sff of deployable nylon 11 structures

Solid Freeform Fabrication of Deployable Nylon 11 Structures

Table of Contents:

Abstract

I. Introduction

A. Project Relevance

1. Use in military, medical, manufacturing

II. Project Description

A. Concept

B. Problem (achieve elastic control)

C. Material Selection (why chosen, mech. properties, availability, low temp.

compliance)

III. SLS Background

A. SLS History & Process

B. Solid Freeform Fabrication Symposium (Austin, TX)

IV. Properties of Nylon 11

A. Relaxation Phase

B. Thermoplastic, Crystal Structure, Glass transition temp.

V. Experimental Parameters

A. GCO function, temperature settings, parts

VI. Experimental Procedures

A. Apparatus, cooling (quenched, natural convection)

B. Deployment Process (pressurization)

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C. CAD Modeling/Issues

VII. Results (Concept Validations)

VIII. Conclusions and Future Areas of Research

Acknowledgments

References

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Solid Freeform Fabrication of Deployable Nylon 11 Structures

Trevor Lawrence Page

The University of Texas at Austin

Mechanical Engineering

1 University Station Stop C2200

Austin, TX 78712

Summer 2005

Abstract

The concept of transforming a condensed or deflated nylon 11 SFF structure,

manufactured in the small build volume currently in the SLS machine into a much larger

deployed structure by heat treatment followed by pressurizing techniques is evaluated.

In the history of the SLS process, strict control over the temperature of each layer in the

build is required to relax crystalline and amorphous regions within nylon build materials

to produce a geometrically accurate prototype. The same techniques that were used in

SLS manufacturing were applied to an annealing process to create compliant nylon 11

structures. Due to the fact that this specific environment temperature is within the melting

range of nylon 11 (170 -191°C), an understanding of the annealing effects up to and

within this melting range was addressed to isolate the critical temperature that alters the

molecular structure of simple nylon 11 parts to the point of controllable deformation.

In this study, the specific annealing temperature causing desirable compliance

was isolated through iterative testing of simple cantilever beam-like structures. First in

Test 0, a physical test implemented on a beam-like structure at ambient temperatures

using human strength to evaluate mechanical properties of nylon 11, determined that the

simple structures are notably incompliant, durable, and low in mass. Test 1, the melting

temperature of the ½ virgin ½ recycled nylon build powder used in the SLS machine was

verified on small cubes to be within the range specified above. During this test, frequent

probing of the cubes indicated surface softening at temperatures leading to the melting

range. The results in Test 1 along with back of the envelope calculations led to testing a

beam-like structure after annealing at a constant 170°C for two and half hours in Test 2.

After annealing and periodic compliant checks, the beam deflected 0.25” from 70psi at its

end. For this rather thick beam of 0.1875” to display any compliance at this temperature

was beneficial, this hinted on the possibility that a desired compliance could be achieved

with thinner beams at higher temperatures in the melting range of nylon 11. Then in Test

3, similar experiments were conducted on beams with an average thickness of 0.140” at a

low 183°C, high 187°C, and then a medium 185°C to determine an optimum annealing

time interval, relaxation temperature, and to control part deformation using a hose clamp.

In the series of experiments in Test 3, the medium 185°C prevailed as the optimal

annealing temperature for its short heat treatment duration, which was based on the

volume of the test part, minimal reduction in volume, and inhibition of crack formations.

Following Test 3, the relaxation temperature was exploited in addressing the deployment

concept with a simple computer aided draft (CAD) model of a condensed basketball

geometry. Using the combination of annealing time intervals based on the deflated

structures volume and pressurization at a methodically designed air inlet the concept was

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determined valid for this deflated basketball shape. In the process of the isolating the

relaxation temperature and creating the solid model, many experimental design and CAD

modeling issues were overcome. The details of these issues are presented in the body of

the report. In the process learning how to find solutions for the unanticipated obstacles in

the research project, many aspects of continued research became apparent for further

concept validation.

IX. Introduction

The concept of creating deployable nylon 11 SFF structures made in an SLS

process has been a topic of discussion amongst the faculty members in the Manufacturing

& Design Department at the University of Texas at Austin. One of the inherent

limitations of the SLS machine is the restricted size of its build volume. Eliminating build

volume size restriction increases SLS capability by enabling development of oversized

nylon 11 components. Capturing this would lead to new paths in SLS manufacturing and

post-process control. For example, large-scale wing cross-sections, load bearing

structures, and functional parts that do not need assembly would be require new SLS

manufacturing techniques with strict post-process control. Having the ability to quickly

produce such large-scale parts would increase demand for SLS manufacturing in a variety

of industries. Not only would the proven concept increase SLS capability, but also

stimulate interest in structural manipulation of nylon components, create a need for

advancing functionality of CAD software, and encourage competitive SFF methods for

improvement. Industries that manufacture products for intricate surgical procedures,

aerospace, automotive braking systems, and oil field applications would benefit from

increased SLS capability [1]. In addition, the military could use the technology for

emergencies where new functional components are needed to be quickly produced [2].

The objective of this research project is to verify the deployment concept for a

simple spherical shaped nylon 11 SFF structure made in the SLS machine. In addition,

the evidence provided in this report serves as a basis for preliminary justification,

allowing the concept to develop into further areas of research. Using conventional design

methodology as the approach to justify the concept, trial and error methods of

experimentation are utilized to obtain a preliminary solution.

