Extrusion Blow Molded Wheelbarrow Tray

Kierstyn Williams – kmw5658@psu.edu (412) 951-0088

April 06, 2015

Extrusion Blow Molded Wheelbarrow Tray

Kierstyn Williams

Penn State Erie, The Behrend College

Abstract

An improved design for a wheelbarrow tray would be

an extrusion blow molded Lupolen 4261 HDPE tray. This

tray will then be assembled to the rails of the wheelbarrow.

The design will be validated by various methods including

weathering, crack, flexural, chemical, and scratch

resistance. Currently, wheelbarrow trays are thermoformed

of steel or plastic. This newly proposed design would

improve the strength, rigidity, and reliability compared to

the current product.

Introduction

A wheelbarrow allows a consumer to transport loads

at small distances manually. They consist of a tray,

typically one wheel, two legs, and two handles. This

improved wheelbarrow tray design will increase a plastic

wheelbarrow’s strength and rigidity to help withstand

forces it will encounter.

Currently, plastic wheelbarrow trays are being sold in

hardware stores across the world. These plastic trays are

typically thermoformed out of a polyethylene sheet. The

specifications advertised for plastic wheelbarrow trays try

to assure customers that it will endure the conditions that a

wheelbarrow is designed to withstand. When reading

reviews online though, it can be determined that plastic

wheelbarrow trays in fact fail prematurely. With an

extrusion blow molded wheelbarrow tray, the dual wall

construction and tack-off features will improve the

wheelbarrow’s strength and rigidity. Additionally, when

compared to a steel wheelbarrow tray, a plastic extrusion

blown molded tray eliminates the risk that rust will form

on the tray. This new design will furthermore weigh less

than a steel wheelbarrow tray, allowing it to be easier to lift

for the consumer (or the consumer can compensate the

reduced weight with a heavier load).

The design’s specifications were analyzed during this

research. It was determined that the wheelbarrow will be

evaluated for adequate weathering, crack, flexural,

chemical, and scratch resistance. The weathering resistance

of a high-density polyethylene (HDPE) will be analyzed

following ASTM G154, accelerated weathering (QUV).

The crack resistance of the HDPE material will also be

examined following ASTM D4226, Gardner Impact. Finite

Element Analysis (FEA) was used to evaluate the flexural

strength of the wheelbarrow tray. ASTM D1693 will be

followed in order to test the HDPE material’s chemical

resistance. Lastly, a scratch resistance test will be

performed on the HDPE material.

Typically, a plastic wheelbarrow is molded out of

high-density polyethylene sheet during thermoforming.

This extrusion blow molded plastic wheelbarrow will be

produced with Lyondell Basell’s Lupolen 4261AG HDPE,

which is an extrusion blow-molding grade of high

molecular weight HDPE. This grade of HDPE retains

excellent impact resistance, even when exposed to extreme

temperatures. When doing lawn work, many of the

components placed inside of a wheelbarrow are not

carefully set in it. With this improved impact resistance

present in the material, the likeliness of cracking will be

reduced. Additionally, Lyondell Basell states that this

HDPE grade has outstanding Environmental Stress

Cracking Resistance (ESCR). This would assist in the

wheelbarrow’s performance when the tray is loaded with

chemicals present on the cargo.

Statement of Theory and Definitions

A wheelbarrow tray sits on top of a wheelbarrow’s

rails and holds a desired load in order to make the load’s

transportation easier. Wheelbarrows were invented in

China as early as 100 B.C.. Initially, wheelbarrows did not

have trays, but just wooden platforms. It was then later

discovered that a basket like design would improve loading

and ease of transportation. Since the initial product, the

wheelbarrow has been modified numerous times in order to

ease labor and improve efficiency. This new extrusion

blow molded design includes a dual wall construction with

tack-offs, in which the nominal wall is 8mm thick. The

space between the dual walls is 10mm. The tack-offs’

locations are placed to anticipate support during heavy

loading. The tray was designed to endure heavy loads,

harsh weathering conditions, crack resistance, various

chemicals, and scratch resistance. It is anticipated to have a

life span of at least 40 months, but can likely last longer

depending on its usage. A household owner who does light

gardening, or a contractor who will carry cement blocks

can use this product. Since the range of consumers is wide

spread, this wheelbarrow tray will have a large anticipated

demand. Wheelbarrows are presently at a high demand,

which can be noted by sold out models on hardware store’s

websites. Additionally, since wheelbarrows have been

produced since 100 B.C., it can be determined that the

demand will always be high. The price for wheelbarrows

can range from $25 to $120 or more. The goal of this

project was to design an improved wheelbarrow tray that

would have an increased strength and rigidity. [1]

Tack-offs are a critical design feature for this product.

