me3263e/tme3263 design for manufacturing &...
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ME3263E/TME3263
DESIGN FOR MANUFACTURING & ASSEMBLY
Homepage: http://courses.nus.edu.sg/course/mpeleeks/personal/index.htm
A/P Lee Kim Seng Deputy Head (Outreach) Mech Mechanical Engg Dept. National Univ. of S’pore
Room: EA 04-17 Tel: 6516 2574 mpeleeks@nus.edu.sg
Design for Assembly of Injection Molded Plastic Parts
A/P Lee Kim Seng
PLASTIC PART DESIGN FOR MANUFACTURING & ASSEMBLY
Contents
A/P Lee Kim Seng
1. General accepted design guidelines
2. Assembly Techniques
Self securing techniques: Press fitting
Riveting (Staking)
Ultrasonic welding
Snap fit
3. Snap fit/joint design considerations
4. Other Assembly techniques: Mechanical Fasteners
Pressure and spin welding
Hot Plate welding
Adhesive bonding
5. Design Considerations for Plastic Parts Assembly
Design Guideline for Injection Molding Components
General accepted design guidelines:
1. Design the main wall of uniform thickness with adequate tapers or draft for easy release from the mold. This will minimize part distortion by facilitating even cooling throughout the part.
2. Choose the material and the main wall thickness for minimum cost. Note: That a more expensive material with greater strength or stiffness may often be the best choice. The thinner wall which this choice allows will reduce material volume to offset the material cost increase. More importantly, the thinner wall will significantly reduce cycle time and hence processing time.
A/P Lee Kim Seng
Design Guideline for Injection Molding Components
General accepted design guidelines:
3. Design the thickness of all projections from the main wall with a preferred value of ½ of the main wall thickness and do not exceed 2/3 of the main wall thickness. This will minimize cooling problems at the junction between the projection and main wall where the section is necessarily thicker.
4. Preferably align projections in the direction of molding or at right angles to the molding direction lying on the parting plane. This will eliminate the need for mold mechanisms..
A/P Lee Kim Seng
Design Guideline for Injection Molding Components
General accepted design guidelines:
5. Avoid depressions on the inner side surfaces of the part which would require ‘Lifters’ (expensive). Through holes on the side surfaces instead of internal depressions are preferred as the former can be produced with sliders (cheaper).
A/P Lee Kim Seng
Design for Assembly of Injection Molded Plastic Parts
Assembly Techniques One of the major advantages of injection molding is its ability to easily incorporate, in the molded parts, effective self-securing techniques.
A/P Lee Kim Seng
Self securing refers to the ability to achieve a secure assembly without the use of separate fasteners or the addition of a separate bonding agent.
Design for Assembly of Injection Molded Plastic Parts
Self-securing Techniques Press Fitting
Riveting The above two self-securing techniques are also widely used with metal parts.
Self-securing techniques unique to plastic parts:
Ultrasonic welding
Snap Joint
A/P Lee Kim Seng
Design for Assembly of Injection Molded Plastic Parts
Press Fitting With press fitting much larger interferences are possible with injection molded parts than is the case with metal parts.
Molded plastic parts such as gears, wheels or bearings are commonly mounted on shafts using a press fit.
A/P Lee Kim Seng
Design for Assembly of Injection Molded Plastic Parts
Press Fitting
The advantage of press fitting is that it requires less precise tolerance control.
A/P Lee Kim Seng
The negative aspect of plastic press fit is that the material is constantly under stress and will invariably relax over a period of time to produce degradation of the joint strength.
Design for Assembly of Injection Molded Plastic Parts
Riveting Integral rivets are easily produced through inexpensive feature additions to the mold.
A/P Lee Kim Seng
Assembly of rivet heads are easily formed by cold heading or by the use of heated forming tools (staking).
Design for Assembly of Injection Molded Plastic Parts
Staking
Typical staked joint: the hole is completely filled by compressing the stud.
Staking by reverse taper: the stud is wider at the base and has no dome.
staking
before after
hole
stud
hole
stud before after
staking
A/P Lee Kim Seng
Design for Assembly of Injection Molded Plastic Parts
Staking
A/P Lee Kim Seng
Design for Assembly of Injection Molded Plastic Parts
Core pin design
Chamfer on the end of the core pin (A) creates two stress risers in the plastic part (B).
Core pin radius (A) is the ‘negative’ of the radius in the molded part (B).
A/P Lee Kim Seng
Design for Assembly of Injection Molded Plastic Parts
Plastic Stud Design
For plastic studs, the hole entrance should be well rounded, and the radius at the end of the hole should protect against stress in the cavity.
A/P Lee Kim Seng
Design for Assembly of Injection Molded Plastic Parts
Ultrasonic welding
Ultrasonic welding equipment involves a special fixture to hold and clamp the parts and through which a high-frequency vibration of approximately 20 kHz is passed.
