49 3 Cost Factors Affecting Productivity A mold’s productivity is ultimately measured by how many good parts it can produce over time. A mold that runs very fast but is frequently down for maintenance or repair will not produce at lowest part cost and highest productivity. Conversely, a mold that runs too slow but produces parts consistently over time is not optimal. The same can be said for each compo- nent in the injection molding system. One way to quantify productivity is to measure the total equipment pro- ductivity (TEP): =×× Production hours (auto cycling) Parts made Scrap parts TEP Available hours Hour Parts produced A good custom molder can achieve TEP’s greater that 80% and good dedicated systems achieve values higher than 90%. 3.1 Where Will the Mold Be Operated? 3.1.1 Condition of Ambient (Shop) Air We tend to assume that the mold will be operated under “ideal” conditions, but this is typically not the case. The environment in the molding shop can vary from very cold to very hot, from dry to very humid, from clean to dusty and dirty. With sudden changes in any of these conditions, a molding operation can be affected significantly. High humidity will affect the mold itself (corrosion) and will affect the cycle time (productivity) of the mold. Rapid temperature changes may even affect the operation of a machine and mold and lead to breakdowns and loss of production. A typical example (A): a molding shop operated eight identical machines in each of two parallel rows; all were molding the same or very similar products with the same type of mold. They all worked fine, except the last machine in one row, which stopped frequently, without apparent reason. After checking for machine problems, such as possible power fluctuations, poor cooling water supply, etc. it was noted that this last machine was close to an emergency exit door, which was supposed to be closed all the time; however, on some days, the workers kept the door jammed open to improve the shop ventilation. The draft from the entering cooler outside air was enough to affect to operation. After ensuring that the door stayed closed at all times, there were no more problems reported with this machine. Figure 3.1 Total equipment productivity (TEP) Figure 3.2 Plan of molding plant in Example A 1281han03.pmd 28.11.2005, 11:0749 50 3 Cost Factors Affecting Productivity Another example: a mold and machine worked perfectly, but occasionally, for several hours, produced pieces with surface blemishes that looked like blisters. Investigation showed that it happened only on very humid days. This particular operation required a rather long mold open cycle. On humid days, the water in the air condensed in tiny droplets on the cold mold cores during the few seconds the mold was open and the cavities and cores were exposed to the shop air; the droplets appeared as blisters on the surface of the product. After slightly increasing the cooling water temperature to bring it above the dew point the problem disappeared. The “penalty” was a slightly longer cycle time, but it ensured continuous production of quality products. Corrosion Prevention It is important to decide how the mold will be protected from corrosion if it is evident that the mold is operated and stored in a humid environment. This can affect the mold cost. A common approach in many shops is to protect the molding surfaces before the mold is put into storage by using silicon spray (“Mold Saver”) or to just apply plain, clean machine oil. Many shops paint the outside of the mold shoe with a permanent oil paint to protect the outside of the mold against corrosion. Another approach is to flash chrome plate the stack parts or to make them from stainless steels; both methods will of course add to the mold cost. For the mold shoe (the mold plates) itself, instead of using oil paint, it can be protected against corrosion with electro-less nickel plating (ENP), which has the additional advantage that it also protects some of the inner surfaces of the mold shoe, which would not normally be covered when the mold is just painted on the outside. ENP also enters the cooling channels to some extent and protects them against corrosion caused by the coolant, but the pene- tration is limited and does not cover the walls of the channels deep inside the plates. ENP is hard (70Rc) but thin and not resistant to scratches and wear. The best method may be to make the entire mold shoe from stainless steel (SS). The basic cost of SS is higher than the cost of mild steels or pre-hardened machinery steels. However, when SS is bought in large quantities, the cost difference can be much less. When molds are expected to run for a long time, the advantage of SS over other steels can justify the higher cost. We must not forget that chrome plating or ENP also cost money. We must also consider the costs of transportation to and from the nickel or chrome plater, the additional time required for these operations, the lack of control over the transport, and the dependence on an outside supplier. Another problem with chrome plating is that any change (requiring re- machining) of a chromed surface requires that the chrome must first be removed from the steel part. This requires shipping the part to the plater for removing the coating by a process similar to plating. After re-machining, the changed part must again be shipped to be plated. This is an expensive and time-consuming procedure. Mold shoe material options:  Pre-hardened plate