There are four locations where a tack-off is placed on this

tray. A tack-off is a small contact area where one side of

the parison will be pushed, and then become compression

welded to the other side of the parison. This is performed

right before the blow phase of the extrusion blow molding

process takes place. These tack-offs will reduce the

deflection of the tray, and increase its strength. Reducing

the deflection and increasing the strength of the tray is

achieved by the increase in modulus due to the additional

stability. Tack-offs may also be used to make a long thin

part, flat when molded. This is again due to its increased

modulus. This concept is similar to the behavior examined

in I beams. [2]

Currently, wheelbarrow trays are thermoformed.

Thermoforming is a process in which a part is created from

a plastic sheet. First, the sheet is heated in an oven, and

then the sheet is transferred to a mold where the part is

formed from a vacuum, a pressure, or a force. Although

hollow parts can be produced during twin sheet

thermoforming, the strength of a part will likely be greater

in extrusion blow molding. This would be due to the sheets

in thermoforming being semi-solid when formed, whereas

the parison in extrusion blow molding will be molten. This

molten state of the parison will allow for a stronger weld

due to the polymer chains having the opportunity to

entangle more. [3]

Extrusion blow molding is a process in which

hollow parts are produced with high production volumes. It

is one of only three possible plastic processes that can

produce hollow parts. A parison (a hollow tube) is first

extruded downward, and then clamped between two mold

halves. Once these mold halves are completely closed and

the mold is sealed, a high pressure of air will blow the

material outward, forcing the molten parison to take the

shape of the mold. Air can be introduced into the part

through a blow pin or needle. A blow pin is typically used

for bottles, whereas blow needles are used for everything

else. This extrusion blow molded wheelbarrow tray design

will have various blow needles located throughout the part

in order to produce the most optimal tray. This is because

the introduction of air through the needles at various

locations will allow the parison to come in contact with the

mold walls at similar times, reducing the final variation in

wall thickness. As the blow phase is taking place, the

blown air also contributes to the parison’s cooling, forming

the part. Initially, extrusion blow molding was a process

that only produced bottles, such as milk jugs or shampoo

bottles. Within the past decade though, extrusion blow

molding has been used in order to improve products and

reduce secondary operations. The extrusion blow molding

process requires a material that has a high melt strength,

meaning when the polymer is molten, it must still acquire

strength. This is critical so the parison will have minimal

sag, and therefore the process will create optimal products.

Parison sag occurs when gravity acts on the parison,

pulling it downward and creating variation in the parison’s

thickness. [4]

Since this improved design will be extrusion blow

molded rather than thermoformed, the orientation in the

tray will be increased. During the thermoforming process,

the polymer sheet’s orientation that was present after

extrusion is relaxed once it is reheated in the oven. This

relaxation makes nearly all the orientation go away.

However, when the sheet begins to sag once heated and

then formed onto the mold while cooling, orientation is

again created. This orientation that results during

thermoforming is still not as strong as the orientation that

exists when extrusion blow molding though. During

extrusion blow molding, orientation is created when the

parison is being extruded and sag occurs on the parison.

Once the parison is put into the mold and blown, additional

orientation of the chains will also result. This increase in

alignment of the polymer chains will improve the strength

of the tray when a load is applied and deformation occurs.

This is because the chains have a higher strength

longitudinally due to the carbon to carbon bonds. This

improved chain alignment will overall allow the tray to

flex and deform more.

There are various design specification that were

determined for this improved wheelbarrow tray design.

This includes the resistance to weathering, cracking,

flexural load, various chemicals, and scratches.

In order to test that the material has adequate

weathering resistance to environmental conditions, an

ASTM G154 will be conducted. This will be performed on

samples of the material. A standard test will be completed

to mimic the conditions in which a wheelbarrow will

experience in the outdoors. This includes the exposure to

Ultraviolet (UV) rays, moisture, and heat. ASTM G154 is

typically performed on an automobile’s components. These

specifications will follow the testing procedure that is

conducted when testing an automobile’s components since

a wheelbarrow may experience similar outdoor exposure as

an automobile. The material will first be exposed to

repetitive cycles of UV rays and moisture under controlled

conditions. This will simulate the accelerated outdoor

exposure that a wheelbarrow will experience in a lifetime.

The UV rays will be produced from fluorescent lamps, and

moisture will be produced from water vapor, which will

create condensation onto the specimen – condensing

humidity. Up to 20 samples will be mounted and tested in

the QUV. They will experience 8 hours of UV exposure at

70ºC followed by 4 hours of condensation at 50ºC.

