development of usb with laser

DTU Mechanical EngineeringDepartment of Mechanical Engineering

TECHNICAL REPORTDevelopment of USB with LASER Design of Plastic Parts - 41737Spring 2016

Groub 13Daniel Sartou s090290Gautham Ramachandren s150906Georgios Pitsilis s152087Piriyankan Arudselvam s152177Sujapragash Ramesh s152182Wahib Abboud s113161

Design of Plastic Parts, S2016

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Table of Contents Introduction 3

1.Methods 4

1.1ProductConceptexplanation 4

1.2FunctionalAnalysis 5

2.ProductDesign 6

2.1Productgeometryrequirements 6

2.2MaterialSelection 7

2.2.1DetailedMaterialSelection 7

2.3FunctionalFeatures 13

2.3.1Button 13

2.3.2Pressfit 14

2.4ComputerAidedDesign 16

2.4.1USBCap 16

2.4.2USBCase 16

2.4.3CrossSection 16

2.4.4ExplodedView 17

2.4.5FinalAssembly 17

2.5DesignforManufacturing 18

3.ProcessDesign 19

3.1ProcessParameters 27

4.MouldDesign 28

4.13-DModels 28

4.2Sprue 29

4.3Runner 30

4.4Gate 31

4.5EjectorPlacement 33

4.6Moulddesigncalculations: 36

References 38


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Introduction This final project has been made during a period of 8 weeks in the course: 41737 Design of Plastic

Parts at Technical University of Denmark. The overall goal is to develop a USB with an additional

function. The overall objective is to work hands-on with the product development process of a

plastic part from idea to mould design, which amongst other include clarifying product geometry

requirements, systematic material selection, design for manufacturing, calculation of functional

features, detailed graphical representation, process design and so on.

Throughout the project, different software has been used. For the material selection, CES Edupack

and CAMPUSplastics have been used to find information about materials. Autodesk Inventor has

been used to carry out the 3D and 2D drawings. Finally, Autodesk Moldflow Adviser has been used

in order to simulate the plastic injection moulding process in order to verify product design.

During the project, the group have followed the concurrent engineering development process,

where the work throughout the project has been carried out based on the methodology of

parallelization.


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1. Methods

1.1 Product Concept explanation The first step the group had to overcome were to brainstorm over which desired addition functions

the final USB product should consist. The following section briefly summarizes the different concepts

for a multifunctional USB, which the group discussed and the final concept selected for further

development. Different ideas were proposed, but only the three best ideas are listed below:

• USB-timer: an USB containing a digital front display allowing the user always to have a clock on him, when carrying the USB, additional the clock would charge by the USB portal.

• USB-pen: a two sided USB, one side containing a pen for writing while the other side containing the USB.

• USB Laser Pointer: same principals as the USB-pen but the pen is substituted with a laser pointer instead.

When choosing the final concept, following considerations were taken into account:

• Simplicity of the product

• Manufacturability

• Manufacturing process and product assembling steps

These considerations narrowed the final choice of product down to the USB laser pointer and the

USB-pen, due to the USB-timer having issues with the simplicity of the product, and implementing

the digital display would increase difficulty in manufacturing the product. In terms of simplicity and

smoother design, the USB laser pointer, made it a favourable final choice over the USB pen. The

USB laser pointer would let us operate under similar dimensions and design a traditional USB case

cover. The final product concept will consist of following parts:

• Cap

• TopCase

• BottomCase

• PCB

• Rechargeablebattery

• Laser


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The purpose and idea behind the product is to have a compact “storage and pointing” device in one

unit. The rechargeable battery for the laser, is charged via the USB port. The button for activating

the laser is hidden beneath the top case, and only indicated by a small icon on the surface. The final

concept is presented on figure 1a.

1.2 Functional Analysis The device consists of two functional parts; the following section will further investigate the function

of each individual part. The functional analysis will be split in two parts one concerning the laser

pointer, and the other concerning the USB storage device.

In terms of the laser pointer, following criteria is important:

• Comfort when holding the device

• Easy “one touch” laser activation option

• Clear indication of where the push button is to activate the laser

• Hidden button to limit particles to interfere with the device

• Smooth surface

• Lux rating of the laser

In terms of design choice, the lens of the laser would be set inside the cover to limit scratches and

particles interfering with the laser lens. Finally, the laser should be rechargeable by the USB storage

output.

In terms of the USB, following criteria is important:

• Dimensions predetermined for the USB

• Sweat resistance.

• Moisture resistance.

• Ability to charge the laser

Figur1a-Finalproductconcept

PRODUCT DESIGN


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2. Product Design

2.1 Product geometry requirements In terms of product geometry, the final product has to follow a set of criteria regarding the dimensions

of the USB; these criteria are showed on figure 2a.

In addition, following constrains must be met:

• Thickness of the green PCB: 1mm

• Largest thickness: 3.5mm

• Thickness of metal part: 4mm

Since, we have to implement an extra module containing the laser module the total length will

increase. However, it is very important to take into account the user experience of a traditional USB.

