Keywords: A compression mold, thermal stability, thermal stability coefficient, molding under pressure.

At design of industrial equipment for molding under pressure of color alloys it is necessary to consider such factors as: properties of the filled-in alloy, a design of castings, type and nature of operation of the equipment and equipment. According to authors of this article, property of the filled-in alloy more influence a final design of a compression mold as they determine the final sizes of details of equipment, and also material of which they will be made, the technological modes of process of molding, etc.
As, now to 50% of preparations of the details received by method of the molding under pressure (MUP) are made of zinc alloys [1], it is necessary to focus the attention on studying of these alloys.
As the reasons good foundry properties of zinc alloys, low temperatures of filling in a casting mold of 380-480 °C, depending on structure of an alloy, and also possibility of drawing various coverings on a surface of the received castings for this purpose serve.
The specified advantages of zinc alloys in the conditions of market economy create serious prerequisites for expansion of application of this technology in the conditions of a mass and large-lot production. However it is interfered by lack of reliable techniques of forecasting of an operational resource of compression molds.

From the theoretical point of view work [2] in which the experimental and settlement technique of an assessment of operational firmness of compression molds is described is important.
In this work the term of operation of a compression mold is broken into 2 periods: the first – before emergence of the first cracks, the second – growth of cracks to the critical sizes or until destruction. Duration of the first period is eventually based on definition of power criterion

where - - the temperature tension operating on compression mold metal in the field of plastic deformations
 - material fluidity limit.
Depending on numerical value of this criterion is defined, what nature of damage. In a case  there is a thermofatigue damage, at quasistatic damage, if addition  of thermofatigue and quasistatic damages is possible. Power criteria gives the chance rather precisely to estimate dependence of behavior of material of a compression mold on tension perceived by it.
For thermofatigue damage:
 
(1) 

For quasistatic damage:
 (2)

For the mixed damage:
 (3)

In these formulas - the size of relative irreversible plastic deformation for a cycle, - the size of the relative saved-up deformation for a cycle, - limit plasticity of material at a temperature for a cycle, m – a curve inclination tangent temperature deformation for the considered material.

Calculation of the period of development of cracks to the critical sizes is carried out on a formula:

where - the current dimensionless measure of damages, 
 - the coefficient considering crack growth rate on i-ohm a loading step
m – curve inclination coefficient “temperature deformation”,
 - the provided coefficient of asymmetry of a cycle on i-ohm a loading step,
i – loading step,
- crack depth for each step of loading.
Thus, operational firmness will be defined by expression

where - quantity of cycles before emergence of the first cracks,
- quantity of cycles before emergence of the critical sizes of cracks.
This calculation procedure with small to deviations was confirmed by practical values. However, its application when using zinc alloys is impossible for the following reasons: calculation of the first period before emergence of the first cracks is made at low-cyclic loading, the second period of growth of cracks to the critical sizes is defined on the basis of the experimental data obtained when using aluminum alloys.
ccording to authors of this work, the provided information allows to draw an unambiguous conclusion on relevance of the problem which is taken out in the name, and also an urgent need of carrying out the theoretical and experimental researches directed on its permission.
In work [1] as the factor influencing thermal stability thermal deformation, i.e. smyaty form-building surfaces of compression molds is considered. At operation these deformations a smyatiya are summarized that leads to emergence of gaps between butt surfaces of compression molds and as a result, to education облоя on the cast products. 
Within a known technique of I.I. Goryunov as criterion of an assessment of thermal stability of materials the compression mold material embrittlement coefficient for one cycle [1] is offered. The physical sense of this coefficient is that when cycling margin of safety and plasticity of metal of a compression mold goes down depending on properties of material and thermal tension. For a case when the maximum temperature tension less strength the formula of calculation of thermal stability has the following appearance:
 (4)
where – thermal stability coefficient which depends on purity of a working surface of a compression mold, sensitivity of material to concentrators of tension, sharpness of cuts (concentrators), weight and amount of castings, etc.
 - temperature tension;
 – general thermal deformation;
- strength of material of a compression mold.
However the data provided in the same source for steel 3Х2В8Ф show that the removed formula doesn’t give the chance rather precisely to predict firmness of a compression mold for LPD of zinc alloys.
According to researches of authors of this work, concentrators of tension and purity of working surfaces of matrixes as these factors define nature of interaction of a stream of the melted metal with compression mold material have the greatest impact on coefficient of thermal stability. 
This statement is basic for a new, advanced technique of forecasting of an operational resource of compression molds of LPD of zinc alloys which key moment is determination of value of coefficient of thermal stability, depending on specific conditions of molding.
Determination of coefficient of thermal stability allows to predict precisely and adequately the size of an operational resource of a compression mold. 
It, in turn, gives the chance to develop practical recommendations about a choice of a design and material of compression molds for molding under pressure of zinc alloys for increase of operational firmness of the most loaded details of industrial equipment.