steel, painted  Plate steel with ENP  Stainless steel Always consider the total costs when comparing mold material costs 1281han03.pmd 28.11.2005, 11:0750 51 3.2 Coolant Supply Note that corrosive plastics such as rigid PVC always require chrome plating or, better yet, SS for the stack parts. The use of full-hardened (or pre-hardened) SS for cavities, cores, and inserts is quite common today, even though the steel cost is higher. When considering the expenses and risks with chrome plating of mold steels and the time saved, the total cost could be more than using SS. Another solution for all these issues is to provide the molding plant and the mold storage facilities with air conditioning or at least with controlled, low humidity air. Some modern molding plants have this equipment, although this means added expense and may not be needed or cannot be justified economically unless in cases where delicate products are mass-produced. Occasionally it can be useful to surround the machine with a shroud to keep the environment immediately around the mold and machine at a desired low humidity with a portable dehumidifier. 3.2 Coolant Supply The available cooling water supply (quantity, quality, and pressure of the coolant) must also be considered. Also, remember, for water-cooling to be effective, the water must flow fast enough to establish turbulent flow. Turbulent flow removes significantly more heat per liter (or gallon) and can be calculated (see [5], Chapter 13). 3.2.1 Is the Coolant Supply Large Enough for the Planned Mold? There is no point to design a mold with an expensive, elaborate cooling system if there is not enough coolant flow and pressure available to take full advantage of it. I have seen some mold plants that developed from only a few to a high number of machines, but neglected to increase the cooling water supply to grow with the rest of the operation. This resulted in the molds running much slower than they could if the cooling water supply had been sufficient. Good cooling of a mold depends not only the coolant temperature but also on the volume of coolant that flows through the mold, measured in liters or gallons per minute. This volume depends essentially on the pressure differen- tial between IN and OUT of the cooling channels in the mold and on the method of distribution through the mold (see [5], Chapter 13). 3.2.2 Is the Cooling Water Clean? Cooling water must be clean, i.e., free from contaminants and/or oxidizers, which corrode the inside of the cooling channels. This is where stainless steel Table 3.1 Calculating Chiller Requirements Resin Chiller lb/h/ton HDPE 30 LDPE 35 PMMA 35 PP 35 PA 40 PPE 40 ABS 50 PS 50 Acetal 50 Tons required = Resin lb/h/ton × lb/h of resin consumed For highest productivity ensure that the cooling channels in the mold are free from sediments (lime, rust, etc.) 1281han03.pmd 28.11.2005, 11:0751 52 3 Cost Factors Affecting Productivity is of great advantage. The coolant must also be free from lime and dirt, which will gradually settle in corners of the cooling system and plug the cooling channels, especially if the channels are small and in elaborate circuits, as is often required in high production molds to cool small mold parts. Under such bad conditions, a mold will probably run satisfactorily and produce as planned for the first few months, but because of buildup of dirt in the cooling channels, the mold will gradually lose its cooling efficiency and run slower than it could with good, clean coolant. Dirt in the water will also require more mold maintenance, as the channels will have to be cleaned from time to time. Such mostly unnecessary costs are often overlooked while worrying about the high initial mold cost. Rust is an insulator and will eventually slow the molding cycle as it builds up. 3.3 Power Supply Electric power supply is not always as stable as required, especially outside the larger industrial areas of North America and Europe. In many parts of the world, especially in developing countries, there are often considerable voltage fluctuations because of weak and overloaded power lines; molders experience occasional, and sometimes even daily, “brownouts” (periods of lower voltage) and are often plagued with complete power failures (blackouts) lasting anywhere from just minutes to many hours. To say the least, these stoppages are annoying, but they can also be very expensive if a mold stops frequently just because of failure of the machine controls. Voltage fluctuations affect molding operations for two main reasons.  Logic controls are sensitive to voltage fluctuations and may require voltage stabilizers. Although this is a machine requirement, it needs to be pointed out. Every time the machine stops, the mold also stops producing. In general, electronics are quite sensitive to high ambient temperature.  