• Designed a polypropylene injection molded plastic box in SolidWorks • Performed the necessary engineering calculations for all snap fits, living hinges, bosses, and draft angles

1. David Sandy
Polypropylene Injection Molded Plastic Box Designed in SolidWorks
DesignGuidelines:
Create a computer drawing or hand sketch of an injection-mold ready plastic box. You
must include all the relevant dimensions on your sketch.
 The box is made from polypropylene
 The box must be 3 in. x 4 in. x 1 in.
 The box must contain a lid that is attached by a living hinge
 The box lid must have a snap feature that holds it closed but must also be able to be opened by hand without tools
 Your design must not exceed the yield strength ofthe plastic
 List any additional design criteria that you used when designing yourbox
 Specify the surface finish for the mold
 Identify possible gate locations consistent with plastic flow and living hinge orientation
 Your box must include at least one dividing rib / section dividing your box into two internal compartments
 Your box must contain at least one boss designed with the proper dimensions to accept a #2-28 plastite screw
Final SolidWorks Designand Calculations

2. Gate Location

3. Surface Finish: Low Gloss Finish
bossO.D.: 3*0.092=.276 in
CBw: 0.167 in
CBd: 0.017857143 in
L: 3*.092=0.276 in
hθ: 0.0771 in
Calculationsfor#2-28 Plastite Screw Boss

4. Recommended Living Hinge Dimensions in Polypropylene:
Thickness: .5mm = .019685in
Width: 1.5mm = .0590551in
Relief Thickness above hinge: .2mm = .00787402in
Snap Fit:

5. Max. deflection from snap fit: Y =
𝑒𝑙2
1.5𝑡
=
.04 ∗.42
1.5∗.03
= .14222 in
Required deflection: y = .05 in
Required Normal Force (P): P =
𝑤𝑡2
𝐸𝑒
6𝑙
=
.6∗.032
∗188549∗.04
6∗.4
= 1.6969 lb
Force (W) needed to engage the snap fit: W = 𝑃
𝜇+tan 𝑎
1−𝜇 tan 𝑎
= 1.6969*
.3+tan 25
1−.3∗tan 25
= 1.51188 lb
Force (W) needed to release the snap fit: W = 𝑃
𝜇+tan 𝑏
1−𝜇 tan 𝑏
= 1.6969*
.3+tan 35
1−.3∗tan 35
= 2.14864 lb
Sources:
http://www.tangram.co.uk/Design%20Guides%20for%20Plastics.PDF for polypropylene living
hinge dimensions (p. 18-19)
http://www.tangram.co.uk/Design%20Guides%20for%20Plastics.PDF for snap fits (p.29-30)
http://www.stanleyengineeredfastening.com/sites/www.emhartamericas.com/files/downloads/Th
readed_Fasteners_for_Plastics_EN.pdf for boss design calculations (p. 18-20)
https://techcenter.lanxess.com/scp/americas/en/docguard/Part_and_Mold_Design_Guide.pdf?doc
Id=77015 for surface finish gate locations
https://www.mcmaster.com/#96001a152/=uh8t9l
Additional Sources:
http://web.mit.edu/2.75/resources/random/Snap-Fit%20Design%20Manual.pdf