Dual Syringe Pump for Silicone 3D Printer
Xinghao Huang,  Haolin He, Jefferson Cam
[An ASME Project, Feb 2019]

Introduction

Since November 2018, the ASME club at UCSB has been funding the Silicone 3D Printer project. Led by Alex Russel, the team started building a 3D printer that uses silicone material as the filament to build 3D object. Some of the concepts were borrowed from the similar project made by Soft Rbotic Toolkit.

We built this dual syringe pump as one the main components in this project. ​The purpose of this syringe pump is to provide a constant, adjustable flow of silicone material for the printer. Here below, I will present our design, test, problems, and modifications. 
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From: Soft Robotic Toolkit

Initial Practice and Prototype

Learning and Practice: 
Because we had experience of syringe pump before, we decided to gain some experience. We first assembled a syringe pump that was made by Kira Puscha, Thomas J.Hintonb, and Adam W. Feinberghad. Their paper "Large volume syringe pump extruder for desktop 3D printers" contained the stl files and BOMs online, so we 3D printed all parts and built it. 
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From: Elsevier
We used built a simple control by Arduino to test it first. The pump ran very well with steady speed. The only problem we saw was that the nuts some could become loose and make two gears not aligned. From this practice, we summarized the following points:
  1. Lead screw pitch should be small to reduce the speed
  2. The infill density for the structure doesn't need to be very high (45% is enough since the load is not big)
  3. The slot for syringe plunger is a very ingenious design
  4. Gear train is good for speed reduction but occupies space.
Initial Prototype:
We then decided to directly use a stepper motor connected with a small-pitch lead screw to drive a pushing block. The lead screw and lead screw nut could transform rotational motion into linear displacement. Haolin suggested that the block could be inserted into two horizontal sliding slots on both sides of the walls, and it could be removed easily. The stepper motor can be placed in-between the syringe, so it "pulls" the block to "push" the syringe plungers

However, this design required the wall to be very thick. The slot might also have relatively large friction due to the sport material during printing. The syringes were separated too much, which increased the overall dimension and exceeded Prusa's print bed size. The lead screw coupler also couldn't hold too much load.
Haolin purposed a slide slot design that is very easy to install and remove. 

[On the left is the pushing block and sliding slot]

Design

After a series of experimentation, we started our design. Below is the materials we used:
Bills of Materials:
  • PLA Filament for 3D printed parts (around 1kg used)
  • 0.4A Phase Current NEMA 17 Bipolar Stepper Motor
  • 150mm 2mm Pitch Lead Screw and corresponding lead screw nut
  • 5mm to 8mm lead screw coupler
  • Two 8mm diameter slide bearings
  • Two 8mm diameter linear guide rod
  • Two 60CC Syringes with luer lock
  • Around 20 M3 socket head bolts and nuts
After the initial prototype, we decided to use two linear slide bearings with two linear guide rods to hold the pushing block. We also move the stepper motor to the side opposite to the syringes so the lead screw nut would "push" the pushing block instead of "pulling". The design made two syringes stay closer to reduce the size. 
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The main frame
Picture
SolidWorks FEA Simulation for the main frame under 20N load
We also learned from the previous syringe pump we built and designed a slots that were easy to remove and lock the syringe plungers. We designed the bottom ending of plunger as a half circular shape. While the slot size is a little bit smaller than the diameter of the plunger's bottom, the curve can make plunger to easily slide into the slot and hold tightly in place.
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Finished Assembly Model
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Details and Interior
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Specification of the Dual Syringe Pump

Assembly and Test
After checking all tolerances, we 3D printed all parts and assembled the pump. We then built a small control system by using an Adafruit Metro Mini Board, A4988 stepper motor driver, and a potentiometer, which could vary the speed. The pushing block moved smoothly.

Modifications

There are two more problems we still need to solve. First, the speed is too fast. The silicone 3D printer requires the pump to extrude material at 26ml / hr. Our stepper motor is too fast for it. 

One way to solve this is to use microstep function on stepper motor driver. However, even though it max microstep deduction can be 1/256 of a full step, the speed is still too fast, and the output flow will be a square wave instead of a continuous function.

Silicone 3D Printer Experiment Setups

Tests (Jan-Mar, 2020)
Xinghao Huang, Haolin He, JP Ramondi, Andrew Furst, Steven Man 

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