The Injection-Molding Process

What machine movements are associated with the individual process phases  

The Injection Molding Machine (Lesson 2) The Plasticating and Injection Unit (Lesson 3) The Mold (Lesson 4) 

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We can explain the injection molding process with reference to the molding cycle illustrated in Fig. 13.2. The cycle consists of four stages injection, packing, cooling, and ejection. 

These newer PET formulations utilize not only a nucleating agent, but also a plasticizing agent [3-9], The crystallization rate of polymeric materials can be broken down into two different regions, i.e. nucleation-controlled and diffusion-controlled (Figure 15.3). In the injection molding process, hot polymer is injected... 

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. 

Rotational flow and pressure flow rate calculations for the screw geometry and process conditions are performed for the injection-molding process in the same manner as for an extrusion process. Since the plasticator of an injection-molding process is not a continuous process, the instantaneous rate must be calculated based on the time that the screw is actually rotating. The instantaneous rate is the rate that is compared to the calculated flow rates for the screw. The instantaneous specific rate is calculated as follows ... 

H.H. Chiang. Simulation and verification of filling and post-filling stages of the injection-molding process. Technical Report 62, Cornell University, Ithaca, 1989. 

G. D. Gilmore and R. S. Spencer, Role of Pressure, Temperature and Time in the Injection Molding Process, Mod. Plast., 27, 143 (April 1950) also, R. S. Spencer and G. D. Gilmore, Residual Strains in Injection Molded Polystyrene, Mod. Plast., 28, 97 (December 1950). 

There are many options for barrel material construction with most extruder barrel designed to withstand up to at least 10,000 psi (69 MPa) internal pressures higher pressure units [30,000 psi (210 MPa)] are manufactured for the injection molding processes since they operate at higher pressures. They have a minimum safety burst pressure of at least 50,000 psi (350 MPa). The need for corrosion and/or wear protection, cost, repair, or their combinations may determine the choice of materials. They can be made from a solid piece of metal. The most common material is carbon steel.6... 

In an all-too-common occurrence throughout the IM industry today, the number of molds that must be set up and optimized for high-volume part production is far outpacing the number of process engineers or trained technicians qualified to do so. It is not uncommon for a molding operation to have a small number of individuals with the education or experience to set up the injection molding process. Even those who can set up the process often do not have time to optimize it... 

Rucinski, P., Manufacturing Solutions for the Automation, Control, and Monitoring of the Injection Molding Process, SPE IMD Newsletter, Issue 59, Spring 2002. 

When filling the mold during the injection molding process, for highest brilliance the flow pattern should be parallel to the surface of the area to be viewed. This permits preferential orientation of the flakes in this area and therefore a better reflection of light. 

These are just a few examples of the enormous range of products made by injection molding. The next section will cover variations to the injection molding process. Some of these variations are so innovative they make possible unique products that would otherwise be enormously expensive or practically impossible to produce. 

With these considerations and with experience demonstrating that there is no standard design for the injection molding process, one is led to the conclusion that each process must be uniquely optimized. 

Kazmer, D.O. Barkan, P. Multi-cavity pressure control in the filling and packing stages of the injection molding process. Polym. Eng. Sci. 1997, 37, 1865-1879. 

Composite density can be expected to vary because of the uneven distribution of filler particles in the manufactured product. This is very typical of the injection molding process where filler is distributed in a complex pattern of flow. In glass reinforced polystyrene parts, manufactured by injection molding, the density varied between 0.9 and 1.4 g/cm depending on the process conditions and locations Ifom which the sample was taken. ... 

The complexity of the injection molding process, and the interdependence of the many variables involved, means that any molding defect may have several different causes, of which more than one may be present at any given time. Table 10.14 shows a list of the defects that one may encounter in injection molding. 

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