This news article was originally written in Spanish. It has been automatically translated for your convenience. Reasonable efforts have been made to provide an accurate translation, however, no automated translation is perfect nor is it intended to replace a human translator. The original article in Spanish can be viewed at Programas para el cálculo del coste de fabricación un molde de inyección y del coste de la pieza restante

Programs for the calculation of the cost of manufacturing a mould of injection and the cost of the remaining piece

Carlos Javierre, Javier Castany, Isabel Claveria. Injection workshop of industry of plastic (TIIP).Unit associated with the CSIC-University of Zaragoza01/02/2002

Companies seek greater every day competitiveness to sell their products at very reasonable prices. One of the factors in the price of the product are the costs of production. Is therefore necessary that the company meet comprehensively the costs involved in its production process, and what is most important, that can anticipate them before even having begun. A work tool that, from a series of data of the piece to be manufactured, calculates and optimizes its costs on the basis of the costs associated with the injection, in a quick and simple process has been developed for this purpose.

One of the most important unknowns that arise in the development of the project of a piece of plastic is the cost of the mold that will inject the piece. To determine this cost in charge of carrying out the project calls for budget of the mold to the moldistas manufacturers in power. Then when on the moldistas lies one of the most difficult tasks and responsibility for: budgeting how much will cost the mold. This work have to be fast and accurately. The moldistas make budgets, primarily on the basis of its experience, based on molds which have already been built. Associate similar parts moulds with similar costs and similar problems, solutions of comparable costs. To cover this aspect in the development of plastic parts has developed other complementary tool to the first presented to the calculation of cost of molds from two different methods; The first through the use of a database of already constructed or purchased and the philosophy of a similar pieces correspond similar molds with similar costs and the second by a method of calculation from mathematical equationsadjusted by coefficients, which estimated separately the costs of different systems of the mould and the total cost as the cost of each of the subsystems that compose.

1 Need of the tools developed

Since that ends the aesthetic and functional design of a piece of plastic injected until that can be obtained in final pieces are a series of stages that schematically we see reflected in the following figure.

Fig. 1. Phases in the development of a piece of plastic injected

Each of these stages are obtained a number of results that are clearing up doubts about the good end of the development of the project. Of this process include the following features:

• Long times of realization

• High investment

• Obtaining pieces at the end of the process

• Check of the success of the project at the end of the process

It can be said that in most cases the development of a piece or a set of plastic parts is a process that can initially begin with great uncertainties, both technical and economic. In an attempt to reduce to the maximum technical uncertainties can turn to experimentation with prototype parts or different programmes of simulation of rheological, durable and functional type, that handled by qualified persons can ensure the technical viability of the project. The two tools presented in this paper have been developed to reduce the uncertainties of economic. "Tool falls for the budget of injection of thermoplastic moulds" and "tool falls for economic optimization of injected plastic parts".

2 Tool falls for the budget of moulds

The calculation of the cost of a mold by means of two methods (Fig 2.) can be done through the program developed. METHOD 1: is a comparative method "for similar pieces" => "will require similar molds" => "with similar costs". From data from molds where we can see their characteristics and costs we can deduct the cost of a new mold. METHOD 2: Calculation from a mathematical model of the cost of the mold. Splits the cost total of cast in different subcostes which can be calculated independently, the total cost will therefore the sum of each of the subcostes.

Fig. 2. Methods for the cálcuo of moulds used by the program costs

2.1 Method 1

The process of calculation using the first method is conceptually very simple. Through the programme can be stack molds appear where reflected their characteristics and costs data. The flow of work through the program could continue the following diagram (fig. 3).

Fig. 3 Flow of application of method 1

Note that the feasibility of the implementation of this first method depends largely on the number of molds built or calculated with our database.

2.2 Method 2

Method 2 is more complex in its implementation. The user must have a certain level of knowledge about injection moulds and must also have a first approach to the design of the mould. Data of departure for the implementation of the programme relating to their dimensions, materials, imprints, feeding, cooling systems, expulsion, movements and other elements that will bring the mold are needed. These data will be required by the programme during its implementation throughout the calculation of each of the subcoste in which the program divides the total cost of the mold. The program performs calculation based on these data and handles large database tatos information on materials for moulds and cost of standardized elements.

The flow of calculation recommended at the time of implementation of the programme is reflected in the following figure:

Fig. 4 Flow of application of method 1

2.3 Results

The results obtained by both methods can be stored according to the format of chips that manages the program in such a way that they move to expand the database of moulds.

For method 2 information obtained over the mold and the results are much broader. These can be broken down into materials and hours of work for each subcoste in different formats.

Fig. 5 Results of method 2

2.4. Adjustment of the method

Method 2 uses equations to calculate the cost of parts in which the mold is divided for its calculation. Adjustment coefficients are used in these equations to ponder every influential factor and to be able to set the method to the production costs of each moldista. These costs were adjusted to a moldista through the analysis of the molds made for two years. Costs were studied both hours and materials of these moulds and General values of these coefficients which are which currently use the program but that they can be adjusted to any moldista were obtained from this study.

Fig. 6 Example of moulds used to obtain the coefficients

2.5 Data item

In order to apply the method 2 is needed a rough design of the mold, its structure, mechanisms and systems that compose. In the chart below we see the starting data used for the quote of a mold of a case of mirror for the automobile and the file of results obtained after the implementation of the programme.

Fig. 7 Data item and results

3 Tool falls for economic optimization of injected plastic parts

The calculation of cost of plastic parts program calculates the cost of a piece of plastic injected as a sum of four costs:

• Cost of the material used

• Cost of the used machine

• Cost of depreciation of the repercutible the piece mould

• Cost of the operator/s associated with the injection of the piece

To calculate these costs, the program uses databases of materials and machines in addition to required information characteristic of the piece and the mold must be entered by the user. Also calculates various parameters such as clamping force, times of cycle and makes an initial selection of the machines that can inject the piece. The scheme of work of the programme is reflected in the chart below.