The report begins by describing the overall approach to the research project, and

then moves into a brief background of SLS manufacturing. Next, the report focuses on

the molecular metamorphoses that define the thermal properties of nylon 11. Then,

details of experimental parameters, procedures, and results are presented, including CAD

modeling issues faced in the design of parts. Finally, the report states conclusions on

concept feasibility and recommends continued areas of research.

X. Project Description

This section describes the proposed general process for validating the deployable

SFF structure concept and provides reasoning for specifically selecting nylon 11 as the

material for this concept.

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Approaching the development of deploying simple nylon 11 SFF structures

begins with visualization of the concept. Deploying a deflated SFF structure is similar to

inflating a balloon, car tire, or a basketball with the addition of SLS pre-processing and

heat treatment post-processing. The approach taken in this study focuses on creating the

deflated or condensed shape of a basketball in CAD software environment. Once the

virtual model of the deflated design is created, the model is transformed into a functional

nylon part via SLS processing. Next, the functional part will go through an annealing

process to relax the nylon 11 material and overcome its elastic properties to prepare it for

inflation. Then the deflated nylon part will be pressurized with air to form the desired

deployed geometric configuration. Upon cooling the part will solidify and form a larger

functional component.

Controlling the elastic modulus of nylon 11 is the key factor for successful SFF

deployment. This parameter will be the focus for conducting preliminary calculations and

annealing experiments on simple shaped nylon 11 spare parts prior to testing the deflated

basketball SFF structure. These tests will reveal specific temperatures and time intervals

at which nylon 11 parts become controllable. After spare parts test iterations, a time and

temperature relationship can be developed, and then implemented on less simple deflated

basketball shaped parts. An ideal method for inflation involves constraining the part,

inflating and maintaining the new form with constant air pressure. Thought this inflation

technique may or may not work, alternative approaches to physical deployment need to

be evaluated. One alternative uses mechanical force from insets, levers or possibly clay to

deploy and secure the condensed region. After the part returns to ambient temperatures,

it is predicted that new inflated form will be achieved. The results of the SFF basketball

part experiments will determine the feasibility of the SFF deployment concept.

SLS manufacturing is able to rapidly produce parts from a range of powdered

materials including metals, nylons, ceramics, alloys and waxes [5]. Half-recycled and

half-virgin mixture of nylon 11 was chosen amongst available SLS build materials for its

mechanical, thermoplastic properties and potential for deployment applications. The ½

virgin ½ recycled nylon powder mixture is assumed to have mechanical properties equal

to virgin nylon 11. With ambient mechanical properties like tensile and compressive

yield strengths of 55 MPa and 69 MPa respectfully, nylon 11 is a reliable material for

SLS manufacturing of precise molds, intricate geometries and functional components like

gears, bearings and coils [3]. In addition, nylon 11 resists abrasive chemicals such as

oils, alcohols and hydrocarbons, and resists physical abrasion, enabling it to function as

self-lubricating material. Nylon 11 is very durable and can withstand harsh environments,

making it ideal for military applications.

Being a thermoplastic material, nylon components can be formed from a variety

of processes like extrusion and drawing, where material temperature has direct affect on

its physical properties. Specific annealing temperatures determine the level of compliance

of nylon 11 parts when subject to the forces and pressures. In addition, nylon 11 begins

glass transitions at low temperatures, making it an ideal material for the elastic control

[4]. An investigation of the nylon-11’s properties at increased temperatures is acquired

through material research and witnessed during preliminary testing.

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XI. SLS Background

In 1986, Dr. Carl R. Deckard introduced SLS as a method of SFF to The

University of Texas at Austin. Dr. Deckard’s invention of SLS ranks high among a

number of SFF techniques such as Stereolithography Apparatus (SLA) and

Photochemical Machining Apparatus (PCM). The SLS method is capable of fabricating

customized functional prototypes, component assemblies, and molds.

Over the last six years, the University of Texas at Austin has fostered the SFF

Symposium. This academic event provides an opportunity for exchange of research

topics and advancement in all areas of and related to SFF technology. Areas of

development include solid modeling, computer controls, system design, beam-materials

interactions, SFF processing and post-processing [6].

The process of nylon SLS rapid prototyping involves CAD modeling, computer to

machine interface, and a sintering process. First, a standard or customized component is

visualized as a solid model, and then a stereolithography (STL) file is created to define

the model’s surface with triangular/rectangular configurations for compatibility with the

SLS machine. Next, the model is introduced to the computer to machine interface.

Imaging software takes the STL model and numerically divides it into thin layers,

creating compounding two-dimensional cross-sections. This step establishes the precise

path for each layer of approximately 0.13-0.30mm to be sintered by a high-powered laser.

New layers of powder cover previously sintered layers of the model and then become

adhered by the sintering effects of the laser. This layering of powder continues in a

sequential manner until the part is complete. During this stage in the SLS process the

temperature of the powder and newly developed layer plays a crucial role in a successful

build. For semi-crystalline materials such as nylon 11, the temperature must be kept

within the melting range to avoid part warpage. The molecular affects of the glass

transition and melting temperatures for nylon 11 are addressed in the following section.

At the conclusion of sintering, parts are slowly cooled and ready for assembly or function

as stand-alone parts. [7]

XII. Crystal Structures of Nylon 11

The internal molecular changes that take place during heat treatment, extrusion,

quenching, controlled cooling and melting processes of nylon 11 directly affect its

material properties. The molecular changes caused by SLS, annealing, and the cooling

processes that the nylon 11 spare and final test parts undergo, must be understood to

accurately design successful experiments.

