Formulation of Multi-Material Molding Cost Estimation
Generic Per-Piece Costs
For a foundation, we first start with the most generic per-piece
manufacturing cost as suggested by Esawi and Ashby's resource-based
modeling [Esa99], [Esa98]. Here the total per-piece cost is split into six
categories: 1) material, 2) tooling, 3) overhead or time, 4) energy, 5)
space, and 6) information. The total cost's units of dollars per piece
are kept consistent by dividing each resource by an appropriate rate.
For example, the overhead per-piece cost is found by dividing the
per-hour cost of overhead by the hourly total assembly production rate.
This is intended to be used for a single process rather than a
manufacturing system such as SMM&A or MMM production lines with
multiple stations (e.g. molding, assembly, inspection, etc.).
There are three essential contributions to cost per unit:
1) a material cost (independent of batch size or production rate),
2) a dedicated production cost (inversely proportional to batch size), and
3) a "gross overhead cost" (inversely proportional to production rate).
Material Costs
The material cost is simply the cost of the raw materials that go into
producing one unit (e.g. resin, fasteners, adhesives, etc.). This cost
is usually quite easily calculated by determining how much material
goes into each product and multiplying it by its respective cost from
the supplier. For example, in injection molding, the total volume of a
part can be used to determine how much resin is needed, which is then
used to determine the cost based on a supplier's resin price.
Dedicated Production Costs
The dedicated production cost (or "tooling cost") is basically the cost
of the specialized equipment (e.g. molds) required to manufacture the
product, amortized over the total production volume. For example, if a
$500,000 mold is used to make a total of 5 million products before it
is retired, the mold itself is contributing a cost of $0.10 to each
product. For injection molding, the tooling includes the mold base, the
machined cavities, the machined cooling system, the ejector system, and
any other special features such as side actions. The cost of each of
these items must be accurately calculated and summed to get the total
tooling cost, which can then be divided by the production quantity.
Gross Overhead Cost
The gross overhead is split into five terms: 1) basic overhead, 2)
capital cost, 3) energy cost, 4) space cost, and 5) information cost.
These all relate to the fact that the manufacture of any product
requires the consumption of all of these five kinds of "resources"
(e.g. labor, capital equipment, power for running equipment, factory
space, and designer's time, respectively).
The reason that the cost of capital equipment is amortized over the
production rate (instead of the production quantity as with the tooling
cost), is because capital equipment (such as an injection molding
machine) is typically used to manufacture many different product lines,
rather than dedicated to a single product. Therefore, it is customary
to amortize such capital costs over a period of time rather than a
production quantity.
The overhead rate can be split into two terms: 1) a basic overhead rate
which covers the cost of labor and other fuzzy expenses, and 2) a total
capital cost which is amortized over a specified write-off time, two,
(usually in years, but expressed in hours for unit consistency). The
write-off time is time is how long the capital cost of equipment takes
to be recovered or paid off, and depends on the conditions of the loan.
There is an additional term contributing to the overhead rate
representing the fraction of time over which the equipment is actually
being productively used. This load factor depends on the factory
conditions and which production lines utilize the equipment. For the
purposes of this model, the load factor and capital write-off time must
be specified by the user. This is because these terms are completely
dependent on the financing practices of the company and cannot be
estimated analytically.
Final Cost
Although every term is important, having the potential to drastically
influence the overall cost, two terms in the gross overhead expression
will be dropped for this model. The first, the cost of energy, will
actually be absorbed into the basic overhead rate. This is because the
primary energy consumption is caused only by the operation of the
injection machines and any auxiliary equipment. These will all have
their own associated run costs based on their power consumption and
other factors. The other term, information cost, while having important
contributions to overall cost, is outside of the scope of the model. It
relates to more administrative cost factors rather than direct
manufacturing costs. While this may seem like gross oversight, it
should not have a large effect in a relative sense, since this value
should be a constant independent of either process variant.
