Planning Basics Additive Manufacturing: Cost Factors and Cost Optimization
Additive manufacturing of components can be an economical solution for many companies. However, there are several cost factors that should be considered.
The cost structure describes the ratio of costs to benefits and is therefore decisive in answering the question of whether series production is economical or not. Since the cost structures of conventional and additive manufacturing processes sometimes differ significantly, the cost factors must be carefully weighed up against each other. An initial cost estimate for additively manufactured products is usually measured by means of cubic centimeters manufactured. The post-processing of the additive components is also a cost factor. Looking at the overall cost structure of additive manufacturing, it is largely determined by the costs for the AM machines, followed by the costs for materials and labor costs. Other factors such as consumables (e.g. gas) and energy costs can be neglected in the overall cost structure. Machines for selective laser sintering (SLS) and laser melting (SLM) are very capital-intensive, which increases the total costs per cubic centimeter. The cost of powder bed-based processes is often compared with that of tool-intensive conventional manufacturing processes such as injection molding and die casting or with machining processes such as CNC milling (Figure 1).
Cost Advantages of Additive Manufacturing by Lower Fixed Costs
Despite the higher variable costs, the use of additive processes in production can be economical, as a glance at fixed costs makes clear. For conventional manufacturing technologies, fixed costs are only allocated to a specific component design. For example, a die can only be used in die casting to manufacture the specific product for which it was developed. Accordingly, the costs of the mold must pay off in terms of the number of parts produced. The costs for setting up a machine tool or changing tools on an injection molding machine must be distributed over the number of parts produced in this production batch until the next set up. If only a few units are produced per batch or small series are produced, the fixed costs can exceed the variable costs. This makes additive production more cost-effective than production with conventional processes.
Additive manufacturing also involves a certain degree of fixed costs, but these costs can be more easily offset by the number of different products that are produced in a single batch. The actual costs incurred per cubic centimeter in an additive construction job depend on a complex combination of technical parameters and the operating conditions of the machines.
- Machine Data
- Construction Job Parameters
- Consumables Data
- Operational Calculations
By changing the parameters, machine manufacturers and operators can reduce the overall costs of additive production. Since laser sintering and laser melting machines are very capital-intensive, the productivity of the machines is a critical parameter that influences the overall costs. The productivity of the machines results from the quotient of the actual throughput of a construction job (i.e. the exact material volume of the finished parts) and the throughput time of the construction job (i.e. the time required to execute the construction job).
It is therefore important to know the structure of the lead time of the construction job in powder bed-based processes, as they are key to optimizing overall costs.
The Lead Time Includes:
- Time for setting up the machine, loading the 3D files and removing the finished components
- Time the machine needs for preheating and cooling in additive production
- Time required for the laser to expose and melt all component surfaces
- Time required to apply and, if necessary, warm up the powder layers
- Time for quality control in the current construction job, if necessary
Cost Factor "Production Time" in the Additive Manufacturing Process SLS
Looking at the SLS process, the actual production time, i.e. the period that is determined exclusively by the activity of the AM machine and is independent of the operator, accounts for about 90 % to 95 % of the total cycle time of the construction job with efficient operation. The production time consists of preheating the machine, building the parts and cooling. The periods required for preheating and cooling are largely constant for each construction job and depend on the size of the installation space of the respective AM machine. The actual construction phase takes up about 60 % to 70 % of the production time, depending on the machine and installation space utilization. For an initial estimation of the space requirement of a component in the installation space, the envelope volume, also known as the bounding box, is often used. During normal operation, 50 % of the construction time consists of exposure / melting and 50 % of heating/application of new powder layers. These figures generally also apply to the SLM process, although the percentages differ considerably in some cases, as the melting of the metal powder takes place much more slowly.
A number of technical and operational measures can reduce the total costs per cubic centimeter. Technical measures on part of the machine manufacturers include the reduction of the time required for exposure and application of powder layers as well as the reduction of preheating and cooling times. This is often achieved by the application of a larger number of lasers and the development of concepts for faster application of the coatings. Figure 3 gives an impression of how much the production time can vary depending on the machine.
