Tài liệu Cost Factors Affecting Productivity P3 pdf

91 Multi-Cavity Molds Especially with multi-cavity molds, it may not be possible to clear all molded pieces out of the molding area in time and some MO time will have to be added. The need for this MO time should be closely scrutinized; while it is sometimes not possible to avoid MO time, it is quite common that the setup persons add extra MO time “just to be sure” that the pieces have cleared, rather than to ensure that the ejection, including air assist, is properly adjusted and timed as intended. The length of the ejection stroke, when to start ejection, when to start and to stop blowing, etc. must be recorded for the next time the mold will be used in the instructions for the setup of this mold, but also in the job file for future reference. Figure 3.51 shows the same cycle as in Fig. 3.50 but with the addition of MO time, providing more time available for ejection but also lengthening the cycle time. Here again it would be possible to use a portion of the closing time of the next cycle to extend the time available for ejection (line L). Following is a list of reasonably achievable MO times to be added for various ejection methods:  Air ejection only none, or 0.5–1.0 s  Machine ejector (hydraulic) 1.0–2.0 s  Robot 1–2.5 s Multiple Ejection Strokes Most molding machines are equipped with the feature of double or multiple ejection strokes. The idea behind this system is that if the original (first) ejection stroke has failed to eject some of the molded pieces, additional strokes will shake these pieces loose from where they hang up so that they fall free to clear the mold before the next cycle will start. A well-designed mold has no need to use this system: the molded pieces must eject on the first stroke. Additional strokes not only add to the cycle time but are also noisy and wear out machine and mold. Figure 3.51 Schematic of ejection timing with added MO time 3.7 Forecasting the Cycle Time Figure 3.52 Large machine with 2-cavity mold, molding two large industrial pails (pail weight 800 g, cycle time 35 s, productivity 205 pieces/h) Don’t use multiple ejection just because it is available. 1281han03.pmd 28.11.2005, 11:0791 Previous Page 92 3 Cost Factors Affecting Productivity Automatic Product Removal Molds with integrated mechanized product removal, which act in mechanical or timed synchronization with the mold opening and closing stroke, will remove the moldings as soon as the mold has opened far enough to allow the take-off member to enter the space opposite the core or cavity. The products are then grabbed (mechanically or with suction cups) and the take- off arm withdraws ahead of the re-closing mold. This method has been used successfully even in multi-cavity molds, cycling as fast as 3 s or 20 shots per minute. Such special ejection methods should be considered, especially when it is required to maintain the orientation of the products outside the mold. These methods also allow shorter strokes thus reducing the time for the products to clear the molding area (swing chutes, guide rails, and other systems). Typically, there is no MO time required. While these integrated systems could add considerably to the mold cost, this is quickly recovered by saving substantial cycle time and by the increase in productivity. It also eliminates the need for subsequent orientation of the molded pieces for stacking, assembling, decorating, etc., often requiring expensive orienting devices further down the line. Robots Robots can often be used to unload large products, or for very large multi- cavity molds for smaller product, especially with relatively long molding cycles. The principle is similar to automatic product removal, except that for safety reasons the mold will have to stay open longer. As a rule, the robot’s “end-of-arm tooling” will not enter the molding area until the mold has stopped in the MO position and the ejection will not start before the end-of- arm tooling is in front of the cores (or cavities) from where the products will be removed. Similarly, the mold will not close before the products and all robot mechanisms are safely out of the molding area. Typically, MO time is 1–2 s, sometimes even