Tooling Corner: Boosting productivity of molds with diamond-like coatings

Diamond-like carbon (DLC) Dylyn coatings provide many answers to mold and die wear and corrosion problems due to their particular properties: A combination of wear protection, improved release, and corrosion resistance that is unique compared to many other surface treatments such as physical vapor deposition (PVD) TiN or electroplated chrome. Hydrogenated diamond-like carbon films are a family of coatings made up primarily of carbon/hydrogen chains in an amorphous structure.

Coatings alone will not solve all wear problems: the design of the tool, the choice of the tool materials, and the preparation of the tool prior to the coating are also important. The higher productivity achieved by diamond-like coatings on molds results from better tool protection yielding longer lifetime and lower tool cost, the reduction of plastic residues, and improved release, allowing less machine downtime and faster cycle times. The materials are useful in extrusion, blowmolding, and injection molding applications.

Deposition Process

DLC are applied by a process called plasma-assisted chemical vapor deposition (PACVD). The PACVD process is done in a high vacuum chamber, in the range of 10-5 mbar. The plastic mold tooling is cleaned to remove all dirt, grease, and oxides. The cleaned tools are attached to fixtures and then suspended from planetary plates so they can freely rotate in the coating chamber for the most uniform coating distribution during the PACVD process. The coating chamber is placed under high vacuum. Prior to deposition the tools are plasma etched. Carrier gases are introduced into the chamber, ions are formed in the plasma and are attracted by the oppositely biased tools, and the film grows by chemical reactions at the surface.

Changing the gases or the deposition parameters will result in different DLC/Dylyn coatings having variations in the hardness, chemistry, surface energy, coefficient of friction, and wear resistance.The tools do not need to be pre-heated to a special temperature in order to form the bond to promote adhesion as in other processes like PV. However, the tools will slowly heat up to a maximum temperature of 200°C due to radiant energy. This is well below the heat treat temperatures of most plastic mold tool materials.

Another consideration for load efficiency and coating distribution is the size and complexity of the tooling. Normal coating thickness is 2 µm, but it can be applied from .2- to 5-µm thick depending on application needs. Although this is not a line of sight process, the ability to coat into deep holes or slots is influenced by how close the walls of the holes are to each other. As the gases penetrate into the close areas, the attraction of the charged particles to the walls reduces the amount of ions as they travel deeper into closed areas causing a reduction in material to form the coating. Very pointed shapes or edges tend to attract more ions, and coating parameters need to be adjusted to control the thickness of the coating. This effect is not anywhere as strong as observed in the electroplating processes, and geometries do not need to be adjusted to compensate for the difference in growth rates.

DLC Coating Properties

DLC/Dylyn offers a unique combination of properties not available in any other single surface treatment. Possessing a low coefficient of friction and a low surface energy approaching that of PTFE, but with a hardness greater than carbide or TiN, DLC/Dylyn offer a unique wear and release combination.

The plastic additives that increase plastic toughness, resist mildew, add fire resistance, and offer many colors, can create a highly abrasive melt that aggressively wears the mold material during injection and/or release of the plastic parts. DLC/Dylyn offer a hardness range from 10 to 25 GPa that significantly increases the surface wear resistance of all plastic mold tool materials. But the DLC/Dylyn coating surface hardness will not change the substrate material?s ability to support a load?under high loading conditions, a hard tool material is necessary to support the DLC/Dylyn coating to offer the wear resistance.

Many of the plastics used today are very sticky, and the effectiveness of plastic part release has a very direct impact on productivity. The DLC/Dylyn offer a coefficient of friction (as measured against steel) ranging from .05 to .1 (compared to .7 for steel) and a surface energy ranging from 25 to 35 mN/m (18 mN/m for PTFE). So DLC coatings offer some very attractive properties to promote better release of plastic parts. However, there are many other factors that could influence release, and looking at the surface energy and coefficient of friction only may not always give the expected properties.

