Mold filling pressure profiles

In order to judge performance capabilities that exist within the controlled variabilities, there must be a reference to measure performance against. As an example, the injection mold cavity pressure profile is a parameter that is easily influenced by variations in the materials. Related to this parameter are four groups of variables that when put together influences the profile (1) melt viscosity and fill rate, (2) boost time, (3) pack and hold pressures, and (4) recovery of plastica-tor. TTius material variations may be directly related to the cavity pressure variation. Details on EQUIPMENT/PROCESSING VARIABLE are in Chapter 8. 

Radial flow between parallel discs is a very common flow type encountered in polymer processing, particularly during injection mold filling. In this section, we seek the velocity profile, flow rates and pressure for this type of flow using the notation presented in Fig. 5.16. 

Pressures above 15 MPa help fill the mold before its components such as the mandrel and casings begin to cool. Above 25 MPa melt pressure, the mold part may crack at the gate. The equipment should allow control of transfer rate and transfer pressure. Once the filling is nearly complete (90%-95%), the transfer pressure should be lowered to half to two-thirds of its initial value to avoid stress concentration in the gate area. The pressure profile has to maximize hold pressure (Fig. 6.57) for packing without crack formation at the gate. 

Figure 10.9. Topical pressure profiles during injection molding. and stand respectively for nozzle and mold cavity pressure. Stages are 1 - dead time, 2 - filling, 3 - packing, 4 - cooling, 5 - ejection.

Fast injection speeds may be used, however to avoid high molded-in stresses profiled, or programmable, speeds are preferred. When molding thick sections, very slow, initial mold filling speeds are recommended these should be followed by relatively high dwell pressures. 

This gives the constant Ci in terms of Q. The pressure profile is then determined from Equation 6.16, and the time to fill the mold for a given gate pressure is found as in the preceding section. 

Figure 11.5 Cavity pressure profiles of solid molded part near gate and near end of fill.

Figure 5 Temperature profile in primary drying of dobutamine HCl-mannitol (1 1), 53 mg solids/mL, 10 mL fill volume. Vials are 5304 molded glass vials (8.3 cm2 cross-sectional area) which are placed in a flat aluminum tray. The heat flux is 42 cal/(cm2 hr), and the chamber pressure is 0.1 torr. (From Ref. 5.)...

The dip BM process bears some resemblance to IBM in that it is a single-stage process performed with a preform on a core/blow pin.The difference is in the way the preform is made. The process uses an accumulator cylinder that is fed by an extruder. The cylinder has an injection ram at one end while the other is a free fit over the blow pin. The blow pin is dipped into the melt so that a neck mold on the pin seals the end of the accumulator cylinder. The injection ram is advanced to fill the neck mold then the blow pin is withdrawn at a controlled rate so that it is coated with a melt layer extruded through the annular gap between the pin and the accumulator cylinder. The thickness of the coating can be varied or profiled to an extent by varying the speed of the blow pin and the pressure on the injection ram. After trimming, the preform is BM in the same manner used for IBM. 

Determine the pressure distribution in a cylindrical uniaxially pressed powder with a length to diameter ratio of 1.5. The powder being pressed is cohesionless with an angle of repose of 30°. The wall of the hardened steel die is well lubricated with stearic acid, which gives an angle of friction with the wall of 1°. The mold was filled and tapped to a density of 54% of theoretical stress then pressed at 100 MPa. Assume that a parabolic stress profile on the die plunger is applied. 

The packing stage typically commences when the mold is filled or nearly filled with the polymer melt and provides additional resin into the mold cavity as the polymer melt cools and contracts. As the packing stage is also crucial to part quality, modern molding machines allow the melt pressure to be profiled as a function of time. The minimum packing pressure is usually set to avoid excessive volumetric shrinkage and... 

Dow developers and partners having first access to these materials have explored the use of OBCs for various industrial applications such as elastic films, fibers, soft touch materials, extmded profiles, injection molded articles, and so on. For example, polyolefin oil-filled systems used to make compounds for soft touch handles, grips, and gaskets benefit from inaeased elastic performance and temperature resistance. Other examples include adhesives that utilize OBCs with low hard block contents to obtain the soft adhesive character but at the same time offer the cohesive strength and heat resistance required for demanding pressure sensitive adhesives. ... 

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