Total Manufacturing Costs
While the per-piece cost of the following equations is a valid and
useful measure for cost estimation purposes, it may be more informative
to see how the total cost varies with the production quantity. This is
because in many cases the optimal process in terms of total cost,
depends on the amount of units made. For this reason, we will look at
total cost as a function of the total production quantity, allowing us
to plot quantity-cost curves and identify break-even points and chose
the proper variant based on estimated production quantity.
Production Parameters
The production parameters relate to the volume and speed of production.
That is, they quantify the number of products produced, and the rate of
production. These parameters directly affect all of the five cost
parameters mentioned above.
Production Quantities
The desired production quantity is the total number of assemblies that
need to be made. This actual number could depend on a customer's order
size or anticipated demand of the product. It is used as the
independent variable for plotting the cost-quantity curves.
The individual production quantities represent the total number of
units processed at each station. For example, denotes the total number
of part B's molded at machine B. In order to ensure that enough
assembly AB's are produced to meet the production demand the actual
production quantity has to be scaled higher to account for defects.
Defect Rates
The defect rates, expressed in a percentage of bad parts to good ones,
are a result of random errors in the manufacturing process which result
in unacceptable components. These could be caused by any number of
reasons such as fluctuations in the controlled parameters (e.g. melt
temperatures and pressures), human errors, or mold malfunctions. These
numbers have to be estimated from historical data and an understanding
of each molding machine's characteristics. However, each molding job is
different, with different part geometries producing different defect
rates. Typically, conservative average defect rates (based on the
plant's production capabilities and adjusted for any geometric issues)
need to be used.
It should be stated, that because the state of today's manufacturing
technology, (including molding machines) has significantly reduced
defect rates, the defect rates used in the above equation will be
rather low (less than 5%). However, it should be notated that because
MMM is both more complex than traditional molding, and the technology
is less mature, there will most likely be higher defect rates
associated with the MMM variant. This is an important difference that
should be accounted for.
The production quantity and defect rate equations have all been
verified as valid through consultation of several molding experts. They
serve to correctly adjust the production quantity to account for the
inevitable production of defective components.
Production Rates
The individual production rates are defined as the inverse of the total
processing time at each corresponding station. For instance, the
production rate of the assembly station is the inverse of the assembly
time. We will make the simplifying assumption that batch processing is
employed throughout the production line. This means that each station
is constantly kept busy by working on batches of the product in various
stages of completion. This eliminates the problem of bottlenecking, and
makes the production rates simpler to compute.If there is more than one
worker at any station, although this would increase the corresponding
station's effective production rate, it would also increase the hourly
labor cost of the station. For instance, if there are two assemblers,
the assembly production rate would double, but so would the hourly
wages paid to the workers. This would cancel out the effect of the
number of laborers on the total cost. Hence, the actual labor
distribution is disregarded in this model.
Inspection and Packaging Times
The inspection, and packaging times must both be estimated based on
historical data and reasonable assumptions about the difficulty of such
tasks based on the product geometry. For example, a simple box-shaped
part would require only a brief look-over by an inspector, while a
complicated part with many features might require several seconds of
inspection. Similar reasoning should be applied to the packaging term.
Molding Time
The total molding time is the time the product is spent in the
injection molding machine. For the SMM&A variant, although both
separate pieces could conceivably be manufactured simultaneously on
different machines (in fact, this would be the preferred method), they
still require dedicated use of both injection machines, which has an
associated cost. Therefore, the molding time for this variant will be
the sum of processing times for parts A and B. On the other hand, the
MMM variant only makes use of one machine. Although the molding
operation involves the injection and cooling of two separate shots, the
actual associated molding time will be only the maximum of the two
molding times. This is because, after the machine has reached steady
state, one finished part AB is ejected from the mold after the required
injection and cooling times. This essentially causes the MMM variant to
have consistently less molding times than the SMM&A variant.
