Originally published April 1996
Thomas L. Miller
Development Chemist, Technical Service and Development
Dow Plastics, Midland, MI
Injection molding is commonly used to manufacture medical parts in large quantities with reliable consistency. Understanding all the variables of injection molding and their impact on successful processing is particularly important for medical device manufacturers, who require tight tolerances and unique performance requirements. Equipment design, material performance, process variables, and part design specifics all contribute to the performance quality of any injection-molded medical part.
In brief, the injection molding cycle can be broken down into four phases: fill, pack, hold, and cooling/plastication. The process begins with the mixing and melting of resin pellets. Molten polymer moves through the barrel of the machine and is forced (injected) into a steel mold. As the plastic fills and packs the mold, the part takes shape and begins to cool. The molded part is then ejected from the mold, ready for finishing steps and assembly.
Equipment. Several types of injection molding machines are available with different methods for blending, melting, and injecting the polymer into the mold. These are available in a range of sizes, offering choices in clamp tonnage, machine capacity, and screw design, depending on the needs of a particular application. Figure 1 shows a typical injection molding machine.
Materials. Depending on the end-use requirements for a device, manufacturers may select from a broad range of engineering plastics. Since processing parameters vary for each material family and resin formulation, the best results are usually achieved by following the handling and processing procedures recommended by the resin manufacturer.
Processing Parameters. While machine selection, material properties, and part design all affect the outcome of injection molding, five processing variables specific to injection molding can have as much or more impact on the success of this process. These variables are: injection velocity, plastic temperature, plastic pressure, and cooling temperature and time. Control of these variables during each of the four phases of the injection molding process can help improve part quality, reduce part variations, and increase overall productivity.
In Phase 1--fill--the screw advances and plastic flows into the mold. Flow characteristics are determined by melt temperature, pressure, and shear rate. Injection velocity--the rate at which the ram (screw) moves--is the most critical variable during fill. A polymer flows more easily as injection velocity is increased. However, injection velocity that is too high can create excessive shear and result in problems such as splay and jetting. More importantly, heat from a higher shear rate can degrade the plastic, which adversely affects the properties of the molded part.
The way in which plastics flow during fill is also affected by their viscosity, or resistance to flow. Polymers with high viscosity are thick and taffylike; those with low viscosity are thinner and flow more easily. Melt temperature affects viscosity and to achieve the best results should be maintained within the temperature range recommended by the supplier.
Plastic pressure, another variable, increases sharply during fill. The molten plastic can, in fact, be under much greater pressure than is indicated by hydraulic pressure (see Figure 2). It is important to understand the flow characteristics during fill of the material being used and to operate the process consistently.
Phase 2--pack--is when the plastic melt is compressed and more material is added to compensate for any shrinkage during cooling. Approximately 95% of the total resin is added during fill, with the remaining 5% added during the pack phase.
Plastic pressure is the primary variable of concern during the pack phase. The screw maintains pressure in the melt, compensating for shrinkage, which can cause sinks and voids. Variations in cavity pressure are a primary cause of deviations in plastic parts.
It is important to completely fill the mold--avoiding overpacking or underpacking--since packing pressure determines part weight and part dimensions. Overpacking can cause dimensional problems and difficulty in ejecting the part, while underpacking can result in short shots, sinks, part-weight variations, and warpage.
Phase 3--hold--is affected by all five of the process variables described earlier: injection velocity, plastic temperature, plastic pressure, and cooling temperature and time. After the mold is packed, the plastic is held in the mold until it is partially solidified and the gate freezes. The drop in plastic pressure reflects the amount of shrinkage that occurs from cooling (see Figure 2). One way to optimize this phase is to decrease the hold time until the part weight changes. At that point, the gate is no longer sealed and resin backflows out of the mold. If hold continues after the gate seals, cycle time increases, using more time and energy to produce the part. The key is to maintain pressure on the plastic until the gate freezes.
Phase 4--cooling and plastication--is generally the longest part of the molding cycle--up to 80% of the cycle time. Optimizing cooling time can yield substantial gains in productivity. Because the gates are sealed during this phase, cooling temperature and time are the only variables at work. The key to optimizing the cooling phase is to balance the desire to cool quickly against the amount of molded-in stress the final part can withstand.
Design Considerations. While designed for functionality, parts should also be designed to maximize overall strength and simplify the manufacturing process. Significant problems in both processing and performance can occur when the basic principles of good design are overlooked. Following basic design guidelines for nominal wall, corner radii, holes, projections, draft, and gating increases the likelihood that the part will process and perform successfully.
Since end-use factors (e.g., sterilization) can also affect material performance, design elements can be used to compensate for certain shifts in material properties. Design factors that increase localized stress should be reviewed with knowledge of the selected material and end-use requirements.
A simple list of design basics (see box, p. 51) can be reviewed at any time during the product development cycle to focus on fundamentals. While good design doesn't always guarantee molding success, it does contribute to processing and assembly ease, part performance, and overall productivity. *
DESIGN GUIDELINES FOR INJECTION-MOLDED PARTS
The following list offers some basic guidelines for part design. While not all-inclusive, it covers design elements common to almost all injection-molded plastic parts.
Nominal Wall. The overall thickness of the part.
* Maintain a uniform nominal wall.
* Avoid overly thick or thin sections.
Corner Radii. The intersection of any two walls. Necessary for part functionality, corner radii act as inherent stress concentrators.
* Avoid sharp corners.
* Design rounded inside and outside edges.
* Maintain a uniform wall even at corners.
Holes. Openings for attaching components or fasteners, or for providing ventilation or light.
* Avoid sharp corners, which can localize stress concentrators, cause weld lines and shear the material during fill, and induce polymer orientation.
Projections. Any features that stand up off the nominal wall: ribs, bosses, gussets, tabs, and standoffs.
* Avoid projections that are too thick or too thin; they can create problems during processing and impede part performance.
Draft. The degree to which the side walls are tapered or angled. The objective of draft is to make part removal as easy as possible.
* Ensure adequate draft for any aspect of a part that is oriented perpendicular to the mold so that it can be freed from the mold.
Gating. The opening through which the polymer melt enters the mold. There are several types of gates--sprue, edge, flash, pin-point, diaphragm, ring, submarine, and tunnel.
* Consider the several factors that will determine the type of gate used: the number of cavities in the mold, the need for symmetrical filling, the size and shape of the part, and how tight the tolerances are for the part.