Overview and Challenges in Cost Estimation


Provided that a given design can be manufactured using either SMM&A or MSM, the next important question is: "Which process is more cost-effective?" The problem entails estimating the cost for both manufacturing alternatives and choosing the better one. Significant theoretical and experiential work has been devoted to estimating the cost of SM injection-molded components. There are a myriad of available methods with varying detail and complexity that can be used to get an estimate of the manufacturing cost of a given quantity of identical SM objects. Some of these more sophisticated estimation models have even been implemented into commercial software such as Seer DFM.

It seems natural that these methods should be generalized to work for MMO's with n different materials. However, at this time there are no established methods for estimating the cost of a MMO. It turns out that there are several difficulties in expanding existing cost-estimation models to incorporate MSM. While some SMM cost-estimation concepts carry over nicely to the domain of MSM, many issues simply cannot be addressed using the same reasoning. The main factors affecting cost are detailed below.

Material Costs

For SMM, material costs are simply estimated by calculating the total weight of material required to form one part and multiplying that by the unit cost of material. More complex analyses work in scrap rates and virgin vs. recycled material costs as well. It turns out that this concept can be easily expanded to include MSM as well. The only difference is the cost of each material subsection of the part must be calculated as before and then summed to get the total cost.

Labor Costs

Calculating labor costs for MSM is also just as straight-forward as that for SMM. After the number of laborers (e.g. machine operators, assemblers, packagers, etc.) is decided upon, the labor cost is estimated by multiplying the hourly labor rate by the time required to produce a finished part. The required work force and work loads may be different for MSM, but the labor costs are estimated in the exact same manner as with SMM.

Equipment Costs

The cost of the equipment required to produce a SM product is usually computed as a simple function of the shot size. The driving factor influencing machine cost is simply the machine size, measured in clamp tonnage. Based on the volume and projected area of the total shot, a mold clamping force can be estimated and from that a proper machine size can be selected. Additionally, a proper injection unit can be selected based on the shot volume and injection pressure requirements.

Unfortunately, given the detailed design of a desired MMO, it is not always readily apparent what equipment is required, and hence the associated cost needed to produce it. An appropriate combination of molding press, injection units, mold-reconfiguration equipment (e.g. a rotary platen), and ancillary equipment needs to be selected based on the product's processing requirements. Unlike SMM, it is not always a simple task in choosing an appropriate molding machine, and in many cases, a proper machine configuration has to be custom made for the specific MSM application. For example, the proper clamp force has to be carefully selected to have enough force to adequately seal the mold while not damaging the mold and its associated mold-reconfiguration equipment. Furthermore, given a product design, it is not always clear which of the three MS processes should be chosen. For instance, it may be possible to use either a rotary platen or an indexing plate to produce the desired MMO. In general, the equipment cost estimation cannot simply be based on the shot size as with SMM.

Tooling Costs

Tooling cost associated with manufacturing a SM mold is a difficult and detailed task in itself. It is a complex function of the mold size, machining and finishing requirements, and the complexity of the desired part. Costly special mold equipment, such as side-actions, core pullers and/or threaders may need to be incorporated into the mold. Gating and venting considerations also add into the tooling costs.

These considerations, along with several unique ones, carry into MSM tooling estimation. A given MS mold is always much more complex than a SMM mold with similar features. MSM requires molds with more cavities and more complex gating systems. MSM always requires hot runner systems, and more sophisticated control schemes. Additionally, specialized subsystems such as rotary platens, index plates, or core toggles must be built into MS molds.

Furthermore, specialized DFM rules have to be considered when designing proper MS molds. Issues such as mold shut-offs that deform partially-complete components ("crush"), interface problems such as flash/weld lines, and more complex flow and cooling characteristics need to be considered. In general, the increased complexity of MS mold tooling requires a revised set of the existing cost-estimation tools based on SMM.

Processing Costs

A final important cost factor that needs to be assessed is the set of costs associated with performing the injection molding process. These costs include power consumption, auxiliary operation costs (such as inspection), and factory overhead costs. For SMM, the most common processing cost estimation scheme simply involves multiplying the total part processing time (the cycle time) by an averaged machine hourly rate (MHR). This rate is usually a simple function of machine size and geographical location. Because there exists an abundance of historic cost-estimation data for SMM, computing a MHR for a given SM machine is simple task of performing a table-lookup. For MSM, this simple scheme could be adapted as well, provided the MHR was known. Unfortunately, no such data yet exists for MSM.

Obtaining this MHR for MSM is a complex task because of the inherent differences in MSM. The multiple injection units and rotating mold components can have significantly different power and other resource consumption rates. Additionally, the secondary operations associated with injection molding (e.g. assembly, inspection, packaging, etc.) can be totally different for SMM and MSM. For example, a MSM version of a product will require no assembly, but may require a more advanced inspection procedure then a SMM version of the same product. These, among several other factors must be considered in calculating an appropriate MHR.

Further complicating matters is the issue of cycle time. Cycle time is perhaps the single most important factor affecting processing cost. With SMM, the cycle time is usually estimated by computing an approximate part cooling time. This cooling time is in turn affected by several parameters such as part volume, wall thickness, and material properties. However, for MSM, the cycle time becomes an even more complex function due to the multiple materials' cooling requirements as well as differences in the process itself. For instance, in a two-material product, the cooling time for each material and the time required to rotate a rotary platen by 180° must be factored into the cycle time, among other things.

Summary

When attempting to justify the use of MSM for a given application, it is crucial to have accurate cost-estimation data based on the conceptual CAD model of the desired object. In order to promote MSM as a viable alternative to SMM&A, a cost-estimation model needs to be developed. The model should be efficient, accurate and insensitive to small changes in the input. An optimal model would use geometric information derived from the CAD model to calculate all the costs associated with producing a certain number of desired pieces. Thus, the problem of cost-estimation is by no means easy and deserves special attention.