Injection Time
The injection time can be approximated using the injection pressure and
power relation provided by BD&K's pressure/power relation. The
injection pressure is set depending on the size of the shot and the
mold filling requirements. The power of the injection unit depends on
its size, and is obtained from the manufacturer's data.
Mold Cooling Time
Perhaps the most accurate method to estimate the mold cooling time is
through CAD cooling simulations such as C-Mold or Pro/E's mold analysis
tools. If this is not feasible, there are two simpler methods of
calculation the cooling time. The mold cooling time may be estimated
based on the maximum wall thickness of the part. This can be done
either by Poli's table-lookup method based on elemental plates and part
complexity or by applying a simple analytical cooling equation.
Mold Resetting Time
The mold resetting time is the time it takes for the mold readied for
the next cycle. This includes both the times required for closing as
well as the operation of any side-actions and/or the rotary platen (for
MMM only). In addition to being affected by the presence of a rotary
platen or side actions, the reset time is greatly affected by the part
height in the direction of the mold opening. This value should be
estimated based on the specifics of the molding press.
Assembly Time
The assembly time specifically refers to the total time required to
assemble the SMM&A variant of the product. This includes both the
handling and insertion times of both material components as well as any
fasteners/adhesives required to secure them. It is of value to
emphasize the fact that the assembly time only applies to SMM&A,
thus causing a potentially significant difference in the cycle times
between SMM&A and MMM. Even if the estimated assembly times have
some inaccuracy, their addition to the cycle time will help to
differentiate the cost values output by the model.
Material Cost
The material cost accounts for all of the separate components of a
product, including resins, fasteners/adhesives, labels, and packaging
materials.
Resin Cost
The resin cost is dependent on the amount of resin used in making one part and the resin supplier's price.
Shot Volume
The shot volume is the total amount of material that is plasticized and
ejected from the mold after injection. This includes the cavity images
themselves (the desired molded part geometry), as well as the hardened
sprue, runners and gating required to fill the cavity.
Part Volume
The part volumes are the actual volume of plastic needed to make each
component (part A or part B) of the final assembly, part AB. These
values can be automatically calculated from the CAD models of the
parts. These volumes are the exact volumes, and should not be confused
with the bounding box volumes of the parts.
Gating Volume
The gating volume accounts for the additional material consumed by the
resin which solidifies inside the sprue, runners and gates. In most
molding situations, the volume of the gate into the cavity is
insignificant compared to the volume of the runners and sprue. Because
of this, the gate volume will be omitted from the total gating volume
calculations. Furthermore, the gate volume is typically included in the
cavity volume, so in those cases, this assumption is accurate.
Therefore, the term "gating volume" will henceforth refer to the
combined volume of the runners and the sprue.
Fasteners Cost
In some SMM&A scenarios, some sort of fasteners must be used to
secure the separate molded pieces together. This could include screws,
pins, clips, and/or adhesives. The fastening method is entirely
dependent on the product design and should be avoided where possible.
For example, a common DFM/DFA technique is to use snap-fit connections
to avoid additional parts in the form of fasteners.
If the use of fasteners is unavoidable, the type and number of them
must be determined for each SMM&A variant of a product design and
then priced according to supplier's rates.
Miscellaneous Material Costs
The miscellaneous costs represent consumable materials such as
packaging, labels and so on. As with the fasteners, each design must be
evaluated on a case-by-case basis to determine the packaging, labeling,
and other miscellaneous material requirements and then price them
accordingly.
Tooling Cost
The tooling cost refers specifically to the total cost of the mold,
including mold base material costs, cavity machining/finishing costs,
and the added costs for manufacturing the cooling system, ejector
system, and any side actions and/or hot-runner systems. Additionally,
the mold has an associated setup cost.
