Justificación de los dispositivos de lamas

Mold designers should verify that the mold can be filled given the cavity geometry and the material properties. However, the filling analyses require the processing conditions including the melt temperature and either the linear velocity or volumetric flow rate of the melt. It is recommended that mold designers assume a melt temperature in the middle of the melt temperature range recommended by the material supplier since this provides the molder with some freedom to adjust temperatures up or down to correct molding problems or reduce cycle time.

The true melt flow rate is not known until after the mold is made and commissioned. The maximum flow rate is typically bounded by the maximum ram velocity of the molding machine, or molding defects caused by high flow rates such as flash, jetting, or burn marks.

The minimum flow rate is typically bounded by the premature solidification of the melt in the mold cavity which results in a short shot. Typical linear velocities of the melt through the mold range from 0.01 to 1 m/s depending on the specifics of the molding application.

105

Thin wall applications will generally have higher linear flow velocities since they require a faster injection to avoid premature solidification, and

their thinness provides for faster linear velocities given the same volumetric flow rate from a molding machine.

Melt flow rates may be estimated by computing the volume of the mold cavities and runners and dividing by the estimated filling time. This approach works well for those practitioners with experience, but may not work well for new molding applications having very different geometries or material properties. Alternatively, additional analysis can lead to a recommended flow rate that balances the amount of shear heating with the heat loss from the melt to the mold. This result should provide not only a reasonable estimate of the melt flow rate, but also a more accurate analysis since it will tend to produce a uniform melt temperature as the melt fills the mold.

The derivation of the melt velocity is provided in Appendix F. For a Newtonian material, the recommended velocity is:

where T_meltand T_wallare the melt and mold wall temperature,κ is the thermal conductivity of the plastic melt, and μ is the Newtonian viscosity. Since the viscosity is a function of the shear rate and velocity, it is necessary to recompute the shear rate and viscosity until the velocity converges.

Example: This analysis will now be applied to the laptop bezel, which has a wall thickness of 1.5 mm and is to be molded of ABS (Cycolac MG47) at a melt temperature of 239 °C and a mold coolant temperature of 60 °C. For the purpose of the analysis, we will initially assume that the linear velocity is 0.5 m/s. At this velocity, the shear rate is computed as:

γ= = ⋅ = ⁻

At this shear rate, the Cross-WLF model provides a melt viscosity of 120 Pa s. This value can then be used to provide a new estimate of the recommended injection velocity:

= − =

Additional iterations are useful to hone in on the recommended velocity. At a velocity of 0.69 m/s, the shear rate is 2,760 1/s. The viscosity at this shear rate is 95.4 Pa s, which in turn suggests a linear melt velocity of 0.77 m/s. A further iteration would yield a shear rate of 3,080 1/s, a viscosity of 88.1 Pa s, and a melt velocity of 0.80 m/s. With additional iterations, the solution will converge to a final velocity of 0.82 m/s. Since the flow length is approximately 0.2 m, the mold cavity for the laptop bezel will fill in approximately 0.25 s

•

•

5.5 Cavity Filling Analyses and Designs

106 5 Cavity Filling Analysis and Design

(not including the runner system). Since the cavity volume is 30 cc, this corresponds to a volumetric flow rate at the nozzle of 125 cc/s.

As implied by the form of Eq. (5.23), the recommended velocity will vary with the melt temperature, the mold temperature, the thermal conductivity of the melt, and the melt viscosity. Higher temperature differences between the melt and wall temperatures, as well as higher thermal conductivity of the polymer melt, require faster melt velocities to maintain a uniform melt front temperature. Lower viscosity materials require a higher melt velocity to generate the shear heating needed to avoid excessive heat loss to the melt.

While the melt velocity does not appear to vary with wall thickness, the effect of wall thickness is considered through the inclusion of the viscosity which is a function of the shear rate. As the wall thickness decreases, the increasing shear rate reduces the viscosity, which thereby requires higher melt velocities to avoid cooling the melt. As expected, higher melt velocities are required as the wall thickness decreases. Figure 5.11 plots the recommended melt velocity for ABS as a function of melt temperature and wall thickness using the analysis. It is observed that the melt velocity can vary from about 0.4 m/s for a molding application with a wall thickness of 3 mm and a melt temperature of 218 °C to about 1.6 m/s for a molding application with a wall thickness of 0.8 mm and a melt temperature of 260 °C.

While there is a significant range in the recommended melt velocity as a function of the molding application, it is important to recognize that the exact melt velocity and flow rate that will actually occur during the molding process is unknown. The objective should be to provide a reasonable estimate of the melt velocity and filling time, and design the mold to operate under a wide variety of conditions. While the foregoing analysis may seem unnecessarily complex compared to simply assuming a filling time based on experience, the analysis is objective and provides a quantitative result that provides insights to the design and use of injection molds.

Figure 5.11: Recommended melt velocity for ABS as a function of wall thickness and temperature

107