This means, it must not be too big and clumsy. Therefore, the USB must not exceed following:

• Total length: 80mm

• Total width: 8mm

Figur2a–PCBdimensionconstrains


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2.2 Material Selection 2.2.1 Detailed Material Selection

Having a LASER pointer attached to the USB makes it quite critical when it comes to cyclic loading

of the USB. There are numerous requirements for the product when selecting a material. But the

most important, or primary requirements for this USB design is that it should be sweat resistant,

opaque, stiff and fatigue proof. In addition, like any other USB device other requirements for the part

are biocompatibility, impact resistant, electrical insulation and have a long life cycle.

Being a handheld device, the USB will be exposed to sweat (pH of 4.5-7), so it must not degrade

over time when exposed. Also enclosing the LASER, the material must not transmit ambient light

into the chamber that interferes with the functioning and performance of the LASER. And the button

for the LASER has been designed to be concealed from being seen and underneath the surface of

one of the cases of the device. Hence the material must be stiff enough (figure 2b) that it is able to

deform elastically when pressed for more than a million cycles and yet not crack. This demands the

stiffness and fatigue strength (Appendix 1)of the part be good.

Assuming that the users will drop the device on the floor on more than 1 occasion during the life

cycle of the part, it would be safe to select a material with good impact strength(Appendix 2).

Also, when the device is used as a USB and the user detaches it from any computer, the user must

not be electrocuted. In other words, the material must be a good insulator. Another interesting feature

of the material is its operation temperature that must be up to 60oC. Theoretically the operation

Young's modulus (GPa)0,2 0,5 1 2 5

Yiel

d st

reng

th (e

last

ic li

mit)

(MPa

)

10

20

50

100

Acrylonitrile butadiene styrene (ABS)

Polyoxymethylene (Acetal, POM)Polyetheretherketone (PEEK)

Phenolics

Polypropylene (PP)

Polyethylene (PE)

Ionomer (I)

Starch-based thermoplastics (TPS)


Figur2b-YieldstrengthinfunctionofYoung’sModulus


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temperature of those devices is close to the room temperature but due to electricity consumption of

the device or of the pc, the temperature can be increased. Taking in account a normal cooling system

for a personal computer, the conducted heat can reach up to 50o C but for safety reasons we

increased the value up to 60o C (safety factor equal to 1.2)(figure 2c).

Figur2c-Elongationasafunctionofmaximumservicetemperature

Being held in one’s hands for long durations at a time, the material must not interfere with their health

meaning the material must also be bio compatible to an extent. Also, the material must sustain a

million cycles without significant changes in its dimensions, hence the material must also possess

good creep strength. Also, from the perspective of aesthetics, the materials must be

miscible/compatible with numerous additives for colour and other stabilisers. Also the material must

be easily injection mouldable(Appendix 3), considering the final production requirement. In addition,

it has been decided that the 2 halves of the USB case will be glued, hence the material must not

react with common adhesives.

Maximum service temperature (°C)40 60 80 100 120 140 160 180 200 220 240 260 280

Elon

gatio

n (%

str

ain)

1

10

100

1000

Polyoxymethylene (Acetal, POM)

Cellulose polymers (CA)

Polytetrafluoroethylene (Teflon, PTFE)

Polyetheretherketone (PEEK)

Phenolics

Polystyrene (PS) Polyester

Epoxies

Polyamides (Nylons, PA)

Polycarbonate (PC)

Polypropylene (PP)

Polyethylene (PE)Polyurethane (tpPUR)


Ionomer (I)



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The 5 step method was used for the material selection procedure. Based on these product

requirements, numerous materials requirements were arrived and they are shown on table 2a.

Product

Requirement Material Requirement Description

Shatter Proof Good Impact strength So that the USB laser does not break when dropped

Cyclic Loading Good Fatigue Strength When the button for the LASER is repeatedly pressed, the USB must not fail

Sweat and water resistant

Excellent chemical resistance

The USB when held in hand must not degrade when exposed to the acid in perspirations

Hazard proof Biocompatibility The laser pointer when held in hand even in the worst case when interacting with the human body must not harm the user’s health

Manufacturing and assembly flexibility

Moldability, gluing (adhesive properties)

The material must be easily injection moldable and easily assembled

Does not let ambient light to pass through

Opaque To ensure that the ambient light does not interfere with the operation of the LASER

Does not conduct electricity

Insulator When the user handles the USB, he/she must not be electrocuted

Stiff Optimal Young’s modulus Due to the thickness of the product and the desired application, the young’s modulus plays an important role in deciding the stiffness

Long life cycle Excellent Creep strength / Fatigue strength

The product must retain its dimensions for at least 3 years

Table 2a – Product requirements, material requirements and description


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Based on the table 2a, the material properties were then listed into 3 categories based on the 5 step

method as:

• Category 1 – Moldability, adhesive properties, biocompatibility, chemical resistance, optical properties, electrical conductivity

• Category 2 – Impact strength, creep strength, fatigue strength

• Category 3 – Stiffness, Cost

A number of plastic materials were available to be chosen from. But only the following amorphous

and semi crystalline plastics were chosen, for the reason that ready and reliable data was available

and also because of the fact that they are some of the commonly used polymers. And based on

categories 1 and 2, the materials were eliminated as shown on table 2b, and the materials chosen

were ABS, SB/SAN and POM, for further analysis.

