An MD&DI April 1998 Column
Four molding techniques open up a variety of new approaches to the design of medical components.
Plastics injection molding and applications development for medical devices are two areas of technology that perpetually leapfrog one another. The increased functional requirements of molded medical components drive the molding technology advances required to fulfill those needs. In turn, the increased capabilities of the injection molding process facilitate innovations in design, manufacturing, and assembly of new medical devices.
In most cases, past and present, conventional injection molding technology has been sufficient to meet the needs of the applications developed. When conventional molding has fallen short, however, novel improvements and modifications to the process have been devised to provide unique solutions to the increasingly sophisticated demands of medical design engineers. There have been many such improvements, but four in particular stand out. The first, pulsed cooling and induction coil heating, takes the conventional molding process and adds to it a greater degree of process control. For two others, gas-assisted injection molding and multi—live feed molding, auxiliary equipment is incorporated into the standard machine design. The last technique, multimaterial molding, requires modified molding machines and tooling for optimal success.
PULSED COOLING AND INDUCTION COIL HEATING
This innovative method for controlling mold temperature (Figure 1) overcomes a previously unavoidable drawback in moldcooling methodology. In the conventional molding process, coolant such as water or oil is circulated through the mold continuously while mold temperature is regulated by a controller such as a thermolator or chiller. This type of temperature control maintains a reasonably consistent water-inlet temperature to the mold.
Figure 1. Pulsed cooling and induction coil heating maintains temperature during the injection stroke by closing manifold valves and stopping water flow.When the injection stops, the valves reopen until the steel cools to the desired temperature. Illustration courtesy of Ken A. Kerouac
When plastic is injected into the mold cavity, the mold temperature rises as heat is transferred from the molten plastic to the mold itself. As the molded part cools and solidifies, heat is conveyed out of the system by transfer to the circulating cooling medium and, to a lesser extent, through the mold base to the ambient environment. Cooling continues through mold opening, part ejection, and mold closing, right up until polymer injection on the subsequent shot. This process has unfortunate implications for the polymer. The melt is now being injected into the mold at what is, from a thermal standpoint, the worst possible moment in the cycle—the point at which the mold is at its coldest temperature. Ideally, coolant should be flowing through the mold only when it is required: from the end of polymer injection to the point where the part is cool enough to be ejected.
The pulsed cooling process is designed to address this problem. A manifold fitted with solenoid-controlled water valves is placed in the supply line between the incoming water source and the mold. The manifold is linked to a controller, which is in turn linked to probes that are inserted into each half of the mold just under the parting line surface. The desired steel temperature (monitored by the probes) is set on the controller. During the injection stroke, the manifold valves are closed, permitting no water flow (and therefore no cooling) to the mold. At the end of injection, a signal is sent to the manifold to open all of the valves and pulse cold water through the system. Once the probes sense that the desired steel temperature is reached, water flow through the mold is again terminated until the next shot.
A unique feature of this system is that with larger, complex molds in which heat transfer from the part will not occur uniformly, more probes may be used in conjunction with manifolds that have a greater number of ports. This will allow for independent zones of control within the mold, so that water flow to areas that are sufficiently cooled will be closed off and diverted to warmer areas of the mold. This degree of control affords the opportunity for better part uniformity and reduced cycle time.
A coinjection molding machine forms parts using two materials (one skin and one core), taking advantage of the benefits of both (Ferromatik Milacron, Cincinnati).
With high-temperature polymers, particularly those that run with hot oil rather than water, the problem is often not losing heat too slowly but too rapidly. A high thermal gradient exists between the hot mold and the ambient air, and the resulting heat transfer may overcool the mold in spite of the benefits of the pulsed water flow. In such cases, the pulsed cooling may be supplemented by, or even replaced by, induction coils inserted into each half of the mold. These coils are also controlled by the unit that regulates the water manifold. Their function is to provide background heating to the mold around its perimeter, thus preventing excessive heat loss and maintaining a better thermal equilibrium within the mold. The drawback to induction coils is that they will only work with ferrous mold materials and therefore are not suited for prototyping efforts with molds made out of aluminum or nonferrous alloys.
