Single Material Molding Cost Estimation
This section covers some of the more commonly used cost estimation
techniques for SMM. It begins with an overview of the methods currently
practiced in the industry. Then, some contributions from the academic
community are discussed. Finally, some commercial cost-estimation
software are described.
All molders use some form of cost estimation before they commit to a
certain production run. In many cases, competing manufacturers all bid
on a job and therefore need to get a fast quote out to the customer.
These quotes need to be formed quickly and accurately so that the bid
is won, and the manufacturer does not lose money on the job (i.e. the
actual manufacturing cost must be less than the quoted price). Because
of this need, there are several back-of-the-envelope methods that are
currently used in the industry to get a representative value for the
molding cost of a specific job in the early stages of product design.
Most of these methods are rather simple, and are only used to get ballpark
estimates of the cost. First, two cost estimation methods posed by
respected experts in the field are detailed. Then, the methodology used
by an actual molding shop is discussed.
Based on years of hands-on injection molding experience, Douglas M.
Bryce offers a simple empirical, yet effective, way to estimate the
cost of an SMM job [Bry96]. His method is used to obtain an estimate of
the per-piece molding cost only. It does not account for the capital
investment costs of the machines and other equipment. Additionally,
Bryce's method does not account for setup costs, tooling costs,
operator costs or maintenance costs. Although, these important items
are briefly touched upon, they are not explicitly incorporated into the
cost estimation equations. According to Bryce, the following input
parameters are required for calculating actual injection-molding
1) Material Costs
2) Raw Material
3) Recycled Material
4) Scrap Allowance
5) Estimated Regrind Buildup
6) Labor Charges (if not included in standard machine rate)
7) Straight Time
9) Hourly Machine Rate
10) Setup Charges
11) Scrap Allowance and Downtime
12) Number of Cavities in Mold
13) Cycle Time, Per Shot
14) Tooling Charges
15) Initial Mold Costs
16) Maintenance Costs
17) Production Volume (for amortization calculations)
Of the above input variables, only some are directly used to calculate the per-piece molding cost. The
other variables are mentioned, but not used in any calculations.
The material cost is the total cost of the resin needed to make one
part, including the material used in the sprue and runners. The
per-piece material cost is calculated by multiplying the total weight
of material used in one injection cycle by the price of the resin, and
then dividing this by the number of cavities in the mold (i.e. the
number of parts produced per cycle):
In many cases, the sprue and runners are removed from the ejected part
and can be reground and used to make more resin. In all injection
molded parts, there is an allowable percentage of this regrind that can
be added to the virgin resin. Bryce offers two simple scenarios that
account for regrind when calculating shot volume:
1) When the percentage of sprue and runner volume ("gating volume") is
less than the allowable regrind percentage then the sprue/runners are
"molded for free" and not included in the shot volume.
2) When the percentage of gating volume is more than the allowable
regrind percentage then the excess volume has to be included in the
total shot volume.
Bryce's method calculates the total processing cost as a simple
function of the geographically-adjusted machine hourly rate (MHR) and
the total cycle time.
The MHR, in [$/hr], is based on the machine clamp tonnage and can be
found in a table. The geographical factor, is a factor which
changes the actual MHR depending on the location of the machine. This
is due to different labor rates, utility rates and other overhead rates
in different parts of the world. For example, in the far Western and
Northeastern United States, 25%-50% should be added to the MHR, while
15%-25% should be deducted to the MHR for central and Southeastern
states [Bry96]. The total gate-to-gate cycle time, in [s/cycle], is
based on the nominal part thickness and is also obtained from a table.