Material Requirement Amorphous Semi Crystalline

PS ABS SB/SAN PVC PMMA PC PE PP POM PA PTFE

Injection Moulding

Adhesion to glue

Additives/Colouring

Acid Resistance (10% H₂SO₄)

Water absorption

Biocompatibility

Optical Property Tsp Op Op Tsl Tsp Tsp Tsl Tsl Op Tsl Tsl

Electrical property In In In In In In In In In In In

Strength (MPa) (Creep) 7 14 11 11 15 9 6 7 19 10 NA

Fatigue Strength (MPa at 10⁷ Cycles)

23 22 NA 26 33 31 23 17 34 66 7

Pass No Yes Yes No No No No No Yes No No

Table 2b – Comparison of polymers

Legend:Tsp–Transparent,Op–Opaque,Tsl–Translucent,In–Insulator,NA–NotAvailable


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Most of the materials have suitable properties, but based on the optical property, creep strength and

fatigue strength desired for the part the 3 materials were chosen. The selected materials were further

filtered down based on category 3, which was based on relative cost (figure 2d) and relative stiffness

of the material.

Also the elongation must be above 2% because over this value a material can be described as non-

brittle. Here the base or reference material used was PP.

MaterialRequirement Amorphous Semi Crystalline

PS ABS SB/SAN PVC PMMA PC PE PP POM PA PTFE

RelativeStiffness 0.84 0.81 0.84 0.72 0.74 0.86 1.2 1 0.67 0.79 1.41

RelativeCost 11 22 14 11 19 38 9 8 26 28 NA

Passed No Yes No No No No No No No No No

Table 2c – Comparison of relative stiffness and relative cost

Using the 5 step method, it was inferred that the best material in terms of relative cost and category

3, would be ABS, as shown on table 2c. One can also argue that SB/SAN is a better choice in terms

of cost. But the creep strength (obtained by multiplying the creep modulus and the maximum

permissible deformation) of the SB/SAN is less than ABS, and that is more important on the long

run. For the simulated condition, that would yield a safety factor of almost 3. In addition, the relative

Price * density2000 5000 10000 20000 50000 100000

Yiel

d st

reng

th (

elas

tic

limit

) (M

Pa)

10

20

50

100




Polymethyl methacrylate (Acrylic, PMMA)

Phenolics

Polyethylene (PE)



Polyhydroxyalkanoates (PHA, PHB)


Ionomer (I)

Figur2d-YieldstrengthinfunctionofDensitytimesPrice


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stiffness of ABS is better than SB/SAN, this would require less material for ABS for the same

stiffness, partially compensating the increased cost factor. Even though POM has better mechanical

properties than both ABS and SB/SAN, one key factor is that it is not acid resistant (sweat in this

case) as much as the others. In order to examine the cost efficiency of the materials, the Young’s

modulus compared with the embodied energy per cubic meter (Mj/kg)( energy required to produce

1 kilogram) in order to present the minimizing of the embodies energy in the polymer while providing

structural functionality, such as the Young’s modulus. Worth noticing is the comparison of the two

graphs before and after the implementation of the filters (elongation, mould ability, maximum

temperature and especially of the transparency) (Figure 2e and figure 2f).

Figur2e–Young’smodulusasafunctionofembodiedenergy(withoutfilters)

Figure2f–Young’smodulusasafunctionofembodiedenergy(withfilters)

Embodied energy, primary production (MJ/kg)50 100 200

Youn

g's

mod

ulus

(GPa

)

0,2

0,5

1

2

5




Phenolics


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Concerning the process, filters were used on Polymer injection moulding properties because our

process must in the most optimized way, including the mass of the USB and the maximum

thickness(Appendix 4) Thus the final processes are the following:

1. Injection blow moulding

2. Injection moulding (thermoplastics)

3. Precision glass moulding

4. Thermoforming

As it was expected the Injection moulding of the thermoplastics is located in a odd position and also

it the most widely used method to product devices like this.

2.3 Functional Features 2.3.1 Button

As mentioned earlier in the report, the button will not be visible as it is placed underneath the top

case. This means, whenever the user wants to activate the laser, a certain force is required. It must

be ensured that the force required is not higher than the strength of the material. A FEM analysis of

the USB has been performed in Autodesk Inventor to clarify whether the design will withstand a

given load condition. It has been determined to apply a force of 10N on the top case. As it can be

seen on figure 2g, maximal the Von Miss Stress is 4,78MPa, meaning the design will not fail as the

material’s strength is higher.