This is a method of joining two or more molded parts through the generation of intermolecular frictional heat at the assembly interface.
A/P Lee Kim Seng
Design for Assembly of Injection Molded Plastic Parts
Ultrasonic welding Ultrasonic welding is a good economic choice where sealed joining is required, since the equipment is relatively inexpensive and the process is fast.
Welding is accomplished typically in 2 s.
A/P Lee Kim Seng
Design for Assembly of Injection Molded Plastic Parts
Ultrasonic welding
A/P Lee Kim Seng
Telephone hand set pre-assembled with snap fits and then ultrasonic welded
Types of Ultrasonic welding
A/P Lee Kim Seng
Design for Assembly of Injection Molded Plastic Parts
Snap Fit/Joint Snap fit is the most widely recognized self-securing method for plastic molded parts.
A/P Lee Kim Seng
Design for Assembly of Injection Molded Plastic Parts
Types of Snap Fit
1. Snap fit developed for mating parts of circular cross-section.
2. Snap fit design involves the use of one or more cantilever snap elements.
A/P Lee Kim Seng
Design for Assembly of Injection Molded Plastic Parts
Annular snap fit design
Cantilever snap fit elements:
A/P Lee Kim Seng
Design for Assembly of Injection Molded Plastic Parts
Cantilever snap fit elements: A/P Lee Kim Seng
Types of Cantilever Snap beams
A/P Lee Kim Seng
One way snap fit, vertically tapered beam: (a) rectangular cross section, (b) trapezoidal cross section
One way snap fit, horizontally tapered beam: (a) rectangular cross section, (b) trapezoidal cross section
Types of Cantilever Snap beams
A/P Lee Kim Seng
Types of Cantilever Snap beams
A/P Lee Kim Seng
Two way snap fit, vertically tapered beam: (a) rectangular cross section, (b) trapezoidal cross section
Two way snap fit, horizontally tapered beam: (a) rectangular cross section, (b) trapezoidal cross section
Types of Cantilever Snap beams
A/P Lee Kim Seng
Types of Cantilever Snap Fits
A/P Lee Kim Seng
Cantilever snap fit U shape snap fit
L shape snap fit Annular snap fit
Design for Assembly of Injection Molded Plastic Parts
Snap Fit/Joint Design Consideration
1. Cantilever snap fit element and its mating undercut should be designed for molding without mold mechanism.
2. Alternative design involves the use of sliders and lifters to mold the undercut.
For a large enough production volume the extra mold costs can be easily outweighed by the subsequent savings in assembly cost.
A/P Lee Kim Seng
Design for Assembly of Injection Molded Plastic Parts
Cantilever snap fit elements: (a) undercuts formed by core rods. (b) undercuts formed by sliders.
A/P Lee Kim Seng
Design for Assembly of Injection Molded Plastic Parts
Cantilever snap beams: undercuts formed by core rods.
A/P Lee Kim Seng
Design for Assembly of Injection Molded Plastic Parts
Cantilever snap beams: undercuts formed by core rods.
A/P Lee Kim Seng
Design for Assembly of Injection Molded Plastic Parts
A/P Lee Kim Seng
Design for Assembly of Injection Molded Plastic Parts
A lifter is used to assist in the ejection of a cantilever snap beam.
A/P Lee Kim Seng
Design for Assembly of Injection Molded Plastic Parts
The cantilever snap beam can be ejected by simply stripping the part from the core.
A/P Lee Kim Seng
Design for Assembly of Injection Molded Plastic Parts
Modified ejector pin can also be used to produce rounded snap beam.
A/P Lee Kim Seng
Snap Fit/Joint Design Consideration
Annular snap fits are commonly used with more flexible polymers. A common application for an annular snap assembly is a push on bottle cap.
Snap assemblies can be designed to be either separable or inseparable.
A/P Lee Kim Seng
Snap Fit/Joint Design Consideration
Cantilever snap beams are commonly used for the assembly of plastic parts.
A variety of both separable and inseparable configurations are possible.
A/P Lee Kim Seng
Snap Fit/Joint Design Consideration
The slotted annular snap assembly is actually a series of cantilever snap beams. This approach is more suitable for rigid polymers.
A/P Lee Kim Seng
Snap Fit/Joint Design Consideration
Cantilever snap beams with a 90° return angle must be manually deflected for separation.
Cantilever snap beams with ramping return angles can be snapped in and out. The angles of inclination and the beam geometry control the relative assembly and disassembly forces.
