Introduction

n the previous reports (Part 1 and Part 2), we have conducted production design for integrated folding hangers to enable mass production in parallel with product design utilizing the compliant mechanism. Compared to folding hangers consisting of multiple parts, the reduction in the number of parts by integration has clear cost advantages, such as the elimination of assembly processes.
The feature of compact folding when not in use is also cost effective in the forming process. This paper compares the manufacturing costs, especially the processing costs, of the proposed folding hanger with those of ordinary hangers for cleaning. The costing method used in this report is also described in detail, as well as the effect of production planning on costs.

Manufacturing cost

Appropriate calculation methods for manufacturing costs are adopted depending on the accounting standards adopted by individual companies and the type of factory or manufacturing department. Here, manufacturing costs are classified into "direct costs" and "indirect costs. In general, direct manufacturing costs consist of direct material costs, direct expenses, and direct labor costs, while indirect costs are allocated to products and parts on a rational basis. In other words, indirect costs are allocated to total cost.

Injection molding equipment is a general-purpose machine and usually does not manufacture only one part. It manufactures multiple types of parts depending on the number of parts produced. Therefore, it is common for molding equipment to set a time rate (unit cost per unit of time used) and allocate costs based on the time, etc., occupied by each of the multiple parts. In this report, however, to avoid complexity, we consider the equipment as a dedicated facility for manufacturing a single part. This is a realistic setting when the number of parts produced is equivalent to the production capacity of the equipment.

Consider direct labor costs. Assuming an automated molding plant, there are tasks such as changeover, material input, changing parts containers, and troubleshooting. In this case, it would be complicated to allocate direct labor costs on a one-to-one basis for each part or manufacturing process, so we will allocate them in the same simple and reasonable ratio as indirect costs. In other words, we will treat them as indirect costs.

As described above, processing cost, which is the base of total cost, is calculated by direct material cost and direct cost. On this base, overhead costs are allocated and added together to arrive at the total manufacturing cost. Please note that the absolute values of the prices shown below do not represent all of the manufacturing costs, as this is not explicitly stated in this report.

Direct material cost

The plastic material used for hangers is the direct material cost. Since plastic molding material (pellets) can be procured in packing units of about 25 kg, the risk of remaining as excess inventory relative to the planned production quantity is small, and there is no problem in simply calculating the material cost per unit from the parts and other materials used.

Assume the yield ratio is 50% [1]. We assume that all runners, defective products generated at startup, and lumpy products that are purged are neither crushed nor reused in the process and are all discarded (free of charge). The volume of the product is 24.42㎤, and based on the yield rate, the average material volume used per molding shot is about 50㎤.
The price of POM raw material [2] is set at 440 yen/kg.

From the above, the direct material cost per hanger can be estimated to be 30 yen/piece.

Other direct cost

Mold

In the previous report, we discussed the design considerations for manufacturing this product by injection molding, primarily based on the physical behavior of the plastic. In this section, we further examine the specifications of the mold and the molding equipment in order to calculate the processing cost. In order to compare and verify the compacting effect of the products, we will compare the following two products

Product O : Molded in the form of a hanger in use (general hangers for dry cleaners)

Product F : Molded in a folded state (integrated folding hanger)

The products O and F are placed inside the rectangle respectively, as shown in the figure below. In order to remove the product from the mold without deforming it, many protruding pins must be evenly distributed throughout the part. Therefore, this rectangle size is the minimum dimension of the ejector plate [3] in the mold, which determines the overall mold size.

The mold structure shall be 2-plate type, cold runner, and selected from the standard mold bases available on the market. The following mold base was selected to place products O and F from the "Blue Book VOL.1.1 (201.10) " published by Futaba Corporation.

Mold L 400 x 550 x 300 (Futaba S4055)
Mold S 200 x 400 x 170 (Futaba S2040)

The figure below shows the dimensions and weight of each mold base. Mold L weighs approximately 500 kg and S weighs approximately 110 kg. By using a folding mold, the amount of steel used for the mold can be reduced to about 1/5.

Consider next the number of products that can be molded at one time in a single mold, or "number of cavities. Up to this point, we have considered a one-cavity mold, but a large mold (L) can accommodate four products (F). The figure below shows each mold and product arrangement. Note that only half of the mold is shown in this figure.

Having determined the general specifications of the three types of molds, we estimated their prices based on the amount of steel used, machining man-hours, and other factors. The following table summarizes the results. Note that prices may increase or decrease depending on detailed specifications such as the type of steel used, heat treatment, required machining accuracy, surface treatment, runner/gate system, cooling pipe layout, etc.

The above is summarized in the following table.

Injection molding machine

Next, select an injection molding machine. Since the projected area of this part is small and the molding pressure is not large relative to the area occupied by the product, there is no need to select a molding machine based on mold clamping force. Selection is based on the mold installation specifications. Specifically, a molding machine with a tie bar spacing [4] larger than this should be selected based on the external dimensions of the mold. Tie bar spacing varies by molding machine manufacturer and model, but we selected the following two models from those available on the market. The price of a complete set of injection molding machines and molding equipment [5], including peripheral equipment, is planned to be 15 million yen and 20 million yen for Machine-50t and Machine-150t, respectively.

Mold L is used in a Machine-150t and mold S is used in a Machine-50t. The table below summarizes the above.