longer. Figure 3.54 shows a typical robot (from the rear of the machine), removing very large industrial pails from a single cavity mold. The end-of-arm tooling (A) grabs the pail with 4 suction cups (B) as it is ejected from the mold. The pail is then raised and moved out of the molding area, where the pail is rotated into a vertical attitude and stacked, opening down, onto the previously ejected pieces (C). Large machines are not only used for large single cavity molds; for example, Fig. 3.52 shows a two-cavity mold for large products with a high production rate; it shows a setup, with a large machine molding two large industrial pails. The pails are ejected randomly, and drop into “pail catchers” below the molding area, from where they are moved by conveyor to stacking and packing stations. Figure 3.55 shows a machine with a 2 × 16 cavity stack mold for small containers. After the mold is open, two parallel arms (A) enter the mold between the two levels (B) to face the cores from where the products are Cavities in the mold can be laid out so that the products are in the proper attitude (and relative relation to each other) for subsequent operations. With suitable take-offs, they can be ejected and transferred to ancillary equipment for stacking, assembling, etc. and save much on unnecessary handling such as unscrambling and rearranging Figure 3.53 High-speed side entry robot for DVD case removal (0.7 s part removal time) 1281han03.pmd 28.11.2005, 11:0792 93 A B C D Figure 3.55 Machine with a 2 × 16 cavity stack mold for small containers A B C Figure 3.54 Typical robot (from the rear of the machine), removing very large industrial pails from a single cavity mold (mass 2.0 kg (PP), 40 s cycle, productivity: 90 pieces/h) ejected into two arrays of 16 receptacles (C) on robot arms, held there by vacuum. The arms retract, and the mold can start the next cycle. Once outside the mold, the plates with the receptacles (C) rotate 90° to deposit the products onto a conveyor (D) that guides them, still oriented as they came out of the cavities, to the next station in the process. 3.7 Forecasting the Cycle Time 1281han03.pmd 28.11.2005, 11:0793 94 3 Cost Factors Affecting Productivity Ejection Stroke, Air Assist The ejection method used in the mold will also affect the time needed for ejection. The mold must be properly designed with sufficient stroke to ensure that the pieces will not hang up on the core (ejector side) or on the ejectors themselves. On the other hand, poor adjustment during setup of the mold will have the same effect as a poorly designed mold. The ejection stroke does not necessarily have to be long, if other methods are used, such as mold- integrated take-offs, robots, or if compressed air is used to eject from the mold. Combined with a short ejection stroke, which will push the molded pieces off the cores just far enough to break a vacuum or its hold on the cores (e.g., products with undercuts, or with very little draft), air assist will blow the product out of the mold. Any “air only” method can be quite tricky and needs experience in designing and setting up to prevent hanging up caused by a vacuum created by the air blowing out of the mold, between the product walls, and the core. With some materials and some shapes, pure air ejection (without any mechanical ejectors) allows the construction of a better, and at the same time, lower cost mold. This is often used for PP and PE containers; these molds can be smaller (no need for an ejector system), they run quieter, faster, and they last longer because there is no wear of moving parts, such as stripper rings, etc. However, the mold maker also needs experience in this design and not all product shapes are suitable for this system. 3.7.8 Ambient Temperatures and Humidity Both ambient temperatures and humidity have an influence on the molding cycle. As already shown in Section 3.1.1, we must mold in an atmosphere above the dew point. In a lower humidity environment, colder coolant could be used and thereby cycle time saved. If the ambient temperature is high, more energy will be used to keep the mold at the preferred, low temperature. Air conditioning or at least humidity control will improve the molding conditions and the productivity. 