The structural characteristic that defines nylon materials is an amide group

(CONH) aligned repetitively throughout the molecular orientation. Amide groups are an

innate phenomenon present in all protein bonds. The term nylon or polyamide (PA-#)

originates from the product of an amine and acid reaction. These semi-crystalline

aliphatic1 polymers are aromatic

2 containing high-performance fibers, giving nylons

strong mechanical properties. Similar to PA-12, PA-11 is composed of one less carbon

atom and x & y monomer. This difference accounts for mechanical and thermal

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properties slightly lower then lose of PA-12, possibly making it an additional material for

SFF deployment. [4]

Measured with x-ray diffraction, the molecular construction of nylon, in

particularly PA-11, is comprised of a two stable crystalline forms, alpha triclinic3 (α-

form) and gamma monoclinic4 (γ-form). At ambient temperatures, dry PA-11 (11-

aminoundecanoic) powder is composed of only 25% α-form semi-crystalline structure

and the other 75%, amorphous5. When temperatures are increased above 95ºC in the

solid state, the α-form crystalline structure changes to a γ-form, resulting a rapid growth

of small crystals. The γ-form has a shorter chain axis and is a fully stable H-bonded

structure. The H-bonds between the amide groups of a material are the determining factor

of the mechanical properties and compliant behavior of all nylon materials. The γ-form

suggests a hexagonal packing, but there is an incomplete rotation about the axes. This

immobilization of crystals caused by the H-bonds maintains methylene sequences and is

given the name pseudo hexagonal. As the material temperature approaches its melting

temperature and the crystal growth rates slow, H-bonds become weak and allow for

greater manipulation of nylon. In addition, during the temperature increase to melting the

weight fraction of the γ-form structure decreases, resulting in lower elastic properties,

leaving the material vulnerable for control. At the onset of melting, the γ-form structure

reverts and back to the α-form. One other crystal structure can be formed, but is

dependent on the thermal history and manufacturing process of the nylon. This form is

termed γ’-form and is obtained by quenching in an ice bath after melting or directly by

drawing between 70°C and 96°C [4]. These processes are indicated by a, h, and e

respectfully in Figure 1.

Figure 1 – Crystal Transitions of PA-11

(a) quenching into ice bath, (b) heating to high temperature (>95ºC), (c) heating to high temperature

(160ºC>x>150ºC), (d) isothermal crystallization at high temperature (>95ºC), (e) drawing at low

temperature (<95ºC), (f) annealing at high temperature (>95ºC), (g) cooling to high temperature (>95ºC),

(h) quenching into ice bath, and (i) isothermal crystallization at high temperature (>ºC). [8 - modified]

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The changes in structure are known as glass transitions and viscoelastic

relaxations. A material's glass transition temperature (Tg) is defined as the temperature

below which molecules have very little mobility. Polymers are rigid and brittle below

their glass transition temperature and can undergo plastic deformation above it [9]. Glass

transitions take place in the crystalline regions of nylon material. For PA-11 the glass

transition temperature is roughly 42°C. This glass transition begins at a rather low

temperature far from PA-11’s melting temperature, indicating that elastic control is

initiated during this change. Other types of transitions occur at temperatures just below

melting are termed viscoelastic relaxations. These relaxations are caused by

electromagnetic phenomenon which stimulates internal motion between molecules and

are only present in the amorphous regions of the material [4].

The relaxation stage nearest to the onset of melting is desired in nylon 11 SLS

process, holding nylon 11 parts close to melting can greatly reduce warpage and

shrinkage [10]. At temperatures near the onset of melting (175 – 191°C), the crystalline

structure is in a state of transition from solid to liquid, while the amorphous region

becomes viscoelastic, enabling control over the material [4]. This stage has been

determined to be optimum for SFF deployment. In the experiments described in the next

section, PA-11 specimens are held at temperatures up to and within the melting range in a

dry air annealing chamber for extended periods of time to make parts compliant for

desired deformation.

XIII. Experimental Design

The design of experiments for justifying nylon 11 SFF structure deployment

concept is completely original to material engineering and SLS processing realms. This

section outlines the steps taken and the parameters involved in validating this concept.

Several experiments need be conducted on PA-11 parts to achieve an

understanding of material behavior from the affects of induced thermal loads. Before any

testing methods could be established, ideas from concept brainstorming sessions help put

together avenues for testing. After much thought and grouping of ideas a general

approach to testing was formed.

This approach begins with an initial introduction to nylon 11 with back of the

envelope calculations (BOE), which utilized PA-11 mechanical properties on beam-like

spare parts. These calculations would provide an estimate of the numerical caliber of the

strength for this material in comparison to well-known materials like steel. Following

the BOE evaluation, tests that employed human senses, like touch, for forceful bending

and texture could be implemented to relate numerical mechanical property data to the

physical environment and stimulate new thoughts and ideas about the actual material.