Figur2g–FEManalysis


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2.3.2 Press fit

In order to ensure that the protective cap does not fall off

unintendedly, small ribs are added to the design.

When the cap is squeezed onto the USB, the width of the

ribs are getting slightly deformed (compression), resulting

in a friction force.

Initially the stresses in the material are high.

σ" = E×ε (1)

While time passes, the stresses are reduced, but by lower

and lower rates. This mechanism is called relaxation.

Another similar mechanism is creep. During creep,

instead of applying a constant strain and waiting for the

stresses to decrease, a constant stress (force) is applied

and the sample will deform over time – again at slower and slower rates.

Since creep data are more commonly provided by the suppliers, it is assumed for these early

calculations that the results from the two methods are sufficiently close to one another, so that it is

safe to solve this issue as if it was creep. Furthermore it has to be assumed that neither the cap or

the USB are deflecting, as well as the ribs are not bending. A final assumption is that the data

presented, which is for tension (unless otherwise mentioned), is also applicable for compression.

Compressive Young’s Modulus and yield strength have been found for ABS[1] (unspecified grade):

E()*+ = 2,5GPaσ2,()*+ = 65MPa

Inserting the values into Eq. (1) gives the highest tolerated deformation within the elastic region:

ε*56 = 2,6%

To stay within the allowed limits, a 1 % deformation during attachment of the cap is investigated.

With the earlier mentioned assumption, data of creep stress needed for a 1 % deformation to occur

after roughly 1 year (10’000 h) will be used to approximate the actual relaxation stress after the same

period of time.

Looking at isochronous stress-strain curves (aka. creep curves) for the material(figure 2i), the

deformation as a function of time and applied stress can be found. The 10’000 h curve at 1 % strain

Figur2h–Front view(Upper)and top view(lower)of

USBinsertedintothecap.


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returns the stress value of 10 MPa, which

can also be expressed as the normal force

by multiplying with the area of interface

between the ribs and the USB.

F9 = σ×A (2)

Then, the friction force can be calculated

by multiplying FN with the coefficient of

friction, µs, which has the general value of

0,1-0,3 for metal against plastics.

F; = µ=×F9orF; = µ=×σ×A

(3)

The first simple approach of computing the area of contact involves a rib thickness of 0,15 mm, a

contact length (in the logitudinal direction) of 10 mm and 6 ribs. This results in the force required to

remove the cap:

F; =0,10,3 ×10MPa×0,15mm×10mm×6

⇓9N ≤ F; ≤ 27N

(4)

For an initial starting point, this result looks really promising, as 5 N (0,5 kg) is the lowest allowable

force.

Following is an iterative process involving customised CAD drawings of parts and moulds, injection

flow analysis, FEM simulations, DFM rules, adjustment of polymer grade and if necessary tradeoffs

in product design. Also tests to determine the actual friction coeficient between the ABS of the cap

and the metal of the USB is required to perfect the design before production of the moulds can begin.

Figur 2i – Isochronous stress-strain curves of Novodur→ P2H-

AT(23℃).Source:Campus→5.2Database


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2.4 Computer Aided Design The USB design has been modelled in 3D using Autodesk Inventor. Furthermore, 2D technical drawings has been made, which can be seen in appendix 5 and appendix 6.

2.4.1 USB Cap

2.4.2 USB Case

2.4.3 Cross Section

Figure2j-Cap

Figure2k-Case

Figure2l-Verticalandhorizontalcross-sectionview


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2.4.4 Exploded View

Figure2m-ExplodedviewoftheUSB

2.4.5 Final Assembly

Figure2n-Finaldesign


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2.5 Design for Manufacturing In order to ensure the USB can be manufactured easily and economically, the product design has been designed in respect to manufacturing(DFM). Different guidelines for plastic parts, given in the lectures, have been used as reference for designing our product [2] . Below it can be seen, which design for manufacturing principles that has been applied for the cap and enclosure.

DFM Principle CAP Enclosure(Base + Top)

Uniform wall thickness Uniform wall thickness of 1mm has been applied, in order to prevent warpage.

Uniform wall thickness of 1mm has been applied, in order to prevent warpage.

Undercuts We have avoided undercuts as it requires special and expensive mould actions.

We have avoided undercuts as it requires special and expensive mould actions.

Draft angles Draft angles has been added on the ribs along the demoulding direction. The recommended value of ≥ 0.5° has been applied.

No draft angles has been applied as the curved profile will generate slip angels that will allow easy demoulding.

Corners and edges All internal and external corners and edges have as a minimum 0.1 radius, except the front interface surface in order to ensure a tight fit between the cap and enclosure.

All internal and external corners and edges have as a minimum 0.1 radius, except the front interface surface in order to ensure a tight fit between the cap and enclosure.

Ribs Ribs, inside the cap, has been designed with a thickness of 0.6 ∙𝑆OPQ to avoid sink marks and warpage.