Melt temperature. Virtually all heaters in molds and molding machines today are electric resistance heaters. The heat output of a resistance heater is proportional to the square of the voltage applied. A drop of just 10% in voltage will reduce the heat output by 20%. While the barrel heaters of the extruder are always thermostatically controlled, a transformer, without feedback, often controls the machine nozzle heaters. With heat controls, any reduction in voltage (and temperature) will be automatically com- pensated by having the heaters ON for longer time periods. In hot runner molds, the hot runner manifold heaters are always equipped with thermocouples; however, because of the high initial costs (in the mold, and for the associated external controls required) many molds do not have heat controls on the nozzle tip heaters and can therefore experience A drop of 10% in voltage will reduce heat output by 20% if not thermo- statically controlled Figure 3.3 Rusted mold components 1281han03.pmd 28.11.2005, 11:0752 53 major temperature variations as the voltage varies. This will lead to trouble in the mold’s performance. Even so, today, about 80% of the high- production hot runner molds are equipped with thermostatically con- trolled nozzles as the added costs can be easily justified with the increased productivity. Cold runner molds: With such molds, power interruptions, while annoying, are not serious. If an interruption is only of short duration – in the order of a few minutes – the plastic in the injection unit is probably still hot enough so that production can resume immediately, without causing problems. If the interruption takes longer, it will take again the time necessary to heat up the injection unit before resuming production after purging. Hot runner molds: With these molds, power interruptions can be more serious. Short interruptions of a minute or two can be tolerated without problems, but any longer stoppage will cause the plastic in the manifold and the hot runner nozzle  to degrade, especially heat-sensitive plastics in the still hot manifold, and  it will freeze sooner, because the masses of the manifold are much smaller than the masses of the extruder. It takes time to heat up the whole system to operating temperatures, and the plastic both within the injection unit and in the hot runner system must be first purged before resuming operation. Note that well designed and built hot runner systems require less time for restarting than poorly designed systems. A good hot runner system should be ready for resuming production in about 10–15 minutes after any inter- ruption. These details are important to understand before deciding on the kind of runner system to select for the mold. A hot runner system may be more suitable than a cold runner mold for a certain application, but may cause endless grief if the power supply is poor. All the well-known and proven advantages of a hot runner system can be lost because of the frequent stoppages due to power supply problems. 3.4 Will the Mold Run in a Variety of Machines or a Single Machine? The mold will often be required to operate in different models of molding machines. This may result in quite some complications in the mold layout and will certainly increase the mold cost. In particular, different locations of the machine ejectors can affect the ejection and the cooling layout of the mold and the overall size of the mold. 3.4 Will the Mold Run in a Variety of Machines? 1281han03.pmd 28.11.2005, 11:0753 54 3 Cost Factors Affecting Productivity The mold must be equipped with all features that are compatible with these various (existing or planned future) machines. This applies to several areas of the mold:  Shut height  Any downstream automation  Mold mounting (including any systems for quick mold changing)  Locating ring size  Sprue bushing size and shape  Machine ejector pin locations  Cooling- and air-circuits  Hydraulic functions  Electrical connectors If a mold is to be designed for one machine only, in one location only, it can result in a simpler mold. For example, there would be no need to provide for various sizes of locating rings and the ejector mechanism and the mold mounting provisions could be designed for the pattern of the selected machine only. 3.5 Is the Mold Planned to Run in a Newly Created Operation? It is a very desirable condition for the mold designer when a mold (or a series of molds) are planned to be operated in a new factory (or in a separate section of an existing factory), because it creates an opportunity for close cooperation of the mold designer with the planning of the whole project. It provides an opportunity to participate in the selection of the most suitable machine for the product to be made, but also to take part in the plant layout, power distribution, cooling water system, and so forth. This is also a good time to introduce standardization of many of the mold elements and mold sizes, of mold mountings (including quick mold changes), power and cooling connections, and any other feature that will affect not only the mold(s) now under consideration, but also future molds for this location. Standardization of mold components, molding machines, and ancillary equipment will not be further discussed here, but they are an important field where savings both in investment (costs of equipment) and increase in productivity can be made. 1281han03.pmd 28.11.2005, 11:0754 55 3.6 Projected Requirements How many pieces of the product will be made from the planned mold? This could be the most important question to ask before deciding on the type of mold required for any job. But this is also often the most difficult question to answer, particularly if the product is new on the market. It is nearly impossible to foresee if the product will find the hoped-for acceptance and increase in sales, or if the product will not be accepted as expected. Also, assuming a total quantity is known, what is the time frame when these quantities are required? If 1,000,000 pieces of a new product are to be molded, the question is:  Is this a limited production run, say within four months (usually as soon as possible) or  Is this quantity needed every year, for a unspecified number of years, or  Is this quantity needed over the expected life of the product, e.g., 5 years, in which case the annual requirement is only 200,000 pieces. 