From Art to Part 2009From Art to Part 2009Plastics Plastics Part DesignPart Design

By: Sidney Wong & KM HoTel: 27667616, 27667620

E-mail: icsidney@polyu.edu.hkickmho@polyu.edu.hk

Intended Learning OutcomesIntended Learning Outcomes

After completing this training session, YOU are expected able to:

� Address the key design concepts for thermoplastic part design

� List out the common thermoplastic part assembly methods , select and applying them in the group methods , select and applying them in the group project

� Apply general plastic design rules/ simple calculation in designing a thermoplastic product

General Design Rules of Injection General Design Rules of Injection MouldMouldeded PartPart

�Uniform wall thickness�Part Geometries�Draft Angle

Radii / Fillet�Radii / Fillet�Structural Ribs Design�Bosses �Snap-Fit Design

Uniform Wall ThicknessUniform Wall Thickness (UWT)(UWT)

� Why Uniform Wall Thickness is important in plastic part design?

� Non-uniform and/or heavy wall thicknesses can cause serious warpage and dimensional control cause serious warpage and dimensional control problems in the injection molded products. Heavy wall sections cause not only internal shrinkage, voids, and surface sink marks, butalso non-uniform shrinkage resulting in poor dimensional control and warpage problems.

Uniform Wall ThicknessUniform Wall Thickness (UWT)(UWT)

Uniform Wall ThicknessUniform Wall Thickness (UWT)(UWT)RIBS

How to achieve UWTHow to achieve UWT

Coring （（（（偷空偷空偷空偷空））））should be employed where possible to eliminate material masses in the part. When wall thickness transitions cannot be avoided, the transition should be made gradually, on the order of 3 to 1.

Part GeometriesPart GeometriesAlthough UWT is achieved, sometimes the part geometries itself will generate moulding problems

Part GeometriesPart Geometries

Task 1Task 1

�Try to modify below part geometry to improve the manufacturability

Draft AngleDraft Angle

Draft angles for internaland external walls areessential to the ejectionof the moulded partsf r o m t h e m o u l d .f r o m t h e m o u l d .External walls requiresmaller draft anglesthan the internal walls.

Draft AngleDraft Angle

Minimum Draft AngleMinimum Draft AngleExternal Wall Internal Wall

W/O Texture

W/ Texture W/O Texture

W/ Texture

Depth < 1” 0° 15′ to 0°30′

(0° 15′ to 0°30′) +

0° 30′ to 1° (0° 30′ to 1°) +30′ 30′) + +

Depth > 1” 1.0° to1.0° 30′

(1.0° to1.0° 30′) +

1.0° 30′ to 3.0°

(1.0° 30′ to 3.0°) +

Radii / FilletRadii / Fillet

Internal sharp corners and notches are the leading cause of failure in injection moulded thermoplastic parts. To avoid the problem occurred, radii / fillet is commonly employed to all “sharp” feature all “sharp” feature

Radii / FilletRadii / Fillet

A fillet radius should be between 25 to 60% the nominal wall thickness. The larger fillet radius is suggested for load carrying features

Radii / FilletRadii / Fillet

The outside corner radius should be equal to the inside radii plus the wall radii plus the wall thickness (R = r + t).

Structural RibStructural Ribss DesignDesign

�Rib is one of the common features used in plastic part design to strength the structure and to reduce the weight of the product.

Structural RibStructural Ribss DesignDesign

� However, for many resins, the use of ribs will produce sink marks on the external surface and this defect becomes very noticeable on the moulded product.

� There are several resins on the market that have good surface appearance behind the rib area, for example, PVC, ABS, PC, LCP, PBT, PETPVC, ABS, PC, LCP, PBT, PET

Structural RibStructural Ribss DesignDesign

Structural RibStructural Ribss DesignDesign•The thickness of the rib at the intersection withthe nominal wall should be 50 to 60% of thenominal wall.•Maximum rib height: h=3 x nominal wall thickness.•Typical draft for ribs is 1 to 1.5°. Minimum draftshould be 1/2° per side.•Spacing between two parallel ribs should be a•Spacing between two parallel ribs should be aminimum of 2 x wall thickness.

