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.

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.

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.

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.

2) Direct labor cost

3) Energy 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.

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.

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.