1. Design for Manufacturing Chapter 4
Professor Joseph Greene
All rights reserved
Copyright 2000

2. Design for Manufacturing
Cost for manufacturing item is dependent upon
Type of machining operation: use general purpose
Material selection: use common materials
Production quantities: higher quantities = lower cost
Design changes: keep the number small
Dimensional accuracy: keep tolerances generous where allowable
CNC Factors versus manual methods
Lead time is reduced
Complex parts routinely produced
Optimize process conditions for feed rates and speeds
Math data taken right from computer to cutter

3. History of DFM Principles of DFM are not new.
Awareness of importance of designing parts for easy manufacturing is key for all time.
Difference is in the use of standards and design guides during the beginning stages of the design and not later.
Eli Whitney (early 1800’s)
Made use of standardizing the design of the lock on the musket so that interchangeable parts could be used
Before him, all muskets were made by craftsmen and no two muskets were the same or used the same parts.
His parts were built to specs and a tolerances
He organized a mass production process for locks

4. History of DFM Henry Ford Value analysis and value engineering
Extensive use of the assembly line
Simplify the manual assembly operation into a series of short-cycle repetitive steps
Concept of Model T revolutionized automobile industry
Simplicity in operation
Absolute reliability
High quality of materials used
Easy serviceability
Value analysis and value engineering
GE (1950’s and 1960’s)sponsored research to design parts with and evaluation of design alternatives based upon value analysis early in the design stage.

5. History of DFM Use of standards
ASME Metals Division (1958) published information on various manufacturing processes and guidelines.
Book edited by Richard Bolz who organized the DFM methodology
DFM was originally called producibility (1960’s) and then manufacturability (1970’s) and then Design for Manufacturability (1985)
Design for Assembly(1968)
Boothroyd and Redford studied automatic assembly
Minimize number of parts in assembly
Combine parts
Eliminate nonessential parts
Make full redesign of assembly

6. DFM Principles Use of standards Use of common components
Design to specifications and tolerances
Use of manufacturing guidelines in the early stages of design that maximize quality of manufactured part
Minimize the use of materials
Minimize the use of floor space in plant
Locate all necessary components near functional operation
Use of automated machining for minimal errors

7. DFM for Injection Molding
Typical characteristics of injection molding

8. DFM for Injection Molding
Effects of shrinkage
Parts are designed with shrinkage included early in the design and before tool build
Shrink rates for common materials
Material Max Shrinkage
Acetal 2.5%
Acrylic 0.8%
ABS 0.8%
Nylon 1.5%
PC 0.7%
PE 5.0%
PP 2.5%
PS 0.6%
PVC rigid 0.5%
PVC flexible 5.0%

9. DFM for Injection Molding
Effects of shrinkage
Sink marks Fig 6.2.3
Closing of U-shaped section Fig 6.2.3
Curving of flat surfaces Fig 6.2.3
Suitable materials
Physical properties- Table 6.2.2

10. DFM for Injection Molding
Design Guidelines
Gate and Ejector pin locations
Edge, Fan, Submarine, Flash, Tunnel, Ring, Diaphragm, disk, or sprue gate (Fig 6.2.6)
Not on a show surface
Not near a structural member or hole or fastener
Minimize flow length
Minimize the number of weld lines
Gate thick to thin
Gate types: Fig 6.2.6
Suggested wall thickness- Table 6.2.3
Have constant wall thickness in part
Have transitions from thick to thin regions if have different thickness. Have gentle no sharp transitions

11. DFM for Injection Molding
Design Guidelines
Holes
Holes are possible with slides but can cause weld lines
Min spacing between two holes or a hole and a sidewall should be 1D (Fig 6.2.9)
Should be located 3D or more from the edge of a part to min stresses
Through hole is preferred to a blind hole because core pin that produces hole can be supported at both ends and is less likely to bend
Holes in bottom of part are better than holes in side, which requires retractable core pins
Blind holes should not be more than 2D deep. (Fig )
Use steps to increase the depth of a deep blind hole (Fig )
For through holes, cutout sections in the part can shorten the length of a small-diameter pin.
Use overlapping and offset mold cavity projections instead of core pins to produce holes parallel to the die parting line (Perpendicular to the mold-movement direction)

12. DFM for Injection Molding
Design Guidelines
Ribs
Ribs should be thinner

13. Sprue Guidelines
The sprue must not freeze before any other cross section. This is necessary to permit sufficient transmission of holding pressure.
The sprue must de-mold easily and reliably.
Dco  tmax mm
Ds  Dn mm
  1º - 2º
tan  = Dco - Ds / 2L

