| 摘要 |
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.
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