Nike - Improving Thermal Conductivity of Resin for Additive Manufacturing of Tooling Inserts

Andrew Hollcraft, Kevin Slonecker, Robert EngelPCE 431: Advanced Materials Processing

March 15, 2016

Presentation Overview● Background● Plan

○ Formulation○ DOE○ Processing Conditions

● Process Troubleshooting and Experimental Revisions ● Characterization

○ Modulated DSC○ Compression○ Hardness○ Challenges

● Benchmark● Results● Conclusions● Future Work and Revisions

Consider this... Every time Nike designers want to test a new shoe design they have to make molds for every shoe size (mens & womens).

http://www.core77.com/posts/4840/a-peek-inside-a-nike-factory-4840 https://jeffreyliving.com/2015/12/22/manufacturing-sport-shoes/

BackgroundProject Sponsor: Nike

Contacts: Bryan Kraft, Kalin Karich

Nike Inc. is a leader of innovation for high quality sports attire and footwear.

● Nike, is seeking a polymer formulation with improved thermal conductivity that can be readily 3D printed and used as a tool insert for in house manufacturing.

● Rapid production of 3D printed inserts is an appealing solution to the high tooling costs and lead times of aluminum or steel molds.

Plan - Formulation ● Higher crystallinity polymers

○ Better heat transfer○ Difficulty in extrusion processing due to reduced melt viscosity

■ Less challenging resin to process desirable

● High heat deflection temperature ○ Analogous to application as injection molding insert

● Nylon 6/6 ○ Challenge: Do not currently have a wide enough MWD for extrusion

■ Proposed solution: Compound inside an injection molder

● Dosing○ Premix slightly more resin than needed to fill the screw

■ Flood fed, but just barely to increase dosing accuracy

Plan - Formulation ● Many highly conductive additives are highly toxic,

expensive, or not readily available○ Primary concern - safety○ Zinc Oxide

■ Moderately conductive ■ Relatively benign ■ Widely used

● Thermal conductivity○ Nylon 6/6 - 0.25 Wm-1K-1

○ Zinc Oxide - 60 Wm-1K-1

○ Ratio of 240 ● 40% loading

○ Triple conductivity○ Towards the upper limit processed by twin

screw extrusion■ Minimize issues during processing to

focus on analysis

Plan - DOE ● 23 factorial with five repeats

○ Repeats would detail shot to shot variation due to axial mixing and dosing

○ Factors■ Additive loading

● 5 and 10% (near upper limit processable by extrusion)■ Barrel temperature

● Limits of nylon 6/6 injection molding processing ■ Shear - back pressure

● Increased dispersion● Limits of nylon 6/6 injection molding processing

Plan - Processing Conditions

Table 1.1 General processing conditions used for most experimental runs. Runs with increased viscosity required slightly different general processing conditions.

Table 1.2 Experimental processing factors and factor levels.

Process Troubleshooting and Experimental Revisions

● Temperature○ Nylon 6/6 - known for drooling

■ Screw retraction to reduce drool○ Frequent cold shuts

■ Reduced residence time

○ Increased thermal conductivity at high loading greatly decreased the working time between excessive drool and cold shuts

■ Temperature factor level - not variable

● Loading○ Loading of 10% was found to be difficult to process

■ Removal of 40% loading○ A control of 0% loading was also regressed

■ Resulting regression: -1(0%), 0 (5%), 1(10%)

Dosing Issues

Characterization - Modulated DSC● Thermal conductivity (K0)

○ Main scope of the investigation ■ Specific modulated heat capacity of thin ~0.5mm and thick (~4mm) sample■ Modulate the temperature at a set rate

Characterization - Yield Compression ● Critical to withstand the cavity pressure of injection molding● Utilized sprue from tensile bar mold for characterization● Since the sprue is drafted, assumed failure at smallest

cross-section diameter● Issue of “seating” influencing the recorded strain value

Characterization - Hardness

● Critical to maintaining tooling surface● Measured hardness of tensile bar at

2 locations○ At end of fill (EOF)○ Near the gate

● Utilized a ¼” ball with 100 kg mass

https://en.wikipedia.org/wiki/Rockwell_scale#/media/File:Rockwell_hardness_tester_001.jpg

