Melt flow compensation in an extrusion apparatus
First Claim
1. In an extrusion apparatus having a liquifier which receives a solid element of a material that exhibits thermal expansion, heats the material, and deposits a flow of the material through a dispensing tip thereof along a predetermined tool path at an output rate, said apparatus using a material advance mechanism to supply the solid element of material to the liquifier at an input rate which controls the output rate, a method for matching the output rate to a predetermined target output rate which is selected to achieve a desired extrusion profile of the material deposited along the tool path, comprising the steps of:
- predicting a melt flow component (QMF) of the output rate for a time step corresponding to a segment of the tool path, the melt flow being a rate of flow attributed to thermal expansion of the material heated in the liquifier; and
commanding the input rate (QiS) for that time step so as to compensate for the predicted melt flow.
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Abstract
Disclosed are a method and apparatus for matching the extrusion rate of a liquifier which extrudes a flow of material to a predetermined target output rate. The present invention adjusts the volumetric rate of material going in to the liquifier to account for a predicted melt flow component of material being extruded from the liquifier. Compensating for the predicted melt flow results in reduced error in an extrusion profile produced by material deposited along a toolpath followed by the liquifier.
187 Citations
13 Claims
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1. In an extrusion apparatus having a liquifier which receives a solid element of a material that exhibits thermal expansion, heats the material, and deposits a flow of the material through a dispensing tip thereof along a predetermined tool path at an output rate, said apparatus using a material advance mechanism to supply the solid element of material to the liquifier at an input rate which controls the output rate, a method for matching the output rate to a predetermined target output rate which is selected to achieve a desired extrusion profile of the material deposited along the tool path, comprising the steps of:
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predicting a melt flow component (QMF) of the output rate for a time step corresponding to a segment of the tool path, the melt flow being a rate of flow attributed to thermal expansion of the material heated in the liquifier; and
commanding the input rate (QiS) for that time step so as to compensate for the predicted melt flow. - View Dependent Claims (2, 3, 4, 5, 6, 7, 8)
repeating the steps of predicting and commanding for subsequent time steps.
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3. The method of claim 1, wherein the melt flow is predicted as a function of a melt flow time constant of the liquifier (τ
- MF) and a percent thermal expansion of the material (%MF).
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4. The method of claim 3, wherein predicting the melt flow component (QMF) of the output rate at the time step comprises adding the melt flow component from a previous time step (QMF t−
- 1) to a predicted change in the melt flow component from the previous time step (Δ
QMF), and wherein the input rate (QiS) for the time step is commanded according to the equation QiS=(1+%MF)(QTarget−
QMF) where Qtarget is the predetermined target output rate.
- 1) to a predicted change in the melt flow component from the previous time step (Δ
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5. The method of claim 4, wherein the predicted change in the melt flow component is given by the difference equation
- Q
MFS = % MF ⋆ Q iS t - 1 1 + % MF - Q MFS t - 1 ⋆ Δ t τ MF , where QiS t−
1 is the input rate from the previous step, and Δ
t is the duration of a time step.
- Q
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6. The method of claim 1, wherein the melt flow over time is predicted using an exponential model.
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7. The method of claim 6, wherein the exponential model is a function of a melt flow time constant of the liquifier (τ
- MF) and a percent thermal expansion of the material (%MF).
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8. The method of claim 7, wherein the exponential model of melt flow over time is given by the equation
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Q iS 1 + % MF ⋆ ( 1 - - t τ MF ) , for a step change in QiS from zero.
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9. An extrusion apparatus comprising:
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a liquifier which receives a solid element of a material that exhibits thermal expansion, heats the material, and deposits a flow of the material through a dispensing tip thereof along a predetermined tool path at an output rate;
a material advance mechanism which supplies the solid element of material to the liquifier at an input rate (QiS) that controls the output rate;
a control for providing control signals to the material advance mechanism, the control signals commanding operation of the material advance mechanism so that the input rate compensates for a predicted melt flow component (QMF) of the output rate. - View Dependent Claims (10, 11, 12, 13)
where QiS t−
1 is the input rate at the previous time step, QMFS t−
1 is the melt flow component of the output rate of solid material at the previous time step, and Δ
t is the duration of a time step.
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12. The extrusion apparatus of claim 9, where melt flow over time is predicted using an exponential model.
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13. The extrusion apparatus of claim 12, wherein the exponential model of melt flow over time is given by the equation
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Q iS 1 + % MF ⋆ ( 1 - - t τ MF ) ·
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Specification