Special Injection Molding Processes

The demands for means of producing good quality complex moldings as cost-effectively as possible, confronted manufacturers of plastics processing machinery with a major task. As this challenge could no longer be met by conventional injection molding methods alone, special processes were called for. Some of these are briefly explained in the subsequent text. 

In an ideal case, an article molded by this process possesses a light, cellular internal structure and a smooth, all enveloping outer skin. This enables high mechanical strength to be achieved, combined with low weight. The process is usually employed with moldings of large volume, e.g. parts for the furniture industry. 

In principle, all injection molding techniques, by which two or more materials are being processed, represent the multi-injection process. As only two materials are employed quite frequently, that is also known as two-component injection molding. 

When producing articles by the sandwich injection molding technique for instance, new material is mostly used for forming the outer skin, whereas recycled material can be employed as core component. This enables a substantial amount of recycling material to be used, without adversely affecting the finished article s appearance. 

With the laminating method, two components are injection molded so as to become bonded to each other by lamination. In this case, the injected materials may be of different grades. Or they can be of an identical component, but differently colored. 

---

TFI is one of the oldest special injection molding processes. This method is used to produce large, thick-walled, rigid parts with a thick outer layer over a foamed core. 

Special Injection Molding Processes Review Questions... 

Special injection molding process for producing decorated parts... 

Special injection molding process, where at least two materials are molded next to each other (butt-on method) or layered, one above the other... 

GIT) Special injection molding process. A polymer is injected into the mold first, followed by an inert gas, which is introduced separately... 

Special injection molding process, with injection taking place while the mold is still partially open. By then closing the mold fuUy at clamping/locking pressure, the melt is compression molded into the respective article... 

Special injection molding process for the production of moldings possessing a cellular internal structure and a smooth external surface... 

IWo specialized injection-molding processes will be considered in some detail. These processes are structural foam and reaction injection molding. In the case of structural foam, a gas is distributed throughout the molded plastic to form a foam. When cooled, the product is a structural thermoplastic foam. The reaction injection-molding process involves mixing two reactive components and injecting them into a mold, where the product is formed. 

A number of specialized injection molding processes also exist and are outlined below. 

The plasticator on an injection-molding machine is a specialized plasticating single-screw extruder. The plasticator has two main differences there is a nonreturn valve on the tip of the screw, and the screw retracts as molten material accumulates between the nonreturn valve and the end of the barrel. Pressure is maintained on the accumulated material by a constant force applied to the shank of the screw via the drive system. This force is typically measured as a pressure applied to the shank and is referred to as the back pressure. During the injection step of the process, the screw is forced forward, the nonreturn valve closes, and the material is injected into the mold. Additional information on the injection-molding process can be obtained elsewhere [Ij. 

Most thermosets are molded with the compression or transfer molding process (tires, dinnerware, electrical components, etc.). Some products, usually iimdler ones, can be made using the injection molding process in specially 1 or modified injection molding machines. Some thermosets are used as Is, applied to sheets or mats of glass and other fibers, and then cured under low) pressure and heat. (Examples are plywood, particle board, structures, etc.)... 

When articles hitherto produced from classic materials (such as wood or metal) are to be made by the injection molding process, it is advisable, to give serious consideration to the special properties of plastics materials, as well as the more salient points of injection molding as a production method. This plastics-oriented procedure requires an understanding of the basic principles of the manufacturing and production processes involved, and also of the plastics material s behaviour. The book provides a comprehensive overview of the principles involved in injection molding. 

Requirements of special performance of plasticizers used in the injection molding process are seldom. In optical disk molding, the material requites mote plasticization during processing than ditring the product use. For this reason carbon dioxide was used as an additional plasticizer. Primary plasticizers were selected in such a manner that they will facilitate dissolution of carbon dioxide in melt. Figitre 14.17 shows that the more carbon dioxide dissolves, the lower is the glass transition tern-process, melt viscosity is decreased and... 

Fig. 8-24 Special two stage foam injection molding process [21].

The transition fi om room temperature to a high processing temperature may decrease a plastic s density by up to 25 percent. Cooling causes possible shrinkage (up to 3 percent) and may cause surface distortions or voiding with internal frozen strains. As discussed in other chapters, this situation can be reduced or eliminated by special techniques, such as cooling under pressure in the injection-molding process (see Chapter S). 

Like thermosets the elastomers will only cross-link in the mold by introducing additional heat. But Thermoplastic Elastomers (TPE) can be processed like thermoplastic materials due to their thermoplastic matrix. The common cross-linked elastomers are highly viscous in the injection unit, with exception of the silicone materials which are processed as liquids (LSR = Liquid Silicone Rubber) in the injection molding process (chapter 2.3 cold runner) using a special injection unit. All elastomers tend to the formation of flashes at the molded parts due to the decreasing viscosity in the... 

The melt temperature is known to be a key variable in the injection molding process that directly affects the materials viscosity, flow behavior, shrinkage, and the resulting part quality. Accurate melt temperature sensors and careful interpretation of the data are critical both for the control and the simulation of the injection molding process. The melt temperature in the machines nozzle is of special interest because it helps characterize the polymers state just prior to entering the mold and is independent of the part and mold geometry. 

Turng, L.-S., Special and Emerging Injection Molding Processes. Journal of injection molding technology, SEPTEMBER 2001. 5(3) p. 160-179. 

The injection molding processing window is rather narrow, and special melt processing and processing parameters are needed to avoid brittleness and achieve acceptable properties. It is necessary to minimize the combined effect of melt residence time and elevated... 

---