Pulsed cooling can be retrofitted to existing molds relatively inexpensively. Because the polymer will fill the cavity under warmer conditions, parts with lower, more uniform molded-in stress can be achieved. This can promote better dimensional stability, including heat-distortion resistance, reduced birefringence in transparent parts, more uniform surface finish, more uniform crystallinity development, and therefore more uniform shrinkage. Studies have also verified cycle time improvements with some applications of between 10 and 20%.
GAS-ASSISTED INJECTION MOLDING
Although the earliest patents on gas-assisted molding were issued in the mid- to late 1970s, the process has only come into its own over the last five years, after a quiet gestation period in the 1980s. Its recent explosive growth has shown it to be a truly enabling technology—things can be achieved with gas-assist that are simply not possible with any other type of molding process.
Gas-assisted molding evolved from a modification of the structural foam process known as gas counterpressure, in which a sealed mold cavity is pressurized with nitrogen before the introduction of the polymer/blowing agent blend. The pressure in the cavity prevents the foaming action of the blowing agent from taking place at the melt front, allowing a smooth skin to be formed at the surface while the cellular structure is created in the core. This process minimizes the surface swirls, splay, and imperfections that often made foamed parts difficult to finish or paint.
Gas-assist, like its parent technology, employs prepressurized nitrogen, but in this case no blowing agent is used in the polymer, and the gas is injected into the interior of the polymer shot, creating a part with a polymer skin that is either entirely hollow or solid with hollow sections, depending on the design (Figure 2). Unlike structural foam, gas-assist produces no cellular structure; the polymer and gas form separate, discrete layers.
Figure 2. Injection, hold, and release sequence for gas-assisted molding of hollow or partially hollow parts. Illustration courtesy of Cinpres, Ltd. (Tamworth, UK).
Suppliers of gas-assist equipment offer a wide choice of hardware and controls, but in all cases the basics of the process are similar. A stand-alone gas-assist unit wired to receive signals from the molding machine draws from a nitrogen source (in most cases either a bottle or a nitrogen generator) and prepares a pressurized charge of nitrogen to be delivered to the mold. As the injection stage of the cycle begins, a signal based on time, pressure, or screw position is sent to the gas unit to trigger gas injection at the appropriate time. Gas may be injected either into the polymer melt through a specially designed molding machine nozzle, which sends the gas through the sprue, runners, and gates before entering the part cavity, or directly into the mold itself through gas nozzles fitted into the mold base.
Once injected into the polymer, the gas bubble finds the path of least resistance, favoring the best combination of high temperature and low pressure. In a properly designed tool run under the proper process conditions, the nitrogen, which is much lower in viscosity than the polymer, remains isolated in the gas channels of the part without bleeding out into any thin-walled areas. The gas channels are those areas that have been thickened to achieve functional utility in the part or to promote better flow of the polymer during filling. Because of the lower viscosity of the gas, the pressure drop across the gas bubble is much lower than across a normal polymer shot. This phenomenon accounts for the ability of gas-assist to provide a higher degree of packing than is possible with the traditional process and to allow the melt to travel longer flow lengths in the mold with fewer gates.
Once the gas has aided the polymer in filling out the cavity, it is held under pressure until the material solidifies, with the gas-hold stage replacing the hold stage in a conventional cycle. The gas is then allowed to vent from the part in order to relieve the internal pressure prior to mold opening. By coring out thick sections with gas, significant cycle time savings can be achieved since there are no thick sections of plastic that require a long time to cool. During the period between the end of gas hold and part ejection, the gas unit is recharged with nitrogen for the next shot.
