Melt flow compensation in an extrusion apparatus

• US 6,547,995 B1
• Filed: 09/21/2001
• Issued: 04/15/2003
• Est. Priority Date: 09/21/2001
• Status: Active Grant

• Current Assignee: Stratasys Incorporated
• Original Assignee: Stratasys Incorporated
• Inventors: Comb, James W.
• Primary Examiner(s): Eashoo, Mark
• Application Number:
  US09/960,133
• Publication Number:
  US 20030064124A1
• Time in Patent Office: 571 Days
• Field of Search: 264/40.1, 264/40.7, 264/177.1, 264/255, 264/308, 425/145, 425/147, 425/166, 425/162, 700/119
• US Class Current: 264/40.1
• CPC Class Codes: B29C 2948/92571 Position, e.g. linear or angular
• CPC Class Codes: B29C 2948/926 Flow or feed rate
• CPC Class Codes: B29C 2948/92904 Die; Nozzle zone
• CPC Class Codes: B29C 48/05 Filamentary, e.g. strands
• CPC Class Codes: B29C 48/92 Measuring, controlling or regulating
• CPC Class Codes: B29C 64/209 Heads; Nozzles
• CPC Class Codes: B29C 64/321 Feeding

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.

Claims

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 (Q_MF) 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 (Q_iS) for that time step so as to compensate for the predicted melt flow.

2. The method of claim 1, and further comprising the step of:

repeating the steps of predicting and commanding for subsequent time steps.

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).

4. The method of claim 3, wherein predicting the melt flow component (Q_MF) of the output rate at the time step comprises adding the melt flow component from a previous time step (Q_{MF t−}

1) to a predicted change in the melt flow component from the previous time step (Δ

Q_MF), and wherein the input rate (Q_iS) for the time step is commanded according to the equation Q_iS=(1+%_MF)(Q_Target−

Q_MF) where Q_target is the predetermined target output rate.

5. The method of claim 4, wherein the predicted change in the melt flow component is given by the difference equation ${\Delta }_{}$

QMFS=%MF⋆

QiSt-11+%MF-QMFSt-1⋆

Δ



 



tτ

MF,

where Q<sub>iS t−

1</sub> is the input rate from the previous step, and Δ

t is the duration of a time step.

6. The method of claim 1, wherein the melt flow over time is predicted using an exponential model.

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).

8. The method of claim 7, wherein the exponential model of melt flow over time is given by the equation $Q_{\mathrm{MFS}}=\frac{{\%}_{\mathrm{MF}}\star }{}$

QiS1+%MF⋆

(1-

-tτ

MF),

for a step change in Q_iS from zero.

9. An extrusion apparatus comprising:

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 (Q_iS) 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 (Q_MF) of the output rate.

10. The extrusion apparatus of claim 9, wherein the control contains an algorithm for predicting the melt flow component as a function of a melt flow time constant of the liquifier (τ

_MF) and a percent thermal expansion of the material (%_MF).

11. The extrusion apparatus of claim 10, wherein the algorithm comprises calculating a predicted change in the melt flow component (Q_MF) from a previous time step to a next time step according to the difference equation ${\Delta }_{}$

QMFS=%MF⋆

QiSt-11+%MF-QMFSt-1⋆

Δ



 



tτ

MF,

where Q<sub>iS t−

1</sub> is the input rate at the previous time step, Q<sub>MFS t−

1</sub> 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.

12. The extrusion apparatus of claim 9, where melt flow over time is predicted using an exponential model.

13. The extrusion apparatus of claim 12, wherein the exponential model of melt flow over time is given by the equation $Q_{\mathrm{MFS}}=\frac{{\%}_{\mathrm{MF}}\star }{}$

QiS1+%MF⋆

(1-

-tτ

MF)·