Flash Formation Research Articles

Injection mold flashing of liquid crystalline polymers

AbstractThermotropic polyesters, such as Vectra (Hoechst Celanese), have excellent moldability for intricate parts that require high precision of form, such as electronic connectors. Two apparently contradictory aspects of molding behavior contribute to the moldability. On the one hand, the low viscosity of the liquid crystalline polymer (LCP) at high shear rates favors ease of filling molds that contain long, thin paths. On the other, parts molded from LCP have little or no flash to interfere with the functioning of the parts.There has apparently been little work on the rheological aspects of flash formation. An approximate analysis is made by considering that the flash is the result of melt being extruded from the mold cavity into a slit at the mold parting line. The driving force for the extrusion is the injection pressure. The flow is assumed to be isothermal until solidification occurs, at a time that depends on the thickness of the slit, on the thermal diffusivity of the melt, the melt and mold temperatures, and on the solidification temperature of the material. The viscosity is assumed to have power‐law dependence on shear rate. It is found that when the aspect ratio (length to thickness) of the flash is small, its length is strongly dependent on the magnitude of the pressure drop at the contraction from the cavity to the slit.At the minimum pressure required to fill a mold, the flash length is predicted to be independent of the rheological and thermal properties of the melt, except for the power‐law exponent. Differences in end correction can, however, account for different tendencies to flash at equal moldability.Comparison of the model with Richardson's analysis of freezing in a cavity suggests a correlation of the thermal properties of the melt with his parameter c, which is related to mold filling ability. Tests of the model and possible refinements are suggested.

普通模型アルミニウムダイカストに生成する鋳ばり

The flash formation times in four types of casting processed by four types of pressure die casting machines (500-1650tons) were studied by measuring the metal flow behavior in the die cavity and the plunger stopping time. The flash in the product area and near the overflow area was formed only when molten metal was filling in the cavity. The flash near the biscuit and runner area, however, occurred after the molten metal had filled in the cavity, as well as occurring while the molten metal was filling in the cavity. This result has also been demonstrated by the cross section structure of the flash.

Rethinking the injection molding process with thermosets and rubber

AbstractTo optimize the processing, a process model has to be developed. To be able to use a model of this kind, we must also have material data (thermodynamic material data, viscosity, processing time) and must install sensors on the machine and in the mould.The flow properties of thermosets/elastomers under processing conditions is determined not only by pressure, temperature and throughput, but also to a considerable extent by the boundary conditions.At a given throughput, it is possible to calculate the pressure loss for passing through a runner system by dividing up the flow into a plug flow and a boundary shear flow. This takes into account the pronounced temperature profile, which brings about a low‐viscosity outer layer and a high‐viscosity core.With thermosets/elastomers, the compounding process influences the flow properties to a considerable extent. This means that the continuous monitoring of the material state must be included in the process control. This monitoring can be carried out during the mould filling period. Since the mould is filled over a wide area at constant velocity, the gate can be regarded as a “rheometer” and the pressure loss between two pressure sensors installed in the gate used as a measure of the viscosity.Starting from this measurement of the flow properties, moulding properties such as weight and surface texture are influenced in the compression phase. The form of the pressure build‐up in the compression phase affects the flash formation; this pressure buildup should, in fact, be carried out in such a way that the pressure maximum is not reached until the outer layers have passed through their viscosity minimum.After the compression, the moulding is crosslinked in the crosslinking phase far enough for it to be demoulded or for the moulding properties, which are connected with a given crosslinking profile over the cross‐section, are reached.