumps, and reduced heat energy for the screw and barrel.

Eliminating the runner eliminates the need for grinders, part separators, and handling equipment. All such auxiliary equipment requires energy: a typical 5-hp motor, for example, uses 3.8 kwh. Secondary operations such as degating may also be eliminated.

The cost of running such motors subtracts from the molder's profit, as does the added maintenance of grinders, part separators, robots, and other equipment.

Part quality. Eliminating the cold runner and sprue eliminates regrind, provides a wider processing window, and can provide more uniform cavity pressure to the part. In a typical two-plate cold-runner mold, plastic flows throughout the cavity from the cavity edge of one side. Cavity pressures are typically higher at the filling end, leading to uneven packing and shrinkage.

Achieving a shorter flow distance can result in a wider processing window. In hot-runner molding, the gate can be located more centrally to the part, which would thus require lower pressures and temperature for filling.

All written specifications for material apply to virgin material. When used in increasing percentages, regrind can reduce part performance and heighten the risk of contaminating material. Many medical parts specify zero regrind. In such cases, runners must be handled and stored for other applications or sold to reprocessors at considerable loss.

Cycle time. Reducing the cycle time of a mold is the most apparent benefit of hot-runner molding. Cycle time may be broken down into the following five steps: mold-close time, cavity-fill time, cooling time, mold-open time, and ejection time. A well-designed hot-runner mold can reduce increments of time in each of these steps.

The distance that a mold must open to allow the parts and runner to eject and clear all mold components determines the time required to open and close the mold. A mold must travel an extra 8 inches when opening and closing to clear a typical 4-in-long cold sprue. Three-plate molding can add even more time because it requires, besides separation of sprues and runners, an extra ejection on stripper plate. In a hot-runner system with direct part gating, only part configuration determines the open-and-close requirements of a mold.

Hot-runner molding also reduces fill time. Omitting the cold runner eliminates the time required for material to flow from runner to part; center-gating the part reduces such time by halving the distance over which the material must flow.

Typically, a cold runner has the largest cross-sectional area so that it can adequately fill and pack the part before freezing off. Hence, a cold runner requires a longer period of time to cool, adding time to the overall cycle.

Justification for the use of hot-runner systems, especially for high-cavitation medical molds, undoubtedly exists. Historically associated with hot-runner systems, however, have been problems of high maintenance and excessive downtime.

Advanced Hot-Runner Design

A hot-runner system, which is simply a high-pressure heated conduit that transfers homogeneous melt from the nozzle of an injection molding machine to the cavities, does nothing to improve melt quality. It can, however, cause several problems if it is poorly designed. Therefore, a hot-runner system must be designed to allow for the following: temperature uniformity, gate control, simplicity, and flexibility.

High-cavitation small-part molds require additional design specifications: leakproof design, individual drop control, balanced fill for dimensional control of parts, and minimum center-line to center-line distance between drops. From these criteria, Kona Corp. has developed its standard Pre-Engineered hot-runner systems specifically for high-cavitation precision molding. How these criteria are met is described below.

Temperature uniformity is achieved primarily by using heat pipes in the manifold and drops. Historically, runner-wall temperatures in hot-runner systems have not been uniform: Electrical resistance heaters cause wide temperature profiles along the lengths of manifold systems. Viscosity, flow balance, and the quality of parts are all influenced by temperature.

The hot-runner system, designed to include heat pipes and highly reliable tubular heaters, effects a near-isothermal condition. Essentially, a heat pipe is a closed evacuated chamber of any shape, filled with a working medium fluid and wick; it functions according to principles of vapor-heat transfer and capillary action. Vapor-heat transfer is, in basic terms, a mass transfer that can transport heat energy over relatively long distances, yet provide nearly uniform resultant temperature. Capillary action is a phenomenon in which surface tension of a liquid is drawn into an evacuated area. A heat pipe is thus an effective means of heat transfer and temperature flattening. It evenly distributes heat provided by electrical resistance heaters in the hot manifold, regardless of the load placed upon the manifold.

In its simplest mode of operation, a heat pipe transfers heat energy from a generic heat source (evaporation section) to any colder area (condenser section). It is important to note the existence of a specific point between evaporation and condensation. Within a vacuum, heating the working fluid to a certain temperature produces a specific vapor pressure, which is uniform throughout the space. Thus, temperature is isothermal within the heat pipe.