Table 2d – Applied DFM principles

PROCESSDESIGN


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3. Process Design

Before getting to our final design and material, an iterative process with a lot of changes has been

done. Both the right design and the right commercial type of ABS plastic has been selected in the

end after a longer process with several reconsiderations. The chosen material is from Styrolution of

the type Novodur HH-106. This material has been selected based on several simulations, where

flow properties and part quality has been the main evaluation criteria. Furthermore, the final design

can be seen on Figure 3a and Figure 3b.

Figure 3a: Final Design of USB Pointer

Figure 3b: Final Design of Case and Cap


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When looking on the main part, it consists of two identical halves. The gate has been placed in the

inside of the part, so the mark from the gate when the part is moulded will not affect the design.

Furthermore, the gate has been placed in the middle to obtain good flow in the part. The case was

made with a wall thickness on 0,25 mm in the beginning, which resulted in a short shot. The design

has then been changed to 1 mm thick part. This can be seen on Figure 3c and Figure 3d:

Figure 3c: Confidence of Fill for 0,25 mm Wall Thickness (Left) and 1 mm Wall Thickness (Right)

Figure 3d: Plastic Flow in 0,25 mm Wall Thickness (Left Upper) and 1 mm Wall Thickness (Other Three)


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These figures from the simulations show that the case with a wall thickness of 1 mm will be filled

100% without any problems.

When looking further on the analysis, the quality of the part can be predicted. This analysis shows a

really good outcome, where only 0,79% of the part is considered to have a medium quality. The rest

is predicted to have a good quality, which means that this analysis accepts the part design. This

analysis can be seen on Figure 3e.

Figure 3e: Quality Prediction of the Case

When this part is run through the simulation software, it shows an injection pressure on 22,53 MPa,

which is the lowest pressure needed to fill the whole part. This injection pressure can be increased

as long as the shot will not create flashes.

Considering air traps and weld lines, it seems quite satisfying. There are obviously some air traps,

which have to be compensated by making air vents in the mold. The size of the air vents should be

in the manner, so the plastic cannot flow through. When looking on weld lines, we have a few ones

mostly around the cylindrical supports, which is due to that two flow fronts meet on the other side of

the injection spot. This is a normal outcome, which is not that important for this part, since it is not

exposed to high forces etc. However, there might be small visible marks at the weld lines, but this is

expected to be small enough to be accepted. See Figure 3f.


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Figure 3f: Air Traps (Left) and Weld Lines (Right) on the Case

The design of the cylindrical supports has also been a process with some minor troubles because

of the sink marks, which is likely to be created on the outer side of the case. This has been

compensated by making the support small enough to not create these sink mark. A smaller diameter

of the supports will compensate for this because of the lower relative shrinkage in those specific

areas. The maximum shrink mark depth is estimated to be 0,0073 mm, which is concluded to be

acceptable. The analysis of the sink marks can be seen on Figure 3g.

Figure 3g: Estimation of Sink Marks on the Part


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Last but not least, the packing of the part has been tested by analyzing the shrinkage and the

warpage. The analysis has been conducted with both 80% packing and 120% packing to see the

difference. The packing time has been set to 15 seconds to ensure that the part is packed until the

gate is frozen off. Time to reach ejection temperature has been used here, which lies within 15

seconds for both cases. The analysis can be seen on Figure 3h.

Figure 3h: Shrinkage and Warpage of the Case with 80% Packing (Left) and 120% Packing (Right)

The analysis shows that there are areas with a maximum shrinkage in the range of 6 - 7,5%.

Furthermore, the maximum deflection caused by warpage is in the range of 0,3 - 0,35 mm. As

already mentioned, the minimum needed injection pressure is 22,53 MPa, which is quite low. By

increasing this and thereby also the packing pressure, it should be possible to compensate more for

the shrinkage and the warpage. This is considered as further work.


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When looking on the cap, the same wall thickness has been to the design is uniform when

assembled. The same analyses have been made for this part as well, which turned out to be less

problematic than the case. First of all, the confidence of filling and the plastic flow was analyzed,

which can be seen on Figure 3i.

Figure 3i: Confidence of Fill and Plastic Flow of the Cap

This analysis shows clearly that the whole part can be filled without any problems. This has lead to

the next step of predicting the quality of the part. This shows areas corresponding to 3,39% of the

whole part, which is expected to end up in a medium quality. This has been accepted since it mostly

appears on the ribs inside the cap. The design of the ribs has still not been completed totally, since

both more calculations, FEA, and more flow simulations have to conducted. This simulation can be

seen on Figure 3j.


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Figure 3j: Quality Prediction of the Cap

When the cap is run through the simulation software, it shows an injection pressure on 13,26 MPa,

which is the lowest pressure needed to fill the whole part. This injection pressure can be increased

as long as the shot will not create flashes.

The analyses for air traps and weld lines look quite satisfying. Air traps can as already mentioned

be prevented by creating air vents in the mold. The air vents should be dimensioned, so the plastic

cannot flow through. Regarding weld lines, we have a few ones because of the ribs. Two flow fronts

from each side of the ribs are meeting and creating these weld lines. As already stated, the final

design of the ribs is still not made, which means that this analysis should be considered for the

redesign as well. The analysis can be seen on Figure 3k.