Sudden impacts will likely take place on the

wheelbarrow tray. This is because the consumers will not

lightly or carefully place heavy materials into the tray. In

order to test for these sudden impacts and crack resistance,

an ASTM D4226 will be performed. This is the standard

testing procedure for Garner Impact. The impact disks used

to perform this test will be heated to a temperature of 35ºC

and chilled to a temperature of 10ºC. Room temperature

disks will also be tested. These temperatures were

determined based upon the temperature range a

wheelbarrow may be used outdoors. The test will

ultimately evaluate the crack resistance of the material,

using the “Bruceton Staircase” method. 20 samples will be

tested for either pass or fail results. The initial height of the

test will be 125mm. Although a wheelbarrow will likely

experience a greater impact height than 125mm, this value

was determined based upon the fact that an impact disk

will have a significantly thinner thickness than the

wheelbarrow tray’s nominal wall. If the impact disk passes,

the height will be increased by 12.70mm for the next run.

If the disk fails, the height is decreased by 25.4mm for the

next run. The mean failure height will be determined to

compare the results of this test. The mean failure height is

calculated once all 20 samples are tested. It is the point

where 50% of the test samples will fail under impact. This

standard test will give a general idea if the material will be

able to withstand sudden impacts and not be easily

cracked. [5]

A sufficient flexural strength in the wheelbarrow

tray is essential in order to determine if the tray can

withstand heavy loads. The flexural strength was analyzed

by a FEA analysis. The wheelbarrow was modeled in Creo

Parametric, and then imported into the FEA software. The

proper constraints were then applied to the model at logical

locations. A force of 1, 245N was applied to the top of the

inside face of the tray. Equation 1 converts the pound mass

to kilogram mass. This kilogram mass was then converted

to newtons, which can be seen in equation 2. This force is

to mimic the force that ten concrete blocks would apply

onto the tray when being carried by the wheelbarrow.

Various plots, such as von-Mises stresses and total

deformation were examined and analyzed in order to

determine if the model will be capable of enduring desired

loads. If the model fails, the tray will have to be redesigned

and analyzed again.

280 𝑙𝑏𝑠 ∗ 0.454 𝑘𝑔

1 𝑙𝑏= 127 𝑘𝑔 (1)

127.12 𝑘𝑔 ∗ 9.8𝑚

𝑠2 = 1,245𝑁 (2)

The wheelbarrow tray will also be exposed to various

chemicals when being used or in storage. These chemicals

include oils, insect killer, lawn fertilizer, weed killer,

cement, and mulch dye. In order to test that the HDPE

material will be able to withstand these chemicals, while

being loaded, a test will be conducted following ASTM

D1693, Environmental Stress-Cracking Resistance

(ESCR). First, the specimen is slowly bent into a fixture,

and then fully immersed in the test chemical. The specimen

will be checked periodically for failure. For this

experiment, the bending stress on the specimen will mimic

the conditions of a wheelbarrow tray, which was about

9MPa (calculated in ANSYS). [6]

Although it is not a critical specification for this

design, the scratch resistance of the material will also be

examined for aesthetic purposes. An operator will simply

push and scrap various materials on the Lupolen HDPE in

order to perform this test. The materials that will be

scraped against the Lupolen will include rock, mulch, and

sand; all of which come into contact with a wheelbarrow

tray regularly.

In the current thermoformed wheelbarrow tray design,

a HDPE sheet is used. HDPE is a type of polyethylene,

which is the most basic thermoplastic material due to its

simple hydrogen carbon bonds. HDPE is denser than

traditional polyethylene due to the polymer chains being

packed together more. The chains are more densely packed

since there is less branching that is present in HDPE,

reducing the interference between chains. HDPE’s basic

chemical structure results in a wide range of material

properties, enabling it to be used in a wide range of

applications. Furthermore, its simple structure and ease of

producing makes polyethylene the least expensive and

most lightweight thermoplastic material. When carrying

loads in a wheelbarrow, it is desired to have the tray not

contribute to the overall weight, allowing the consumer to

carry more products. HDPE is fully recyclable, supporting

environmental sustainability. It is also extremely impact

resistant, which is beneficial for sudden impacts on

wheelbarrow trays. Moreover, HDPE is rot and corrosion

resistant, which is an advantage over steel wheelbarrow

trays. When compared to a steel wheelbarrow tray, this

resistance will possibly result in an extended life span.

