Procedure for EstimatingMold Costs

Therefore the goal of a procedure to estimate mold costs must - raise the certainty and accuracy of a cost calculation, - reduce the time consumption for the calculation, - make it possi

3 P r o c e d u r e f o r E s t i m a t i n g

M o l d C o s t s

3 1 G e n e r a l O u t l i n e

Injection molds are made with the highest precision because they have to meet a variety

of requirements and are generally unique or made as very few pieces

They are to some extent produced by very time- and cost-consuming procedures and are therefore a decisive factor in calculating the costs of a molded product The mold costs for small series often affect the introduction of a new product as a deciding criterion [3.1] In spite of this, in many shops, calculation not have the place to which it should be entitled

The respective mold costs are often not computed at all but estimated based on experience or in comparison with molds made in the past This is also a consequence of the fact that the number of orders is only 5% of the number of quotations The necessarily resulting uncertainties in such a situation are compensated by an extra charge for safety, which is determined by subjective criteria [3.2] This leads to differences in quotations which render the customer uncertain

Therefore the goal of a procedure to estimate mold costs must

- raise the certainty and accuracy of a cost calculation,

- reduce the time consumption for the calculation,

- make it possible to calculate costs of molds for which there is not yet any experience,

- ensure a reliable cost calculation even without many years of experience [3.3] Extreme caution is in order if molds are quoted considerably less expensive than the result of such a calculation would call for Crucial working steps may have been omitted, which would result in irreparable shortcomings in use

3 2 P r o c e d u r e s f o r E s t i m a t i n g M o l d C o s t s

Mold cost can be computed in two different ways, either on the basis of the data of production planning or based on a forecast procedure

The first procedure assigns costs to each working step and to the used material The high accuracy of this procedure is opposed by many disadvantages and difficulties The method is time consuming and requires from the accountant detailed knowledge of working hours and costs in mold making Besides this, it can be applied only after the mold design has been finalized

A basis for estimating the costs of injection molds was worked out by the Fachverband Technische Teile im Gesamtverband Kunststoffverarbeitende Industrie (GKV) (Professional Association Technical Parts in General Association of the Plastics Processing Industry) [3.4] This should facilitate estimating the costs of molds It is based on practical experience, e.g working time for runners (Figure 3.1) If such costs

Figure 3.1 Time for machining runners [3.4] The stated times relate to milling in one platen

only (cross section B), excluding set-up time The set-up time is 30 min for platen up to 100 mm diagonal or in diameter, 35 min for up to 250 mm and 40 min for up to 500 mm

are combined with those of standard mold bases and other standards taken from the cata-logues of producers of standards and the costs of outside work and design, then the costs

of an injection mold is the result A form as it is also proposed by GKV is best used for the compilation (Figure 3.2)

In mold-making shops the quotations are generally determined with the help of prognosis procedures From the literature two general basic methods for predicting costs are known (Figure 3.3) [3.5]: cost function and costs similarity The first method, the cost function starts with the assumption that there is a dependency between the costs of

a mold and its characteristics This dependency is expressed in a mathematical function The characteristics are the independent variables or affecting quantities, which determine the costs

The second method is the costs similarity Starting with an injection mold to be calculated and its characteristics, another existing mold with similar characteristics is looked for in the shop The costs for this mold are generally known and can now be used for the new object In doing so one can fall back on existing data such as the system of classification, which is described in [3.6]

Both methods have their advantages and disadvantages The cost function provides accurate results only if the affecting quantities have nearly the same effect on costs This

is rarely the case with the variety of injection molds today [3.3]

With the similarity method one can only fall back on molds which are designed in the same way and, thus, have similar cost-effective quantities To make use of the specific benefits of both methods a combination of them presents itself (Figure 3.4) This can be achieved by grouping similar injection molds or structural components of the same kind together and determining a cost function within each group [3.5]

There is a proposal [3.7], therefore, to divide the total calculation into four cost groups related to their corresponding functions (Figure 3.5)

Minutes

Cross section B

Figure 3.2 Blank form for computing mold costs as suggested by IKV (Institute for Plastics

Processing) [3.4]

s a b c

Combination

Cost function Cost similarity

Category 1 Category 2 Category n

Effective quantity Effective quantity Effective quantity

Figure 3.4 Combination of cost function and cost similarity [3.3]

Costs are determined for each cost group and added to the total costs The systematic work on the individual groups and the additive structure reduces the risk of a miscalculation and its effect on the total costs [3.7]