14. Runner Guidelines Common runners Full-round runner Trapezoidal runner
Modified trapezoidal runner (a combination of round and trapezoidal runner)
Half-round runner
Rectangular runner

15. Gate Design Gate Design Overview Single vs. multiple gates
Single gate is usually desirable because multiple gates have weld lines
Gate dimension
The gate thickness is usually two-thirds the part thickness.
The gate thickness controls packing time
Chose a larger gate if you're aiming for appearance, low residual stress, and better dimensional stability.
Gate location
Position the gate away from load-bearing areas.
Position the gate away from the thin section areas, or regions of sudden thickness change to avoid hesitation and sink marks

16. Gate Design Gate Design Overview Gate Types Manually trimmed
Requires an operator to separate parts from runners during a secondary operation
Types include sprue, tab, edge, overlap, fan, disk, ring, film, diaphragm, spider
Automatically trimmed gates
Automatically trimmed gates incorporate features in the tool to break or shear the gate
Should be used to
Avoid gate removal as a secondary operation
Maintain consistent cycle times for all shots
Minimize gate scars
Types include Pin, Submarine, hot-runner, and valve

17. Gate Design Design Rules Gate location Gate Length
Should be at the thickest area of the part, preferably at a spot where the function and appearance of the part are not impaired
Should be central so that flow lengths are equal to each extremity of the part
Gate symmetrically to avoid warpage
Vent properly to prevent air traps
Enlarge the gate to avoid jetting
Position weld and meld lines carefully
Gate Length
Gate length should be as short as possible to reduce an excessive pressure drop across the gate. Ranges from 1 to 1.5 mm (0.04 to 0.06 inches)
The gate thickness is normally 50 to 80 percent of the gated wall section thickness. Pin and submarine gates range from mm (0.01”- 0.08”)
The freeze-off time at the gate is the max effective cavity packing time.
Fiber-filled materials require larger gates to minimize breakage of the fibers

18. Boosting structural integrity with ribs
Structural integrity: the goal of every design
The major component of designing for structural integrity, in many cases, is to design the structure to be stiff enough to withstand expected loads. Increasing the thickness to achieve this is self-defeating, since it will:
Increase part weight and cost proportional to the increase in thickness.
Increase molding cycle time required to cool the larger mass of material.
Increase the probability of sink marks.
Well-designed ribs can overcome these disadvantages with only a marginal increase in part weight.
Typical uses for ribs
Covers, cabinets and body components with long, wide surfaces that must have good appearance with low weight.
Rollers and guides for paper handling, where the surface must be cylindrical.
Gear bodies, where the design calls for wide bearing surfaces on the center shaft and on the gear teeth.
Frames and supports.

19. Ribs Design Rules Keep part thickness as thin and uniform as possible.
This will shorten the cycle time, improve dimensional stability, and eliminate surface defects. .
If greater stiffness is required, reduce the spacing between ribs, which enables you to add more ribs.
Rib geometry
Rib thickness, height, and draft angle are related: excessive thickness will produce sinks on the opposite surface whereas small thickness and too great a draft will thin the rib tip too much for acceptable filling.
Ribs should be tapered (drafted) at one degree per side.
Less draft can be used, to one-half degree per side, if the steel that forms the sides of the rib is carefully polished.
The draft will increase the rib thickness from the tip to the root, by about mm per centimeter of rib height, for each degree of draft angle.
The maximum recommended rib thickness, at the root, is 0.8 times the thickness of the base to which it is attached.
The typical root thickness ranges from 0.5 to 0.8 times the base thickness.

20. Recommended Rib Design Parameters.
See Figure 1 for recommended design parameters.

21. Ribs Design Rules Location of ribs, bosses, and gussets
Ribs aligned in the direction of the mold opening are the least expensive design option to tool.
As illustrated in Figure 1, a boss should not be placed next to a parallel wall; instead, offset the boss and use gussets to strengthen it.
Gussets can be used to support bosses that are away from the walls. The same design rules that apply for ribs also apply for gussets.
Alternative configurations
As shown in Below, ribs can take the shape of corrugations.
The advantage is that the wall thickness will be uniform and the draft angle can be placed on the opposite side of the mold, thereby avoiding the problem of the thinning rib tip.
Honeycomb ribbing attached to a flat surface provides excellent resistance to bending in all directions.
A hexagonal array of interconnected ribs will be more effective than a square array,with the same volume of material in the ribs.