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 de un molde de inyección y del coste de la pieza resultante

Programs for the calculation of the cost of manufacturing of a mold of injection and the cost of the resulting piece

Carlos Javierre, Javier Castany, Isabel Claveria
Workshop of Injection of the Industry of the plastics (TIIP)
Unit Associated to the CSIC - University of Saragossa
15/03/2002

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.

1. INTRODUCTION

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.

2 NEED FOR THE TOOLS DEVELOPED

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

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.

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 Time of realisation
high Investments
Obtaining of pieces at the end of the process
Comprobación of the success of the project at the end of the process

Can say that in the majority of the cases the development of a piece or a group of pieces of plastic is a process that initially can begin with big uncertainties, so much technical like economic. To try diminish to the maximum the uncertainties of technical type can resort to the experimentation with pieces prototype or to the different programs of simulation of type reológico, resistant and functional, that handled by people qualified can ensure the technical feasibility of the project. To reduce the uncertainties of economic type have developed the two tools that present in the following work. “Tool FALLS for the budget of moulds of injection of thermoplastic” and “Tool FALLS for the economic optimisation of pieces of plastic injected”.

3 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

3.1 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 scheme.

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.

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

Fig. 3 Flow of application of method 1

3.2 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 2

The method 2 is more complex in his application. The user has to have a true level of knowledges on moulds of injection and besides has to have a first approximation to the design of the mould

Fig. 5 Results of method 2

3.3 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.

3.4. 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.

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

Fig. 6 Example of moulds used to obtain the coefficients

3.5 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.

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.

Fig. 7 Data item and results

4 TOOL FALLS FOR ECONOMIC OPTIMIZATION OF INJECTED PLASTIC PIECES.

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 machine used
Cost of the amortización of the mould repercutible to the piece
Cost of the operario/s associated to 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.

After the implementation of the programme are two tables of results which include data such as; cycle time, cost of the piece (broken down by different factors and expressed in percentage and total cost), time of production, machine capable of injecting mold...