Mold Base Cost
The mold base is the unfinished set of steel (or aluminum) plates that
will house all of the mold components, including the cavities/cores,
the gating system, the ejector system, and etc. It is typically bought
from a specialized dealer, built to the specifications of the molding
job. Then the molder finishes manufacturing the mold by machining and
adding the required mold components into the base. The base itself can
be rather costly due to its complexity and precision. The number and
type of mold bases, and hence total cost, depend on the molding process
being used.
Mold Base Dimensions
The mold base dimensions depend directly on the part size as well as
any special mold features such as side actions. They can be calculated
based on the part's bounding box dimensions by using a modified version
of Poli's equations.
Mold Wall Clearance Factors
The mold wall clearance factors, represent the additional percentage of
the cavity size that should be added to the mold area and height,
respectively. These factors depend on the required mold strength which
is a function of the mold material, part size, and injection pressure.
They must be conservatively chosen so as to ensure safe mold operation
while not requiring excessive mold size.
Additional Area and Height Required by Side Actions
The additional mold base area and height required for housing the side
actions or other accessories depends on the nature of the device and
must be appropriately chosen. However for estimation purposes, a
constant length factor can be added onto the mold at the appropriate
location.
Part Bounding Box Dimensions
The part bounding box dimensions are simply the maximum cavity length,
width, and height corresponding to the respective dimensions of the
mold base. These dimensions can be automatically extracted from the CAD
model of the part, after its orientation relative to the mold base has
been chosen.
Mold Machining Cost
Assuming the mold cores and cavities are machined into the mold base
in-house, the total cost of the required machining operations can be
estimated as the product of total machining time and the machine shop's
hourly tooling rate.
Machine Shop Hourly Rate
The actual machining rate depends on the process capabilities of the
machine shop and must be specified based on historical job data. If
this rate is difficult to determine, an appropriate value can be
estimated based on current national averages (for example, at the time
of this writing, a quick internet search of "machine shop rates"
returned values between $40/hr to $80/hr).
Total Mold Machining Time
The total mold machining time is the sum of the individual machining
times required for cores, cavities, and gating. As is customary for
most molds, it will be assumed here that the cores and cavities are
embodied as inserts which are secured into pockets of the mold base.
This facilitates machining, mold repair, and even potentially allows
some mold bases to be reused with different core/cavity insert sets.
Cavity and Core Insert Machining Time Calculations
The times required for machining the core and cavity inserts of the
mold are complex functions of the gross part size, part
geometry/complexity, and machining process capabilities. If the exact
mold design is completely specified along with the process planning,
the machining time can accurately and easily be predicated through CAM
simulations. Unfortunately, in the early stages of design the exact
mold configuration has most likely not been determined and hence it
becomes tricky to estimate this time.
Gating Machining Time Calculations
As well as the machining required to produce the desired cores/cavities
in the mold base, additional machining is needed for the mold gating
system. In an ideal situation, the completely-specified cavity layout
would be input into a cutter path generation program to obtain the
total milling time in a similar manner to predicting the core/cavity
milling times. If this is not possible, a rough estimate can still be
obtained through volumetric considerations.
Tolerance Costs
"Tolerance costs" refer to the costs associated with achieving the
required dimensional tolerances specified in the plastic components'
design. Obviously it will cost more to mold a part that has tight
tolerances, because the tooling used to produce it must have tight
tolerances itself. Thus, we can account for this added cost by
estimating the additional machining time required to achieve the
desired tolerances in the core and cavity inserts of the mold. It may
be assumed that desired core/cavity dimensional tolerances are produced
by additional precision milling operations. Typically, any milling
process is performed in a series of increasingly precise (but slower)
machining steps, using smaller and smaller tools. The initial steps are
referred to as "roughing" operations and the final step is referred to
as a "finishing", or here, "tolerancing" operation.
Hourly Tolerance Machining Rate
The hourly machining rate applied to tolerancing operations is similar
to the other hourly rates previously discussed. It is a representative
hourly cost that accounts for the labor and machine tools (milling
machines) used to perform the tolerancing. The actual value depends on
the rates charged by the particular machine shop and should readily be
calculated based on historical machining data.