3.6.1 Making Prototype or Experimental Molds 3.6.1.1 Prototype Molds Prototype molds are required to make samples of a new product for evalua- tion of a newly developed shape, to see how the product appeals to the eye and/or to the touch. Molded samples can be subjected to the expected stresses and wear and the results are better than testing a hand made (machined, or assembled) model. The result also could be more accurate (and possibly cheaper) than a computer simulation. Because it is only important to mold the overall shape of the product, without worrying about productivity of the mold, shortcuts can be taken everywhere: mold materials such as mild steel, aluminum, even plastics (epoxy, etc.) can be selected, as long as they are sufficiently strong and resistant to the heat and the pressure of the injected plastic. Working to close tolerances is usually not necessary. Generally, there will be no need to worry about surface appearance (polish, engraving, even flashing). There is no need for cooling channels; it will take just a little longer to cool the plastic before being able to remove the molded sample from the mold. In many cases there is also no need for an ejector mechanism. An air jet directed against the edge of the product at the parting line, or a few simple ejector pins that can be manually pushed to eject, may be all that is required. Other features of the product, such as internal or external threads, can be produced by using loose inserts in the mold that can be ejected with the product and then unscrewed by hand. Loose inserts can also be used for odd shapes in the sides of the product, which would otherwise require side cores. Round holes or simple openings in the sidewalls could be machined after the molded piece is cold. These are just some of the mold features that can 3.6 Projected Requirements Figure 3.4 Typical prototype mold for a lid, capable of 4 in to 8 in lid prototyping (Courtesy: Husky) Figure 3.5 Single-cavity prototype mold for production 2×2 system. In this case, the prototype stack was used as a spare in the production mold (Courtesy: Husky) Projecting the number of molded pieces is often the most important and difficult question to answer 1281han03.pmd 28.11.2005, 11:0755 56 3 Cost Factors Affecting Productivity be omitted to simplify the stack and to reduce the cost of the prototype mold. If prototypes are frequently required, the stacks could be mounted in a common mold shoe, thus saving even more costs. The runner system would normally consist of a simple sprue gate directly into the product or a sprue and short runner could be used for edge gating. The gate will then be cut manually. 3.6.1.2 Experimental Mold This type of mold is different from the prototype mold: it will be used mostly to establish the behavior of the plastic in a newly developed product during injection. Some of the above cited shortcuts to save costs can be used, but in general, the mold would be closer to a simple, single-cavity production mold. The gate should be located as planned for the production mold. The mold could also be used to establish the most suitable location of the gate and the method of gating for the product. Such a mold would normally require the proper finished appearance of the product. Note that especially in thin walled products, the finish affects the flow of plastic through the cavity space. Cooling efficiency is not as important as in a production mold, but some cooling should be provided to maintain a stable mold temperature. Because the quality of a molded piece depends very much on the accurate repetitiveness of cycle time, an ejector mechanism should be provided rather than manual product removal to eliminate any operator-created variations in ejection (and cycle) time. An important feature of an experimental mold is often the facility with which some stack parts can be changed. This adds costs but will make experimentation easier. Experiments with such molds can also determine the effect on molding cycles when areas of the mold are not cooled, little cooled, or well cooled. Such information can be valuable before an expensive, multi-cavity production mold is designed. The difference between “ordinary” and “exceptional” cooling could mean much in engineering the production mold. Reduction in cycle time achieved by exceptional cooling could be insignificant and not worth the additional costs and complications to the mold. 3.6.1.3 Combination of Prototype and Experimental Mold This applies when an inexpensive mold is required to establish the shape of the product, but at the same time it is planned to explore market acceptance of such product by manufacturing a few hundred or even thousand of samples for field testing. Typically, such molds should run “fully automatic,” but there is no need to achieve maximum efficiency in molding, as with better cooling, better runner system, etc., and without special finish or most engravings. Such molds can also be used to establish shrinkage conditions. I remember a case where a client wanted a very simple prototype mold to see how a newly designed LDPE cover would fit as a shield over a metal product he had been selling for years. The prototype mold was supposed Figure 3.6 Typical 