Task 2Task 2

�Try to integrate the concept of “Draft”, “Fillet” & Rips to enhance the design

BossesBosses

�Bosses are thermoplastic cylinders attached to a side wall or end corners. Special self-tapping screws are used to mount other components. The boss’s outside and hole’s inside diameters are outside and hole’s inside diameters are based on size, depth and type of screws, pullout torque requirements, resin modulus of elasticity, creep, and boss weld line strength.

BossesBosses

BossesBosses

BossesBosses

� Typically the boss OD = 2 ID.� The wall thickness at the base of

the boss should remain less than 60% of the nominal wall thickness.

� The boss height should be less than 3 ´ OD.than 3 ´ OD.

� Draft on the OD is 1/2° and ID is 1/4° Min.

� Keep the minimum distance of twice the nominal wall thickness between 2 bosses.

SnapSnap--Fit DesignFit Design

� Snap fits are commonly used as an assembly method for injection molded parts. Snap fits are very useful because they eliminate screws, clips, adhesives, or other joining methods. The snaps are molded into the product, so additional parts are molded into the product, so additional parts are not needed to join them together.

� There are three main types of snap fits: Annular, Cantilever, and Torsional.

http://engr.bd.psu.edu/pkoch/plasticdesign/snap_design.htm

Annular Snap FitAnnular Snap Fit

� ASJs are generally stronger, but need greater assembly force than their cantilevered counterparts.

� Annular Snap Fit are basically interference rings. � Annular Snap Fit are basically interference rings.

http://machinedesign.com/ContentItem/61167/FundamentalsofAnnularSnapFitJoints.aspx

Annular Snap Fit Annular Snap Fit –– childchild--proof bottlesproof bottles

The ridge geometry of the annular snap-fit plug determines the assembly force F needed to engage the joint. A shallow return angle of 30°A shallow return angle of 30°easily separates while a 90°angle is permanent. Designers of child-proof bottles employ a clever trick to transform a permanent snap joint into one that easily disengages.

http://machinedesign.com/ContentItem/61167/FundamentalsofAnnularSnapFitJoints.aspx

Key points of applying Annular Snap FitKey points of applying Annular Snap Fit

� Don’t use cylindrical snap-fits with very stiff materials, making the plug from a more rigid material than its mating female hub.

� Use an engagement angle of 20° to 30° and a Use an engagement angle of 20 to 30 and a release angle of 40° to 50°.

� Place the undercut near the open end of the hub.� Size the undercut so that the design stress

figure is not exceeded.

Cantilever Snap FitCantilever Snap Fit

Cantilever snap fits are the most widely used type of snap fit. There is a considerable amount of calculation and of calculation and engineering that goes into designing a good snap fit

http://engr.bd.psu.edu/pkoch/plasticdesign/snap_design.htm

Cantilever Snap FitCantilever Snap Fit

http://engr.bd.psu.edu/pkoch/plasticdesign/snap_design.htm

Key points of applying Cantilever Snap FitKey points of applying Cantilever Snap Fit

� Keep within the allowable strain figure.� If the calculated allowable deflection is too small,

try increasing the snap hook length.� Design so that the snap hook is no longer flexed

after it has clicked into the catchafter it has clicked into the catch� Snap-fits are meant to be used either once or

just a few times, so fatigue and wear can be neglected.

� Radius the root of the snap hook to reduce stress concentration.

Torsional Snap FitTorsional Snap Fit

� The torsional snap-fit relies for its spring effect on twisting rather than flexing like the other types.

� It is a good way of fastening a hinged lid on a box or container.box or container.

Key points of applying Torsional Snap FitKey points of applying Torsional Snap Fit

� Use torsional snap-fits when you want to be able to release the catch easily.

� Include a design feature to show where to press.� Design a stop feature to prevent excessive

torsion.torsion.� Do not make the catch lever length too short

otherwise the twist angle and torsion becomes too great.

� Reduce the opening force by making the length of the opening lever longer than the catch lever.