Mechanics of Injection Molding

This section gives an account of the injection molding, process, particularly those aspects that are important in a drug delivery/dosage form design context. The reader is encouraged to consult engineering textbooks on injection molding for more comprehensive accounts.

The following sequence of operations comprises the manufacturing cycle (Fig. 12.3):

• Materials are fed from a hopper (funnel) into a heated barrel with a reciprocating screw (extruder).

• Materials are melted, mixed, and advanced into an injection chamber at the end of the barrel.

• The screw-ram pushes the screw; the nonreturn valve seals the chamber.

• The molten mix in the chamber is pushed through a nozzle into a mold where it solidifies under hold pressure to ensure complete filling and compensate for shrinkage.

• The mold opens and the product is ejected. The time span between mold openings is the Cycle Time, an important processing variable (the shorter the time, the more units are produced).

12.2.1.1 Feed Step

As represented in Fig. 12.3, material(s) in the hopper (feeder) (3) can be fed to the barrel (6) passively (gravimetrically) or by dosing screws, vibrators, grinders, regrinders, etc. A single hopper or an array of hoppers to feed more than one material concurrently may be employed. Ingredients may be also granulated in a screw-extruder and fed as pellets. Hoppers can be placed at different positions along the barrel, so that (for example) a heat-sensitive material can be added closer to the nozzle, minimizing the risk of thermal degradation. Accuracy of dosing is high, machine manufacturers claiming accuracies in the 0.1-1% range.

12.2.1.2 Melt Formation

The barrel (Fig. 12.3) can be heated by enveloping mantles (heaters) (4) with temperature usually rising toward the nozzle end (8). A motor and gears (2) rotate the reciprocating screw, advancing material toward the nozzle; a nonreturn valve (7) is fitted at the end of the screw. Material, possibly at its Glass Transition Temperature, is trapped in the space between the valve and the nozzle. A cylinder or motor (1) pushes the screw forward forcing the material into the cavity (or cavities). Material is cooled and solidifies while the cylinder maintains pressure (retention). This ensures complete filling of the cavity as many plastics shrink while cooling.

The path from the nozzle to the cavity (runner) results in formation of an extra, residual, part, the cold runner, in each cycle. This cold runner can be recycled but in applications where this could pose a problem (such as a risk associated with reprocessing a drug), a hot-runner might be used. As the name implies, it is heated so that the material remains fluid next to the cavity. Thus, when the mold opens, the plastic breaks leaving a smaller "scar" than if using cold runners. The hot-runner may also be gated so the break is cleaner and neater. The gating might be thermal (Fig. 12.4) (the heat shears the material after cooling but prior to the opening of the mold) or mechanical (Fig. 12.5) (a pin seals the opening). The benefits of hot-runners are many and can influence mold design, cycle time, product finish/elegance, etc., and usually outweigh the added cost.

12.2.1.3 Fusion of Melt

The product is formed when the molten material filling the cavity is allowed to solidify by cooling. Cavity shape defines the shape of the fused melt. There can be multiple cavities in the mold space, producing identical or different items. Molds must be balanced to ensure a good fill pattern. In the case of identical cavities (producing many identical items in each cycle), the number is a multiple of 2 to make the mold symmetric (Fig. 12.6) [6].

Fig. 12.4 Thermal Gating. The TIP is heated c

Fig. 12.4 Thermal Gating. The TIP is heated

Fig. 12.5 Mechanical gating. The nozzle is sealed when the mold opens

12.2.1.4 Mold Design and Mode of Operation (Material Flow)

The simplest mold (Fig. 12.3) consists of two parts: The Injection Mold (10), which is attached to the stationary platen (9), and the Ejector Mold (11), which is attached to the Movable Platen or Rear Platen (12). The platens are connected by 4 (in some cases just 2) Tie Bars or Tie Rods (13). The Movable Platen is able to slide back and

Fig. 12.6 Examples of fill patterns, note the symmetry

forth, driven by the Clamping Cylinder (14), and guided by the Tie Bars. The maximum mold size (high/wide) is limited by the distance between the Tie Bars. This distance and the Clamping Force (defined as the injection pressure multiplied by the total cavity projected area) are the parameters used for comparison of machines.

Molten material enters the mold through a Sprue where the nozzle docks. Channels transverse the mold for circulation of coolants (usually cooled or tempered water), thereby enabling appropriate solidification and optimum cycle time. When the content has solidified, the mold is opened (by moving the Ejector away from the Injector). Product remains on the Ejector, and the Ejector Pins push it (and eventual cold runners) out of the mold. As the molten materials are pushed into the cavity, the air present there either compresses and/or prevents complete filling, or, if the mold is well designed, escapes through air vents. Figure 12.6 provides a schematic of various "feed and fill" configurations.

Mold complexity reflects product complexity. Design modifications include lateral movement capability, transverse Ejector Pins, multilayer molds, Slides, etc. Cavities are usually engraved in changeable Inserts, which are mounted on the mold.

Some molds are designed to produce products made of two (or more) distinct materials, e.g. differing in color, chemical composition, or function. The Egalet® technology is an example, where a "Matrix-in-a-tube" is produced in one machine in one cycle. Such a machine has two (or more) injection units, and the molding process is repeated, often sequentially. In other cases, distinct injection processes are executed simultaneously.

Prototype molds can be made from mild steel, aluminum or nickel, and epoxy. Production molds are made of tool steel, hardened steel or beryllium-copper alloys. Production molds made of medical grade steel are also available.

Note: As the introductory paragraphs indicate, injection molding is a well-developed and mature technology in industrial operations other than pharmaceutical products. Consequently, specific terminologies have evolved and are widely used in the thermoplastics lexicon, some of which have equivalents in pharmaceutical technology. To avoid confusion, such terms are italicized throughout the text for clarity.

A good example of the problems related to terminology is the potentially confusing term "Extruder". In the minds of many pharmaceutical scientists it is a machine that forces wet material through a perforated screen ("wet granulation"), or performs a similar operation. In this chapter, only THERMOPLASTIC processes are addressed. An extruder as used in the plastics industry for ther-moforming is termed a Hot-Melt Extruder in pharmaceutical operations, or a Compounder, if different materials are fed to form the extrudate (see: Pharmaceutical Extrusion Technology, Drugs and the Pharmaceutical Sciences, Vol. 133, 2003 I. Ghebre-Selassie and C. Martin Editors. Marcel Dekker).

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