Gate control is achieved by properly applying one of eight tip styles. The design objective at the gate is to produce gate control over the full range of normal processing temperatures for each material. Therefore, drop temperature is not limited to a narrow operating range in order to produce a clean vestige, but can be set over the full range to optimize part quality. Each gate style has been designed with the aid of CAE finite element heat-transfer analysis to provide, at the gate, a sharp interface between hot and cold: The result is a clean vestige break over the full range of normal operating temperatures for each resin.

Gate control and isolation are vital because two very distinct processing temperatures, those of the hot melt and the cold mold, are required. The temperature profile at the break point of the gate is critical: If the temperature profile is too hot, drooling and stringing will occur; if too cold, a freeze-off condition may result. Various tip geometries and materials of construction were analyzed, using CAE finite element technology, to produce the best temperature profile in the critical gate area. Although the standard tip styles are adequate for most applications, the CAE program is useful on a case-by-case basis for special applications. Valve gates can also be used for applications that require zero vestige.

Simplicity is achieved by using the lowest number of control zones necessary for attaining temperature uniformity within the manifold system. When each leg of the manifold has only one thermocouple wire and one heater wire, fewer zones allow easier system installation and operation. The use of the threaded drop design eliminates the needs for troublesome O-rings and tight preload requirements.

Flexibility is achieved by offering standard manifolds with fixed lengths, widths, and thicknesses. Because of such molding variables as size of mold and number of cavities, the variables of the hot-runner design must fit within a given fixed dimension. Variables of hot-runner design include runner diameter, for optimizing shear rate and residence time; number of drops; and gate spacing limited only by minimum center-to-center distance for each drop style.

Leakproof design is accomplished by using bushings that thread directly into the manifold, thus ensuring a self-contained system that does not permit O-ring leakage. The bushings are designed to be located in-line with the cavity in the mold and on a diameter at the gate; the bushing body is allowed to flex within conservative limits during thermal expansion. The manifold is of one-piece design, which again eliminates the need for O-rings and surface-to-surface contact.

Individual-bushing thermal shutoff control. Long-life and reliable tube heaters are pre-installed on each of the bushings, which contain heat pipes that uniformly distribute temperature throughout the bushings. Because of the thread's limited contact with the manifold, the temperature of the bushing is not affected by the temperature of the manifold; a cavity can thus be shut off with the flick of a switch. The ability to shut down a cavity is often desirable because of the close tolerance and fine detail required in medical molding: If a cavity goes out of tolerance, the molder may not want to shut down production in order to resolve the problem.

Balance within the molding process is indicated by consistent flow rates, times, and pressures to each cavity. Two methods of balancing are acceptable within a hot-runner system-flow branching and pressure-drop balancing. Balance is crucial for high-precision multicavity molds: For each 1000-psi difference in cavity pressure, a differential in part shrinkage of 0.5% to 0.75% is possible. Flow-branch balance, often considered the best method of balancing, can be used for molds that have 2n cavities. However, there are other factors that must be considered. For example, flow branching usually doubles residence time within the manifold and increases total pressure-drop throughout the system.

Pressure-drop balance can be utilized with any number of gates and provides for minimum residence times and pressure drop. Because balance denotes consistent flow rates, times, and pressures to each cavity, one may surmise that the only difference from one cavity to another is the difference in total pressure drop that is due to varying flow lengths. Equalizing pressure to each cavity by varying runner-channel diameters can produce accurate flow balance.

Because temperature can greatly affect plastic viscosity, uniformity of temperature is critical in either type of balance. To determine the best method for balance, each application must be evaluated.

Cavity spacing is determined by part detail, proper mold cooling, any "action" required within the mold, and the minimum center-to-center distance provided by the manifold bushing. The bushing described in this article was designed to meet all requirements for small-part medical molding while maintaining a minimum 1.500-in C/L to C/L distance.

The hot-runner design described above has been developed specifically for small-part medical and closure molding. By providing high reliability, simplicity, and reduced maintenance, it allows the molder to benefit from the economic advantages of runnerless molding.
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Title Annotation:Engineering resins: innovation, development, realization
Author:Hume, William J.
Publication:Plastics Engineering
Date:May 1, 1990
Words:1939
Previous Article:Materials: extruding thermoplastic foams with a non-CFC blowing agent.
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