The improved wheelbarrow tray design will also be

made of HDPE. However, the HDPE will be a high

molecular weight grade and in pellet form. The HDPE

grade chosen for this design is Lyondell Basell’s Lupolen

4261AG. When comparing the sheet versus the pellet form,

there are few material property differences. The orientation

that results from extrusion blow molding compared to

thermoforming higher. The increased molecular weight of

this grade will improve impact resistance of the tray as

well. This is because the high molecular weight of this

HDPE grade will result in more entanglement of the

polymer chains, providing more strength to the tray. HDPE

is also chosen because of its flexibility. Deformation will

occur in the tray when a load is applied. With a flexible

material chosen to be used, this material will not fail when

deformed to a certain extent. Although this may seem like

a risk that can possibly cause failure, it should be

understood that it would not. This is because the failure

will only be determined based upon the calculated

maximum stress. Therefore, this deformation is possible

without failure occurring due to HDPE’s flexibility. [7]

Design Validation

The success of the design will be judged on the

wheelbarrow tray meeting the required specifications and

its ability to compete or excel against the current

wheelbarrow trays. The design of the extrusion blow

molded wheelbarrow tray must meet specified

requirements that allow for direct competition with

thermoformed wheelbarrow trays. The design will be

evaluated on the resistance of weathering, cracking,

flexural strength, chemical resistance, and scratch

resistance.

The resistance of weathering on the wheelbarrow tray

will be judged based upon the ASTM G154 test. This test

will not be performed on the final product, but just samples

of the Lupolen HDPE material. This standardized test

includes exposure to UV rays, moisture, and heat, all of

which a wheelbarrow tray will experience while outdoors.

It is critical that the tray can withstand these conditions in

order to be a successful design. The ASTM G154 is

typically conducted on components of automobiles. These

conditions that are exhibited in this test simulate outdoor

exposure at an accelerated rate, acquiring results more

quickly. Since automobiles are outside like wheelbarrows,

testing in these conditions will also be useful for a

wheelbarrow tray design. The HDPE material will first be

exposed to repetitive cycles of UV exposure and moisture.

20 samples will be tested to ensure accurate results. The

samples will be experience 8 hours of UV exposure at

70˚C, followed by four hours of condensation at 50˚C. In

order for this material to be considered successful, it must

retain its properties for a total of ten QUV sessions. This

value was determined based upon the assumption of a

desired life span for a wheelbarrow to be five years. If a

typical season that a wheelbarrow is used is eight months,

then the desired life span of a wheelbarrow is about 40

months. Assuming there are four weeks in a month and the

wheelbarrow is used once a week for five hours, the

wheelbarrow will be used and exposed to environmental

conditions a total of at least 800 hours in its life. Since

each QUV session will give a UV exposure of eight hours,

a minimum of ten sessions must be performed on the tray.

Crack resistance will also determine the success of the

wheelbarrow tray’s design. The crack resistance of the

material will be determined based upon the ASTM D4226,

Gardner Impact test. This test mimics sudden impacts that

the wheelbarrow tray will experience when loaded. Impact

disks produced of Lupolen HDPE will be tested. In order

to test for the variation in outdoor temperatures, the disk

will be tested at 10˚C, 23˚C, and 35˚C. This temperature

range was determined based upon common extreme

temperatures that a wheelbarrow may be used. The

Gardner Impact test is either pass or fail. It is desired for

the material to pass at a height of at least 125mm. This

height was determined based upon the common height an

object may be dropped into a wheelbarrow tray. The

thicknesses of the impact disks were taken into

consideration. Since an impact disk’s thickness is

3.125mm, this indicates that the nominal wall of the

wheelbarrow tray (8mm) is about 2.56 times thicker than

an impact disk. It will be assumed for this specification

that the nominal wall will therefore be able to withstand a

sudden impact at a height 2.56 times higher than the disks.

Therefore, if the mean failure height is at least 125mm, it

will be assumed that the nominal wall of the wheelbarrow

can likely withstand a sudden impact at a height of

320mm. The test will be performed on 20 samples to

ensure accuracy.

Flexural strength will also test the design for

success. In order to test the flexural strength of the tray, a

FEA was performed. Importing the Parasolid model into

ANSYS was the first step. Lastly, the force was applied to

the top of the inside face of the wheelbarrow. Symmetry

was applied to the model in order to simplify the solution.

The mid plane tool was then applied in order to simulate

the thin dual walls. The model was then constrained at 2

different points. Force C was applied to the top of the

inside face of the tray. A force of 1,245.5N was determined

based upon two conclusions. When researching plastic

wheelbarrow’s strength online, the force that the

wheelbarrow could typically withstand was about 1,023N.

Additionally, a calculation was created in order to support

this research. It was determined that one of the heaviest

loads that a wheelbarrow will need to possibly withstand

will be concrete blocks. A 203.2mm by 203.2mm by

406.4mm concrete block was found to be 127kg. The

maximum number of concrete blocks one would likely

load into the wheelbarrow will be about ten. Therefore, the

maximum load in the wheelbarrow was therefore

determined to be about 1,245N.