In the following the individual groups for estimating costs are presented in detail

3 3 C o s t G r o u p I: C a v i t y

With cost group I the costs for making the cavity are computed

They are essentially dependent on the contour of the part, the required precision and the desired surface finish The costs are determined by the time consumption for making the cavity and the respective hourly wages

Figure 3.3 Method of

cost forecast [3.3]

Method of cost forecast

Cost function Cost similarity

New mold

Characteristics X 1 X 2 ^x n

Existing mold with similar characteristics Costs

New mold

Characteristics X 1 X 2 ^x n

Cost function

Costs

They generally result from

(3.1) Costs for cavity,

Time spent on cavity,

Time spent on EDM,

Average machine and labor costs,

Additional material costs (inserts, electrodes, etc.) can often be neglected in face of total costs

3.3.1 C o m p u t a t i o n of W o r k i n g H o u r s for Cavities

The time tc required to produce a cavity can be calculated on the basis of statistically or analytically determined parameters

(3.2) [3.8] Machining procedure,

Depth of cavity,

Surface area of cavity,

Shape of parting line,

Surface quality,

Number of cores,

Tolerances,

Figure 3.5 Cost classes for estimating

mold costs [3.3]

Cost class I Cavity

Cost class n Basic design Standard moldSplit-cavity mold

Cost class I Basic functiont

elements

Runners and gates Cooling system Ejection system

Cost class IV Special functional elements

Three-plate mold Slide mold Unscrewing device

Mold costs

CDD Degree of difficulty,

CN Number of cavities

CD, CA, Cc are real working hours, the remainders are time factors

The following correlations are used to determine the individual times or time factors

3.3.2 T i m e F a c t o r for M a c h i n i n g P r o c e d u r e

The shares of individual kinds of machining for making the male and the female half of

a cavity are identified as a percentage and multiplied by the machining factor fM

(Table 3.1) This factor has been found in practice and is a quantity for the speed differences of various techniques in machining the contours of cavities

(3.3)

(3.4)

CM Time factor machining procedure,

fMi Machining factor (Table 3.1),

a { Percentage of the respective machining procedure,

nM Number of machining procedures

Table 3.1 Machining factor fMi

Milling EDM Duplicate milling Turning Grinding Manual labor 0.85 1.35 1.0 to 1.35 0.4 0.8 to 1.2 0.8

3.3.3 M a c h i n e T i m e for Cavity D e p t h

If one looks at a molding above and below a suitably selected parting line, one has to distinguish between elevations (E) and depressions (D)

The time consumption resulting from the depth of the cavity is determined by the mean of elevations and depressions above (1) and below (2) the parting line In doing so

it seems practical to establish the elevations as their projected area on the plane through the parting line If the core is not machined from the solid material but made as an insert, the result for one cavity half is

Elevation with depression; core machined

from solid Herein is

Elevation with depression; core made as insert

(3.5)

CD(1) Time consumption for one half of cavity [h]

mR Averageremoval = [1 m m I r1] ,

fEP Ratio between area of elevation 1 and

fDP Ratio between area of depression J projected area

CD(2) is computed analogously

(3.6)

CD Time consumption for depth of cavity [h]

3.3.4 T i m e C o n s u m p t i o n for Cavity S u r f a c e

The surface of the cavity or the molding respectively is the second basic quantity after the depth which affects the machine time directly It is

(3.7)

with score factor share of turning fs

(3.8)

Cs Time consumption for cavity surface [h]

AM Surface area of molding [mm2 • E-02]

&j Turning as share of machining [-]

(elevation)

(depression)

3.3.5 T i m e F a c t o r for P a r t i n g Line

Steps in the parting line are considered by the time factor Cp.

Number of steps Cp for plane faces Cp for curved faces

0 1.00 1.10

1 1.05 1.15

2 1.10 1.20

3 1.15 1.25

3 3 6 T i m e F a c t o r f o r S u r f a c e Q u a l i t y

The quality of the surface is as important for the appearance of the molding as for its troublefree release The surface quality factor cs is affected by the roughness height, which can be achieved with certain machining procedures It can be taken from Table 3.3.