Next, the melting point for pure nylon 11 was found within a range from many property

experts, so this had to be verified for the ½ virgin (pure) and ½ recycled powders. Then,

the first annealing test to directly compare the BOE calculations would be set up to obtain

an agreement with the calculated data and to gather initial compliant responses of nylon

11 at temperatures below its melting range. From this point on, details of further testing

could not be strictly planned and would be and implemented based on the results from the

first annealing test. To help develop further testing, valuable knowledge about the

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temperature dependent behavior of nylon 11 would be accumulated from material

research. Specifically, the information attained about nylon 11 SLS temperature control

attributed to further testing within the melting range of nylon 11. The most important PA-

11 material property involved in testing was the melting temperature. The melting

temperature parameter placed bounds on experimental procedure was given by a majority

of material property resources to be within the range of 175 – 191°C. This range calls for

test iterations to isolate an accurate temperature for adequate material deformability. In

all materials, mechanical properties are time and temperature dependent. For example,

the elastic modulus of PA-11 decreases with increasing temperature over some allotted

time frame. The question focused around the SFF deployment concept, is at what

temperature will a part become compliant enough for creative plastic deformation? By

far, the time and temperature that a nylon part is held at are the most crucial parameters

for these experiments. This valuable information would motive a series of tests that

would reveal pertinent knowledge of how to gain elastic control over nylon 11, therefore

validating the concept by achieving desired compliance for part geometry manipulation.

An even larger obstacle to overcome is in designing the deflated basketball

structure, inflation, and cooling methods to prove the concept. The details of this design

are presented in the Final CAD Model Testing section of the report.

Necessary aide in designing testing experiments came from evaluating the

physical parameters involved in experiments. The most crucial parameter is the heat

treatment oven performance. This factor attributed to successful experimentation and is

explicitly described in the following paragraph.

Since the level compliance of PA-11 parts is a function of the temperature at

which a heat treatment conducted, understanding oven operation is pertinent. The

Yamato DX300 Gravity Convection Oven (GCO) with a maximum temperature of 300°C

was used to perform various annealing experiments on the several PA-11 specimens.

This oven is equipped with an over-heat protection (OHP) sensor which shuts off heat

input, should the oven surpass the desired or “fixed temperature” set by the user. First,

the Yamato Instruction Manual was thoroughly read to ensure proper default operation

[11]. Once operation was understood, two practice runs were made where the time and

actual or “measured temperature” obtained from the digital readout (received from two

thermocouples inside the oven) were recorded to graphically display the oven’s increase

in temperature or “ramp”. Data recorded indicated a linear temperature ramp for this

oven, where the desired temperature is reached after an average of twenty-three minutes.

Next, the oven was tested for accurate thermocouple temperature reading with a simple

calibration test. Two tests were conducted, one at a high temperature and the other at a

low temperature. A long thermometer was inserted into one of the evaporation vents and

compared with the measured temperature indicated in the digital readout. Each test

showed only a ± 0.5°C deviation, suggesting that the thermocouples are accurately

calibrated. Based on the information and experience gained from evaluating the DX300’s

performance, the oven proved to have an intuitive operation, great consistency, and

desirable temperature stabilization capabilities. Only one notable problem was faced, the

OH protection sensor began to malfunction when set only 12°C plus the fixed

temperature during testing, so it was then set to roughly 50°C plus the fixed temperature

for future tests to avoid complications. During oven testing, it was determined that PA-11

specimens will be placed in the oven after it has reached a stable condition to keep parts

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from unnecessary heat treatment over the ~23 minutes of pre-heating. As noted in the

product manual, the oven is equipped with evaporator vents to keep the interior free form

moisture, although the PA-11 specimens should not contain any moisture, the vents were

kept slightly open to maintain good working condition.

XIV. Concept Evaluation

This section covers the initial steps and experiments taken to evaluate the SFF

deployment concept. First, BOE formed a basic understanding of PA-11 material

properties and helped determine where to begin designing how to deploy the basketball

SFF structure. Following the calculations, preliminary experiments were conducted on

spare or used PA-11 parts in form of small cantilever beam-like configurations to isolate

the optimum annealing time and temperature for desired compliance. Then, final tests

implementing new PA-11 deflated basketball shaped parts are focused upon for concept

validation. Each section contains results and conceptual conclusions formed for each test.

6.1 Back of the Envelope Calculations

Preceding experimentation, BOE calculations were performed on the principle

stresses found in a spherical pressure vessel and deflections of a cantilever beam. Similar

to the basketball SFF structure, a spherical pressure vessel would tolerate a specific

uniform internal pressure before the walls began to yield. For determining rough

estimates of wall thickness the spherical pressure vessel was assumed to have no

seams/welds, inlets/outlets, or supportive structures. Without these assumptions the

pressurization of the vessels would create stress concentrations in localized areas [12].

The analysis of stress concentrations is beyond the scope of this project, but must be

carried out for complex geometries in future research. The equation below was used to

determine wall thickness, using a yield tensile strength for PA-11 at 140°C from [13].

t

pr

2 [12] σ = tensile stress, p = internal or gage pressure

r = inner radius, t = wall thickness

The necessary wall thickness for an internal pressure of 6.6psi was estimated at

0.25”. This wall thickness was thought to be an accurate starting point for thickness of

the basketball SFF structure, but the internal pressure seemed low for nylon 11 basketball

inflation. Other BOE estimations include maximum stress and strain, crack formations

and are located in the supplemental journal.

The remaining BOE calculations focused on predicting deflection behavior of

spare PA-11 cantilever beam-like structures. The equation below was used in the method

of superposition for determining the load required to deflect a modeled beam 0.25”.

