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In this paper is described the investigation process and development of a new mixture for foundry cores, in which a factorial complete at two levels experimental design for mixtures with relationships between the components and an operation variable is used. The mixture that is developed uses the traditional process sodium silicate - CO2 and a new element has been employed: the carbonaceous schist as to facilitate the breaking down of the pieces from de moulds. A nomogram was made, in which it is represented the region where the relationships between components show the best properties, as resistance to the compression and breaking down. Also, it was carried out the integral hygroscopic characterization of the mixture selected as the best, which show excellent hygroscopic properties, compared with the similar ones that use for breaking down the cores, mixtures with sugary compounds.En este artículo se describe el proceso de investigación y desarrollo de una nueva mezcla para machos de fundición, en el que se emplea un diseño de experimento para mezclas, factorial completo a dos niveles con relaciones de componentes y variable de operación. La mezcla que se desarrolla utiliza el tradicional proceso silicato de sodio (vidrio líquido) - CO2 y como elemento novedoso se tiene el empleo de los esquistos carbonosos como aditivo desarenante. Se confecciona un nomograma que presenta la región donde las relaciones entre componentes muestran las mejores propiedades de resistencia a la compresión y desmoldeabilidad para la mezcla en estudio. Además, se realizó la caracterización higroscópica integral de la mezcla seleccionada como de mejores propiedades, la cual mostró excelentes propiedades higroscópicas, comparada con sus similares que emplean desarenantes del tipo azucarado.

The moisture diffusion in molding mixtures employing in foundry processes is described. Using gravimetric techniques the humectation kinetic behavior of different types of molding mixtures for foundry cores were registered. The mixture higroscopicity is compared using the porcentual increase in wet mass. The kinetic study is ending with the calculations effective coefficients of diffusion of the atmospheric water in the mixtures to different bleakness. For this goal the fitting through non lineal methods was used. The influences of the agglutinative and draw materials in the hygroscope properties of these were analyzed too.Se describe la difusión de humedad en mezclas de moldeo utilizadas en los procesos de fundición. Empleando técnicas gravimétricas se registra el comportamiento de la cinética de humectación de diferentes tipos de mezclas de moldeo para machos de fundición. La higroscopicidad de las mezclas es comparada usando el criterio del incremento porcentual en masa húmeda. El estudio cinético se concluye con los cálculos de los coeficientes efectivos de difusión del agua atmosférica en las mezclas a distintas intemperies. Para este propósito se emplean métodos de ajuste no lineal. También se analiza la influencia de los materiales aglutinantes y desarenantes en las propiedades higroscópicas de estas mezclas.

Studies in several organisms show that recombination and replication interact closely. Recombinational repair usually requires associated replication at some stage; moreover, additional replication can induce recombination through either homologous or illegitimate events. In prokaryotes, stimulation of recombination by replication is more dramatic when rolling circle replication is employed. In contrast, theta-type replication induces only a modest increase in recombination frequency. In this article, we show that induction of theta-type replication from a supernumerary origin in the symbiotic plasmid (pSym) of Rhizobium etli leads to a 1000-fold increase in deletion formation on this plasmid. These deletions span 120 kb (the symbiotic region) and have as endpoints the reiterated nitrogenase operons. We have named this phenomenon RER, for recombination enhancement by replication. RER is not affected by the position of the replication origin in the pSym, the direction of advance of the replication fork, or the distance from the origin to the recombining repeats. On the other hand, RER is dependent on an active recA allele, indicating that it is due to homologous recombination. RER displays a strong regionality restricted to the symbiotic region. The similarities and differences of RER with the recombination process observed at the terminus of replication of the Escherichia coli chromosome are discussed.

To gain insight into the extent of exact DNA repeats in sequenced bacterial genomes and their plasmids, we analyzed the collection of completely sequenced bacterial genomes available at GenBank using the program Miropeats. This program draws graphical representations of exact DNA repeats in whole genomes. In this work, we present maps showing the extent and type (inverted or direct) of exact DNA repeats longer than 300 bp for the whole collection. These repeats may participate in a variety of events relevant for bacterial genome plasticity, such as amplifications, deletions, inversions, and translocations (via homologous recombination), as well as transposition. Additionally, we review recent data showing that high-frequency architectural variations in genomic structure occur at both the interspecies and interstrain levels.