Number of Surfaces Requiring Tolerancing
The number of surfaces requiring tolerancing are simply the number of
part surfaces that have specified dimensional tolerances that are
tighter than those provided by regular rough machining. Each one of
these surfaces has a corresponding surface in the core or cavity insert
of the mold.
Surface Area
The surface area is the total area on the core or cavity's surface that
possesses the tolerance value trying to be obtained. The entire surface
typically has to be engaged by the finishing tool, increasing the total
machining time. These surface area values are readily obtained from the
CAD model and are required inputs to the model.
Surface Milling Feed Rate
The tolerancing feed rate is also a process-specific input variable and
is obtained in a similar manner as the regular volumetric feed rate
used in rough machining. This value depends on the milling operation
and can be obtained from a machinist's handbook [44]. The actual feed
values are typically much lower speeds than regular machining feed
rates, and are based on the process capabilities and tools of the
individual milling machine.
Surface Finishing Costs
Surface finishing costs will here refer the combined cost of two
separate actions: 1) hand polishing the mold to produce the desired
appearance on the part's surface, and 2) incorporation of textures onto
the mold surfaces. These two operations are performed independently,
but they are both carried out to enhance the surface texture of the
part, so are rolled into one cost.
Hourly Rates of Mold Polishing and Texturing
As with the other tooling operations such as machining and tolerancing,
both types of surface finishing operations have associated hourly rates
that deal with the costs of labor and equipment operation throughout
the tooling process. These values are company-specific and are input to
the model based on historical tooling cost data. The only difference
here is that since mold texturing is usually outsourced to specialized
companies, the hourly rates can be directly quoted from them.
Mold Polishing Time
Mold polishing is a costly operation, performed by hand on the mold
core and cavity surfaces. Typically, a special type of sand paper is
used to buff the mold surface until the desired finish is produced. The
total time required for this action is proportional to the surface
area, part complexity, and desired surface finish. Surface finishes are
typically divided into seven distinct categories, each with an
associated Society of Plastics Engineers (SPE) grade.
Mold Texturing Time
While the mold texturing time could possibly be estimated using similar
reasoning as that for the tolerancing and polishing times, it will be a
bit different because there are many specialized textures, each with
their own associated application processes and costs.
Ejector System Costs
The process of preparing the mold base to receive the ejector system is
a lengthy (and costly) task, and unfortunately, very difficult to
predict in the early product design stages. This makes the total cost
of ejector system hard to estimate with any certainty. According to
Boothroyd, although the cost is directly related to the number of
ejector pins, which in turn, is dependent on the part size, core depth,
rib geometry, and other part-complexity features, no strong
mathematical relationships between these factors and cost could be
established.
Cooling System Costs
Most typical molds employ a cooling system which is simply a series of
connected cylindrical channels that run throughout the mold, close to
the cavity and core surfaces. These channels circulate cold water
during the cooling phase of injection molding, helping cool the molten
resin. There are other specialized cooling devices that can be built
into molds in order to help meet special cooling requirements and reach
difficult mold areas. Some such devices are baffles and water jets. For
simplicity, often these specialized cooling devices are not considered
in this model.
Cooling channels are usually machined into the molds as a series of
deep drilling operations. Then certain channels are plugged on one or
both ends to create a closed circuit. A simple example of this is
illustrated in Figure 1. An easy way to calculate the machining costs
of these drilled holes is by realizing that the cost is directly
proportional to the machining time.
Drilling Hourly Shop Rates
The hourly cost of drilling is just another hourly machine shop cost
associated with the operation of drill presses and the associated labor
costs.
Drilling Time
The time required for drilling the holes is directly proportional to
the channel size, which in turn, is directly proportional to their
length and diameter.