EI

PL

3

3

[12] δ = deflection (0.25”) , P = load , L = length (2.25”)

E = elastic modulus (180 psi), Ix = moment of inertia

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A localized pressure or load required to deflect the beam a minimum of 0.25” was

estimated at 0.13psi. This resulting load also seems to be on the low side, which is mostly

likely the cause of the assumed elastic modulus (E) at elevated temperatures. The BOE

analysis estimates a low caliber of forces and stresses placed on spare PA-11 parts.

Turning this problem around could prove useful when solving for E after annealing a

beam-like specimen at a know temperature. This could be an approximate method of

determining the relationship between temperature and the elastic modulus of PA-11.

This concept is compared with actual deflection testing in Test 2.

6.2 Preliminary Testing

The remainder of this section describes procedures and results of several

preliminary experiments using spare or used PA-11 parts. The three preliminary tests

accumulate information about the two main factors, time and temperature, for PA-11 part

manipulation. Tests also provide relevant information in agreement with the authors of

[4] dealing with material properties of PA-11 upon induced heat treatment. During

testing, CAD models for the deployable basketball were being developed and results of

the design and obstacles faced are presented in the final testing sections. Note:

Experiment apparatus, tools, and specific procedures for these tests are explicitly

explained in the supplemental journal.

6.2.1 Test 0 – Hands-On

This test was a completely hands-on, sensory-based experiment, where conceptual

thoughts, ideas, and recommendations for further experiments was established. The

color, texture, smell, ambient compliance, weight, and machinability of a single beam-

like used PA-11 specimen were studied.

Color – cloudy white; non-reflective; not nearly as bright as the snow colored PA-11

powder

Texture – rough with few smooth surfaces, grainy; mostly like a result of the direction in

the SLS build process; further machining could be used to create a smoother surface

Smell – similar to a plastic with a kind of sweet synthetic odor that would be apparent if

melted

Ambient Compliance – very strong material; not even close to breaking with my own

hands; surface is also very hard suggesting a high impact strength

Weight – very, very light material; similar to plastics, possibly lighter

Machinability – easily cut with a band saw; part frayed at ends

This test provided a practical sense of the mechanical properties of PA-11 and

was useful for setting up the physical apparatus in Test 2. In ambient conditions nylon 11

is remarkably incompliant, suggesting that affects of temperature must be significant.

6.2.2 Test 1 – Melting Point

The first test was conducted to verify the melting temperature of the ½ virgin ½

recycled PA-11 parts. The time frame at which melting occurred was recorded for further

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tests. Two sugar cube size parts were used. A small cube with a volume of ~ 0.0175 in3

and ~0.375” thickness, and a larger cube with a volume of ~0.234 in3 and ~0.5”

thickness. Parts were tested one at a time and placed on a small aluminum sheet in the

center of the oven shelf. The larger PA-11 cube was inserted into the oven at ~27°C, and

then the oven was fixed at 180°C to observe the effects of incremental heating. Melting

was clearly present when the cube bonded to the aluminum sheet at 178°C after twenty-

seven minutes. The small specimen was inserted once the oven reached a stable 194°C.

After approximately fifteen minutes the small cube had adhered to the aluminum sheet at

180°C, indicating melting just like the larger cube. Much more surface melting was

observed in this smaller cube, but no compete melting. The results of this test verified

that the melting temperature is within the range of 175 - 191°C as stated by material

property excerpts.

Results indicate that incremental heating required more time to reach surface

melting. These results reinforce the idea to place parts in the oven after it stabilizes and

suggests that it will take a longer annealing time for the temperature at the center of

thicker parts to reach the fixed temperature. The longer time frame can be attributed to

PA-11’s low thermal conductivity of 0.23 W/m-K, impeding heat transfer, notably for the

thicker of the two cubes. In the remaining tests, specimens will be placed in the oven

once upon stabilizing at the fixed temperature (inducing a larger temperature difference)

to reduce heat treatment time. Both specimens reverted to very hard states after quickly

cooling and had turned to a yellowish resin pigment. The yellowish pigment indicates

surface oxidation from the air annealing process [4]. The test provided useful information

on the annealing time of very small parts and verified the melting range of ½ and ½ parts.

6.2.3 Test 2 – Deflection/Permanent Deformation

The focus of the second experiment was to determine the annealing time interval

and temperature a thin PA-11 beam-like structure must reach to deflect one inch and

compare results with BOE data. This is the first test toward isolating the annealing time

interval and temperature for achieving compliance of PA-11 material.

To complete this test a small pressure regulator was connected to a compressed

air source to be used as the force driving beam deflection. The experimental apparatus is

complete with a widely incremented pressure gage for clear measurement and an air gun

for accurate pressure release. Safety goggles should always be worn when using

compressed air. These materials and apparatus along with others are used in this

experiment and the remaining tests for deflection, deployment, and cooling.

A small beam of 2.25” in length and 0.1875” in thickness was placed in the oven

for annealing at 170°C. The 170°C temperature was chosen from the specified 140°C

compliance at 66psi by [13] and was increased to just below the range of melting for

greater compliance. The beam was then periodically checked for its level of compliance

by bending and rated on an ordinal scale from one to ten. In the ordinal scale, a rating of

one represents incompliant or equal to ambient conditions and ten represents that the part

has reached the desired level of compliance. Subjective bending over the heat treatment

time frame indicated a decrease in elasticity with increasing temperature, which is in

agreement with material research. After three hours at 170°C the beam was subjected to

40psi at its end, causing a minimal deflection of 0.125”, then increased to 70psi, and only

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reaching 0.25”. The beam was stronger than predicted at this temperature and failed to

deflect one inch.

