D esign for lnj ection Molding
FICURE 8.18
Annular ring design.
2.
thick, and the boss projections are 12 mm outer diameter, 6 mm inner diameter
and 30 mm high. The material proposed for the design is high-density polyethylene,
which is a very inexpensive polymer. However the poor material mechanical
properties give rise to the substantial wall thickness.
a. Estimate the material cost for the part, assuming the runner system will be
reused in the machine (use the data in Tables 8.1 and 8.3). b. Estimate the appropriate machine size from the list in Table 8.4.
c. Estimate the cycle time (use the data in Table 8.3). d. Estimate the process cost per part using the machine rate in Table 8.4. Assume
in this calculation that the molding shop has 85% plant efficiency; that is,
machines are typically stopped at L5% of the time for batch setup, adjustments,
and miscellaneous stoppages. Increase the process cost appropriately
to account for this.
Continuing from Problem 1, for the ring design in Figure 8.18, you propose a
design change to use polyethylene terephthalate (pET) with 30% glasi tlit. pEr
is used for soda bottles, and you have located a quality recycled material supplier
who can deliver the glass filled PET for $2.25/kg. Using the data in Table g.1,
calculate the main wall thickness required to give the same wall bending stiffness
as for the high-density polyethylene part in the original design proposal.
Decrease the wall thickness of the bosses in the same proportion.
Recalculate the material and process cost for this proposed redesigru using the
same steps as in problem 1. Is your redesign idea a sound economic proposal? If
the recycled material was unavailable and you had to pay the full supplier price
of $3.74/kgwould the change still be worthwhile?
(Hint: recall from Chapter 2 that for equivalent bending stiffness the wall thickness
should change in proportion to the cube root of the modulus)
A cable company is considering the possibility of changing their current die cast
utility pole mounted distribution box to an injection molded one. The motivation
is to eliminate the costs of trimming, plating (for corrosion resistance) and
machining of screw threaded holes for securing. In additiory the new design will
have snap-fit hinges and an integral snap-fit latch plate for self securing. The
specifications of the box cover are estimated from concept sketches, and calculations
of equivalent strength and stiffness of the die cast model. The cover is to be
369
Design for lnjection Molding
b. Estimate the cost of a single-cavity mold for the distribution box cover. The
design requires no mold mechanisms. Compare your estimated number of
7a\ manufacturing hours with a quick estimate from Figure 8.10.
( 4 ,f particular- manufacturing company uses a simple internal cost accounting
\21method in which all the machines of a particular type are given the same hourf
rate. In addition, in the company’s internal tool-making division, no account is
made for the savings in making multiple identical items of tooling. Under this
costing system all of the different injection molding machines of different sizes
are assigned an hourly rate of C,=h$/h, and the cost of an n-caviry mold is n
times that of a single-cavity mold (i.e., m = !).
By making appropriate changes to Equations in Section 8.1O show that this
cost accounting system would lead to the conclusion that the optimum number of
cavities for a particular part would be obtained when the mold cost per part was
equal to the process cost per part.
5. The base part of the piston assembly illustrated in Figure 8.16 is shown in Figure
8.19. The part outer dimensions are: length 76 mm, width 64 mm, height 30 mm.
The part volume is 29 cm3 and it is to be molded from ABS. The maximum wall
thickness is 3.0 mm.
You have obtained a quote of $8000 for a single-cavity mold for the part.
However, large quantities of the part are required justifying the use of a 4-civity
mold. For the proposed 4-cavity mold production, estimate the following l
a’ The appropriate machine size from the selection in Table 8.4, miking an
approximate allowance for the projected area of the runner system as well as
the four cavities.
b. The likely molding machine cycle time.
c. The processing.oit pu. pu.t, assuming 85% plant efficiency and the machine
rate in Table 8.4.
d. The likely 4-cavity mold cost assuming an 85% progress curve in the manufacture
oi ttt” fo,ri cavities.
6. Figure 8.20 shows two views of a hose clamp which is to be injection molded
using 30% glass-filled polycarbonate; see Table g.3. The envelope size of the
part is 60 x 80 mm and the wall thickness is 3.0 mm. A total of 4 million clamps
is required over the mold amortization period of 30 months. You have already
estimated the cycle time to be 1,9.6 s for a single-cavity mold operation on a 300 kN
FIGURE B.20
Pair of identical injection-molded hose clamps.
371
418 Product Design for Manufacture and Assembly
Design
FIGURE
changes
9.38
of a three-hole
ffiKK
bracket for minimization of manufactured scrap.
held to tolerances of approximately +Q.Q5 mm However, as part size increases, precision
is more difficult to control, and for a part with dimensions as large as 50 cm peimissible
tolerances are in the range of +0.5 mm. The requirement for tolerances much tlghter than
these guideline values may call for features to be machined at greatly increasel cost. For
formed parts, or formed features, variation tends to be larger and minimum tolerances
attainable are in the range of +0.25 mm for small parts. This includes bending when
dedicated bending dies are used. Thus, a tight tolerance between punched holes, which
are on parallel surfaces separated by bends, would require the holes to be punched after
bending at greater expense. If the holes are on nonparallel surfaces, then mlchining may
be necessary to obtain the required accuracy. Finally, in the design of turret press pirts to
be bent on press brakes, it should be noted that the inaccuracies oi this bending process are
substantially worse than with dedicated dies. Attainable tolerances between 6ent surfaces
and other surfaces, or features on other surfaces, range from +0.75 mm for small parts up
to +1.5 mm for large ones.