4-cavity experimental mold that will emulate the behavior of the production mold (48 up to 144 cavities) (Courtesy: Husky) 1281han03.pmd 28.11.2005, 11:0756 57 to produce about 100 samples. We made a mold with a very simple cavity and core, all mild steel, with a few ejectors and a simple through-shooting gate right into the center of the product; some cooling channels, no polish, no engraving. There was hardly a simpler mold possible. The client promised that if the new idea was accepted in the field, he would buy a production mold. After a few months, I called to ask him how the idea took on, and he told me that the mold has already produced several thousand pieces and was still in perfect condition, and that he wont need another mold. A properly designed production mold would surely run faster – i.e. produce more pieces per hour – but with really small quantities this is not worth the extra cost. 3.6.2 Production Molds Production molds are any type of molds other than prototype and experi- mental molds. At this point in the planning for a new mold it becomes necessary to have basic information on  How many pieces will be required?  What will be the molding cycle? Once these data are available, there should be not much difficulty to proceed, but both these data are usually difficult to ascertain. Since the mold type and number of cavities will depend primarily on the quantities required to be molded, we must first differentiate between the various possibilities as they present themselves, before deciding on the kind of mold that will be most appropriate. 3.6.2.1 New Products The new, untried product is a common case and can be part of a new “invention” or an existing product previously made from a different material. Will the market accept it as is in its new shape, made from injection-molded plastics? Will it require modifications after complaints or suggestions from the field after it was launched on the market? Or will it be a disappointment for the seller, and soon disappear? Unfortunately, the “entrepreneur” takes all the risk when investing in the required mold. Of course it would be convenient to keep mold cost as low as possible, but we know that this may increase the cost of the products in the long run. The cost of a high cavitations mold may also affect the timing of the launching of the product. Should a large production be anticipated, which will require a multi-cavity mold of high quality? In this case, if the product is not accepted in the field, the loss could be substantial. But there is also another, just as serious problem when launching a new product: the investor was overly cautious and is waiting for 3.6 Projected Requirements Conclusion: There is no clear answer to the above questions. It may depend on the expected life of the product, which is often just as difficult to estimate. Some products are seasonal and the demand finds an early saturation point. Some products increase in demand until some competitive, similar or even better product comes along, in which case demand for the original pro- duct could sooner or later disappear. One possible advice is to build a mold for the initially estimated volume, and add 25% for surge demands, unanticipated stoppage, and some growth 1281han03.pmd 28.11.2005, 11:0757 58 3 Cost Factors Affecting Productivity the acceptance in the field. If the product is a great success, the first mold was probably not designed for the unanticipated, high demand. What would be the best strategy at this point? Make another mold (or even several molds) similar to the first one and run them side-by-side? This approach may have the advantage of lower additional investment while providing more flexibility. It is easier to find several smaller machines than larger machines. However, a larger system, using a high-production mold, with more cavities, better runner systems, better cooling, better ejection, more automation, and therefore higher up time, will result in the lowest cost of the products. 3.6.2.2 Existing Product, Large Quantities Some products are “timeless”, meaning that their annual quantities are more or less constant and known. Their use may vary within seasons and even with the economy in general, but they remain essentially unchanged. This applies to many technical articles, as well as to many packaging products, such as food containers and to medical products. In these cases, it is not difficult to establish annual requirements and a projection for how long the product will be in demand. In addition, it is always important to consider the whole system, i.e., machine, mold, and any after-molding operation (automation, product handling, packaging, assembling, etc.,) that will yield the lowest-cost product. With long and high production runs, even high mold cost is insignificant per unit produced and helps lower the product cost, provided it runs faster, longer, and with higher quality products. 