The specification in [13] did not give any value of deflection or state geometry,

perhaps only suggesting that deflection will occur. This beam is rather thick, so the sign

of any deflection was a positive one. In addition, the beam cooled quickly like the cubes,

stiffened and became once again incompliant. The beam also became very discolored like

the cubes, but was much harder. The increase in hardness is one of many material

properties that increase after annealing of PA-11 material. The annealing process is

primarily used for reducing residual stresses caused by thermal and crystallinity gradients

while maintaining material properties. Annealing can increase the strength and elastic

modulus of nylons while maintaining other material properties when slowly cooled back

to ambient temperature. The slow controlled cooling process will decrease risk of

inducing any new residual stresses [4].

The results persuaded increasing the temperature to within the melting range

and cooling by natural convection in the next test to overcome the elastic properties and

control shape while increasing strength of a PA-11 part. The idea of a wall thickness less

than 0.25” for the CAD basketball model became a clear necessity.

As expected, comparisons of the tests results to the BOE calculations were

completely erroneous. Effects of stiffening from rapid cooling and the assumed modulus

(180psi) in the BOE calculations may have been the source of error in this comparison.

6.2.4 Test 3 – Relaxation State

As previously stated, there are viscoelastic relaxation states that PA-11

experiences at elevated temperatures. The SLS process takes advantage of these states to

eliminate shrinkage and warpage of parts during a build. For this method of annealing,

the temperature was increased within the melting range of PA-11. This test prepares to

isolate the relaxation temperature degree with three controlled experiments involving

thin, small strip-like PA-11 spare parts.

The first test used a nylon 11 strip ~4.451” in length and ~0.136” in thickness

held a low-end temperature of 183°C. The objective of the test was to heat the part to

183°C, place it in a confined shape and then place back in the oven for a second heat

treatment. Once the second annealing procedure completed, the strip was left out of the

oven to cool by natural convection. The

thin strip reached adequate compliance

after forty minutes of annealing, and then

was bent 180 degrees until the strip ends

made contact and braced be a hose clamp

as shown in Figure 2. No time was spared

for the second heat treatment lasting 120

minutes, making sure that the strip would

maintain the new bent shape. Once the

part was removed from the oven it cooled

naturally after seven minutes.

When the part cooled the braced

was removed and the part held its new

shape. The results of this test isolated a

Figure 2 – Clamped Annealing

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low-end temperature of 183°C that effectively reshaped the PA-11 strip with a second

heat treatment, but caused discoloration and embrittlement. Many consequences of

lengthy annealing were reveled during the process. For instance, while bending and

placing the constraint on the strip, cracks began to form at the bend signifying that the

part had become brittle. This phenomenon takes place when nylons are held close to there

melting points for extended periods. Annealing at lower temperatures may reduce

residual stresses and keep the material from becoming brittle [4]. Fatigue stress induced

during the periodic compliant checks may also have be the cause of the crack formation.

Fatigue stress may also be an area of concern. If parts show signs of fatigue, compliant

checks will have to be eliminated and the deployment process may have to be completed

in a single pressurization step. In all, this material behavior tells us that the deployment

process must be kept short and simple to maintain desirable structural stability for use in

a real-time applications.

The next test employs the KISS method engineering, by following that same

procedure as the first, but changing the temperature to a high-end 187°C, eliminating the

second heat treatment, and reducing the number of checks. Two strips were used with

similar dimensions as in the first test, but added a smaller control strip, which was

inserted to evaluate changes in volume from annealing. All three pieces were placed in

the oven at the same time. The first was to be removed early in the heat treatment,

assuming it would be compliant based on results of the first test, while the second

remained in the oven longer to see if the strip would in fact become brittle and crack.

Test strip #1 was held in the oven for an hour and a half, and bent successfully

with minimal or no crack formation and not placed back in the oven for re-heating. Test

strip #2 was removed forty-five minutes later and responded with heavy crack formation

upon bending. Both parts cooled by natural convection, both taking approximately

twelve minutes to cool. After two hours the smaller control piece (#3) was removed and

was not bent, but cooled in eighty seconds by a constant 40psi airflow. The control strip’s

volume reduced by 5.89 %.

The results provided even more understanding of PA-11 behavior. The strips both

bent successfully with minimal periodic compliance checks during significant annealing,

although cracks formed on parts held for longer periods. The second heat treatment is not

necessary to achieve a new shape and there is an apparent reduction in volume, mainly in

the length dimension, at this high-end temperature. The reduction in volume and cracks,

represent part shrinkage or warpage, indicating that this temperature and time frame

reduced strain and possibly other material properties. The direction of part shrinkage is

effected by the specific direction of the build process for that part [14]. The direction of

build for these parts may have also contributed to the crack formation.

The third of the series of relaxation tests follows the same procedure as the second

test, but reduces the fixed temperature to 185°C and significantly shortens the annealing

process to eliminate crack formation and reductions in volume. Checks for part

compliance commenced much earlier in the annealing process. Strip #4 was removed at

the first check, after twelve and a half minutes and bent successfully with minimal crack

formation. A control strip was removed and only had a 1.1% in volume reduction.

At 185°C the PA-11 strips exhibited excellent physical behavior with an

annealing time of ten to twelve minutes for totally compliant parts and only small

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reduction in volume. These parts also maintained their strength and elastic properties

after cooling and were not discolored by oxidation inside the oven as shown in Figure 3.