3.7.9 Comparing Molding Cycles of the Same Product in New Molds Having the cycle times recorded from a mold for the same product, which ran in a machine and environment other than planned for the new mold provides accurate production figures (shots per hour). However, the influence of all the factors which may be different for the new mold must be considered. The new mold may have more (or fewer) cavities; the molding machine may have a different dry cycle or faster injection; the ambient shop air may be different, and so on. The new conditions could be better or worse and must be considered. Any improvements in ejection, runner system, and cooling, etc. can make dramatic improvements in productivity. Figure 3.56 Air ejected medicine cups (2×32) into a high-speed robot (Courtesy: Stackteck) 1281han03.pmd 28.11.2005, 11:0794 95 Conclusion Many factors affect the cycle time. We have learned what affects it in a positive and in a negative way, but we still are left with these questions:  What is the expected cycle time?  How can we proceed in selecting the proper size mold?  What is the proper number of cavities? To state it bluntly, there is no definite, correct answer; we must rely heavily on the memory and opinion of experienced molders and on records of the operation of earlier, similar molds. All we can say is that compared with a known mold for a similar product, and by knowing how such mold was constructed and operated and on which machine it has been producing, we will be able to claim that we can do better (or differently) by using different features and equipment in the planned mold that will improve the cycle time (and productivity) of the mold at a reasonable cost. 3.8 Number of Cavities Required To determine the number of cavities required the following data or values based on assumptions are needed:  The annual production quantities required of the mold, or the output of the mold if required only for a shorter period  A conservatively estimated cycle time. Factors affecting cycle time have been discussed in Section 3.3. 3.8.1 Available Operating Time There are 8,760 hours in a full year (365 days × 24 hours). Obviously, there will be non-productive (“lost”) hours of inactivity, required for maintenance, or caused by breakdowns. There are two commonly used approaches to determine the actually used hours of a machine, but molders also often approach to this issue their own way. A more conservative approach (often by a custom molder) may select the following method: 50 weeks of 5 days at 24 hours equals 6,000 hours. By allowing 10% for lost time, there will be 5,400h/year available per machine. This gives a “buffer” of 8,760 – 5,400 = 3,360 hours, in case that working more hours should be required. This buffer can come in handy when deciding on the number of 3.8 Number of Cavities Required A good estimate for available hours per year is 5,400 1281han03.pmd 28.11.2005, 11:0795 96 3 Cost Factors Affecting Productivity cavities, e.g., when, by working a few extra days or even weeks, a smaller mold (fewer cavities) could be satisfactory for the expected production requirements. A more aggressive approach (often used by “dedicated molders”) plans on using more hours: 50 weeks of 7 days of 24 hours = 8,400 h/year, and by allowing about10% for shutdowns, will assume 7,560 h/year available per machine The examples of production shown later in this book are based on the more conservative number, 5400 h/year 3.8.2 The Minimum Number of Cavities The calculation for the number of cavities is simple:  Establish the productivity of the planned mold per hour (shots per hour), based on the estimated cycle time (t est ) in seconds (s). 3,600 s (1 hour) divided by t est (s) equals the number of shots per hour (shots/h).  