The chemical resistance will also be tested for the

tray’s material. The wheelbarrow tray must have adequate

chemical resistance in order to avoid cracks, and therefore

failure. Lupolen 4261AG is clamed to have excellent

ESCR, which is an advantage to this design. In order to

determine the chemical resistance, ASTM D1693 will be

performed. This is an ESCR test method for polymers. The

specimen of the Lupolen 4261AG HDPE will be bent into

a fixture that results in a stress of 9MPa. This stress was

determined by the FEA results. Once the specimen are in

the fixtures with a proper strain applied, they will be

submergeed into various chemicals. The chemicals that the

tray will come into contact with may be during use, or

while in storage. These chemicals include lawnmower

motor oil, insect repellant, lawn fertilizer, weed killer,

cement, and mulch dye. Once fully submerged, it will be

desired that the HDPE can withstand cracking for a half

hour. This time was determined based on the maximum

time a load may be held in the wheelbarrow tray. If the

material does not crack within a half hour while immersed

in the chemical at a strain, it may be concluded that the

material is a good choice for this application. If the

material cracks before a half hour, an alternative material

may be considered in order to meet specifications.

The scratch resistance of the Lupolen 4261AG

HDPE material will also be examined. This would only be

done in order to make sure that the tray would have good

aesthetics after use. It would not be ideal for the tray to

look damaged and overused looking when it has only been

used once.

Design Procedure

This design was developed when using a wheelbarrow

one day while mulching. The plastic wheelbarrow was

flimsy and it can be immediately noticed that the

maximum strength it could withstand wound not be

sufficient for carrying heavy loads. The shelled out shape

of the wheelbarrow gave the initial idea that it would be a

good product to extrusion blow mold.

The design initially was intended to produce two

wheelbarrow trays at one time. This would have been

accomplished by a creating a single walled part. An initial

sketch can be viewed in Figure 1: Initial Wheelbarrow

Tray Sketch. This design would have improved production

rates.

Figure 1: Initial Wheelbarrow Tray Sketch

The tray’s design was then modified after considering

the inadequate strength that was present in a single walled

wheelbarrow tray. The idea of designing a dual wall

constructed tray was created. The intentions were to

improve the wheelbarrow tray’s strength and prevent

premature failures from occurring. The sketch can be seen

in Figure 2: Modified Design with Dual Walls.

Figure 2: Modified Design with Dual Walls

Lastly, the concept of having tack-offs in the tray’s

design was decided. The intentions of adding tack-off

features to the tray is to make the construction have more

strength when loaded. Tack-offs will improve the strength

and rigidity of the tray. The tack-off features can be seen

below in Figure 3: Tack-off Feature.

Figure 3: Tack-off Feature

After the final sketch was decided on, the model was

created in PTC Creo 2.0 Parametric.

The general dimensions of a wheelbarrow tray were

researched. These basic dimensions were then used to

create an extruded box for the body of the tray. Rounds

were the added to all edges of the box, except at the parting

line. The extreme rounds were added to the side edges in

order to create a tray that can easily be emptied/dumped.

The extruded part was then shelled out to a thickness of

26mm. Since the design was desired to be dual walled, the

part was cut in half vertically, and then shelled again along

its inside edge at 10mm. This generated the dual wall

feature. After, tack-offs needed to be added. It was

determined that four tack-offs would be added to improve

the strength. This was simply done by revolving a 8mm

wide box that extended to both dual walls. This model can

be seen in Figure 4: Modeled Half Tray. The model was

then mirrored over the cutting plane.

Figure 4: Modeled Half Tray

By mirroring the model over the cutting plane, a

whole wheelbarrow tray was generated. Draft was added to

the final model. This model can be seen in Figure 5: Final

Model.

Figure 5: Final Model

The height, depth, and width of the tray are. The dual

walls are each 8mm thick, and space between them is

10mm. The tack-offs are the same as nominal wall, 8mm.

A FEA stress analysis was performed on the

wheelbarrow tray. This was done by saving the Creo files

as a parasolid, and then importing and generating the

model in FEA Workbench. The mesh was modified and

refined once, to ensure the most accurate results. The

refinement was at around the sharp edges of the tack-offs,

which produced singularity in the model. The mesh

statistics for the initial mesh and refined mesh can be seen

in Figure 6: Mesh Statistics.

Elements Nodes

Initial 16,485 15,711

Refined 17,339 16,422

Figure 6: Mesh Statistics

Constraints were added to the model in FEA. This was

to prevent rigid body motion of the wheelbarrow tray once

the force is applied. The constraints that were added can be

seen in Figure 7: FEA Constraints. Displacement tags

show where the two constraints are located on the model.