Figure 3 – Stress Relaxation Strip Four

The thickness of the parts was believed to have to greatest influence on the level

part compliance. The small thickness provided a great starting point for the wall thickness

of the SFF basketball in the final tests. Unavoidable problems with these tests deal with

part and oven heat loss when opening the oven for temporarily removing parts during

compliance checks. An ideal annealing chamber would be equipped with inverted gloves

and an observation window for parts examination while maintaining oven temperature.

6.3 Final CAD Model Testing

The final stages of testing incorporated the relaxation temperature established in

the last preliminary test into deflated basketball shaped SFF structures. Time and PA-11

build powder became limited, so experiments were not able to evolve into an iterative

process for developing a sound method of predicting deflated geometry. Therefore, these

final two tests were conducted to verify the concept of deployable structures. The final

tests required development of simple CAD basketball models capable of sustaining

internal pressures and design of an air pressure inlet for effective deployment.

Three-dimensional solid modeling software was used to create the model of the

condensed SFF structures. The design of the deflated geometry is very important, given

that it directly affects the post geometry of a deployable SFF structure. SolidWorks

proved to be the most useful among Pro-E and AutoCAD’s Inventor 9 for constructing

simple solid models in a compressed/condensed/deflated state, due to its perceptive

guided user interface and collection of useful modeling features. The simple SFF

structure implemented in the final stages of this project is a model of a deflated

basketball. The basketball shape was chosen for its simple geometric configuration and

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predicted symmetrical deployment. In SolidWorks, a Deform feature was used, which

enables users to easily manipulate any geometry in a three-dimensional manner. This

feature proved somewhat useful for predicting deflated basketball geometry, but

reconfigured the wall thickness previously established. Rendering of the figure, made the

wall thickness non-uniform and incapable of producing constructive results. The wall

thickness affects the level compliance once a part is heated, so it was decided that the

deformed shape was to be mimicked using conventional drawing techniques to retain a

uniform wall thickness. Had time permitted, solid models were to be modified based on

results of experimental iterations, where the configuration and dimensions of the

condensed solid models would be continually modified to address new insights for

predicting geometry, dimensional limitations, and experimental parameter adjustment.

The basketball model created is just the beginning of models towards complete

comprehension of predictive geometry for deployable SFF structures. Further approaches

to pre-deployment geometry are discussed in the concluding section of this report.

Although, the concept behind this research seeks to increase to capability of the

SLS machine by developing parts larger than the current build volume, models must start

out small. Small sizes are easier to build and perform practical tests on without creating

unnecessary complexity. Thin wall thickness of SFF structures become compliant after

short annealing durations, as concluded in the Test 3. The wall thickness of the parts in

Test 3 were just above an eighth of an inch, so a thickness slightly below an eighth was

used to ensure a compliant response to air pressure deployment. Wall thickness, radial,

and height of the basketball SFF are shown in Figure 4.

Figure 4 – Deflated Basketball Cross-section

Supportive

base

Air pressure inlet

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The thicker area shown supporting the model was as designed as a base for the

user to grip and easily constrain the part during inflation. The model begins development

of CAD models designed to verify the deployment concept at a basic level, and must be

elaborated upon in further research to reduce extra features such the base.

The design of an effective air inlet is another important function required for

successful deployment. The ideal deployment process of SFF structures utilizes air

pressure to uniformly inflate a deflated geometric shape. Inflation using air pressure is

quick, safe, and is practical for post heat treatment processing. One of the coherent

problems faced when dealing with pressurization, is creating an effective seal between

the air source and the deflated structure. Consideration of how the PA-11 object will

behave after annealing persuaded the design to incorporate a constant seal, keeping the

structure deployed upon pressurization until cooling and stiffening, thus becoming self-

supporting. Shown on Figure 5, air is delayed from exiting the interior cavity by an

inward protruding inlet creating a physical barrier to detour the air.

Figure 5 – Air Pressure Inlet Design

The guided radial nozzle conforms to the tapered rubber extension piece

connected at the end of the air gun, which is firmly held in place to create a tight seal.

The diffuser shaped interior opening disperses air throughout the interior air cavity.

Note, a nozzle shaped interior design may inhibit air from exiting better than the diffuser,

and should be evaluated in further inlet designs.

As for testing, two new PA-11 deflated basketball models were annealed at the

compliant temperature of 185°C. The first of the two identical deflated structures were

annealed for an hour with compliance checks after ten and thirty minutes in the oven.

Checks were restricted to physically placing force on the inner deflated wall without

attempting to deploy the structure with air pressure. At one hour the first test part was

ready for deployment using the tools in Figure 6.

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Figure 6 – Deployment Tools

Pressurized deployment of the first part began by firmly creating a seal between

the nozzle end and the chamfered opening of the air inlet. Pressurization started at 35psi,

but was quickly increased to 60psi due on insufficient inflation. When the initial 35psi

was released the part “exhaled” as it proceeded to return its deflated state. The part only

partially reverted to its initial deflated state, because the symmetrical geometry had

already has begun to induce structural forces to form the convex dome, (inverse of the

initial concave deflated form), shown in Figure 7. The part was very hot when first

removed from the oven and took an hour to cool by natural convection. The second

deflated part was annealed in the same manner as the first, but was not checked for

compliance and removed fifteen minutes earlier and cooled by forced convection.