Surface quality Roughness um Quality factor Cs Note

Coarse Ra ^ 100 0.8-1.0 Faces transverse to

demolding direction Standard 10 ^ Ra< 100 1.0-1.2 EDM roughness

Fine 1 ^ Ra< 10 1.2-1.4 Technically smooth High grade Ra< 1 1.4-1.6 Superfinish

3 3 7 M a c h i n i n g T i m e f o r F i x e d C o r e s

The machining and fitting of cores in both cavity halves is covered by the time factor C c This work becomes more difficult with increasing deviation of the fitting area from a circular shape The contour factor is multiplied by the number of cores with equal fitting area.

tB Time base = 1 [h]

(3.9)

Figure 3.6 Time valuation for tolerance

Percentage of medium fine tolerances requirements [3.7]

Close tolerances as well as critical tolerances for bearings (centricity, accuracy of angle, parallelism, flatness, freedom from displacement) considerably increase the time needed for producing the cavity

3.3.9 T i m e Factor for D e g r e e of Difficulty

a n d Multifariousness

A departure from an average degree of difficulty (CDD = 1) is considered the special effort for an extreme length/diameter ratio of cores, their large number in a small area and complex surfaces For large plane parts without openings the time factor is reduced (CDD < 1) Table 3.5 shows relevant criteria with their corresponding factor

r CT

fCF Contour factor (Table 3.4) [-]

n Number of cores with equal fitting area [-]

j Number of different fitting areas [-]

Contour factor fCF Fitting area

1 Circular o

2 Angular D

4 Circular, large O

8 Angular, large D

10 Curved contour g

3 3 8 T i m e F a c t o r f o r T o l e r a n c e s

Close tolerances raise costs To produce moldings economically no closer tolerances should be considered than necessary for the technical function

A standard for making precision molds implies that mold tolerances should not exceed about 10% of those of the finished molding The factor for dimensional tolerances CT

comprises the expected expenditure for required accuracy and posttreatment (Figure 3.6)

3.3.11 C o m p u t a t i o n of W o r k i n g H o u r s f o r E D M E l e c t r o d e s

Because the geometry of the electrode surface corresponds to the contour of the part, the working time can be calculated in the same manner as was done for the cavities (Equation 3.2).

(3.10)

CM Like 3.3.2,

CD Like Section 3.3.3 (elevations and depressions have to b e exchanged),

CA Like Section 3.3.4,

aE Share of E D M for producing cavity,

C-DD

0.7

0.8

1.0

1.2

1.4

1.6

Difficulty

Very simple

Simple

Medium

Difficult

Extremely

difficult

Criteria

Large, plane areas, circular parts Rectangular parts, areas with some open-ings; mount depth/diameter: L/D ^ 1 Circular and angular openings, LZD=I

Shift possible, L/D « 1 - 5 small parts High density of cores L/D « 5, complex surface

Very high density of cores

5 ^ L/D ^ 15, complex spherical faces

3 3 - 1 0 T i m e F a c t o r f o r N u m b e r o f C a v i t i e s

For a larger number of equal cavity inserts or several equal cavities an allowance per cavity has to be considered based on the fabrication in series The correlation between the time factor CN and the number of cavities nc is presented with Figure 3.7.

Figure 3.7 Score factor for number of cavities

[3.7]

r CN

Number of cavities N

CT Like Section 3.3.8,

CDD Like Section 3.3.9,

CN Like Section 3.3.10

3 4 C o s t G r o u p I I : B a s i c M o l d s

The basic mold retains the cavities, the basic functional components (runner, heat-exchange and ejector system) and any necessary special functional elements (three-plate mold, slides, unscrewing unit) As far as self-made basic molds are concerned, it is practical to distinguish different quality grades

A basic mold of grade I is for a small number of moldings with little precision, for test series etc It is not hardened

A basic mold grade II has case-hardened plates, additional alignment, heat insulation

on the stationary half and, if of round design, is equipped with three leader pins It is assumed to produce technical parts and medium-sized quantities

A basic mold grade III is largely hardened and made with large quantities, high precision and reliability in mind [3.7]

However, injection molds are largely built with mold standards Thus the total costs

of the basic mold are primarily the costs for the readily usable standards, expenses for specific machining operations not included (Figure 3.8) It is suggested, though, to consult the catalogs of suppliers for up-to-date prices and the availability of mold bases

of a different design like such with floating plates, etc

Standard basic molds are all of the highest quality and differ only in the steel grade being employed, which affects the service life of the mold, its polishability or its corrosion resistance The costs presented in Figure 3.9 are based on the use of AISI4130 type steel The bases are supplied preheat-treated and precision ground

Figure 3.8 Total basic costs [3.3]

Mold standards

Treatment of standards

Drilling

Heat treatment

Coordinate-table grinding

Assembly

Basic costs