Figure 3k: Air Traps and Weld Lines on the Part


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One of the most important things to consider about the ribs is the sink marks, which possibly can be

created on the outer side of the ribs. The difficult thing here is to fulfill both the requirements of the

press fit calculations while also following the guidelines for compensating sink marks. However, it

has been possible to come up with an acceptable solution, which can be seen on Figure 3l.

Figure 3l: Sink Marks Estimation on the Cap

As it can be seen on Figure 3l, there are green areas on the outer side of the side ribs. The rib design

has still been accepted at this point, since these sink marks are estimated to have a depth of

approximately 0,087 mm. This is considered as low. However, this analysis will also be kept in mind

when redesigning the ribs in the future development.

Last but not least, the packing has also been analyzed for the cap. Exactly the same packing

pressure and packing time as used for the case has been used for the cap. This needs also

modifications like for the case, since the injection pressure is quite low. This might also need to be

increased together with the packing pressure. The analysis for the cap with a packing pressure on

120% for 15 seconds has given a maximum shrinkage of 5.572% and a maximum deflection on

0,1209 mm due to warpage. This might also be possible to optimize, which is considered as further

work. This analysis can be seen on Figure 3m.


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Figure 3m: Shrinkage and Warpage of the Cap

3.1 Process Parameters As a summary, the process parameters for both the case and the cap can be seen on Table 3a.

These values are the ones, which are extracted at the current stage with the development done so

far.

Case Cap

Injection Pressure (Minimum) 22,53 MPa 13,26 MPa

Packing Pressure (Test) 120% 120%

Packing Time (Minimum) > 7 s > 9 s

Filling Time < 1 s < 0,5 s

Cycle Time (Expected) 15 - 20 s 15 - 20 s

Table 3a: Process Parameters for the Case and the Cap

The cycle time is based on assumptions and expectations from the simulation software, which may

vary a lot.

MOULDDESIGN


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4. Mould Design A well designed mould would aid in the manufacture of technically sound plastic products of the

highest quality. In our case, we have assumed for a very high productivity rate and hence we decided

to use a hot runner mould. The preferred material for the mould would be tool steel with a hardness

of at least 55 HRC. And we have chosen to make a 3 plate hot - runner mould because of the

geometry of the part, and also the runner geometry, the cavity orientation in the die and increased

productivity.

4.1 3-D Models The 3D mould design with the sprue, runners, gates, cavities, cores and ejector pins for the cap and

casing are as shown in figures 4a and 4b respectively. For both, the cap and the case, the plate

numbered 1 is the ejector plate that carries the ejector pins, and has a hole big enough to

accommodate the injection nozzle. Mould numbered 2 carries the sprue and a half of the runner

while mould numbered 3 carries the other half of the runner and the gate, with the core on the other

side. Mould numbered 4 has the cavities. For the sake of simplicity and ease of explanation, both

the moulds have been designed with only 4 cavities in them.

Figure4aSideviewofassembledmoulds-cap

1 2 3 4

Ejector pins

Sprue

Runner

Pin Gate Core

Cavity

Injection

Nozzle


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Figure4bSideviewofassembledmoulds–case

In the following sections, the design considerations and the location of sprue, runners, gates and

ejectors are discussed in detail. The analytical method used for designing the dimensions for the

same have been explained in detail in section 4.6.

4.2 Sprue The sprue used here is a tapered sprue, to maintain the desired pressure during the injection

moulding process. The sprue that has been used for both the casing and the cap is as shown in

figures 4c and 4d respectively. Since the mould is a hot runner mould, the problem of part ejection

with respect to the sprue is not significant. Also, the only difference between the sprue for the casing

and cap is only in terms of dimensions and not the geometry.

1 2 3 4

Pin Gate

Core

Runner

Ejector Pins

Sprue Cavity

Injection

Nozzle


30

4.3 Runner The runner used for the case and the cap are the same, that is the geometry of the runners used

here are circular runners. Though costly, the constrain posed by the manufacturing process for

making the die for the given cavities, and also considering that the mould is a hot runner mould,

huge material savings can be made.

Figure4eMouldswithsprue,runnerandgate-cap

Figure4dCloseupofsprue-cap Figure4cCloseupofsprue-case

Runner

Sprue

23

Gate

Gate

2 2

Ejector

Holes


31

Figure 4e shows the two runner halves that are cut into the two mould havles for the mould. The

mould shown here is for the cap and the runner system looks exactly similar for the case as well, as

shown in figure 4f.

Figure4fMouldswithsprue,runnerandgate-case

4.4 Gate The gate used for the cap and casing are pin point gate. This makes the degating process easy. The

positioning of the gate has been optimised for and based on the simulations run for the same. The

positioning of the gate was also so chosen that the aesthetics is not affected by it. It is located on

the inner surface of the cap and case, hence this will also help conceal the gate mark after degating.