Finally, an important consideration in the design of any sheet metal part should be the
minimization of manufactured scrap. This is accomplished by designing part profiles so
that they can be nested together as closely as possible on the strip or sheei. Alio, if individual
dies are to be used, then the part should be designed if poisibte for cut-off or partoff
operations. Figure 9.38 illustrates the type of design changes that should always be
considered. The cut-off design lacks the elegance of the iounded-end profiles. Neverthlless,
the acute sharp corner is removed during deburring, and for manylpphcations this type
of design may be perfectly functional.
PROBLEMS
In all of the questions below use the data from the Tables for material costs, material
properties, machine performance, and machine hourly rates. Assume a rate of $40/h for
all die-making cost estimates.
in Figure 9.25, whose dimensions are given in mm, and
12 gage commercial quality low-carbon steel, determine the
a. The likely cost of a progressive die assuming a total production volume of
15Q000 parts;
b. The required press force for the external shearing, internal hole punching
and the bending operation;
c. The press cycle time and the processing cost per part assuming 15% downtime
for batch setup and miscellaneous stoppages; d. The manufactured cost per paft, including material cost, but neglecting the
small return on manufactured scrap for the low-carbon steel. Assirme thl die
is amortized over the total production volume.
2. The part illustrated in Figure 9.39 is the tool part of a barbeque spatula which is to
be assembled to a wooden handle. The head is 15 cm long, and has a width of 9 cm
‘or the part shown
which is made from
following:
420
FIGURE 9.41
Machine support bracket.
FICURE 9.42
Deep drawn aluminum rotor-primary shape.
Product Design for Manufacture and Assembly
c. The processing cost per part prior to the bending operatiory assuming sets of
20 are made from custom-sized sheets measuring 800 x 840 mm.
d. The press force required for the bending operation.
e. The cycle time and operation cost per bracket for the press brake operation.
4. If the machine support bracket illustrated in Figure 9.41 is to be made from
annealed low-carbon commercial quality steel with properties as in Table 9.2,
what is the smallest inside bend radius you would recommend? If for the structural
requirements of the desigru you wish to use half-hard commercial quality
steel (not annealed after the finish rolling passes of the strip) for which the maximum
allowable strain is reduced to 0.1O to what value would you need to increase
r\the inside bend radius?
( 5. lThe body of a rotor component shown in Figure 9.42 Is to be made by deep drawing v/from 2.03 mm thick A1 3003 aluminum alloy sheet. The part is 13 cm high. The large
diameter is 20 cm which steps down to t7.5 crr., at distance 6.5 cm from the base.
a. Estimate the required blank diameter to produce this part and determine that
it is possible to produce the initial 20 cm diameter cup in a single drawing
operation, so that only one redraw operation would be needed.
b. The set of operations required to make the rotor includes: blank, draw redraw,
and trim. Estimate the costs of the dies for each of these operations.
SS s:\-s=
D esign for Sheet Metalworking
FIGURE 9.43
Deep drawn aluminum rotor-finished part.
Estimate the press forces required for each of the operations.
Estimate the cycle times for each of the operations. Assume that hand loading
and unloading are required for each operation except blanking.
e. Using rates for the appropriate machines from the Table 9.4, estimate the
processing cost for the set of four primary operations.
6. The completed rotor component involves the production of the body, as described
in problem 5, with the addition of a centered 12.5 cm diameter hole, surrounded
by twelve 1.5 cm diameter holes as shown in Figure 9.43.
a. Estimate the press force required to punch these 13 holes simultaneously, and
select an appropriate press from Table 9.4.
b. Estimate the cost o{ the required die.
c. Estimate the cycle time and process cost for this piercing operation.
c.
d.
421,
References
Lange, K. Handbook of Metal Forming, McGraw-Hill, New York, NY, 1985.
Tschaetsch, H. Metal Forming Practice, Springer, Berlin,2009.
Kalpakjian, S. and Schmid, S.R. Manufacturing Engineering and Technology,6th Ed., PrenticeHall,
Englewood Cliffs, NJ,2010.
4. DeGarmo, E.P., Black, ].T., and Kohser, R.A. Materials and Manufacturing Processes,9th Ed.,
Wiley, New York, NY, 2003.
5. Geng, H. Manufacturing Engineering Handbook, McGraw-Hill, New York, NY,2004.
6. Eary, D.F. and Reed, E.A. Techniques of Pressworking Sheet Metal, Znd Ed., Prentice-Hall,
Englewood C1iffs, NJ, 1974.
7. Zenger, D. and Dewhurst, P. Early Assessment of Tooling Costs in the Design of Sheet Metal Parts,
Report No. 29,Department of Industrial and Manufacturing Engineering, University of Rhode
Island, Kingston, August 1988.
8. Zenger, D.C. Methodology for Early Material/Process Cost of Estimating, Ph.D. Thesis, University
of Rhode Island, Kingston, 1989.
9. Nordquist, W.N. D,e Designing and Estimating,4thEd., Huebner Publishing
