Molding processes mold temperature

Rotomolding. Nylon-6, nylon-11, and nylon-12 can be used in rotomolding and are generally suppHed for these appHcations as a powder or with a small pellet si2e. The process involves tumbling the resin in a heated mold to form large, thin-walled mol dings. Nylon-11 and nylon-12 use mold temperatures of 230—280°C and nylon-6 is processed at over 300°C. An inert gas atmosphere is preferred to avoid oxidation. 

The PEEK resia is gray, crystalline, and has excellent chemical resistance T is ca 185°C, and it melts at 288°C. The unfilled resia has an HPT of 165°C, which can be iacreased to near its melting poiat by incorporating glass filler. The resia is thermally stable, and maintains ductiUty for over one week after being heated to 320°C it can be kept for years at 200°C. Hydrolytic stabiUty is excellent. The resia is flame retardant, has low smoke emission, and can be processed at 340—400°C. Crystallinity is a function of mold temperature and can reach 30—35% at mold temperatures of 160°C. Recycled material can be safely processed. Properties are given ia Table 16. 

The standard injection molding machine used had a screw diameter of 30 mm and the aspect ratio of 23.70. The barrel temperature profile was 270, 280, 290, and 295°C. The mold temperature was about 90°C. The injection molded tensile samples were processed according to the CAMPUS specification (Computer Aided Materials Preselection by Uniform Standards) [24] and DIN 53455 Form 3. To obtain the different flow conditions, four groups of samples were injection molded by varying melt... 

A distinction should be made between machine conditions and processing variables. Machine conditions are basically temperature, pressure, and processing time (such as screw rotation/rpm, and so on) in the case of a screw plasticator, die and mold temperature and pressure, machine output rate (lb./hr), and the like. Processing variables are more specific such as the melt temperature in the die or mold, melt flow rate, and pressure used. 

The basics observed in molded products are always the same only the extent of the features varies depending on the process variables, material properties, and cavity contour. That is the inherent hydrodynamic skin-core structure characteristic of all IM products. However, the ratio of skin thickness to core thickness will vary basically with process conditions and material characteristics, flow rate, and melt-mold temperature difference. These inherent features have given rise to an increase in novel commercial products and applications via coinjection, gas-assisted, low pressure, fusible-core, in-mold decorating, etc. 

Minimizing the cycle time in filament wound composites can be critical to the economic success of the process. The process parameters that influence the cycle time are winding speed, molding temperature and polymer formulation. To optimize the process, a finite element analysis (FEA) was used to characterize the effect of each process parameter on the cycle time. The FEA simultaneously solved equations of mass and energy which were coupled through the temperature and conversion dependent reaction rate. The rate expression accounting for polymer cure rate was derived from a mechanistic kinetic model. 

Many physical and process constraints limit the cycle time, where cycle time was defined as the time to the maximum exotherm temperature. The obvious solution was to wind and heat the mold as fast and as hot as possible and to use the polymer formulation that cures most rapidly. Process constraints resulted in a maximum wind time of 3.8 minutes where wind time was defined as the time to wind the part plus the delay before the press. Process experiments revealed that inferior parts were produced if the part gelled before being pressed. Early gelation plus the 3.8 minute wind time constrained the maximum mold temperature. The last constraint was based upon reaction wave polymerization theory where part stress during the cure is minimized if the reaction waves are symmetric or in this case intersect in the center of the part (8). The epoxide to amine formulation was based upon satisfying physical properties constraints. This formulation was an molar equivalent amine to epoxide (A/E) ratio of 1.05. 

At an A/E ratio of 1.05, wind time of 3.8 minutes and molding temperature of llO C, the curing profiles of the part were simulated varying the press temperature until the maximum exotherm temperature occurred at the center of the part. This condition was achieved at a press temperature of 135 C. The minimal cycle time at the optimal processing conditions was simulated to be eight minutes. 

The activated carbonyl of anhydrides can acylate alcohols or amines at the temperatures necessary for polymer processing. These reactions have been verified by HPLC using the polymer system described in Table 2. An examination of the HPLC chromatograms in Fig. 25 indicates that the phthalic anhydride peak (3.2 min) diminishes with increasing injection-molding temperatures and that two new peaks (4.6 and 6.9 min) increase in intensity. These new peaks corresponded to the half phthalate esters of 1,6-hexanediol and trans-... 

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. 

FIRE RETARDANT FILLERS. The next major fire retardant development resulted from the need for an acceptable fire retardant system for such new thermoplastics as polyethylene, polypropylene and nylon. The plasticizer approach of CP or the use of a reactive monomer were not applicable to these polymers because the crystallinity upon which their desirable properties were dependent were reduced or destroyed in the process of adding the fire retardant. Additionally, most halogen additives, such as CP, were thermally unstable at the high molding temperatures required. The introduction of inert fire retardant fillers in 1965 defined two novel approaches to fire retardant polymers. 

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. 

BPA polycarbonate becomes plastic at temperatures around 220°C, The viscosity decreases as the temperature increases, exhibiting Newtonian behavior, with the melt viscosity essentially independent of the shear rate, At the normal injection molding temperature of 270-3l5°C, the melt viscosity drops from 1.100 to 360 Pa j s (11.000 to 3,600 poise). Because the viscosity of polycarbonate can only be reduced by increasing the temperature, the ultimate limit on molecular weight is controlled by the processing conditions and the thermal stability of the polymer. 

For the polyurethane composition considered here the maximum (adiabatic) temperature jump is equal to 25 K. This means that at ordinary working temperatures of the mold, i.e., 40 - 80°C, the maximum increase in the material temperature does not reach an unacceptable level of close to 200°C, where thermal degradation of the polymer can begin. We can restrict the maximum temperature growth Tmax by using the mathematical model based on Eqs. (4.10) - (4.13) and then constructing Tmax - vs - to curves for various mold temperatures. These provides a means of choosing the optimum process parameters when the temperature must not exceed Tmax. 

If for any real situation K < Kiim, as determined from Fig. 4.62, then the pressure droji during processing will not exceed the permissible level. Fig. 4.62 also shows that decreasing Gz 1 and increasing the mold temperature results in a considerable increase in Kiim... 

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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