Successful deployment of the designed deflated SFF structures verifies the

concept, even though the deployed shape is not spherical. This spherical shape in Figure

7 did not require a constant pressure to keep it inflated. Once the parts were partially

inflated, the spherical geometry became self-supporting. This reinforces the need for

continuing the development of predictive deployment geometry, since other more

complex shapes will not have this advantage. The annealing time interval parts were held

at, agrees with the heat transfer principles based on volume or thickness. Although, these

deflated structures trapped a large volume of unsintered PA-11 powder, which added to

the time frame for heat transfer due to a thicker non-hollow wall. Powder evacuation is

another obstacle that must be addressed in future work. From a practical perspective, the

time for the heat treatment and deployment process are fairly short and ideal for

emergency situations.

Air regulator

with large

incremented

pressure gage Safety

goggles

and heat

protective

gloves Air gun with

black rubber

nozzle

Constrained

base plate

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Figure 7 – Deployed SFF Vessel

As previously mentioned, the direction of the SLS build may have as affect on

part shrinkage. Bottom to top build direction was utilized for these parts to create a more

symmetric geometry, structural stability, and to avoid any unwanted stress

concentrations. In addition, the cooling methods might have an affect on volume

reduction, stress concentrations, and PA-11 material properties. Slow cooling is

recommend for retaining material properties, but what about the forced cooling

implemented in many of the tests. A common form of rapid cooling nylons is quenching,

where a parts are immersed in an ice bath. Quenching, causes a rapid change in

temperature and forms a γ’ crystalline structure [4]. Using forced convection cannot be

compared to this type of rapid cooling method, for two reasons. One, there is a very large

temperature difference between an ice bath and compressed air. Two, the ice bath causes

changes in crystalline structure because of the large temperature difference between the

part and ice bath, such a difference does not exist in forced convection cooling method.

XV. Conclusions and Recommended Research

Thorough research on the thermal behavior of nylon 11 and preliminary testing, a

stress relaxation state was revealed at an annealing temperature of 185°C. Nylon 11 parts

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annealed at this relaxation temperature achieved desired compliance after begin held for a

time interval proportional to their volume. The concept of deployable solid freeform

structures has been physically addressed with the designed models presented and is it

realized that the concept is valid.

Although the concept has been physically validated, further proof of concept

remains as the next objective toward professional acknowledgment. Many avenues of

research sparked interest while designing tests to isolate a stress relaxation temperature

and creating a deflated geometrical model of a deployable structure. One of the main

areas was predicting what geometry would result in an accurately defined inflated form.

Other noteworthy areas of research that would have been very useful during the design of

experiments include complex geometry deployment, stress concentration analysis, CAD

software development, alternate materials (PA-12), and seal formations.

Accurately predicting the geometry of deflated structure to produce an accurate

inflated figure will take time and effort. An alternate approach to successful deployment

to a desired shape is presented. This concept simply follows the need for maintaining a

specific wall thickness and minimizing elastic deformations during deployment. First, the

desired geometry is created. Second, the original deflated concept is developed and the

surface area of the desired shape and the deflated geometry, no matter how complex,

must be calculated. CAD software may be capable of calculating values for any

geometry. Third, the deflated design area must equal the desired surface area. This third

step will be very challenging in modifying the original geometry to equal that of the

desire shape. An example of this concept is illustrated in Figure 8.

Figure 8 – Example of Predictive Deflation Geometry

This example represents a truly condensed geometry. The length of the condensed

barrier equals the circumference of a cross-section (circle) of the sphere. This area of

focus would also stimulate CAD software development. Software capable of predicting

geometric manipulations would be ideal for this research. Simulations of deflated and

inflated figures developed in such software would enable users to view the deployment

process of an original deflated design to determine its potential.

Another area mentioned in the BOE section is evaluation of stress concentrations

in complex figures. This analysis would be very involved and would require complex

1. Desired Shape - Sphere

Original Surface Area = ?

Desired Surface Area = 4πr2

2. Original Deflated Concept

Cross-section of the

condensed shape

3. 4πr2 = Original Surface Area

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calculations adaptable to various applications. Further development may consult help

from finite element software like Ansys or CosmosWorks to analyze impractical

geometric configurations.

Once a more in-depth study of the relaxed state of PA-11 at 185°C or a more

accurate temperature is completed, the process of deployable solid freeform fabrication

can be defined, written, and then modified to a science.

Acknowledgments

The author would like to express immense gratitude to all the individuals that

contributed to this project. Special thanks to Dr. Carolyn Seepersad for providing this

wonderful opportunity to study such an interesting material and for her full support

during this project, Dr. David Bourell for providing the GCO oven and lab where testing

commenced, Dr. Scott Evans for his enlightenment on approaching this research project,

engineering design knowledge, and resources for this project, Don Artieschoufsky and

the ME machine shop staff for manufacturing assistance and equipment resources,

special thanks to Mario Faustini and Jeremy Murphy for the spare and new test parts,

along with resources and help with understanding SLS operation.

Definitions [15]

1. Aliphatic - having carbon atoms linked in open chains

2. Aromatic - (chemistry) of or relating to or containing one or more benzene rings; "an

aromatic organic compound"

3. Triclinic - having three unequal crystal axes intersecting at oblique angles

4. Monoclinic - having three unequal crystal axes with one oblique intersection

5. Amorphous - without real or apparent crystalline form

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sff of deployable nylon 11 structures

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