Injection molding holding pressure

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. 

A typical injection pack/hold pressure during injection molding of polyamide 66 components is 1,000 bar and the injection temperature is 280°C. The gate freezes shut when the average temperature inside the mold reaches 225°C. 

For the injection molding machine setter, internal mold pressure curves of this type are an invaluable aid for finding the optimum times and pressures for the injection- and holding pressure-phases. This kind of graph enables one to find much data regarding the quality of production run and moldings alike. 

For the material within the mold cavity, the holding pressure phase is concluded as soon as the sprue solidifies-in other words, as soon as it is sealed. From this point on, no more material can enter the cavity. During the injection and holding pressure phases, the material in the cavity has already begun to cool against the relatively cold mold wall. The outer layers have soldified very quickly. The cooling time therefore starts as early as injection. 

Melt temperature Mold temperature Injection pressure Injection speed -Holding pressure ... 

Keywords injection molding, processing parameters, mold temperature, melt temperature, injection speed, holding pressure, skin-core morphol-ogy, spherulites, molecular orientation, a phase, 3 phase. Young s modulus, structure-property relationships, SCORIM, pol)nnorphism. 

Melt temperature Mold temperature injection time holding pressure time holding pressure... 

In micromolding, both surface patterns were replicated to the ratio HR over 0.95 at standard molding condition. HR decreased with increasing distance from the gate in the case of 50 and 100 pm in depth. We could conclude that replication properties were affected by cooling rate and final polymer flow. Moreover, HR increased with increase of injection speed, holding pressure, injection temperature, and mold temperature. 

Suspension Polymers. Methacrylate suspension polymers are characterized by thek composition and particle-size distribution. Screen analysis is the most common method for determining particle size. Melt-flow characteristics under various conditions of heat and pressure are important for polymers intended for extmsion or injection molding appHcations. Suspension polymers prepared as ion-exchange resins are characterized by thek ion-exchange capacity, density (apparent and wet), solvent sweUing, moisture holding capacity, porosity, and salt-spHtting characteristics (105). 

Injection molding involves two distinct processes. The first is melt generation, mixing, and pressurization and flow, which is carried out in the injection unit of the molding machine. The second is product shaping, which takes place in the mold cavity. Injection molders, therefore, have two distinct parts the injection unit and the mold/clamping unit. The function of the injection unit is to melt the polymer and inject it into the mold the function of the clamping unit is to hold the mold, open and close the mold, and eject the finished product. 

The injection molding cycle is depicting in Figure 7.74. We can begin the cycle at any point we wish, but let us start at the point the screw moves forward and fills the mold with polymer melt. The screw moves forward and fills the mold with melt and maintains the injected melt under pressure, during what is called the hold time. To ensure that polymer does not flow backward, a check valve is attached to the end... 

An injection molding machine is operated by hydraulic power and equipped with an dectric motor and hydraulic pump. The maximum hydraulic oil pressure is ca 14 MPa (2000 psi). A hydraulic cylinder opens and closes the mold and holds the mold dosed during injection another cylinder forces the screw forward, thereby injecting the mdt into the mold. A separate hydraulic motor turns the screw to plasticate, homogenize, and pressurize the mdt. Control of these movements is a combined function of the hydraulic and electrical systems (35—37). 

The sequence of events during the injection molding of a plastic part, as shown in Fig. 3.40, is called the injection molding cycle. The cycle begins when the mold closes, followed by the injection of the polymer into the mold cavity. Once the cavity is filled, a holding pressure is maintained to compensate for material shrinkage. In the next step, the screw turns, feeding the next shot to the front of the screw. This causes the screw to retract as the next shot is prepared. Once the part is sufficiently cool, the mold opens and the part is ejected. 

Various amounts of filler contents are available typically, 33% glass fiber is used. Tg of 143°C and melting temperature of 342°C, with a continuous use temperature of 260°C, are observed without thermal degradation [11], This material dissolves in concentrated sulfuric acid and is not chemically attacked by water or pressurized steam. Most importantly, it can be injection molded and is capable of holding... 

Increase in or effect Melt temperature Holding pressure Holding time Injection speed Mold temperature... 

A typical arrangement for a direct screw-transfer injection-molding machine for thermosets is shown in Figure 2.16. The machine has two sections mounted on a common base. One section constitutes the plasticizing and injection unit, which includes the feed hopper, the heated barrel that en closes the screw, the hydraulic cylinder which pushes the screw forward to inject the plasticized material into the mold, and a motor to rotate the screw. The other section clamps and holds the mold halves together under pressure during the injection of the hot plastic melt into the mold. 

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