A/P Lee Kim Seng
Snap Fit/Joint Design Consideration
Cable ignition bracket
Cable ignition bracket - 2D view
A/P Lee Kim Seng
Snap Fit/Joint Design Consideration
A/P Lee Kim Seng
Design for Assembly of Injection Molded Plastic Parts
Other Assembly Techniques:
Pressure and spin welding
Mechanical Fasteners
Adhesive bonding
A/P Lee Kim Seng
Hot Plate welding
Mechanical Fasteners
Design for Assembly of Injection Molded Plastic Parts
A/P Lee Kim Seng
Mechanical Fasteners
Design for Assembly of Injection Molded Plastic Parts
A/P Lee Kim Seng
Mechanical Fasteners
A/P Lee Kim Seng
Design for Assembly of Injection Molded Plastic Parts
Excessive thick and thin section at the base of the blind boss should be avoided.
A/P Lee Kim Seng
Mechanical Fasteners
Design for Assembly of Injection Molded Plastic Parts
A/P Lee Kim Seng
Wall too thick Redesign – even wall thickness
Mechanical Fasteners
Design for Assembly of Injection Molded Plastic Parts
A/P Lee Kim Seng
Design for Assembly of Injection Molded Plastic Parts
Pressure and spin welding
A/P Lee Kim Seng
Design for Assembly of Injection Molded Plastic Parts
Hot Plate welding
A/P Lee Kim Seng
Hot plate welding is achieved by placing a heating strip between the parts to be assembled. When the edges of the parts become soft, the strip is removed and the parts are quickly brought together.
Design for Assembly of Injection Molded Plastic Parts
Hot Plate welding
A/P Lee Kim Seng
Materials included: Acrylic lens to ABS lamp housing; Polycarbonate lens to same lamp housing PC lens to ABS housing.
(a) Automotive coolant system pressure reservoir & (b) Brake fluid reservoir made of polypropylene and assembled with hot plate welding process.
Design for Assembly of Injection Molded Plastic Parts
Hot Plate welding
A/P Lee Kim Seng
Materials included: Acrylic lens to ABS lamp housing; Polycarbonate lens to same lamp housing PC lens to ABS housing.
Design Considerations for Plastic Parts Assembly
Poor appearance as a result of mismatch can be avoided by deliberately designing one part larger than the other.
A/P Lee Kim Seng
Design Considerations for Plastic Parts Assembly
A/P Lee Kim Seng
Design for Assembly of Injection Molded Plastic Parts
Whenever possible, bosses should be free standing, or attached to sidewalls using ribs in order to minimize the potential for sink marks and shrinkage voids.
A/P Lee Kim Seng
Recommended Design Parameters.
A/P Lee Kim Seng
DFM for Injection Molding
Parts are designed with shrinkage included early in the design and before mould was built Shrinkage rates for some common materials
Material Max Shrinkage 1. Acetal 2.5% 2. Acrylic 0.8% 3. ABS 0.8% 4. Nylon 1.5% 5. PC 0.7% 6. PE 5.0% 7. PP 2.5% 8. PS 0.6% 9. PVC rigid 0.5% 10.PVC flexible 5.0%
A/P Lee Kim Seng
Design Rules for Shrinkage
The following design rules provide some guidelines for developing low-shrinkage, warp-free parts One can reduce or control shrinkage and warpage by
properly designing the part, mold, and process, as well as through careful material selection. Wall thickness: Avoid non-uniform wall thickness or
design a transition length of three times the thickness of the thinner region
A/P Lee Kim Seng
Design Rules for Shrinkage (Cont..)
Thick Sections: Alter the design to replace thick sections that cause significant shrinkage and lead to sink marks or internal voids. • A thin, uniform wall with ribs provides for uniform shrinkage,
strength to weight ratio, and cost effectiveness.
A/P Lee Kim Seng
Shrinkage and Warpage
These causes are described more fully below. Non-uniform mold cooling across the part thickness or over
the part Cooling rates that differ because of Part thickness variation
A/P Lee Kim Seng
Shrinkage and Warpage (Cont..)
These causes are described more fully below.
Part geometry asymmetry or curvature
A/P Lee Kim Seng
Warpage in molded parts results from differential shrinkage.
Variation in shrinkage can be caused by molecular and fiber orientation, temperature variations within the molded part, and by variable packing, such as over-packing at gates and under-packing at remote locations, or different pressure levels as material solidifies across the part thickness.
Differences in filled and unfilled materials
Shrinkage and Warpage (Cont..)
A/P Lee Kim Seng
Weld Lines
Venting of Plastic Part
A/P Lee Kim Seng
e-mold Design
Product designers
Mold designers
Email 3D file E-Drawing
Video Conferencing Net Meeting
A/P Lee Kim Seng
Key DFMA principles
• Minimize Part Count
• Minimize Overall Part Wall Thickness
• Standardize parts and materials
• Create Modular assembly
• Design for Efficient joining/Assembly
• Minimize Reorientation of parts during Assembly and/or machining
• Simplify and Reduce the number of Manufacturing Opearations
• Specify “Acceptable” surface Finishes for functionality
A/P Lee Kim Seng
Design for Recycling
A/P Lee Kim Seng
Thank You
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