The clamp tonnage is a function of the fluid pressure exerted on the
walls of the mold. Because the mold must remain closed, the clamp
tonnage must be slightly greater than the surface force on the mold due
to the injection pressure, yet not too large as to damage the mold. The
required clamp tonnage can be calculated as a function of part size and
The Rosato family is perhaps the most respected authority in the fields
of injection molding and plastics processing. Their Injection Molding
Handbook includes comprehensive guidelines for estimating the costs of
injection molding [Ros00]. In general, Rosato breaks the cost of molding
down into two categories: 1) fixed costs, and 2) variable costs. These
are further broken down into the following areas:
1) Material cost
2) Direct labor cost
3) Energy cost
1) Main machine cost
2) Auxiliary equipment cost
3) Tooling cost
4) Building cost
5) Overhead labor cost
6) Maintenance cost
7) Cost of capital
While it is fairly obvious that all of the above factors do indeed
contribute to the total molding cost, it is not readily apparent how to
obtain these values and form an estimate of the total cost. Rosato
offers some widely used methods for estimating some of the variable
cost parameters. These methods are described below.
"Material Times Two"
The first approximation that can be used simply involves estimating the
total cost as a constant multiple of the raw material cost. In many
cases, the multiple is two, and Rosato claims the cost is within 30% of
the actual cost [Ros01]. Although this method has the advantage of
simplicity, it is severely limited due to its neglect of two major
parameters: 1) cycle time, and 2) production volume. The next method
tries to partially correct these shortcomings.
"Material Costs Plus Shop Time"
One of the most commonly employed techniques adds the costs of
processing time to the material costs. Given good estimates of the
cycle time and machine rent, Rosato claims the cost can be accurately
estimated within 10%. Although this method now accounts for the cycle
time and is also relatively simple, it still does not account for the
production volume. Additionally, it can be difficult to estimate the
machine rent and cycle time.
"Material Costs Plus Loaded Shop Time"
This final technique further refines the previous two techniques by
splitting the cost of shop time into a labor element and a direct
burden on the labor rate. The advantage of this technique is that the
effect of production volume is finally taken into account, while the
equation remains quite simple to evaluate. Unfortunately, the burden
function and cycle times are not easily computed, and no method for
estimating these quantities is presented.
In addition to estimating the variable costs associated with SMM,
Rosato offers some guidelines for estimating the fixed machine cost as
well. The simplest method is to estimate it as the ratio of the total
machine investment and the annual quantity of parts produced. However,
this is under the assumption that the equipment is dedicated; that is,
utilization is at 100%. For situations in which full utilization is not
required or when many different parts are produced on the same machine,
this is not applicable. So a corrected version involves multiplying the
total annual investment by a fraction representing the utilization of
Cost of Capital
In addition to the fixed machine cost, the cost of capital needs to be
calculated explicitly. This accounts for the time value of money and is
obtained by the well-known simple-interest capital recovery equation.
In summary, Rosato provides a comprehensive list of all of the fixed
and variable factors affecting injection molding cost. Each factor is
discussed in detail, and several of them have explicit equations.
Unfortunately, many of the important factors have no convenient means
of calculation or even estimation. Furthermore, the cost estimation
guidelines presented are not tied together into one concrete
step-by-step method as in Bryce's work. Hence, with the exception of
the cost of capital concepts, which aren't unique to his method, none
of Rosato's cost estimation models are of much practical significance.
In an effort to accurately distill the art of injection molding cost
estimation down to a concrete methodology, several academic researchers
have conducted work in the field. Their research has resulted in some
unique cost estimation methods based on advanced variations of the
previously discussed industrial techniques. Although loaded with
assumptions and generalizations, these methods take into considerations
many more factors and can help provide a much clearer picture of the
various costs associated with a particular molding job. Two separate
academic cost estimation methods will first be discussed and then a
third method built as a simpler hybrid of the two will be detailed.
Boothroyd, Dewhurst and Knight's Method
Geoffrey Boothroyd, Peter Dewhurst, and Winston Knight's (BD&K)
manual, Product Design for Manufacture and Assembly, provides cost
estimation techniques to be used during conceptual design for several
manufacturing process, including SMM [Boo02]. In particular, they offer
separate methodologies for estimating the required molding machine
size, the total cycle time, the cost to build the mold, and the optimal
number of cavities per mold. These methodologies are detailed below.
With similar reasoning as Bryce's, BD&K state that the required
molding machine size is a function of the required clamp force. This in
turn, is a function of both the projected cavity area and the maximum
fluid pressure exerted on the mold during the filling stage. Although
the projected area of the runner system should be included, they
provide a simple method to account for the runners' area as a
percentage of the part volume.