Gas-assist applications often fall into one of three categories:
- Short-shot molding. A process in which certain features such as ribs or thick walls are cored out with gas in an otherwise solid molded part. This process gets its name from the method of only partially filling the cavity during the polymer injection phase of the cycle and then relying on the gas injection phase to fill out the remainder of the cavity with the material the gas bubble is displacing from the core.
- Full-shot molding. A process in which the mold is completely filled during the plastic injection phase. Gas is introduced into the cavity in this case only to provide local packing and to compensate for the effects of polymer volumetric shrinkage as the part cools.
- Hollow molding. A process in which all or nearly all of the part is cored out by the gas, in effect making the part itself the gas channel. This is the method most often used to make parts with large cross sections such as rods, tubes, and handles.
With the gas-assist process, thick parts can be molded without cycle time sacrifices and associated issues such as sink marks and voids. Parts cored out with gas will be lighter, with a higher stiffness-to-weight ratio, resulting in material savings and reduced costs. Large parts can be filled more easily, creating parts with less molded-in stress and better dimensional stability. The surface appearance of gas-assist parts is much improved over that of structural foam. Many items produced by gas-assist do not require paint or any other kind of secondary finishing step.
MULTI—LIVE FEED MOLDING
The conventional injection molding process imposes severe limitations on how the polymer melt may be manipulated once it has filled the cavity. Between the end of first-stage injection and final part ejection only two external effects can act on the melt. The first is heat removal through a combination of conduction and convection. The second is the relatively static pack-and-hold pressures applied by the screw. Used to compensate for volumetric shrinkage and to prevent sinks from forming as the part solidifies, these pressures can only be applied until the gate freezes off. The multi—live feed (MLF) process (Figure 3) employs auxiliary injection molding equipment to achieve dynamic oscillation of the polymer melt within the mold cavity. This oscillation creates a shearing effect on the polymer that prolongs the time the material remains in the molten state and promotes improved orientation and weld line characteristics.
Figure 3. The multiple live feed process, showing (A) out-of-phase oscillation (fill), (B) in-phase oscillation (pack), and (C) holding phase. Illustration courtesy of Eliot M. Grossman (Cinpres-Scorim, Ltd., Tamworth, UK).
Weld lines are molding defects created when multiple polymer flow paths converge in a cavity, such as when melt fronts from different gates meet each other or when flow from a single gate is forced to split and recombine around a pin or other projection in the tool. Weld lines can create appearance problems, since their visibility on the surface may require a costly painting step, or they may be structurally objectionable, since weld lines are inherently weak areas of the parts that can act as stress concentrators and possible failure points in impact or flex situations.
The heart of the MLF process is the head, which mounts to the front of the injection barrel. Within this head, the polymer shot is divided into two independent flow paths and delivered to the mold through two nozzles rather than one, as is done conventionally. A modified sprue bushing with two sprues is required, as is a runner layout that maintains separation of the flow paths.
The head contains hydraulically actuated pistons that act on the melt and provide the oscillation and subsequent pack-and- hold phases employed by the process. Piston motion is controlled by a stand-alone unit synchronized with the injection molding machine by a signal such as first-to-second-stage transfer time or position on each shot.
The MLF process employs three modes of operation:
- The A mode uses an out-of-phase motion of the two pistons; when one piston is moving forward, driving the melt ahead of it, the other piston is retracting, creating volume for the melt to move into. This mode most often begins immediately after first-stage injection, although it can be triggered to begin during injection as well.
- In the B mode, both pistons move in the same direction simultaneously, packing out the cavity. Depending on the capability of the molding machine, the level of packing pressure the MLF unit can provide may be higher than what the machine can deliver.
- In the final C mode, both pistons move forward once and remain in the forward holding position until gate freeze-off. The B and C modes in the process take the place of the pack-and-hold stages in the conventional molding cycle.
The MLF unit controls five specific parameters:
- Temperature of the head. Thermocouples in the head regulate the operation of cartridge heaters to maintain a desired temperature.
