Molds importance factors

In addition to the process parameters, there are large numbers of factors that decide the success of an injection molding operation. Mold design, product design, position, and size of gates, type and positioning of runners, etc., are extremely important factors to be taken care of. A detail consideration regarding these factors has been discussed below [213]. 

Common problems like insufficient filling-packing and poor dimensional control are often related to the gate size and design. Similarly, gate location is another important factor. They should be located in areas having heaviest cross-section of the part to assure fill-out and elimination of sink marks. Also their position should not facilitate the residual molded stress formation in the part, knit line formation. 

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. 

In building mathematical models of product formation in a mold it is possible to treat a polymeric material as motionless (or quasi-solid), because the viscosity grows very rapidly with the formation of a linear or network polymer thus, hydrodynamic phenomena can be neglected. In this situation, the polymerization process itself becomes the most important factor, and it is worth noting that the process occurs in nonisothermal conditions. 

All experimental results reported in this paper were obtained with molded samples for a theoretical reason. These experiments should be repeated with pellet samples so that the experimental results could be more directly applied to actual extrusion operations. The energy efficiency will become an important factor in the future in designing a new extruder for a given output rate as energy cost increases. 

The parts are obtained by injection of molten aluminum into steel molds. The molds weigh some tons and are expensive to produce. Maximization of their lifetime is an important factor. 

Viscosity is an important factor. Standard PDMS oils spread well and form an efficient release film on the mold surface, but they do not perform in this application when paintability of the demolded articles is required. 

The process as it is practiced now actually produces a ceramic shell of modest thickness (about 0.2-0.5 in. thick) around the wax pattern. The relatively thin shell when compared to solid mold casting not only saves material but also allows for more uniform and rapid cooling of the metal parts. This is an important factor in maintaining fine grain size that results in stronger parts. 

Another important factor in mold temperature control is the material the mold is made from. Beryllium copper has a high thermal conductivity, about twice that of steel and four times that of stainless steel. A beryllium copper cavity should thus cool about four times as fast as a stainless steel one. A mold made of beryllium copper would therefore run significantly faster than one of stainless steel. 

Since ejection involves overcoming the forces of adhesion between the mold and the plastic, the area provided for the knockout (KO) is an important factor. If the area is too small, the KO force will be concentrated, resulting in severe stresses on the part. As a result, the part may fail immediately or in later service. In materials such as ABS and high-impact polystyrene, the severe stresses can also discolor the plastic. 

Perhaps the most important factor to a process engineer in predicting extrusion or molding behavior is melt viscosity. Several methods are used to obtain the viscosity of polymer solutions and melts experimentally as a function of shear rate [19]. Instruments for making such measurements must necessarily accomplish two things (1) the fluid must be sheared at measurable rates, and (2) the stress developed must be known. Two kinds of instruments having simple geometry and wide use a rotational viscometer and capillary or extrusion rheometer. 

Typical thermoplastic binders which are found in literature for injection molding of ceramic bodies are, styrene-butadiene, polyethylene, polypropylene, polybutene, ethylene vinyl acetate, polymethylmethacrylate and polyoxymethylene. When selecting one of these binders for thermoplastic extrusion of ceramic bodies, it should be noted that the shrinkage of par-tially-crystalline polymers is higher than for amorphous polymers, and hence warping during cooling is more critical in the former case. This is, however, not the only criterion for selection price and processability at adequate temperatures are also important factors to consider. 

The increasing use of plastics with abrasive fillers and reinforcements created a demand for an even more abrasion resistant barrel than the standard iron/boron type. The use of glass fiber reinforced compounds for injection molding has been the single most important factor since a fabricator would be lucky if they could reach 6 months of continuous operation. This need has been successfully answered by the development of liner materials containing metallic carbides such as tungsten carbide and titanium carbide extending their life. 

Stripping The designer must consider whether the product can be stripped from the threads. Stripping is the easiest (and often the lowest cost) solution for ejecting molded products however, the ease of stripping depends on many equally important factors. 

Shrinkage during drying is one of the most important factors for the quality control of molded products. It is not only related to the deformation and the generation of crack, warp. 

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