Drilling Speed
The drilling feed rate, or speed relates how fast holes can be drilled,
that is, how fast the drill can travel through the mold core and cavity
plates per unit time. This value is highly dependent on the hole
diameter, mold material (typically steel), and most importantly, the
machine shop's drilling equipment and capabilities.
Cooling Channel Length
The total cooling channel length is the most important parameter in
terms of cooling system cost and is also, unfortunately, the most
difficult to determine at an early design stage. This is because just
from looking at the CAD model of the part, it may be difficult to
determine the best cooling strategy.
Fortunately, our fundamental assumption that the MMM variant is
strictly the rotary platen MSM method slightly simplifies the cooling
channel considerations because both shots have the same core, and
hence, the same cooling channels in the core side of the mold. In terms
of the cavity side of the mold, we can also assume the cooling channel
layout for both cavities A and B will be the same in either process
variant. Because the cavities should be nearly identical in the SM mold
and the MS mold, this assumption is justified. These assumptions reduce
the total number of unique cooling channel layouts to four, as listed
below and illustrated in Figure 2:
1) A layout for the core side of mold A, and the common core side of mold AB
2) A layout for the cavity side of mold A. and the cavity side of mold AB - shot A
3) A layout for the core side of mold B
4) A layout for the cavity side of mold B and mold AB, shot B
Side Action Systems Costs
If the part geometry absolutely requires undercuts with respect to the
mold parting direction, side actions will have to be utilized on each
side of the mold cavity that contains said undercuts. The cost of
integrating side actions into a mold base depends directly on the type
of side action mechanism used, which is in turn, depends of the number
and nature of the undercuts.
Hot Runner system Costs
Hot runner systems are required in the MMM variant's mold and optional
in both or either of the SMM&A's molds. Typically hot runner
systems are custom-built by a specialized company to meet the molder's
needs. Resultantly, the associated hot runner system costs are
controlled directly by their manufacturer. However, linear size-based
assumption can be used to estimate representative costs for relative
comparison purposes.
Mold Setup Costs
The mold setup cost is a one-time cost associated with installing the
mold into the molding press and preparing it for production. The cost
is incurred simply because it takes time and manpower to hoist a mold
onto the press and ensure that it works properly. This also includes
the time required for setting the controller for proper operation as
well as putting the machine through any necessary dry-runs and getting
it up to steady-state production. The total setup cost can be expressed
as the sum of the individual setup costs for either process variant.
Basic Overhead Costs
The term "basic overhead" will be used here to refer to the hourly cost
of running the production line, including labor costs and machine
rates. The direct labor and machine costs will be adjusted to account
for indirect overhead costs, by applying appropriate overhead rates.
The overall overhead rate is split into a sum of 1) direct labor costs,
and 2) molding machine/s operational costs, which are then adjusted by
corresponding overhead rates.
Labor Rates
The labore rates account for the different hourly wages paid to
different laborers involved with production. The actual values for
these rates depends on the individual company's practices as well as
the level of skill required for the product under consideration. These
numbers can be calculated from historical and geographical data.
Machine Rates
The machine rates refers to the hourly cost of running the injection
molding machines. This accounts for the power consumption and other
utilities used during their operation.
Capital Investment Costs
The total capital investment cost is simply the cost of all the
required production line equipment amortized over a period of time. It
is amortized over a time rather than a production quantity because the
equipment is typically used throughout several product lines, rather
than dedicated to a single product. For instance, the same molding
machine will be used to manufacture many different parts.
Equipment Costs
The equipment simply refers to all of the machinery required to
manufacture and assemble the product. This includes the molding
machines, assembly equipment, and other miscellaneous items (e.g.
packaging equipment). This also includes the rotary platen required for
the MMM variant. Table 1 lists some common capital equipment that
must be priced, including both required and optional equipment:
Table 1 - Common Equipment for MMM and SMM&A
While Table 1 is by no means a comprehensive list, it shows all of
the required equipment as well as some typical auxiliary equipment that
should be considered for production of either product variant.