The structure of DLC/Dylyn is amorphous and would be similar to glass. These coatings are very dense and exhibit very little porosity, thereby offering excellent corrosion protection. Since the carbon-based coatings are extremely chemically stable, strong acids or bases have little or no effect. Dylyn coated parts were compared to nickel boron and PVD TiN surface treatments in a 168-hr continuous salt spray test per ASTM B 117-97 parameters (see photo, p. 26). The results were quite startling, showing a dramatic improvement of corrosion resistance by Dylyn.

What Tool Materials Can Be Coated?

The choice of mold tool material is very critical for optimum life and mold cycle performance. To decrease mold cycle times, highly thermally conductive materials like H-13 carbon steel, aluminum, and beryllium copper are used. To improve wear, materials like D-2 and A-2 hardened steels are used. To improve corrosion resistance, materials like 300 and 400 series stainless steels are used. DLC/Dylyn can be applied to all these tool materials because of the low coating temperature and the ability to adhere well to all these tool materials. Many other surface treatment processes involve chemistries or temperatures that can have an adverse effect of the tool materials causing corrosion, hardness changes or alter the size of the tool. In addition, DLC/Dylyn can be stripped from all these materials with very little or no changes in the surface finish by a controlled etching process that does not involve any chemicals to remove the coating.

Specific Applications

The first application area DLC/Dylyn coatings have shown to be quite effective in is blowmold tooling. The most effective use comes from coating all components, base, top, bottom, inserts, grips, and pushups. There were two major benefits. First, the formation of plastic residue due to outgassing of the polycarbonate preform is slowed down dramatically by the presence of the DLC/Dylyn coating. Secondly, blowmolders have found the frequency of cleaning has gone from once a day to scheduled weekly mold cleaning with at least a 50% reduction in the time to clean. The net result is improved overall cycle time and more containers being produced during the same time frame.

The injection molding of polycarbonate plastic at 525°F to make preform PET cores is another DLC/Dylyn coating application. The factor controlling cycle time is the ability of the polycarbonate PET cores to solidify enough to be ejected without deforming. The design of the tooling requires a core made of 420 stainless or H-13 steels with little or no draft over a few inches.

The industry started to work on reducing cycle time by first trying to improve the core finish to a #1 finish, but later introduced controlled texturing by draw stoning or glass beading for best release. They also looked at common surface treatments like nickel boron and TiN, which did have a positive impact on release, but did not give the corrosion protection from mold sweating they had hoped for. When DLC/Dylyn coatings were implemented, the molder observed up to a 10% faster cycle time, less maintenance downtime, and longer tool life. One particular processor reported faster cycle times lasting well over a year with once a week carbon dioxide cleaning and monthly Class 2 cleaning, producing well over one million preforms.

The extrusion of white PVC plastics for furniture and electrical applications is another DLC/Dylyn coating application. The white color in PVC products is generally created by adding TiO2 to the PVC melt, and the highly abrasive TiO2 filler acts like a liquid hone causing severe wear to the 420 stainless steel tooling. Extruding PVC yields corrosive attacks on the die steel, so changing to harder, more wear-resistant steel is not possible. The DLC/Dylyn coating brought both wear and corrosion resistance to the extrusion die.

In the production of plastic closures, the molder uses unscrewing thread-cores for parts such as seal caps. After production, the seal caps have to be decoratively galvanized, and the presence of lubricants is not allowed because they interfere with the galvanizing process. Molding the seal caps without lubrication creates a big flashing and wear problem, but DLC/Dylyn helps with efficient production.

One of the best, and often overlooked, applications is the problem of sliding parts in the mold tooling hanging up and causing unplanned downtime due to flashing, plastic particles, and dirt. Many times these sliding mold parts are never even in contact with the plastic melt, but the outgassing of the melted plastic leads to thickening of the lubricant, increasing the deforming force. DLC/Dylyn coatings reduce wear that causes flashing and with an increased hardness in combination with an extreme low coefficient of friction, reduce the effect of dirt in the sliding mold tooling.