Injection molding

Injection molding of plastics is analogous to die casting of metals. The plastic is heated to a fluid state in one chamber, then forced at high speed into a relatively cold, closed mold where it cools and solidifies to the desired shape. This method of processing plastics became a commercial reality in the early 1930s. It is the fastest and most economical of all commercial processes for the molding of thermoplastic materials, and is applicable to the production of articles of intricate as well as simple design.

A slightly modified version of injection molding known as "jet molding" is applicable to the molding of thermosetting materials. The principal difference between the two processes is the function of the temperature of the nozzle and mold on the material. Injection molding employs a relatively cold mold to solidify thermoplastics by chilling the mass below the melting point. Jet molding employs a relatively hot nozzle and mold to harden the thermosetting material by completing the cure.

The Process

The sequence of operation known as the molding cycle is as follows:

1. Two mold halves which, when closed together, combine to form one or more negative forms of the article to be molded, are tightly clamped between the platens of an injection-molding machine.

2. The closed mold is brought into contact with the nozzle orifice of a heating chamber. The heating chamber, known as the plastifying cylinder, is of sufficient size to carry an inventory of material equal to several volumes required to fill the mold. This permits gradual heating of the plastic to fluidity.

3. An automatically weighed or measured quantity of granulated thermoplastic, sufficient to fill the mold cavity, is fed into the rear of the plas-tifying cylinder.

4. A reciprocating plunger actuated by a hydraulically operated piston forces the material into the plastify-ing cylinder. An equal quantity of fluid plastic is thus forced out of the front of the cylinder through the nozzle orifice and into the mold.

5. A pressure of several thousand megapascals is maintained on the material within the mold until the plastic cools and solidifies.

6. After the molded item has hardened sufficiently to permit removal from the mold without distortion, the mold is opened and the part ejected.

Injection molding offers several advantages over other methods of molding. Some of the more important of these are as follows:

1. The process lends itself to complete automation for the molding of a great number of parts.

2. Mold parts require little if any post-molding operations.

3. High rates of production are made possible by the high thermal efficiency of the operation and by the short molding cycles possible.

4. There is a low ratio of mold-to-part cost in large volume production.

5. Long tool and machine life requires a minimum of maintenance and relatively low amortization costs.

6. Reuse of material is possible in most applications.

Molding machines vary considerably in design as well as capacity. For example, the clamp mechanism may be hydraulically operated, hydraulic-mechanically operated, or entirely mechanical. The injection of the material into the mold may be accomplished by a rotating screw as well as a plunger.

Machines are rated according to the weight of plastic that can be injected into the mold (the shot) with one stroke of the injection plunger. The capacity of commercially available machines covers a range from a fraction of a gram to 12.5 kg.

Limitations

The injection molding process is subject to the following limitations:

Material. Any thermoplastic material may be a candidate for injection molding, if upon heating it can be rendered sufficiently fluid to permit injection into a mold and the resulting molded article retains all the desired properties.

Geometry of Part. Any part, regardless of geometry, that can be removed from a mold without damage to the mold or part is moldable.

Weight of Part. The weight of the part must be within the "shot" rating of the particular machine being used.

Projected Area and Wall Thickness of Part. The limitation on these dimensions will be governed by several factors including the relative fluidity of the plastic being molded, the pressure necessary to fill the mold, the rigidity of the part to permit ejection from the mold without deformation, and sufficient clamp force available to hold the mold in a closed position when the necessary injection pressure is developed within the cavity.

Production Economy. Economy can be realized only when relatively large quantities are produced. In addition to the material consumed and the cost of the molding operation, the number of pieces produced must also bear the cost of the mold in which they are cast. Depending on the size and complexity of the geometry of the part to be molded, mold costs will vary from a few hundred to many thousands of dollars. Again depending on size and complexity of the part, the rate of production will vary from a few minutes for a single part to several hundred per minute.

In addition to the high production rates made possible by ejection molding, articles of high quality and relatively precise dimensions can be produced. The production of industrial parts held to dimensional tolerances of ±0.002 in./in. is quite common.

The injection-molding process has made possible the development of an extremely large family of plastics. The basic types of thermoplastics and the human-modified compositions available are analogous to metals and alloys. New compositions as well as process variations are constantly being developed to meet the demands of particular situations.

Molds

The molds used in injection-molding machines for producing such large automotive components as body panels can now be made more compact and lighter than ever before. What makes this possible is nickel vapor deposition.

For car manufacturers, this means the capital invested in production molding machines can be significantly reduced, or, in more practical terms, it means smaller equipment is needed to handle the molds.

In the manufacture of shell molds nickel carbonyl vapor is fed into a low-pressure chamber where, at a temperature of 180°C, the vapor decomposes and nickel is deposited, atom by atom, onto a heated metallic master, or mandrel. Carbon monoxide and any unused carbonyl gases are recycled.

The process creates a layer of nickel that is 99.9% pure, at a deposition rate of 0.25 mm/h, faithfully reproducing all surface features. Delivery time for the production of large molds is greatly reduced, compared with conventional mold-making techniques. Uniform wall thicknesses of 1.5 to 25 mm have been achieved using the vapor deposition method.

A surface hardness of up to 42 Rc to a depth of 0.25 mm gives the shell molds excellent wear resistance.