Displacement A is located at the edge on the bottom of the

wheelbarrow tray. This constrains the model in the y

direction from deformation. This A Displacement

constraint is used to mimic the rails that would be present

in the final wheelbarrow product. Displacement B is

located at the edge of symmetry in the model. This

constraint indicates to ANSYS that the model cannot be

moved at the symmetric cut. This constraint also forces the

z direction to be constrained in the model, preventing rigid

body motion.

Figure 7: FEA Constraints

Presentation of Design

Initially, the design of this improved wheelbarrow tray

may look almost visually identical to a current

wheelbarrow tray design. However, when examined more

closely, it can be noted that it is a dual wall construction

with tack-off features. The current design for wheelbarrow

trays only have a single wall thickness of about 5mm.

Other than the handrails used as a frame for the

wheelbarrow, there are no other support features for the

tray. It can be predetermined that this single wall thickness

is not sufficient for a wheelbarrow’s needs. This can be

seen from multiple consumers’ experiences with plastic

wheelbarrow trays. This flaw in a wheelbarrow tray’s

design led to the new extrusion blow molded design.

With this improved design, strength will be enhanced

and the product will become more reliable for consumers.

Ideally, the impact resistance of the tray will also be

improved, eliminating the cracking that occurs when

loaded. The same material will continue to be used to

produce the tray, due to it being able to obtain the

necessary specification that the tray needs.

FEA Stress Analysis In order to ensure that the model has sufficient

strength for the desired maximum loads, a FEA analysis

was run on ANSYS. By performing an FEA analysis on

the part before producing it, all necessary modifications

can be made on the final design. The FEA analysis is used

to take precaution before spending the money on possibly

producing a part that will fail. After the parasolid model

was imported in to the program, the material properties

were entered in the program. Proper constraints were then

applied to the wheelbarrow tray. This prevents rigid body

motion when the force is applied. Next, a downward force

of 1,245N was applied to the top face on the inside of the

wheelbarrow tray. This force was calculated based upon

the maximum load that the tray may carry.

The maximum stress in the wheelbarrow tray was

examined in the FEA analysis. The force acting on the top

inside surface of the tray resulted in stresses, mostly

located in the middle of the surface. This stress distribution

can be seen in Figure 8: Von-Mises Stresses. The

maximum stress that occurs in the tray is 13.25MPa.

However, the maximum stress results from the singularity

at a sharp edge around the tack-offs. Improving and

refining the mesh at that tack-offs’ edges may reduce this

stress, but it will still be present regardless due to the

singularity. The color of the stresses that are on the scale

will have to be compared to the colors present on the

model. When comparing the scale of the stresses to the

model in the von-Mises plot, it can be concluded that the

maximum stress in the wheelbarrow tray will only be about

8.84MPa. This stress will occur in the middle of the tray,

where there is not a tack-off modeled. It should also be

noted that more of the stresses occur on the inside wall of

the tray when compared to the outer wall of the tray.

Figure 9: Von-Mises Stresses Bottom shows the stresses

that occurred on the outer surface of the wheelbarrow tray.

Figure 8: Von-Mises Stresses

Figure 9: Von-Mises Stresses Bottom

When a load is applied to the inside of the tray, it

can be determined that most of the stresses act on the

inside wall of the tray, with the outer wall acting as

support. This additional support will increase the strength

in the tray, as desired. If the tray were only a single wall,

this stress would be much more concerning since there

would not be another wall to support the top wall.

In order to determine if the tray has adequate

flexural strength, the 8.84MPa von-Mises stress is to be

compared to Lupolen 4261AG HDPE’s maximum flexural

strength. The maximum flexural strength for Lupolen

HDPE is 23.10MPa. Since the calculated von-Mises stress

was less than the maximum allowable flexural stress, it can

be concluded that the flexural strength in the newly

designed extrusion blow molded wheelbarrow tray is

sufficient for proper use.

Nominal Wall Thickness When designing this product, it was realized that the

nominal wall thickness would significantly affect the

strength of the tray. In order to ensure the designed wall

thickness is adequate, an FEA analysis was conducted on

ANSYS. The results of the analysis can be viewed in

Figure 8: Von-Mises Stresses and Figure 9: Von-Mises

Stresses Bottom, which verify that the nominal wall

thickness is sufficient. This is because the maximum stress

calculated will be less than the maximum allowable stress.

FEA Deformation

The deformation of the wheelbarrow tray was also

calculated. ANSYS was used in order to acquire the results

of the tray’s deformation when a force was applied to the

inner surface. This mimics the loads that the wheelbarrow

is designed to carry. It is desired for the tray to not

dramatically deform and cause catastrophic stresses. The

deformation results can be seen in Figure 10: Deformation

for the improved wheelbarrow tray. It can be noted that the

maximum deformation in the tray was calculated to be

11.21mm. This deformation primarily occurs in the middle

of the tray since it does not have additional support from a

tack-off. The deformation would be reduced if the material

had a higher modulus, or if there was an additional tack-off

added to the middle of the tray. Although the deformation

may appear high, it can be realized that most of the

deformation only occurs on the top face of the tray.

