Mold cavity temperatures

Time, temperature, and pressure are the primary factors influencing the surface flnish of the molded part. Time factors include rate of injection, duration of ram pressure, time of cooling, cross-linking time, and rotation of the screw (RPM). Pressure factors are applied pressure (high or low), back pressure on the extruder screw, and pressure loss before the charge enters the cavity, which can be caused by a variety of restrictions in the mold cavity. Temperature factors are mold (cavity and core), barrel, and nozzle temperatures, as well as the melt temperature because of back pressure, screw speed, and frictional heat. 

Figure 14.16 Common probes for monitoring mold body and mold cavity temperatures and pressure. (Plastics Technology Laboratory, Northern Illinois University, DeKalb, IL).

In any case, a uniform mold cavity temperature is important when manufacturing SMC parts. For instance, Barone and Caulk [21] have shown that... 

These fabrication techniques are used primarily with thermosetting resins. With both, the molding compound is heated to soften it enough to flow under pressure into a mold cavity. Temperatures around 150"C are typical for the most commonly used materials. The compound then is held in the mold at pressures around 2000 psi or more for a sufficient lime to promote cross-linking and polymerization. This results in a hard, rigid product. In many cases the mold charge is preheated to shorten the cure time. 

Typical injection molding stock temperatures for Vamac compounds at the nozzle should be in the range of 70°C-85°C. The cold runner systems should be set in the same range of 70°C-85°C. Scorch can be a serious problem if the temperature exceeds 100°C-110°C. Mold cavity temperatures are typically 175°C-190°C for injection molding cycle times of 1-3 min. 

Research has also focused on applying various control techniques to vary the part cavity temperature. Catic [6] suggested that mold thermal control should be based on the measurement of the mold cavity temperature instead of the inlet and outlet coolant temperatures. Gao [7] investigated the control of part average temperature over the cooling cycle. A Dahlin controller was implemented that used coolant temperature as the manipulated variable to obtain good control performance of the part average temperature. 

Weld line Weld or knit fines where two parts of a melt join while flowing into the mold cavity can result in problems. The quality of the weld depends on the temperature of the material at the weld point and the pressure present in the melt after flowing from the gate. The higher the temperature and pressure, the more complete the weld and the better the product performance and appearance. Bringing the material to the weld point at a higher temperature and pressure requires rapid filling of the mold cavity (Chapter 3, BASIC FEATURE, Weld Line). 

As an example the shrinkage and in turn its tolerance of injection molded TPs will be affected as follows. (1) Higher cavity pressures will cause lower shrinkages. (2) Thick sections will shrink more than thin ones. (3) A cooler at the time of the product being ejected from the mold cavity will bring about a lower shrinkage. (4) A melt temperature of the material at the lower end of the recommended... 

Even though most of the literature on processing specifically identifies or refers to thermoplastics (TPs) as in this book, some thermosets (TSs) are used (TS polyesters, phenolics, epoxy, etc.). The TPs reach maximum heat prior to entering cool mold cavities, whereas the TSs reach their maximum temperature in hot molds (Fig. 6-3). 

Contact molding Also called open molding or contact pressure molding. It is a process for molding RPs in which the reinforcement and plastic are placed in a mold cavity. Cure is either at room temperature using a catalyst-promoter system or by heating... 

Metal molds and cores are used in permanent mold casting. The process works best in continuous operation so that the mold temperature can be maintained within a fixed operating range. The operating temperature of the mold is one of the most important factors in successful permanent mold casting. Mold cavities are machined from solid blocks of graphite. Mold life is the major cost factor in permanent mold casting. 

The flow characteristics can be determined using the model shown in Figure 7.79. During the time interval nnder consideration, it is assumed that the polymer has been heated to a uniform temperatnre, T , that is equivalent to the mold wall temperature. As long as the preform radius, R, is less than the radius of the outer wall of the mold cavity, Ro, we can treat the problem as an isothermal radial flow of an incompressible power law flnid flowing between two disks that approach each other at a constant rate, h. In this way, the velocity field, Vr z, r, t), the pressnre distribution in the mold, P z, r, t), and the plnnger force, Fn z, r, t) can be obtained as follows ... 

Press type. Pasadena Hydraulics, Inc., Model P-210 Mold cavity. 6 X 7 X 0.083 inch Press temperature. 180°C. 

The time required to cure the materials is dependent entirely on the method of molding, mold temperature, and material temperature when introduced into the mold cavity. There are now four basic methods of molding thermoset molding compounds (1) Compression, (2) Transfer, (3) Injection, and (4) Extrusion—with the method most commonly used as rated. Very small quantities are processed by extrusion. All methods of molding may be done automatically or semi-automatically. In most cases, the injection method is practically all done automatically. 

Figs. 4.63 - 4.66 illustrate the location of lines of constant values of temperature, degree of conversion, velocity and viscosity for five consecutive positions of the front of a stream, which correspond to the following values of the axial coordinate xf 0.2, 0.4, 0.6, 0.8, and 1.0. These lines of constant values of the process variables are calculated for the flow and property values designated by the point D in Fig. 4.61. In this case, the mold temperature Tm = 70°C, the initial temperature of the reactive mix To = 40°C, and the initial temperature of the insert Ti = 20°C. An area above the horizontal line of symmetry of the mold cavity (i.e., the upper part of the cavity) contacts the "hot" surface of the mold and the lower part is in contact with the surface of the cooler metal insert. Thus, we can conclude that the distributions of temperature, degree of conversion, viscosity and velocity of movement of the reactive mix along the mold are related to the ratios between the transfer rate and the chemical reaction, which are characterized by the values of the Da and Gz Numbers. 

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