Les traitements de surfaces appliqués aux moules et outils : une évolution permanente des performances

Surface treatments applied to moulds and tools : “constant performance progress”

H.E.F. R&D – H.E.F., Groupe Hydromécanique et Frottement, Andrézieux-Bouthéon

Résumé

Abstract

Moulding technology facilitates the production of components with complex designs. In France the turnover of the moulding industry doubles each ten years and the main sector of activity of this industry is polymer moulding, which accounts for 58 % of the total turnover. Because of fast tool wear caused by the severe conditions experienced during injection processes increased tool performance is needed.

Methodical investigations show tool life limitations. Complementary industrial points of view indicate that tool life is often limited by surface wear phenomena (see annexes 1 and 2). Abrasion, corrosion and sticking phenomena can cause tool surface damage whilst mechanical stresses can induce fatigue and plastic deformations. Tool performance must be adapted to different solicitations on it’s core volume and surface. Surface engineering sciences, numeric simulations and surface analyses are necessary to establish correct diagnostics which will allow for developments enabling a significant increase in tool life. Core material improvement and better surfaces treatments are also needed. New designs and new metallurgical choices could be industrialised to produce short series or prototypes.

Surface phenomena should also be examined using a microscopic approach. A surface may be seen as the limit between solid and gas (see annexe 3). With careful observation of the surface from the outer limit to the inner undamaged metal structure one can identify several layers such as adsorption layers, oxide layers and brittle metal layers. An oxide layer whose growth rate and stability are dependent on thermodynamic conditions covers most metallic surfaces. Topographic analysis is a complementary approach to understanding surface phenomena such as sticking or seizure. Reflectivity and colour measurements can provide very efficient though indirect quantification of surface morphology and roughness (see annexe 4). Tribological tests confirm the relation between the coefficient of friction a surface and it’s reflectivity.

Numeric simulations can make tool design optimisation easier. New composite materials covered with metallic layers can be used to make moulds for the production of prototypes. The HIP (Hot Isostatic Pressure) technology is competitive for the production of moulds with complex designs. Aluminium moulds covered by protective layers are attractive for medium series. Specific developments of PVD processes are necessary to keep good adhesion at low temperature (150°C). Recent progress in steel metallurgy offers improvements on machining, welding and mechanical resistance. Powder metallurgy, laser and plasma technologies are efficient means for restoring tools. Super alloys can solve glass moulding problems. In the case where several types of solicitations are present, coating properties complement core material properties. Thermal treatments and thermo-chemical treatments can improve resistance to thermal and mechanical fatigue. Thick metallic layers can offer efficient protection against corrosion. PVD coatings produce a large quantity of materials, which can offer good resistance to abrasion and oxidation or offer a low friction coefficient (see annexe 5). Multilayers can combine several of these properties. Specific surface finishing and recent process progress allow this technology to be carried out successfully. Multi-technological coatings are industrially used in the case of severe corrosion + abrasion or fatigue + abrasion.