Multiply the number of shots per hour (sh/h) with the available number of hours per year (5,400) to get shots/year. This is the number of molded pieces produced by one cavity  Divide the projected annual quantity Q by the number of shots/year. This gives the approximate minimum number of cavities required for the job. Example 3.11 For example, if 2 million pieces per year are needed and the cycle time is estimated to be 10 s: 3,600 s/h ÷ 10 s/sh = 360 sh/h. This will result in 5,400 h · 360 sh/h = 1,944,000 sh/year. This is just shy of the required 2 million and one cavity could be satisfactory, provided other considerations justify a single cavity mold. However, if for example, this requirement of 2 million pieces must be delivered in 4 months, we have only 5,400 h ÷ 12 · 4 = 1,800 h available. We now calculate 1,800 h · 360 sh/h = 648,000 shots during this shorter time span, and by dividing 2,000,000 ÷ 648,000 we get 3.09, that means we need at least 3 cavities. Since we have a buffer (by working more than 5,400 hours), and the difference is small (0.09), the choice of three cavities could be acceptable, provided the product could be laid out practically in a mold. In some cases, a 3-cavity mold is practical; otherwise, a 4-cavity mold would be the preferred choice. This provides also some provision for future growth. Many dedicated molders use 7,560 hours/year 1281han03.pmd 28.11.2005, 11:0796 97 These calculations give us the minimum number of cavities required for a job, but they do not tell us whether it is the most economical number of cavities. For this, we must bring in another important consideration: the machine hour cost. 3.8.3 Machine Hour Cost per Unit Molded Before proceeding, we must define and recognize the term “machine hour cost” and understand how the cost of the molding machine will affect the cost of the product we intend to mold on the machine considered for the production. Machine hour cost is the total of the actual cost of the machine, its accessories, the cost of installation of the machine in its location in the plant, and the connection to the services, plus the cost of money (interest of loans for the purchase of the machine etc.) spread over the number of hours (or years) the machine is expected to produce. There is really no definite life expectancy (in years) of a machine, as long as it is well maintained and as long as the molding technology does not change. For practical reasons, and as permitted by tax legislation, the cost of the machine is usually written of in 5, 7, or 10 years, even though most machines can (and do) work satisfactorily for much longer time periods before being replaced. We should also consider the case where a machine could be acquired for a one-time special molding application and would not be suitable for other molding operations. This machine could possibly be required for a much shorter time and should in fact be considered as an extension of the mold. That means the machine should be depreciated entirely over this single project and this special product should then be priced accordingly. For a standard type injection-molding machine, we must first know the actual total price paid and then assume the number of years for writing it off. For example, if the total cost (as defined above) is $200,000 and we plan to write off the machine in 7 years, we calculate: $200,000 ÷ (7 years · 5,400 h) = $5.29/h of depreciation. This is the actual machine hour cost per hour for this machine. Obviously, by writing it off in fewer years, the cost will be higher. By writing it off over a longer period of time or by using it more hours/year, the machine-hour cost will be lower. The amount is relatively small and for simplicity of cost accounting, groups of machines of various sizes and costs or even the total of all machines in a plant are lumped together into one machine hour cost. To this cost, the costs of overhead (cooling plant, compressed air supply, power, plant space, etc.) are added to define a plant-wide machine hour cost to be used for costing of any product made there. The machine hour cost 3.8 Number of Cavities Required 1281han03.pmd 28.11.2005, 11:0797 98 3 Cost Factors Affecting Productivity used for cost calculations may be $25.00, $40.00, $60.00, or more, even though the actual machine hour cost may only be in the order of $5.00. Note that the machine hour cost should really not include the cost of power, compressed air, and cooling water for machine and mold, because these costs are directly proportional to the number of pieces molded (the amount of plastic “converted”). However, for simplicity of accounting it is usually included in the