Cost Optimization: In-House Production or External Procurement of Additively Manufactured Components
Investment in Own Additive Manufacturing Machines
For in-house production, a company invests in AM machines and operates them for their own production. In order for the machines to pay off, high capacity utilization is necessary. These measures should also be taken into account in order to keep the overall costs as low as possible:
- 1. To ensure a tight filling of the installation space with components
- 2. Maximizing the share of actual exposure time in the overall throughput time, for example through high installation space utilization
In laser melting, tight packing can only take place on the horizontal plane of the machine, since in this process, the components cannot be produced in several layers on top of each other. The laser sintering process achieves the best economic efficiency if the components are manufactured tightly packed in several layers on top of each other. Figure 4 illustrates the effects of a high packing density on the unit cost of a small lever. It is apparent how the total costs largely stabilize after the first level has been filled. For efficient operation of laser-sintering machines, at least one dense level of components should be achieved. The discontinuities in the graph result from the fact that each time a new layer is started, the total time for applying the layers in the construction job increases. However, due to the shape of the components, the height of the components in the mounting direction (z-direction) usually does not use the entire available horizontal space (x-y-direction) of the machine. Accordingly, the increase in time and cost of construction is not fully spread over a new densely packed horizontal layer. It is therefore recommended that the height of the superstructure is planned as evenly as possible for the entire construction job. Some areas that reach higher z-coordinates unnecessarily increase the costs of the construction job.
Furthermore, it should be noted that in additive manufacturing, the factors time and component quality must be weighed against each other, since higher quality always goes hand in hand with an increased time required for production. In this case, the selected parameters are decisive, e.g. layer thickness. Thinner layers enable greater accuracy of detail and better surface quality but require more production time and thus increase overall costs. In laser sintering in particular, the factor of powder recycling also plays an important role in construction costs. The higher the proportion of recycled used powder, the lower the costs for the construction job. However, too much used powder can influence the flowability and deteriorate the construction quality or even lead to defects in the finished components.
Purchase of Additively Manufactured Components from External Service Providers
An alternative to in-house production is external procurement (buy). External procurement is the simplest method for a company to gain access to additive manufacturing technologies. It does not require any specific knowledge about the operation of the machines or any major investments in advance. The decision for external procurement also means lower risks and price fluctuations in production for the company, as the efficient use of AM machines is the supplier's responsibility.
The prices for external procurement depend on the total material volume of the order (in cm3) as well as on the effect of the component design on the installation space utilization. For example, bulky lightweight structures can take up a lot of space on the construction panel, although they only have a small volume of material. Orders with larger material volumes and higher packing density are cheaper because they are easier to produce on the supplier’s machine. Figure 5 gives an overview of average prices for external procurement for SLS and SLM procedures.
According to a study conducted in 2014, two different price strategies can be identified for AM service providers. The study compares 21 offers from different AM service providers worldwide. Their results are shown in Figure 6 for prices per cubic centimeter when ordering in quantities of 1 and 100. The dashed line marks prices that are the same for quantities of 1 and 100. The AM service providers can be divided into two categories (A and B):
Category A (purple ellipse):
The AM service providers demand similar prices per cm3 for quantities of 1 and 100, which suggests that the suppliers in category A want to ensure optimum space utilization of their machine by combining orders from different customers. This procedure allows the supplier to offer stable prices per cubic centimeter.
Category B (green ellipse):
AM service providers charge higher prices for small quantities (between 5 and 10 euros per cm3) than for larger quantities (between 0.5 and 1 euro per cm3). Category B service providers obviously prioritize fast delivery and do not combine orders or only subordinate orders. The resulting cost situation is more similar to the one that would occur in the case of in-house production by the company, i.e. inversely proportional to the quantity of the ordered component volume.
The awareness of these two strategies of the service providers allows the planners of the company to optimize their purchasing methods and thus minimize the total costs of additive production.
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