- Sequence of operation. In most cases, the sequence of the modes will be in the order described in the previous section. However, it is possible to change the order of operating modes or to eliminate one or two of them entirely.
- Pressure. The forward and return pressure of the pistons can be controlled. The property that most often dictates pressure settings will be the viscosity of the material.
- Frequency. The number of piston oscillations per second can be set. The shear sensitivity and thermal diffusivity of the material may dictate this parameter, since it is the shearing effect of the oscillations that prolong the molten state of the polymer.
- Duration. The length of time each mode is employed within a given cycle controls orientation. If high orientation is desired (with fiber-reinforced parts, for instance), parts with thicker walls may require longer duration, since a larger percentage of the cross section is relatively unoriented polymer, compared to the highly oriented layers closer to the part surface.
The MLF process may eliminate the need to paint parts by reducing the surface weld-line severity to the point where the weld lines are virtually invisible. Whether the surface weld lines are eliminated or not, the internal ones may be altered to a degree that will make the part stronger than it would be with weld lines formed by conventional processes. In fiber-reinforced parts, orientation of the fibers is improved, achieving improved strength in the fiber direction. Filling of thin-wall parts may be facilitated if the oscillation is initiated during polymer fill, creating the shearing effect that may postpone the onset of rapid cooling and solidification that can limit the success of thin-wall molding.
Unlike the other three processes reviewed above, which superimpose auxiliary equipment and some mold modifications onto otherwise standard molding machines, multimaterial molding requires specialized molding equipment in and of itself. A number of different methods fall under the broad umbrella of multimaterial molding: coinjection, overmolding, two-shot molding, and sandwich molding. In all cases, the basic premise of multimaterial molding is to take economical advantage of two or more materials with uniquely different properties by incorporating them into one molded component.
The fundamental equipment requirement for multimaterial molding is to have as many plasticating units (machine barrels) as there are materials to incorporate into the molded part. With some machine designs, multiple barrels are mounted onto the machine base itself; with other systems, the design is more modular, allowing barrels to be added or removed as needed and aligned in different configurations (even vertically) to accommodate more or fewer materials or to adapt for floor space or ceiling height limitations.
Coinjection and Sandwich Molding. These two similar processes (coinjection is sometimes classified as a subset of sandwich molding) form parts with a skin of one material and a core of another, taking advantage of the benefits of both. The critical material requirement in this kind of molding is compatibility. If the two polymers are not compatible, they will not adhere to each other, resulting in delamination at the interface between the two polymer layers and failure of the part.
Figure 4. The coinjection process works by first injecting the skin material (A) then switching to the core material (B). A small amount of skin material can seal the gate to finish the process. Illustration courtesy of Battenfeld of America (West Warwick, RI).
In coinjection molding, two barrels are joined together by a common manifold and nozzle, through which both materials flow before entering the cavity (Figure 4). The nozzle is designed with a shutoff feature that allows only one of the materials to flow through at any given time. To set up the process, the relative percentage of skin to core material is determined, most often by trial and error, and the two barrels are each programmed with the appropriate shot size. On injection, the injection unit with the skin material (often called the A barrel or A side) injects the set amount of polymer. This is followed by the core material in the B barrel which, in a manner similar to gas-assist, penetrates the skin polymer and completes filling of the cavity without breaking through to the skin surface. Since plastics flow in injection molding is laminar, the two materials can be molded in this skin-core configuration without mixing with each other.
Coinjection molding often has a third stage, in which a small amount of skin material completes the injection stage. This is known as an A-B-A injection sequence and accomplishes the task of completely encapsulating the core material, thus protecting it from any weathering, chemicals, or other exposures to which it might be susceptible.
Figure 5. Sandwich molding's multiple plasticating units serve as extruders that allow fixed amounts of material into a single injection unit. Illustration courtesy of Ferromatik Milacron (Cincinnati).