overhead. This (rather artificial) machine hour cost must now be apportioned to the cost of the product. If we have a single cavity mold, running at a 10 s cycle, we produce 3,600 s ÷ 10 s = 360 pieces per hour. Assuming a machine hour cost of $40.00, the machine hour cost per unit molded is therefore $40.00/h ÷ 360 pieces/ h = $0.111 per unit. By selecting a 2-cavity mold instead, the production doubles to 720 pieces per hour, while the machine hour cost per unit is reduced by half, to $0.0555. If another machine could operate the same mold at an 8 s cycle, the figures would be different: 3,600 s/h ÷ 8 s/sh = 450 sh/h, and with the same machine hour cost of $40.00, the machine hour cost per unit for a 1-cavity mold would be $40.00 ÷ 450 = $0.0889 per unit. Similarly, a 2-cavity mold cycling at the faster speed would result in a machine hour cost per unit of $0.0444. Therefore, the more cavities are in the mold, the lower is the machine hour cost per unit molded; but also, the faster the machine cycles, the lower is the machine hour cost. This change in machine hour cost per unit can be charted as follows, based on an assumed machine hour cost of $40.00 The incremental gain, i.e., the reduction in machine hour cost per cavity, becomes smaller as the number of cavities increases and as the cycle time decreases. For example, with a 10 s cycle, increasing the number of cavities from 1 to 2, we reduced the machine hour cost from $0.11 to $0.055. By selecting 4, 8, or 16 cavities, this cost will reduce to $0.0275, $0.0138, and $0.0069, respectively. With larger number of cavities, these incremental amounts are very small and would in most cases not influence the choice of the number of cavities. With large quantities, (100 millions or more), the deciding factor will usually be the highest productivity required of the mold. Typical custom molding rates can be found in trade magazines. They can range from $30–$100/hour, depending on machine size and location Table 3.1 Cost of Machine Hours in $ per Unit Molded Cycle time shots/ hour 1 cavity 2 cavities 4 cavities 8 cavities 16 cavities 12 s 300 $0.1333 $0.0667 $0.0333 $0.0167 $0.0083 10 s 360 $0.1111 $0.0556 $0.0278 $0.0139 $0.0069 8 s 450 $0.0889 $0.0444 $0.0222 $0.0111 $0.0056 With large quantities, the deciding factor will usually be the largest cavitations approach 1281han03.pmd 28.11.2005, 11:0798 99 3.8.4 Mold Cost per Unit Molded The productivity of a mold increases in proportion to the number of cavities. But a mold with more cavities will be more expensive and will require a larger machine with more clamp force and more injection and plasticizing capacity. The cost of two identical stacks will be about twice as much as the cost of one stack, but this relation can change. The more identical stacks are made at the same time, the more attention can be given to better “mass- production” mold manufacturing methods by providing special tooling, jigs, and fixtures. Also, programming for CNC machines is the same whether one or any number of cavities are produced, so that the first cavity will cost more than additional cavities. Obviously, the mold shoe will be larger as the number of cavities increases, but not necessarily proportional to the number of cavities. Only a tentative layout will make it possible to indicate how much larger the mold shoe will have to be. For the purpose of this discussion, we will assume that a 2-cavity mold will be 1.8 times as expensive as a 1-cavity mold for the same product and we will use the same increase for larger molds. The factor of 1.8 has been arbitrarily chosen from experience and would apply to a fairly complicated stack; this factor cannot and must not be used as a universally applicable factor. It is used here only to indicate that doubling the number of cavities does not mean that the mold will be twice as expensive. With simpler stacks, this factor could be smaller, such as 1.6 or 1.5. When the cavities and cores are very simple and can be machined right out of the cavity and core plates and when cold runners are planned, there could be only little difference between the cost of making 1, 2, 4, or even more cavities. Figure 3.57 shows a simple cold-runner, multi-cavity layout for