2 3

Runner

Sprue

Gate

Gate


32

Figure4gSectionedviewofgate-cap

Figure4hSectionedviewofgate–case

3 Core

3Core

Gate

Gate


33

4.5 Ejector Placement The case and the cap both have cores. And when the plastic shrinks at the end of the injection

moulding cycle, they tend to cling on to the cores. The ejector pins have been placed taking this into

consideration. Hence the ejector pins have their axis aligned with axis of the cores. Also the plate

has been located on the injection side, as shown in figures 4a and 4b. A total of 4 ejector pins have

been used per cavity for the cap, while 6 ejector pins per cavity have been used for the case. If there

were odd number of pins, the ejection force will not be completely balanced and the ejectors would

result in distortion of the finished product. Hence the choice for even number of ejectors.

The ejectors pins here have been designed with a diameter that is less than diameter of the ejector

holes in the moulds, so that there is a small clearance or gap that they can act as vent holes and

can facilitate the expulsion of air and other gases from within the mould cavity. But this can be

simulated and optimised for the exact dimensions so that the plastic is not wasted.

The ejector plate for the cap and the case are shown in figure 4i and 4j respectively. In figures 4k

and 4l, the mould with the ejector holes are shown along with a close up view of one of the cores.

In addition, detailed views of moulds 3 and 4 are shown in figures 4m and 4n.

Figure4iEjectorPlate-Cap Figure4jEjectorPlate-Case

1 1


34

Figure4kEjectorholes–Cap(Closeupofonecore)

Figure4lEjectorholes–Case(Closeupofonecore)

EjectorHoles

Ejector

Holes

3

3


35

Figure4mExplodedviewofmoulds3and4-Cap

Figure4nExplodedviewofmoulds3and4-Case

3

3 4

4

Cavity

Gate

Ejectorholes

Core

CavityCore

Gate

EjectorHoles


36

4.6 Mould design calculations: In our case, we have primarily designed the sprue, runners and gate dimensions for the USB casing

and cap, based on simple assumptions and logic. Here, we used the shear rate as the limiting factor.

In other words, the mould is so designed that the shear rate of the polymer in the cavity is less than

the maximum allowed shear rate for ABS. The maximum allowed shear rate for ABS is 50000 s-1.

The following dimensions were calculated based on the final process parameters from moldflow

simulation. Also the mould has been decided to be a hot runner mould. This was done because of

the high production volumes that would be needed for the part. Though costly, material loss can be

reduced significantly, in terms of the sprue, runner and gates. And also degating becomes easy.

The volumetric flow rate can be calculated using the formula [3]:

𝑄 =𝐶𝑎𝑣𝑖𝑡𝑦𝑉𝑜𝑙𝑢𝑚𝑒

𝐹𝑖𝑙𝑙𝑡𝑖𝑚𝑒×𝑁𝑢𝑚𝑏𝑒𝑟𝑜𝑓𝐶𝑎𝑣𝑖𝑡𝑖𝑒𝑠

Cavity volume for casing = 1.187542 cm3

Assuming a fill time for casing = 0.5 s

Number of cavities = 4

Based on these values, 𝑄 = 0.593771𝑐𝑚f

Using the formula for shear rate for a round gate as

𝛾 =32×𝑄𝜋×𝑑f

Where d is the diameter of the gate. The maximum allowed shear rate (𝛾) for ABS is 50000 s-1. Here

we assume the gate diameter to be 1 mm which is the same as the wall thickness. We get a

calculated shear rate as 6000 s-1. While for 0.5 mm the shear rate is 50000 s-1. Hence any dimension

between these values is a safe dimension for the gate. A very high diameter is not advisable [4], and

the maximum diameter was fixed at 1mm and minimum at 0.5mm. It was decided that the pinpoint

gate with a circular cross section is the best solution. And a circular runner was chosen as well, and

the diameter of the runner was calculated using the formula

𝐷 = 𝑠OPQ + 1.5

Here the thickness 𝑠OPQis 1mm. Hence this resulted in a circular runner of 2.5 mm.

The volumetric flow rate for the entire mould can be calculated as


37

𝑄 =𝑁𝑜. 𝑜𝑓𝑐𝑎𝑣𝑖𝑡𝑦×𝑂𝑛𝑒𝑐𝑎𝑣𝑖𝑡𝑦𝑣𝑜𝑙𝑢𝑚𝑒

𝐹𝑖𝑙𝑙𝑡𝑖𝑚𝑒

Since it has been decided to have 4 cavities in the mould, flow rate was𝑄 = 9.5𝑐𝑚f. Based on this

value, the dimension for the circular runner was calculated using the shear rate formula𝛾 =

32×𝑄/𝜋×𝑑f, and the diameter for the shear rate of 6000 s-1 was 2.5 mm, the same as the runner.

