Additive manufacturing and rapid prototyping of parts using polymeric materials

1. Purpose

Extrusion of thermoplastic polymers can be used to illustrate the importance of understanding the viscous nature of materials. Rapid prototyping (3-D printing) and additive manufacturing demonstrate the effects of thermal conditioning, wettability and part quality in parts processing. The student will produce a number of extruded parts at different nozzle temperatures and analyze how temperature affects flow and quality of samples.

2. Extrusion

Extrusion is a manufacturing method of a part using a low viscosity material which has been subjected to high shear rates and temperatures in order to reduce the viscosity so that it will freely flow into a mold or a stage.

2.1 Injection Molding

In injection molding, a plastic is heated to a molten state and then forced through a nozzle into a heated mold. The plastic enters the mold through the sprue, then passes through runners which distribute the molten plastic evenly to all cavities in the mold. The mold cavity is connected to the runners via a gate (Figure 7.1).[1] Extra polymer filling the channels is later removed, chopped up and recycled back into the machine (called regrind). The mold, usually machined from stainless steel, is used to create the part geometry and may consist of two or even three pieces.[2]

Injection molding typically uses a screw plunger or plunger. The plastic, usually in the form of pellets, is fed in through a hopper in the top of the barrel and melted in the barrel. It is next forced into the mold through the heated barrel.2

In the screw plunger method, the faster of the two methods (Figure 7.2a), a screw is used to evenly distribute the heat to the plastic pellets, causing them to melt more quickly. The screw can also be used to mix in any additives that could cause the polymer to change color.2

In plunger injection molding, a ram is used to force the molten plastic into a mold through the nozzle (Figure 7.2b). At the end of either process, the plastic part is removed from the mold (Figure 7.3).2 The molds must be able to withstand high temperatures and pressures “and produce parts with close tolerances after numerous rapid operations.”1

Table 7.1 contains commonly used thermoplastics that can be used in injection molding.[3] In thermoset polymers, the individual chains are covalently linked and cannot be reformed upon heating. Unlike thermoplastics polymers, thermosets are usually not used in injection molding because after they are formed their cross-linked networks resist heat softening and reforming.1

Table 7.1: Commonly used Thermoplastics used in Injection Molding3

Name / Abbreviation
Acrylonitrile-Butadiene-Styrene / ABS
Nylon / PA
Polycarbonate / PC
Polypropylene / PP
Polystyrene / PS

2.2 Additive manufacturing

Additive manufacturing, also known as 3-D printing, has been adopted over the past two decades as a means of rapid prototyping and low volume production throughout a number of industries to create complex and intricate customized parts. The value of additive manufacturing arises from not having to closely adhere to traditional “design-for-manufacturing” protocols[4]. That is to say, that when considering production of a part the engineer has the freedom to design merely for form, fit and function without having to worry about equipment limitations; i.e. machining capabilities.

Unlike subtractive manufacturing, where material is removed to develop a part, additive manufacturing build parts from the bottom up, layer by layer. Complex parts containing internal channels, undercuts or overhangs, which may require secondary processing with traditional manufacturing methods, are easily achieved. Furthermore, by producing a multi-component part all in one process, the manufacturing cost are reduced. Tooling and redesign cost are eliminated because each part is produced very close to meeting end use tolerances. If a prototype is tested and needs to be modified, these can be reproduced easily without the need for tool modification, and instead merely require adjusting the electronic design file (stereo lithography or STL).

Part production has always been limited by the capacity of the instrument creating the part: lathe, end mill, computer numerical control router (CNC). Currently, Fused Deposition Modeling (FDM) devices have the capacity to build a single part of dimensions ca. 100 x 100 x 65 cm3. Parts bigger than the capacity of the instrument, must still be produced in multiple stages, and then hot air welded or bonded to give them the strength and functionality of a single part.

A few of the more commonly used thermoplastics used in the production of rapid prototyping include: acrylonitrile butadiene styrene (ABS), nylon (polyamide) (PA), polycarbonate (PC), polylactic acid* (PLA), polycaprolactone* (PCL), as well as ABS/PC and other polymer blends. (*biodegradable)

Fused deposition modeling (3-D printing) lays material down in layers[5]. The polymer material is drawn from a supply spool through a heated nozzle, which decreases the viscosity of the polymer though thermal and shear forces, Fig. 7.4. The nozzle maintains the polymer at a temperature just above its melting point with the aid of resistive heaters (liquifiers). This allows the polymer to easily flow through the nozzle and form each layer. Each deposited layer hardens immediately, after being extruded from the nozzle, and bonds to the layer below. Layer thickness and vertical dimension is determined by the extruder die diameter, ranging from ca. 120 to 320 mm. In the X-Y plane, 25 mm resolution is possible. Once a layer is built, the platform lowers, and the extrusion nozzle deposits another layer.