BD&K split the total cycle time into three sequential stages: 1)
injection time, 2) cooling time, and 3) mold resetting time. The
resetting time is the time it takes the mold to get ready for the next
shot, i.e. the time it takes to open the mold, eject the part, and then
close the mold.
Although it can be quite difficult to predict the total cost of a mold
in the early design stages of a part because the exact mold
configuration has not been specified, BD&K have posed a method to
estimate the total mold cost based on the size and geometric features
of the desired part. Their method assumes the mold base will be
purchased by a specialized supplier and then the required tooling will
be performed in-house by the molder. This includes drilling the cooling
lines, and milling pockets to receive the core and cavity inserts.
The cost of the mold base was found by Dewhurst and Kuppurajan to be a
function of the surface area of the mold base plates and the combined
thickness of the cavity and core plates. Both of these parameters
depend on the part size and number of cavities as well as any
side-action or threading mechanisms needed to produce the part.
BD&K offer some suggestions for calculating the minimum values of
both base plate area and core/cavity plate thickness. These guidelines
are based on the minimum clearances between adjacent cavities and the
minimum thicknesses of the side walls containing the cavities. They
recommend a minimum clearance of 7.5 cm. If side actions are to be
used, then twice this clearance value should be added to the mold
length or width to be used in the area calculation.
Unlike estimating the mold base cost, the tooling cost is much harder
to predict. In many cases it is hard to estimate how long the tooling
will take for a mold simply based on the part requirements. However,
BD&K offer a systematic method of tallying up mold tooling hours,
or "points", based on the geometric complexity of the mold cavity. The
sum of these points can then be multiplied by an appropriate machining
labor rate to get the tooling cost of the mold. The mold tooling time can be split into the following tasks:
1) Ejection system manufacturing time
2) Cavity/core complexity-driven machining time
3) Cavity/core size-driven machining time
4) Side-action mechanism manufacturing time
5) Internal lifter mechanism manufacturing time
6) Unscrewing threading core manufacturing time
7) Surface finishing time
8) Extra machining time required to achieve desired tolerances
9) Surface texturing time
10) Parting plane machining time
The time required to machine the actual mold geometry into both the
core and cavity inserts is a function of the part size and geometric
complexity. The total core/cavity machining time can be split into two
separate times to account for this.. The geometric
complexity is a function of the number of surfaces that need to be
machined onto the cavity or core.
Optimal Number of Cavities
The effect of the chosen number of identical cavities in a mold can
have a dramatic effect on the per-piece part cost as well as the mold
tooling cost and required machine size. In general, a tradeoff analysis
needs to be conducted to minimize all of these quantities as a function
of number of cavities.
In summary, BD&K present a novel and systematic approach to
estimating the required machine size, cycle time, optimal number of
cavities, and most importantly, the mold cost. However, other important
cost parameters such as resin cost, processing cost, and capital costs
are not addressed. Many of BD&K's ideas are commonly implemented in
various forms in other cost estimation models and commercial software.
The theories are well-suited to adaptation to MMM and will be used
extensively in the development of the cost estimation model.
Design for Manufacturing, A Structured Approach by Corrado Poli,
presents extensive DFM guidelines as well as comprehensive
cost-estimation techniques for many popular manufacturing process,
including SMM [Pol01]. In fact, compared to the previously described
methodologies, Poli's method is the most systematic and detailed, in
the sense that it accounts for many more important factors. On the
other hand, it is the most complex and lengthy of the methods,
requiring more input parameters as well as more detailed computations.
Total Costs and Relative Costs
According to Poli, the total cost to produce one part, in [$/part], is a
function of the mold tooling cost, the processing cost, and the resin
cost. However, the cost to produce the exact same part in different
geographical locations or different times or with different equipment
can cause the cost to vary significantly. To remedy this, Poli
introduces the concept or relative costs. Relative costs are simply
true costs divided by the costs of a reference part. Similarly, the
concept of relative time, length, volume, etc. can be developed. In of
all Poli's equations, the reference part was a flat washer with a 1mm
thickness and inner/outer diameters of 60mm/72mm, respectively.