Also, the excellent thermal conductivity of nickel reduces cycle times in the injection molding process because heat is quickly transferred from the shell to water-cooling channels in the mold assembly.

Nickel has other engineering advantages in shell molds, including its outstanding weldabil-ity, the uniform wall thicknesses possible, and the absence of residual stresses.

Recently, a 68,080-kg injection mold was created for the side panels of a prototype Composite Concept Vehicle. Larger components, up to 1.2 by 2.5 m, can be easily accommodated.

Updated Processes

Injection-compression molding (ICM) has been around for years — in fact, it is how old vinyl records were made. However, with the recent availability of high-speed microprocessors and advanced software to precisely control the molding cycle, the process now works with long-fiber-reinforced thermoplastics. This offers the prospect of stronger and lighter parts, lower costs, and better part-to-part consistency. While standard injection-molding produces reinforced-thermoplastic parts with good success, ICM can optimize the performance of long-fiber-reinforced thermoplastic materials (see Figure I.2).

The difference between ICM and conventional injection molding is that the shot is injected at low pressure into a partially open tool, as opposed to a closed one. The mold closes to compress and distribute the melt into the far reaches of the cavity, thus completing the filling and packing phase. This eliminates molded-in stresses resulting from high-injection pressures.

There are two basic types of ICM: sequential and simultaneous. In the latter, compression can begin at any point during injection, and cycle times are similar to those for conventional injection molding. With sequential ICM, the injection stroke ends before compression begins, and cycle times are 1 to 2 s longer to accommodate secondary clamping motions.

ICM uses significantly lower injection pressures than standard injection molding so longer fibers remain in the finished part. That translates into better mechanical properties. For example, ICM maintains the 12.7 mm fiber length of commercial composites, resulting in finished parts with higher impact strength and more iso-tropic mechanical properties. In typical applications, impact strength improves 15 to 20% in 3.2-mm wall thickness and over 50% in 1.5-mm wall thickness.

Thus, one major benefit of ICM is that it can produce thinner-walled, long-fiber-reinforced parts previously unattainable with injection molding. This is an important consideration in automotive applications where companies want to reduce weight and still maintain high stiffness and impact strength.

However, ICM start-up costs may be higher. The tooling, for example, depending on part and size could cost 10 to 15% more. ICM also requires a press with a second-stage compression stroke, as well as precise clamping, accurate shot-size control, and speed control during secondary clamping.

Costs to convert standard injection-molding presses to ICM vary with the type of machine, age, sophistication of the controller, and whether it has precision linear-position encoders to determine clamp and screw locations. In most cases the cost to upgrade a newer injection-molding machine is justified by the reduction in part cost.

Parts can also incorporate ribs, bosses, gussets, and through-holes. ICM is particularly suited to relatively flat parts, such as automobile load floors, sunroof liners, seat backs, and door panels.

Another new process is the plastic injection molding method of forming in which heat-softened material is forced under pressure into a cavity, where it cools and takes the shape of the cavity. The molding operation can be completed in one step because details such as screw threads and ribs may be easily integrated into the mold. High process repeatability is important to economical operation, because it means that few variables must be monitored and adjusted during the process. The key benefit of electric molding technology (EMT) is that it delivers this repeatability, while improving productivity and quality.

FIGURE I.2 Injection-Compression Molding.

Repeatability improvement is the reason that will likely also pull EMT into the mainstream of injection molding. This success is based on the fact that electric machines can control variables much more closely than is possible at comparable cost with hydraulic machinery. This tighter process capability translates into a variety of benefits, including less scrap, lower labor costs, and higher quality.

The essential "repeatability potential" for EMT is inherently higher than that of hydraulic power. The reason for this is that hydraulic drives are typically distributed systems, involving a compressible fluid and a complex network of hoses, tubes, and valves that enable one or two pumps to drive all machine axes. Therefore, any conditions that affect fluid or flow properties also affect positioning of the machine.

By contrast, an electric machine has a motor for each axis. In this case, "axis" may be defined as any motion that is controlled through a feedback loop on the basis of variables such as time, position, pressure, or velocity. On an injection-molding machine, the main linear axes involve clamping, ejection, injection, and sled pull-in. Rotary axes may control extrusion, and in some cases, die-height adjustment.

An all-electric powertrain on one of these axes may consist of a belt, two pulleys, and a ballscrew. With a separate motor for each axis, electric machines have the ability to drive and coordinate all axes of motion simultaneously, a significant advantage over hydraulic machines in some applications. Whatever its configuration, the electromechanical powertrain is rigid, "solid-on-solid."

Currently, EMT is more expensive than hydraulic machines, as is true with most newer technology. However, when comparing prices with hydraulic machines, specifications and capabilities should be balanced. By the time circuits and controls on a hydraulic machine are enhanced to approach the performance of a general-application electric, the cost difference narrows significantly — and hydraulic technology remains less precise.

Because it impacts molding costs in so many ways, hydraulic oil will be seen as a business liability as well as an environmental hazard. Any increase in electricity prices will also drive demand for EMT. In the mold-building industry, a corresponding increase is anticipated in the production of servo-electric systems to actuate core pulls and other functions. The environment of the molding plant will likely change considerably in a relatively few years, along with the standard of quality that is expected from the process.