simple, small cavities and cores that can be cut right into the cavity and core plate. In Table 3.2, if a single cavity mold costs $20,000, we assume that a 2-cavity mold will cost $36,000, and a 4-cavity mold could cost 1.8 · $36,000, or $64,800, and so on. Figure 3.57 Simple cold-runner, multi-cavity layout A: balanced; B: not balanced Table 3.2 Relationship Between Mold Cost per Unit and Total Production Requirements Estimated total production # of cavities Mold cost 10,000 100,000 1,000,000 10,000,000 1 $20,000 $2.0000 $0.2000 $0.0200 $0.0020 2 $36,000 $3.6000 $0.3600 $0.0360 $0.0036 4 $64,800 $6.4800 $0.6480 $0.0648 $0.0065 8 $116,640 $11.1600 $1.1160 $0.1116 $0.0112 3.8 Number of Cavities Required 1281han03.pmd 28.11.2005, 11:0799 100 3 Cost Factors Affecting Productivity It can be easily seen that the larger the number of pieces to be produced, the smaller will be the mold cost per unit. A simple table based on the above assumptions and with assumed total quantities of 10,000, 100,000, 1,000,000, and 10,000,000 will show this relationship. Table 3.2 shows that the mold cost per unit increases dramatically when the requirements are relatively small and that in such cases a multi-cavity mold does not make much sense, unless the total requirement must be delivered in a very short time. On the other hand, for large quantities, the actual mold cost per unit is very small and a mold with more cavities should be selected. These simple calculations, which take only minutes to perform, must always be made before deciding on the number of cavities planned for a new mold. In general, it can be stated that for very large quantities, the best quality mold (most solid construction, best cooling, best injection system, best ejection method, etc.) and the largest number of cavities possible is the best choice for a new mold and will yield the lowest product cost, even though the initial investment is high. As we mentioned earlier, in most products, the cost of plastic usually constitutes the largest portion of the piece cost. If, for example, the molds in the above example produce pieces with a mass of 100 g and the cost of the plastic is $1.00 per kg, the cost of material would be $0.10 per unit. In most cases, it would not make much difference whether the mold cost per unit of $0.002 or $0.0064 was added to the material cost, but we gain much in productivity of the mold with higher cavitation and at the same time dramatically reduce the total cost by using fewer machine hours. More about the total estimated product cost will follow. 3.8.5 Calculation of the Required Clamp Size We must calculate the projected area of the product and then make an assumption of the average pressure the cavity space will see during injection. This is difficult and can be calculated by a computer (mold flow) analysis of the product. But from past experience we know that thick-walled products need lower pressure to fill the mold, and thin-walled products need higher pressures. Also, if good surface definition is required, maximum pressures will be needed. For the purpose of quick but conservative estimating, the following values can be used:  Thick-walled products: Use 28,000 kPa pressure (3,000 to 4,000 psi)  Thin-walled products: Use 35,000 to 55,000 kPa (5,000 to 8,000 psi) Note that we speak here of the average injection pressure within the cavity, not the indicated injection pressure at the machine nozzle, which could be in the order of 140,000 kPa (20,000 psi) or more. If the machine cannot provide these required high pressures, the product cannot be molded successfully on The mold cost per unit is calculated by dividing the mold cost (price) by the total number of pieces that are expected to be made from this mold Pail Molder Resin 51% Molds 7% Machines 6% Automation 1% Labor Direct 3% Labor Indirect, Admin & Insurance 22% Maintenance 1% Building and Sub-Systems 6% Electricity & Water 3% Figure 3.58 The cost of plastic usually constitutes the largest portion of the piece cost 1281han03.pmd 28.11.2005, 11:07100 [...]