Additionally, without the dual wall design of the tray, tack-

offs would not be able to be used. This would decrease the

support in the tray, and increase the deformation.

The flexibility present in Lupolen HDPE allows

for this deformation, without damaging the material. It can

be concluded that there is an allowable deformation present

in the tray. This improved dual wall construction of the

tray also reduces the amount of deformation.

Since the improved design will be extrusion blow

molded instead of thermoformed, more orientation will be

present in the tray. This increased orientation will increase

the strength of the tray when a load is applied and

deformation occurs.

Figure 10: Deformation

Lifetime Analysis

For many consumers, wheelbarrows are only used in

the spring through fall seasons. By owning a wheelbarrow,

outdoor labored work becomes easier. It is desired by the

consumers to be able to own a wheelbarrow for a few

years, before having a concern about product failure. For

this design, it was assumed that a wheelbarrow’s lifespan

would be at least 800 hours.

Since a wheelbarrow will be used primarily outdoors,

it is critical that the tray can withstand environmental

conditions. These conditions include UV rays, moisture,

and heat. UV rays are proven to degrade polymers and

decrease the lifespan of a material. This especially holds

true with crystalline structures because the densely packed

regions hold the UV radicals longer, exposing the polymer

to the rays longer. HDPE is extremely water resistance due

to its non-polar structure. This would allow the tray to be

water resistant, but if heat is involved in the exposure, it is

possible the moisture will cause degradation. This is

because the heat expands the polymer, allowing more free

volume between polymer chains and the water molecules

can attack the polymer. With the results of the ASTM

G154, it can be determined if the material will be suitable

for this application. The test will be run ten times to ensure

the environmental conditions will be exposed to the

material for at least 800 hours. It is likely however that

HDPE will be a good material due to its high percentage of

crystallinity present in the polymer chain. These dense

regions will make it more difficult for degradation to result

and weaken the chain.

It is also important that the tray will not fail due to the

material’s lack of chemical resistance. It is typical for a

wheelbarrow to be exposed to various chemicals while

outdoors. This is why the testing of the HDPE’s chemical

resistance will be evaluated and analyzed before producing

the product. It is expected that HDPE will be a suitable

material for this application due to it good chemical

resistance. The chemical resistance that is present in HDPE

is due to it crystalline structure, which prevents chemicals

from penetrating the chains of the polymer. Some of the

chemicals that would be tested include oils, insect killer,

lawn fertilizer, weed killer, cement, and mulch dye.

Conclusion

This improved wheelbarrow tray design will provide a

reliable solution to transporting loads manually. The

current thermoformed single wall design is proven to be

weak, and fail prematurely. The thermoforming process

does not allow the chains to orientate as much as they

would be able to when going through the extrusion blow

molding process. The thin single wall construction cannot

withstand necessary forces or conditions when the

wheelbarrow is used as well. Since a wheelbarrow tray is

not designed to be handled carefully, it is vital that the tray

upholds its desired specifications. Although more material

will have to be used when producing this improved

product, consumers will notice the improved reliability of

this wheelbarrow tray. If this tray upholds its claims,

unlike the current plastic wheelbarrow trays, consumers

would be willing to spend the extra money.

Since this wheelbarrow tray will be extrusion blow

molded rather than thermoformed, orientation will be

increased in the polymer. This is because more orientation

occurs during the extrusion blow molding process when

compared to the thermoforming process. With more

orientation present in the polymer, the tray will be stronger

due to its increased alignment of the polymer chains. The

chains are stronger when pulled longitudinally due the

strength present from the carbon hydrogen bonds. With

this increase in alignment of the polymer chains, the tray

will be able withstand a greater force than the current

thermoformed tray.

The tray must withstand various environmental

conditions since it will be exposed to them every time it is

used. ASTM G154 can test this specification. The tray

must maintain its properties when exposed to UV rays,

moisture, and heat. Although these conditions do not

necessarily provide a risk for the HDPE material when

individually exposed, when all three factors are exposed to

the material, a high risk of degradation is present. It will be

made sure that the testing will be ran for at least 800 hours

in order to calculate for the lifetime analysis.

The flexural strength in the tray is vital. It is necessary

for the deformation and stresses to be determined before

the mold is produced and the tray are made. This will save

money for the company incase there are complications in

the design. If the maximum stress is higher than the

material’s maximum flexural strength, then an alternative

material will have to be chosen or an additional tack-off

will have to be added to the design.