For a diameter of 1.3 mm the shear rate is close to 50000 s-1. Hence a tapered sprue with a maximum

diameter of 2.5 mm and a minimum of 1.5 mm.

In a similar manner, the cavity volume for the cap was calculated to be 0.963 cm3.

Based on this volume and the previously used equations, the dimensions for the different aspects of

the mould for the cap was calculated as well. Since the volume is quite close to the volume of the

case, the gate dimension was retained between a maximum of 1 mm and minimum of 0.5 mm.


38

References

Internet

1. http://www.matweb.com/reference/compressivestrength.aspx,

accessed: 09 May 2016.

2. .https://www.campusnet.dtu.dk/cnnet/filesharing/SADownload.aspx?FileId=4081956&FolderId=

974398&ElementId=506619,

Accessed 08 May 2016

3. http://www.pcn.org/Technical%20Notes%20%20Gates%20and%20runners.htm,


4. http://www.teijin.com/products/resin/technical/images/pc_sekkei_gate_pinpoint.jpg,


Software

1. CES Edupack

2. CAMPUSplastics

3. Autodesk Inventor

4. Autodesk Moldflow Adviser

DTU Mechanical EngineeringDepartment of Mechanical Engineering

APPENDICESDevelopment of USB with LASER Design of Plastic Parts - 41737Spring 2016

Groub 13Daniel Sartou s090290Gautham Ramachandren s150906Georgios Pitsilis s152087Piriyankan Arudselvam s152177Sujapragash Ramesh s152182Wahib Abboud s113161


2

Table of Contents Appendix1 3

APPENDIX2 3

APPENDIX3 3

APPENDIX4 4

APPENDIX6 5

APPENDIX7 6


3

Appendix 1

Appendix 1 - Fatigue strength in function of Yield strength.

APPENDIX 2

Appendix 2 - Impact strength in function of the price for the ABS products.

Yield strength (elastic limit) (MPa)10 20 50 100

Fatig

ue st

reng

th a

t 10^

7 cy

cles (

MPa

)

5

10

20

50





Polyurethane (tpPUR)


Polypropylene (PP)


Polyester

Epoxies


Polyethylene (PE)

Ionomer (I)

Price (EUR/kg)2 5 10 20 50 100 200

Impa

ct st

reng

th, u

nnot

ched

23

°C (k

J/m

^2)

10

100

1000

10000

ABS (heat resistant, injection molding)


4

APPENDIX 3

Appendix 3 - Moldability.

APPENDIX 4

Appendix 4 - Relative cost index of processes in function of Economic bath size.

Moldability

3

3,5

4

4,5

5


Polyester


Phenolics

Polystyrene (PS)

Polyethylene terephthalate (PET)Polycarbonate (PC)

Ionomer (I)


Economic batch size (units)1 10 100 1000 10000 100000 1e6 1e7

Rela

tive

cost

inde

x (p

er u

nit)

(EUR

)Lo

ad F

acto

r=0,

5, C

apita

l Writ

e-of

f Tim

e=5y

rs, O

verh

ead

Rate

=82,

4EUR

/hr,

Batc

h Si

ze=1

e3, C

ompo

nent

Mas

s=1k

g, M

ater

ial C

ost=

7,49

EUR/

kg, C

ompo

nent

Len

gth=

1m

10

100

1000

10000

Injection molding (thermosets)

Injection molding (thermoplastics)

Injection blow molding

Extrusion blow molding

Rotational molding

Glass blow molding

Transfer molding

Reaction injection moldingThermoforming

Polymer casting

Precision glass molding

SECTION A-A

SCALE 4 : 1

SECTION B-B

SCALE 4 : 1

A A

B

B

1

1

2

2

3

3

4

4

5

5

6

6

A A

B B

C C

D D

SHEET 1 OF 1

DRAWN

CHECKED

QA

MFG

APPROVED

Group 13

10-05-2016

DWG NO

2

TITLE

USB with laser - Cap

SIZE

A3

SCALE

Technical University of Denmark

REV

4 : 1

2.40

R.10

11.80

R.10

R16.77

16.40

R.1021.80

R.10

13.00

R.10

R.10

1.00

5.00

9.00

10.00

12.00

2.50

15.50

16.50

.50

175.0°

.26

All internal corners:

0,1mm radius

Symmetry Line

10.00

2.50

DETAIL B

SCALE 5 : 1

DETAIL A

SCALE 5 : 1

BA

1

1

2

2

3

3

4

4

5

5

6

6

A A

B B

C C

D D

SHEET 1 OF 1

DRAWN

CHECKED

QA

MFG

APPROVED

Group 13

10-05-2016

DWG NO

1

TITLE

USB with laser - Case

SIZE

A3

SCALE

Technical University of Denmark

REV

2 : 1

55.00

22.00

5.00

31.10

8.50

5.00

8.50

12.00 5.005.00

R.50

R.50

3.50

5.00

8.50

5.00

27.50

R.10

3.85

Centerline

1.00

R40.02

.80

1.00

R.60

development of usb with laser

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