3.6.2.3 Limited Quantities Sometimes, a product is required in a limited quantity or for a one-time occasion only. This may be the case where a molded piece is designed for a special occasion or application. The quantities are relatively small but usually known. Frequently, a molded piece will be required as a promotional item, such as giveaway items to retail customers. Such promotions are usually limited in time and the requirements are stipulated at the beginning of an advertising campaign. Usually, such promotion needs fast delivery of the molded pieces, and the total amount of pieces in a very short time span. A decision will have to be made: Should the order be produced on a large, multi-cavity mold? This will yield the best piece cost but will require a larger machine, which may not always be available at the time the mold is ready for production. The mold cost will be higher but the cost per molded piece is probably insignificant. The problem is that such larger molds will take longer to build and there may not be enough time. Also, it leaves the molder vulnerable, in case of machine or mold breakdowns, in which case there could be no production at all. As an alternative, several smaller, identical molds could be built which are simpler and can be made faster by contracting out to more than one mold maker if necessary. These (smaller) molds can be built faster and then be run on smaller machines, which are also usually easier to locate; if necessary, Conclusion: Investing in the best possible mold is usually the key to a successful operation Figure 3.7 High volume production mold for a stadium cup (2×12 cavities, air eject, modular construction) (Courtesy: Husky) 1281han03.pmd 28.11.2005, 11:0758 [...]... injection molding The simplest approach to increase productivity is to increase the number of cavities; while this obviously has improved the productivity, the main target of research is how to reduce the cooling time (note: from the early beginnings of these Figure 3.13 A part is cooled very efficiently when dropped into cold water 64 3 Cost Factors Affecting Productivity molds, the number of cavities has... hazardous gases created when working with machine tools 62 3 Cost Factors Affecting Productivity 3.7.4 Efficiency of Cooling The purpose of mold cooling is to remove, in the shortest possible time, the heat energy that entered into the cavity space during the injection of the hot plastic melt The higher the efficiency of removing the heat, the higher the productivity of the mold Molds for Small Production (Fewer... will require the best possible cooling methods, which are more costly to design and to manufacture These molds use the most suitable (and sometimes expensive) mold materials to facilitate the rapid removing of the heat The higher costs incurred will usually be worthwhile, because they result in a mold with higher productivity and in lower costs per molded piece Molds for Large Production of Heavy-Walled... time 3,600 s (1 hour) divided by the cycle time (in seconds) equals the number of shots per hour Shots per hour times the number of cavities equals the number of pieces produced per hour 60 3 Cost Factors Affecting Productivity 3.7.1 Type of Plastic Molded There are several issues to consider: Melt temperature required to be able to inject and to fill the cavity space Higher melt temperatures require... Turret gear Stationary platen Tiebar nut Tension plate Runner block Mold stroke cylinder (4) Guide rail Clamp base Tiebar (4) Figure 3.16 Schematic of the indexing clamp (Courtesy: Husky) 66 3 Cost Factors Affecting Productivity Molds for Most Other Products Molds for most other products are equipped with a cooling system somewhere between these extremes Often, in molds for intricate shapes requiring many... there are also other, mainly older machines with dry cycles up to 20 s! It obvious that for large production Molding cycle (s) = Dry cycle + Injection + Cooling + Ejection + Mold open 68 3 Cost Factors Affecting Productivity and a mold with a short molding cycle, the length of the dry cycle is much more important than with a mold that requires a long molding cycle Figure 3.18 Schematic showing the... accurate estimate of the probable cycle time In case of similar or even identical products and molds, the cycle time can vary considerably when run on different make and size machines Machine factors affecting the mold productivity are dry cycle, injection speed/pressure, tonnage, and recovery time The molding cycle time is the dry cycle time plus the time required to inject and cool the molded piece(s)...59 3.7 Forecasting the Cycle Time even at different custom molders This will probably increase the piece cost, (a) because the smaller and less expensive molds are not as productive as a larger mold, and (b) because of the added cost of dealing with more than one source However, this approach will also ensure that any breakdown will be less serious to the customer In... system of continuous vent gaps (C), venting grooves (D), and channels (E) to permit fast filing of the cavities The productivity of the mold at 6.0 s cycles yields 2,400 tubs per hour 3.7.6 Effect of Molding Machine on Cycle Time Several features of the molding machine affect the mold productivity and will be discussed in the following It is important to be familiar with the machine for which the mold... small, there are two alternatives: Individual molds, with the least amount of “high productivity features (especially good cooling, hot runners, etc.) or Making inserts for so-called “universal mold shoes” which are listed in most of the mold supply house catalogues Such mold inserts for universal mold shoes usually do not cost much more than the mold stack for a regular mold They can be mounted in the . 49 3 Cost Factors Affecting Productivity A mold’s productivity is ultimately measured by how many good. Total equipment productivity (TEP) Figure 3.2 Plan of molding plant in Example A 1281han03.pmd 28.11.2005, 11:0749 50 3 Cost Factors Affecting Productivity Another