Sandwich molding also results in a skin-core structure, but the mechanics are slightly different (Figure 5). In this process, multiple plasticating units are used only as extruders to feed their percentage of the total shot to a single injection unit. The injection unit itself also accounts for one of the polymer layers. Prior to injection, then, the injection unit has built up a shot consisting of as many layers as there are materials. Again, because of the laminar nature of polymer flow in the injection molding process, these layers do not mix with each other.
Upon injection, the last material fed to the injection unit, which is at the front of the barrel, becomes the skin of the part, and all of the subsequent layers form beneath it, working toward the center, until the last material in, which was that material plasticized by the injection unit itself, becomes the very core. Because all of the injection is done through only one injection unit, there is no opportunity for an A-B-A sequence, as in coinjection; therefore, complete encapsulation of the core is not possible. The core will be exposed at the gate leading into the cavity, which is often not a problem if the gate is on an area of the part that will be hidden or if the material in the core is resistant to the detrimental effects of environmental or chemical exposure.
Coinjection and sandwich molding can provide both economical and functional advantages to a molder. Economically, material costs can be reduced by having the premium or more expensive material only on the outside of the part where it is visible, while the inside may be filled with a less expensive resin. The core of a coinjected part may also be an excellent outlet for regrind generated in production operations. Functionally, parts can combine both structure and utility. A part requiring surface features such as a soft touch, or a 100% color match to eliminate paint, may be molded with a more rigid core material, or even a filled one, to provide a good combination of a soft, pliable surface and high rigidity.
Overmolding. With sandwich or coinjection molding, it may not always be apparent that two materials are being used. Overmolding, or two-shot molding, results in parts in which it is clearly evident that more than one material is being used. In these processes, only part of a product is molded in one material, and that molded piece is manipulated so the second material can be molded around, over, under, or through it to complete the final part. This method is sometimes referred to as in-mold assembly, since the resulting part effectively acts as an assembly of two materials rather than as a layered structure.
Tool design is a critical element for successful overmolding. In some designs, the first component is molded in the first material; then sliding action in the tool creates an additional cavity space that can be filled by the second material to complete the in-mold assembly of the part. The mold cavity also has to be transported, either by mold rotation or mold shuttling, to a position of alignment with the second barrel so it can complete filling of the mold. In other cases, the first molded component has to be physically removed from its cavity and placed in another cavity for molding with the second material. The second molding stage may be another barrel on the same machine, in which the same sort of rotating or shuttling tooling may be employed, or it may be an entirely different mold on an entirely different machine, which saves tooling costs but ties up two machines instead of one.
In most cases, good compatibility of the different materials is required to promote good adhesion and to prevent delamination and part failure. In some cases, however, incompatible materials may be deliberately molded with each other for applications in which relative motion between the two subcomponents is desired. It is possible, for instance, to create a jointed part with a ball molded in one material and a socket molded in a second, incompatible material. The parts can be assembled right in the mold, and since the materials are incompatible, the ball and socket will not adhere, allowing free movement between the components.
The primary benefit of overmolding is savings on assembly costs. Downstream assembly operations may be eliminated, and time and expense are reduced if mechanical fasteners or adhesives do not have to be purchased, installed, or applied. One of the more commercially visible overmolding applications is toothbrushes that have a soft section built into the handle to achieve a better grip. This application would not be practical if it required any kind of mechanical fastener, and human oral contact may limit the kinds of adhesives that may be considered.
An article such as this can only touch upon the many innovations in molding technology that contribute to the success of medical device manufacturing. As unmet needs continue to be identified, new materials in new designs for new applications will continue to be developed. Injection molding technology will continue to keep pace with these developments and provide the manufacturing capabilities necessary to bring these products to life.
Ken A. Kerouac is a specialist with the applied fabrication technology team in the technical service and development (TS&D) department and Peter F. Grelle is a development leader in the automotive technical service and development group at Dow Plastics (Midland, MI).