Sudden impacts are also important for the tray to

withstand. These sudden impacts when loading heavy

components can cause cracking in the tray. This is the most

common complaint when reading online reviews of plastic

wheelbarrow trays. Wheelbarrows are not designed to be

handled carefully, and sudden heavy impacts will occur on

the tray. Once these impacts are applied to the tray, it is

necessary for the tray to be able to withstand cracking.

ASTM D4226 will be performed on the tray to get an idea

of the tray’s impact and crack resistance.

In its life span, wheelbarrows are exposed to several

chemicals. It is important to indicate that the designed tray

will not fail due to its weak resistance to chemicals that it

will likely be exposed to. HDPE does retain excellent

chemical resistance due to its highly crystalline structure.

The highly folded areas of the chain make it difficult for

the chemical to penetrate the amorphous regions of the

chain.

Scratch resistance will also be evaluated for the design

of this wheelbarrow tray. This is mainly for aesthetic

reasoning. If a consumer spends a great amount of money

on a wheelbarrow, they would be disappointed if it looks

messed up after one use. It is important to design a product

that is aesthetically appealing to the customers, and that

these products maintain their appearances for some time.

Future Work

Multiple steps will still need to be implemented before

the final wheelbarrow tray is produced. A prototype mold

would be ideal in order to make sure that there are no

major problems when molding the tray. If there are

complications, they can be analyzed and fixed in the design

before thousands of dollars are spent on creating an actual

mold. Additionally, the predetermined testing will need to

be implemented before the final product is produced.

Again, this is to ensure that no major complication will

occur with the suggested design. It is also an option to

choose an alternative material if they do not meet the

specifications for the predetermined tests.

After performing the FEA analysis, it determined that

the tray was capable of withstanding the appropriate loads.

When analyzing the results, it can be noted that the

maximum stress and deformation occurs in the middle of

the tray. By adding an additional tack-off in that region, the

stresses and deformation will be reduced. This can be

assumed because there are not high stresses or deformation

at the tack-offs that are present in the current FEA model.

Additionally, the tack-offs increase the rigidity of the tray,

improving its strength and maximum load. With this

reduced stress and deformation, it is likely that the

wheelbarrow will be able to ensure more force and the

strength will be increased.

Acknowledgements

I would like to thank and acknowledge Mr. David

Johnson, Mr. Jonathan Meckley, and Mr. Gary Smith. Mr.

David Johnson assisted with FEA analysis and determining

the proper constraints for the model. Mr. Meckley provided

the opportunity to conduct this project. Mr. Smith provided

input about the proper material selection for the

wheelbarrow tray.

[1] R. Secrest, “Wheelbarrow,” How Products are Made,

Advameg Inc., (15 August 1998). [Online]. Available

http://www.madehow.com/Volume-5/Wheelbarrow.html

[Accessed: 18 March 2015].

[2] GE Plastics. “Engineered Blow Molding Part Design,”

N/A. [Online]. Available:

https://cms.psu.edu/section/default.asp?id=201415SPBD%

5F%5F%5FRPL%5FET323%5F004 [Accessed: 02 April

2015].

[3] Custom Part Net, “Thermoforming,” Custom Part Net,

(2009). [Online]. Available:

http://www.custompartnet.com/wu/thermoforming.

[Accessed: 06 April 2015].

[4] Conaplat Machinery Division, “PET Moulding,”

Conaplat, (N/A). [Online]. Available:

http://www.conaplat.com.ar/pet/type_of_blow_moulding.h

tm. [Accessed: 02 April 2015].

[5] Intertek, “Accelerated Weathering (QUV),” Intertek

Group Plc. Available:

http://www.intertek.com/polymers/testlopedia/accelerated-

weathering-by-quv/. [Accessed: 20 March 2015].

[6] Prospector, “Environmental Stress Cracking Resistance

(ESCR) Plastics Test Standard,” UL Prospector. Available:

http://www2.ulprospector.com/property_descriptions/AST

MD1693.asp. [Accessed: 20 March 2015].

[7] Lyondell Basell, “Lupolen 4162AG HDPE,” Lyondell

Basell. Available:

https://polymers.lyondellbasell.com/portal/site/basell/templ

ate.PAGE/menuitem.f1ff80c98625adef62e408fde5548a0c/

?VCMChannelID=2daf473032095110VgnVCM10000064

6f3c14____&javax.portlet.tpst=7bc706488cd9f36418c369

1026f6c341&javax.portlet.pst=7bc706488cd9f36418c3691

026f6c341_ws_MX&javax.portlet.prp_7bc706488cd9f364

18c3691026f6c341_viewID=single_product_grade&VCM

ContentID=e22e95b162337010VgnVCM100000646f3c14

RCRD. [Accessed: 04 April 2015].