Characterization Challenges ● Heat deflection temperature

○ ASTM - Three point bend■ Load of ⅓ lb

○ Compression

■ More closely model the actual part application

■ Extension rods too short

● Thermogravimetric analysis○ Determine resulting inorganic loading

■ Indicative of dosing control ○ Tare issue

■ One hour per sample■ Misloads result in days of testing

○ Oxidation of polymer

■ Jumping of pan coupling into the laser level

Control Samples● Processed with no Zinc loading (coded -1) and back pressure of 100 psi

(coded 0) ● High error in thermal conductivity so results of DOE will be evaluated as %

increase between test samples and control

Literature Value Mean Measured Value Mean %error

Thermal Conductivity (W/m*K)

.245 .385 57.1%

Yield Compression Strength (psi)

12500 12390 0.88%

Hardness (Pa) 80 EOF: 68.1 Gate: 71.2

EOF: 14.9%Gate: 11.0%

(Literature values from http://www.gplastics.com/pdf/extruded-nylon-66.pdf)

Results● Regression analysis conducted for each of three responses (95% confidence)● Statistically significant factors

○ Thermal Conductivity- %Zinc (p = 0.0018)○ Compression- Interaction between %Zinc and back pressure (p = 0.0490)○ Hardness (EOF)

■ %Zinc and Backpressure full regression (p = 0.00008, p = 0.0468 respectively)■ Main effects only %Zinc (p = 0.0001)

○ Hardness (Gate)- %Zinc (p = 0.0019)

Discussion

● Thermal Conductivity○ Decaying significance of % loading of Zinc

■ 0%-5% Zinc loading: 60.9% mean increase in thermal conductivity■ 5%-10% Zinc loading: 11.0 % mean increase in thermal conductivity

○ More data points for % Zinc loading could be added below 5% and between 5-10% to develop a full graphical correlation to thermal conductivity of Nylon

● Compressive Strength○ Standard deviation of high factor levels calculated

○ Determined that back pressure had positive linear correlation to compressive strength at a high zinc loading

Conclusions● Regression results show that Zinc Oxide is a viable additive for increasing the

thermal conductivity of a semi-crystalline base polymer● Compounding with Zinc Oxide had no negative effects on yield compression

strength and was shown to increase the hardness of Nylon 66 ● Can compounding be performed inside an injection molder?

○ Cannot starve feed - variability in loading○ Mess control - powder additive

■ Dosing accuracy - masterbatch

Future Work and Recommendations ● Compounding in Twin Screw Extruder (higher MWD grade required)● Analyze more factor levels for additive loading to develop predictive model● Characterization time estimate

○ Modulated DSC - 14 hours of instrument time

● Modulated DSC ○ Standard operating procedure

● Sampling● Assumptions - when various equations are applicable● Methodology

● Heat deflection temperature - compression○ Longer extension rods

● TGA to determine actual additive loading and subsequent variation

References1. Huang, Xingyi, Pingkai Jiang, and Toshikatsu Tanaka. "A Review of Dielectric Polymer Composites with High Thermal

Conductivity." IEEE Electrical Insulation Magazine IEEE Electr. Insul. Mag. 27.4 (2011): 8-16. Web. <http://ieeexplore.ieee.org/stamp/stamp.jsp?tp=&arnumber=5954064&tag=1>.

2. Dupont, “Dupont™ Zytel® Nylon Resin Product Information,” 42A NC010 datasheet, 2007. <http://www.t-link.com.hk/MaterialSpec/Nylon66/ZYTEL42ANC010.pdf>.

3. "Deflection Temperature Testing of Plastics." MatWeb. Web. <http://www.matweb.com/reference/deflection-temperature.aspx>.

4. "Manufacturing – Sport Shoes." Jeffrey Living - Sport Shoes. 2015. Web. 2016. <https://jeffreyliving.com/2015/12/22/manufacturing-sport-shoes/>.

5. "A Peek inside a Nike Factory." Core77. Web. 2016. <http://www.core77.com/posts/4840/a-peek-inside-a-nike-factory-4840>. 6. "Rockwell Hardness Scale." Web. 2016. <https://en.wikipedia.org/wiki/Rockwell_scale#/media/File:

Rockwell_hardness_tester_001.jpg>.