... function of mold cost, number of cavities, cycle time, and plastic weight, plus any post-molding handling and operations 108 3 Cost Factors Affecting Productivity Table 3.7 Cost of a Product Made with Different Numbers of Cavities with 10 s Cycle # of cavities Cost per unit 2 Mold cost Machine cost Plastic $0.3600 78.9 $0.0560 12.3 $0.0400 8.8 Total $0.4560 $0.1320 $0.0996 Mold cost Machine cost Plastic... $0.0279 106 3 Cost Factors Affecting Productivity Table 3.4 Unit Costs of Product with 10 s Cycle # of cavities Cost per unit 1 Estimated total production $2.0000 $0.1111 $0.2000 $0.1111 $0.0200 $0.1111 $0.0020 $0.1111 $2.1111 $0.3111 $0.1311 $0.1131 Mold cost Machine cost $3.6000 $0.0556 $0.3600 $0.0556 $0.0360 $0.0556 $0.0036 $0.0556 $3.6556 $0.4156 $0.0916 $0.0592 Mold cost Machine cost $6.4800 $0.0278... $2.0889 $0.2889 $0.1089 $0.0909 Mold cost Machine cost $3.6000 $0.0444 $0.3600 $0.0444 $0.0360 $0.0444 $0.0036 $0.0444 $3.6444 $0.4044 $0.0804 $0.0480 Mold cost Machine cost $6.4800 $0.0222 $0.6480 $0.0222 $0.0648 $0.0222 $0.0065 $0.0222 Total 8 Mold cost Machine cost Total 4 100,000 Total 2 10,000 1,000,000 10,000,000 $6.5022 $0.6702 $0.0870 $0.0287 Mold cost Machine cost $11.1600 $0.0111 $1.1160 $0.0111... for mold costs per unit and machine hour cost per unit as in Tables 3.1 and 3.2 were used Unit costs of product attributable to mold cost and machine hour cost (referring back to mold costs shown in Table 3.1 and based on a machine hour cost of $40.00) are shown in Tables 3.3 to 3.5 Instead of the unit values given in Tables 3.3 to 3.5, we could also consider the sum-total of the estimated mold cost plus... $31,110 $131,100 $1,131,000 Mold cost Machine cost $36,000 $556 $36,000 $5,560 $36,000 $55,600 $36,000 $556,000 $36,556 $41,560 $91,600 $592,000 Mold cost Machine cost $64,800 $278 $64,800 $2,780 $64,800 $27,800 $64,800 $278,000 $65,078 $67,580 $92,600 $342,800 Mold cost Machine cost $116,640 $139 $116,640 $1,390 $116,640 $13,900 $116,640 $139,000 Total 8 Mold cost Machine cost Total 4 1,000,000 10,000,000... $2.1333 $0.3333 $0.1533 $0.1353 Mold cost Machine cost $3.6000 $0.0667 $0.3600 $0.0667 $0.0360 $0.0667 $0.0036 $0.0667 $3.6667 $0.4267 $0.1027 $0.0703 Mold cost Machine cost $6.4800 $0.0333 $0.6480 $0.0333 $0.0648 $0.0333 $0.0065 $0.0333 Total 8 Mold cost Machine cost Total 4 100,000 Total 2 10,000 1,000,000 10,000,000 $6.5133 $0.6813 $0.0981 $0.0398 Mold cost Machine cost $11.1600 $0.0167 $1.1160 $0.0167... resulting in a slow cycle time In addition, the ejection mechanism fails frequently and causes downtime and scrap Cost of molds Cycle time Up-time Scrap Machine time cost Material cost Product weight Productivity (pieces per hour) Machine hour cost per unit (A) Plastic cost per unit (B) Scrap cost per unit (C) Mold 1 $10,000 10 s 80% 5% $40 $1/kg 100 g 1,152 $0.0347 $0.1000 $0.0050 Mold 2 $20,000 6s 90%... $0.0648 $0.0278 $0.0065 $0.0278 Total $6.5078 $0.6758 $0.0926 $0.0343 Mold cost Machine cost $11.1600 $0.0139 $1.1160 $0.0139 $0.1116 $0.0139 $0.0112 $0.0139 Total 8 Mold cost Machine cost Total 4 100,000 Total 2 10,000 1,000,000 10,000,000 $11.1739 $1.1299 $0.1255 $0.0251 Table 3.5 Unit Costs of Product with 8 s Cycle # of cavities Cost per unit 1 Estimated total production $2.0000 $0.0889 $0.2000 $0.0889... $255,640 To calculate the cost of the product, we must now add the cost of plastic, the cost of handling directly chargeable to the product, and the cost of any postmolding operations Example 3.12 Assuming a mold is required for a product with a mass of 40 g, made from PP, at $1.00 per kg The estimated cycle time is 10 s The cost of plastic 40 g · $1.00 ÷ 1,000 g/kg = $0.04 per unit The cost of the product... area of the runners must be used to calculate the clamping force Whenever possible, attempt to inject with lower melt temperature and higher injection pressure for highest productivity and lowest cost 102 3 Cost Factors Affecting Productivity 3.8.6 Shot Size Shot size was discussed earlier in more detail, here only the essentials will be repeated: Once the actual or estimated mass of the planned product . 11:07107 108 3 Cost Factors Affecting Productivity To arrive at the actual cost of the product, we add the mold cost/ unit and machine hour cost/ unit from. 11:07105 106 3 Cost Factors Affecting Productivity Table 3.5 Unit Costs of Product with 8 s Cycle Estimated total production # of cavities Cost per unit