Depreciation

In the previous section, we estimated the prices of molds and molding equipment, which are classified as direct manufacturing expenses. we tabulated the initial investment costs (purchase price) for the three manufacturing specifications and calculated the depreciation for each. The straight-line method was used for the individual k, and the useful life was set at 2 years for the mold (depreciation rate of 0.500) and 8 years for the molding equipment (depreciation rate of 0.125). The following table summarizes the results. This is the direct cost (in this case, processing cost).

Possible production quantity

The cooling time was estimated to be 11 s from the previous report, so the molding cycle including dry cycle is planned to be 20 s for all three types of molds. This gives a production capacity of 180 shots/hour. Assuming an operating time of 23 hours/day, 28 days/month, the maximum production quantity would be as shown in the table below. For reference, the table below shows a simple calculation of the unit cost of processing, assuming that exactly the same quantity can be sold as the maximum production quantity.

Production (Sales) Plan

We have calculated the processing cost from the direct costs required for manufacturing, but once the equipment and facilities are installed, these costs are fixed. It is a fixed cost that cannot be changed by production volume. Therefore, it is the sales volume that determines the manufacturing cost, and the cost will vary depending on the production performance. Below we assume specific sales volume (production volume), calculate the cost, and compare the difference depending on the equipment specifications.

(1) One million parts per year

Equipment	Manufacturing cost	Production quantity	Processing unit price
S-Fx1	3.075 million-yen/year	1 million-pcs/year	3.01 yen/piece
L-Ox1	4.000 million-yen/year	1 million-pcs/year	4.00 yen/piece
L-Fx4	4.800 million-yen/year	1 million-pcs/year	4.80 yen/piece

In this quantity, folding hangers can be produced at 75% of the processing cost of ordinary hangers. This is an advantage due to the compact size.

As for the 4-cavity mold (L-Fx4) for folding hangers, it is not realistic to make it a dedicated equipment for this quantity. If it were truly a dedicated facility, we would not have L-Fx4-specification equipment, so there would be no point in comparing it to others.

In reality, the equipment would be shared with other products instead of being dedicated equipment. Roughly speaking, in the case of L-Fx4, the operating hours will be less than 1/5 of the capacity, so the processing cost will be about ³/piece. In this case, the number of manufacturing changeovers will vary depending on the delivery lead time and production lot size, so the utilization ratio must also be considered. These factors have a significant impact on manufacturing costs.

(2) Five million parts per year

In this case, a set of equipment using a single-cavity mold will not provide the required production volume, so multiple sets of the same mold and molding equipment must be prepared.

Equipment	Manufacturing cost	Number of equipments	Production quantity	Processing unit price
S-Fx1	3.075 million-yen/year	4 sets	5 million-pcs/year	2.46 yen/piece
L-Ox1	4.000 million-yen/year	4 sets	5 million-pcs/year	3.20 yen/piece
L-Fx4	4.800 million-yen/year	1 set	5 million-pcs/year	0.96 yen/piece

For the single-cavity mold, the processing cost of the folding hanger is 77% of that of the standard hanger.
Using the same size mold and molding machine, the folding hanger (4-cavities) can be produced at 30% of the processing cost of the standard hanger (single-cavity). The higher the production volume, the more the effectiveness of the compact design of the folding hanger becomes apparent.

(3) Relation between production volume and cost

The figure shows the relationship between production quantity and processing cost for each equipment specification. The processing cost of folding hangers (L-Fx4) with single-cavity molds is much lower than others. Product compactness allows downsizing of molds and molding machines, contributing significantly to cost reduction.

Considerations

For this product, direct material cost dominates the manufacturing cost. There is room to reduce the material cost by half to 15 JPY by using hot runners, minimizing the runner volume, and crushing and reusing the material in the process. Design changes to general-purpose plastics (e.g., PP) are even more effective. Material selection (design) and procurement capabilities have a significant impact on cost

Although processing costs account for a small percentage of manufacturing costs, any deviation from the production volume plan will result in a significant change in costs. Molds and molding equipment, which are fixed assets, are expensive and therefore risky as cash. In contrast, material costs have a smaller risk to sales performance because the amount purchased can be adjusted based on the amount used.

Although assembly and processing costs were not mentioned in this report, product integration (combining multiple parts into one) can lower the investment cost of molds and molding machines and also significantly contribute to lower assembly and processing costs. In addition, the volume occupied is directly reflected in lower transportation costs. Furthermore, storage space when not in use will be reduced, which will be beneficial for use in smaller stores, for example.

Conclusion

A folding hanger was used as an example as an application of compliant mechanism. Since it was a relatively small part, the absolute value of processing cost became small, but the cost reduction effect as a comparison was confirmed to be significant. If the product were to be even larger, the cost reduction effect of downsizing would be even greater due to the increased cycle time.

In addition, products such as hangers are products that require the use of recycled materials and the establishment of collection schemes. In other words, the selection of materials has a significant impact not only on cost but also on the product concept. From the circular economy perspective, which will be the mainstream in the future, not only resource recycling but also energy consumption by transportation, i.e., low carbon dioxide generation, will become a new value. Compactness of products, including during production, and product design that applies compliant mechanisms to achieve this will be indispensable technologies for the creation of new products and services in the future.