Relative Tooling Cost
The tooling cost relative to the reference part's tooling cost is a weighted function of the raw die material costs and die
Relative Processing Cost
The actual per-piece processing cost, in [$/part], is simply the product of the effective cycle time and the MHR.
Relative Material Cost
Similar to the relative processing cost, the relative material (resin)
cost is simply the cost of the resin needed to make one part divided by
the cost of the resin for a reference part.
Relative Total Cycle Time
The relative cycle time is defined as the total cycle time
divided by the cycle time of a reference part. Total cycle time
consists of a base cycle time that depends on the maximum wall
thickness and geometry of the part, additional time required for metal
inserts or internal threads (i.e. "insert molding"), and a penalty
factor resulting from specified surface finishes and tolerances.
In summary, Poli provides a comprehensive and systematic quantitative
methodology for relative cost estimation of a SMM job. Many important
cost parameters, such as the relative mold tooling cost, are considered
in his method. However, it is important to remember that these values
are all relative; that is they must be compared against a reference
part. If the costs associated with the reference part are well known,
then it may be possible to obtain accurate values for absolute costs.
Compared to BD&K's method, Poli's method is more comprehensive, but
more involved as well. The relative nature of the calculations may
yield more accuracy when a reference part's cost estimate is accurate.
Although the relative comparison theories of Poli's method seem well
suited to a relative cost estimation/comparison model, they are not
that widely used as they are rather complicated, highly empirical, and
difficult to adapt due to the highly numerical and tabular structure.
Fagade and Kazmer's Method
Adekunle Fagade and David Kazmer (F&K) present a simple and
effective approach to SMM cost estimation in their article "Early Cost
Estimation for Injection Molded Components" [Fag00]. Most notably, they
offer simple empirical equations to predict the cost of the mold and
the lead time required to manufacture the mold. Additionally, they
present methods for material and processing cost estimation . Their
methods are detailed below.
As with most authors, F&K agree that the total per-piece cost of
injection molding, c [$/part], is the sum of three separate factors,
namely, material cost, processing cost and molding cost respectively.
Mold Cost and Lead Time
Along with BD&K and Poli, F&K agree that the mold cost is a
directly correlated to the part's complexity. However, unlike the
previous authors, F&K's definition of "complexity" is much simpler.
In stead of going through multiple and sometimes ambiguous steps in
determining a part's complexity, F&K's method uses the following
five complexity-determining input values in their cost estimation
1) Number of unique dimensions needed to define the part
2) Volume of part bounding box
3) The number of actuators required (e.g. side cores)
4) Whether or not the mold requires a "high polish finish"
5) Whether or not the mold requires a "high tolerances"
For comparison purposes, F&K compared their mold cost estimation
method with BD&K's method and Poli's method. According to them, the BD&K underestimates the mold costs and
under-predicts the relative sensitivity between the two different
designs. The Poli model exhibits greater range than the observed mold
quotes, but is likely the best estimator for these two test parts.
F&K's proposed model over-predicts the mold cost and doesn't
exhibit adequate sensitivity. F&K point out however, that their
method is the simplest, requiring only several easy-to-obtain input
parameters. Besides just supplying an estimation method for mold cost,
F&K also present a similar analysis for estimation the total lead
time it would take to receive the mold from the manufacturer. Similar
to the previous material cost estimation methods, F&K provide two
simple equations to estimate material cost and processing cost
In Summary, F&K provide a simple and straightforward method for
estimating the cost of an injection molding job as well as a simple
estimate for the mold manufacture lead time. However, their method does
not take into account multiple cavity molds and was shown to be less
than accurate for two test parts when compared with other available
cost estimation models.
Although F&K's method can be quite useful for obtaining a rough
cost estimate in the early design stages, it is rather simple and
empirical. Additionally, it would be difficult to directly adapt these
theories to MMM cost estimation, hence they will not be used anymore.