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.


Industrial Techniques

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.


Bryce's method

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 manufacturing costs:

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

8) Overtime

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.
Material Cost
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.
Processing Cost
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 injection pressure.


Rosato's Method

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:
Variable Costs
1) Material cost

2) Direct labor cost

3) Energy cost
Fixed Costs
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.
Machine Cost
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 the machine.
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.
Summary
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.


Academic Work

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.
Machine Size
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.
Cycle Time
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.
Mold Cost
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.
Summary
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.


Poli's Method

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 [$/part], is a function of the mold tooling cost, the processing cost, and the resin cost. However, the cost to produce the exact same part in different geographical locations or different times or with different equipment can cause the cost to vary significantly. To remedy this, Poli introduces the concept or relative costs. Relative costs are simply true costs divided by the costs of a reference part. Similarly, the concept of relative time, length, volume, etc. can be developed. In of all Poli's equations, the reference part was a flat washer with a 1mm thickness and inner/outer diameters of 60mm/72mm, respectively.
Relative Tooling Cost
The tooling cost relative to the reference part's tooling cost is a weighted function of the raw die material costs and die construction costs.
Relative Processing Cost
The actual per-piece processing cost, in [$/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.
Summary
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.
Total Cost
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 method:

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 respectively.
Summary
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.