Molding Machine Cost
The cost of each molding machine includes the cost of the press and the necessary injection unit/s.
Molding Press Cost
The molding press refers to the part of the molding machine which holds
the clamp mechanism that controls the opening/closing of the mold. The
price of the press is a function of the press size, which is a function
of the required clamping force.
Injection Unit Cost
Like the molding press, the cost of the injection unit is a function of
its size. The required size is in turn, a function of the total shot
volume. The injection unit's size is characterized by the volume of the
plasticizing barrel. According to Bryce, the ideal barrel size is twice
the volume of one shot [Boo02].
Assembly Station Cost
The cost of the assembly station for the SMM&A variant includes the
cost of any tools, fixtures, and workbenches required at the assembly
station. The total cost is entirely dependent on the station's
configuration and must be input to the model by the user. The station
could consist of a simple table and chair, or an elaborate assembly
cell with drop-down tools and conveyors, etc.
Rotary Platen Cost
The rotary platen adds a significant cost to the MMM process variant. A
typical platen can be in the ballpark of $50K-$60K. The cost of the
platen is a direct function of its size, which is in turn, a function
of the size of the mold base that it rotates. The mold core side of the
mold base essentially bolts directly to the rotary platen, so the
platen should be large enough to accommodate the base. As a general
rule, the platen's diameter should be slightly larger than the length
of the diagonal of the mold base.
Auxiliary Equipment Cost
The auxiliary equipment term is intended to include all the other
optional equipment outside of the direct molding/assembly items. This
could include things such as resin dehumidifiers, packaging equipment,
etc. The presence of such extra equipment is case-specific and the
costs must be supplied by the user.
Capital Investment Parameters
The capital investment parameters, namely the load factor and write off
time, are highly dependent on the investment strategy of each
individual company and their accounting practices. These values have to
be determined based on the company's specific needs in order to be
input to the model. Interest Factor
The "interest factor" is a term added to include the cost of capital;
that is, the interest incurred throughout the repayment of the loan.
Write-off Time
The write-off time is simply a time period over which the capital cost
of the equipment is to be recovered. This would typically be the length
of the loan taken out to pay for the equipment.
Load Factor
The load factor is used to represent how effectively the equipment is
used. It can account for things such as breakdowns, scheduled downtimes
(for maintenance etc.), and the possibility that the equipment is used
on several product lines rather than dedicated to the production run
being estimated. This actual number can be difficult to estimate,
especially in the early product design stages, as there are many
uncertainties associated with running a production line. This number
can be conservatively estimated based on historical data, or using more
sophisticated means such as discrete event simulation.
Plant Floor Space Costs
Since the entire production line will be taking up space in some
manufacturing plant, a rather significant cost is incurred as a result.
This is because it simply costs money to rent and maintain facilities.
Bulky molding machines take up precious floor space that could
otherwise be used on other production lines or as storage room. While
this cost could be absorbed into the basic overhead as with several
other factors, it may be independently considered here for greater
emphasis and ease of calculation.
Floor Space Used by Production Lines
The total floor space is the sum of all the space used by every station
in the production line, including molding machines (and their
associated auxiliary equipment), assembly stations, and packaging
stations. Here it makes sense to emphasize the significant difference
in floor space usage between the two process variants; that is, the
fact that SMM&A requires two machines and an assembly station,
whereas MMM requires only one (although usually larger) machine. This
difference could significantly affect the relative costs between the
two processes. It should be noted that the footprint of the MMM machine
is highly dependent on the layout of the injection units. 3D views of
these same layouts are shown below in Figure 3 for quick reference.
Hourly Cost of Space
The hourly cost of space must be determined based on the cost of
renting the entire manufacturing facility as well as other factors such
as upkeep. These values are company-specific and should be readily
calculated based on historical data. 
