US 4,613,471 A · Extruded plastic foam density control system and method

Abstract

A system and method for the control of the density of extruded foam controls foaming based on the volume per unit time of extrudate being formed. A gear pump controls the flow of melt to the die. The pump speed provides an accurate measure of the volume per unit time of melt moving to the die. The speed and cross-sectional dimensions of the extrudate are measured to determine the volume per unit time of the extrudate. Since the unfoamed melt density times the volume per unit time of the unfoamed melt being delivered to the die is equal to the density of the foamed extrudate times its volume per unit time, a ratio of the density of foamed extrudate to unfoamed melt can be determined from the two volume per unit time calculations. Once the ratio for an appropriate foam density is determined empirically, density-affecting characteristics of the extruder can be controlled to bring the calculated ratio towards the desired ratio. If the actual density of the unfoamed melt is known, the actual density of the foamed extrudate can be determined. In that case, the extruder is controlled to bring the calculated density towards a desired density. With the gear pump run at constant speed, measurements determinative of volume per unit time of the extrudate are sufficient to control density. Alternatively, if the volume per unit time of extrudate being produced is held constant then foam density can be controlled by controlling the rate of delivery of unfoamed melt to the die.

Claims48

Dependent claims

A method of controlling the extrusion of foamed extrudate in an extrusion line having an extruder including the steps of:

measuring at least one characteristic of the foamed extrudate that is dependent on the density thereof,
controlling the flow of the unfoamed melt in the extruder to provide a known volume per unit time of the unfoamed melt being delivered to the extrusion end of the extruder,
developing from the measured characteristic of the foamed extrudate and the known volume being delivered to the extrusion end a control value dependent on the density of the foamed extrudate, and
controlling at least one operating characteristic of the extrusion line affecting density of the foamed extrudate based upon the control value.

1 more element

An electrical control system for control of density in foam extrusion in an extrusion line including an extruder that has means for the introduction of material into the extruder to form melt therein, means for introduction of

a foaming agent into the melt, a die, a barrel, means for moving the melt in the barrel towards the die, heat exchange means in association with the barrel, metering means for controlling the amount of unfoamed melt delivered to the die, and means for measuring, downstream of the die, at least one dimensional characteristic of the extruded foam,
the electrical control system including means responsive to the means for measuring at least one dimensional characteristic and exclusive of any nondimensional measured variable of the extrudate, to produce a first electrical indication that is dependent, at least in part, on the density of the extruded foam,
means for providing a second electrical indication of the amount of the unfoamed melt being supplied to the die by the metering means,
means for deriving, from the first electrical indication dependent on foam density as derived exclusively from dimensional measurement, and the second electrical indication of the amount of unfoamed melt delivered to the die as controlled by the metering means, a signal indicative of the density of the foam, and
feedback means to produce a density control signal as a function of the density indicative signal and suitable for controlling at least one density determinative operating parameter of the extrusion line to maintain the density of extruded foam within predetermined limits;
whereby extruder density control is based upon dimensional measurement as the only variable measured downstream of the die.

3 more elements

An electrical control system for control of density in foam extrusion by an extrusion line having an extruder that includes means for the introduction of material into the extruder to foam melt therein, means for the introduction of a foaming agent into the melt, a die, a barrel, means for moving melt in the barrel towards the die, heat exchange means in association with the barrel, and means for delivering a metered amount of unfoamed melt to the die, the system including:

means for measuring from the foam being extruded at least one characteristic determinative of volume per unit time of the extruded foam,
computation means having an input from the means for measuring from the foam,
the computation means comprising means for developing, based only on measurements determinative of volume per unit time, including said input, an electrically represented value dependent upon the density of the foam, and
control means responsive to the electrically represented value dependent upon the density of the foam to provide a control signal operative to alter at least one operating characteristic of the extrusion line effective to vary the foam density.

1 more element

A foam extrusion line having an extruder including means for the introduction of material into the extruder to form melt therein, means for the introduction of a foaming agent into the melt, a die, a barrel, means for moving the melt in the barrel towards the die, heat exchange means in association with the barrel, metering means for controlling the amount of unfoamed melt delivered to the die, means for measuring downstream of the die at least one characteristic of foamed extrudate being extruded determinative of the volume per unit time of the foamed extrudate, and means in controlling relation to at least one foam density affecting operating variable of the extrusion line to control the density of the foamed extrudate in dependance upon the measurement of the characteristic of the foamed extrudate, measurement downstream of the die relating to volume per unit time being the only downstream measurement used by said means in controlling relation to control density.

Dependencies are inferred from the claim text.

Extruded plastic foam density control system and method

Life of this patent

Extruded plastic foam density control system and method

Life of this patent

Abstract

Claims48