3. Experimental Procedure


3. Experiment

The exercise is carried out using MakerBot Replicator 2 software and 2x Fuse-Deposition Modeling 3-D printers. The feedstock for the printers is polylactic acid (PLA) filament with color additives; stock filament diameter is 1.75mm.

3.1 Making the Part

  1. Open the Makerbot desktop software. The platform for the part will appear on the screen.
  2. Click on the PREPARE tab (3rd from left).
  3. Turn on power to printer (back, bottom right).
  4. Add object file from “240 prints” file: Cube.STL
  5. A box will appear on the platform
  6. Click on TURN tab (2x) to open window with turn commands
  7. Use +/- 90° buttons to lay box flat (bottom down)

(if orientation is not what you wanted, click on RESET ROTATION)

  1. Click on MOVE button to lower part onto platform.
  2. Click ON PLATFORM and CENTER
  3. Return to VIEW tab to see part on platform
  4. Open SETTINGS (top right drop down menus)
  5. Click on ADVANCED OPTIONS
  6. Quality: Material – Makerbot PLA

- Low Resolution (no rafts or supports)

- infil 10%

- # shells: 2

- layer height: 0.30mm

  1. Temperature tab
  2. Set extruder temperature to 190°C.
  3. Speed
  4. While extruding 90 mm/s
  5. While travelling 150 mm/s
  6. SAVE SETTINGS
  7. Click PRINT (top right) – there may be a delay while the computer slices the part into layers and prepares G-code for nozzle)
  8. An estimate for time and filament consumed will be given
  9. PRINT PREVIEW can be used to telescope through different layers of part as these are laid down.

7.  Click PRINT and printer will begin to warm up nozzle in preparation for production. The display on the front of the printer will display the actual temperature of the nozzle.

  1. Repeat procedure, starting with step 5, to produce samples at each of the temperatures of interest: 210, 230, 250, 270°C.
  2. After you have collected all of your samples, view these under the stereomicroscope (in 3355 Hoover) and collect images that qualitatively illustrate the viscosity of the polymer at each temperature and the quality of the parts produced.

4. Assignment

4.1 Use the images you collected with the stereoscope to qualitatively discuss the effect of temperature has on the viscosity and quality of the parts you made. Which temperature would you recommend a client use to produce parts quickly and with high quality?

4.2 The nozzle temperature of the barrel was set to 270°C. If this is the temperature at which the viscosity of you polymer is 50% of the zero-shear viscosity, ho, calculate the zero-shear viscosity temperature of the polymer. The activation energy of viscous flow is given by E= 75.80 kJ/mol.

4.3 Discuss how selection of the molecular weight of a commercial polymer can lower the extrusion temperature requirements of your extruder. How does the zero-shear viscosity of a polymer with Mw=250,000 g/mol compare to that of a polymer of Mw=192,000 g/mol? How do these considerations affect production cost of parts?

4.4 Discuss the difference in flow rate, Q, of the a molten polymer extruded through two different printers, one of which has a nozzle aperture of diameter 1.5 mm and the other of 0.5 mm (assume applied pressures and nozzle channel length are equal for both). Now consider the system if the pressure remains the same for both extruders, and you wish to produce samples at the same rate with both instruments. What modification to your nozzle length must be made?

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[1] Fried, J. R., “Polymer Science and Technology”, Prentice Hall PTR, Upper Saddle River, NJ, 1995

[2] IST Amatrol, “Manufacturing Processes 3. LAP1: Introduction to Injection Molding and Operations.” Amatrol, Inc., 1999

[3] BFP, “Injection Moulding” http://www.bpf.co.uk/bpfindustry/process_plastics_injection_moulding.cfm

[4] Camuel, B., 3-D printing design: five tips to achieve additive manufacturability, Industry Week, Technology, Oct. 15, 2014

[5] http://www.custompartnet.com/wu/fused-deposition-modeling 11/14/2014