Improve Mold Life Using Thin Film Metal Coatings and Ion Nitriding
There are a number of publications advocating the use of thin film, hard metal coatings to improve mold life. Hard coatings and ion nitriding can protect the surface of molds and mold components from erosion caused by engineered resins containing abrasive fillers like fiberglass and carbon-containing fillers. These surface treatments can withstand the elevated melt temperatures that these resins require for the injection process. The erosion effects mentioned are caused by the injection process. Chemical reactions in certain resins cause failures in the steel surfaces of molds and components.
Suitable surface treatments can improve the performance and lifetime of mold components. Plasma pulse ion nitriding and hard coatings deposited by thermal, physical, chemical and plasma assisted methods can protect the surface from attacks by resins and injection pressures.
Various vacuum coatings and methods to deposit these coatings - that exhibit a low coefficient of friction on steel - are available in the marketplace.
Physical vapor deposition (PVD) using a cathodic arc or sputtering process can produce various titanium, titanium aluminum and chromium nitride or chromium carbide films. These films are normally deposited at temperatures below 480°C/ 900°F. Films produced using the PVD process exhibit a smooth surface suitable for most mold applications. Further polishing techniques can be employed if the application requires.
Titanium-based films can be chemically removed prior to major mold repairs and re-coated afterward. Titanium-based films typically are three to five microns thick and have hardness values of 2500-4500 Vickers hardness. Chromium-based films must be mechanically removed or stripped in acid- based solutions. Chromium-based films are slightly thicker than titanium films ranging on average from five to eight microns.
Chemical Vapor Deposition (CVD) films are produced in atmosphere or vacuum and are typically titanium-based nitrides or carbides. Aluminum oxide also can be produced in the CVD process. The process temperatures vary, but generally are high - ranging from 850°C to 1,100°C/1,560°F to 2,370°F.
CVD films have excellent wear characteristics and can be produced in multiple layers. Normal coating thickness for these films is five to eight microns. They can be removed using the chemical method and re-coated as needed. Since the CVD process is high temperature, the heat treating and tempering process must be performed again on tool steel materials. Carbide tooling requires no further processing after the CVD process. Surface finishes on CVD coatings require pre-coat and post-coat polishing.
Plasma Assisted Chemical Vapor Deposition (PACVD) coatings are now available. These extremely smooth coatings are processed at 480°C to 550°C/900°F to 1,020°F. They have the traditional titanium and titanium-aluminum-based coatings and some newly developed boron coatings can be produced in the PACVD process. PACVD is a lower temperature gas process. An advantage to using PACVD in mold applications is coatings can be applied into deep cavities and cores since the techniques aren't line-of-sight dependent. Most mold steels can be coated using this process provided they have a tempering range above 900°F.
PACVD coatings are suitable for highly polished surfaces, these surfaces will remain as finished. In most cases, coatings perform best on surfaces of 16 microfinish or smoother. Rougher surfaces tend to have peaks that may chip - causing premature coating failure.
Most coatings have a threshold for operating temperatures normally 500°C to 600°C. Coatings that contain aluminum, boron or chromium can operate at elevated temperatures above 600°C for extended periods of time without failure.
PVD and PACVD coatings can be applied in conjunction with ion nitrided surfaces. Some surface preparation is necessary to ensure proper bonding of the coating to the steel surface.
Ion nitriding is a case-hardening technique used on a variety of mold steels and is performed in a vacuum furnace at temperatures from 850°F to 1,100°F for most steels. Many tool builders that use pre-hardened molds steels for improved machining have a cost-effective option to harden the surface of tooling and still maintain the core properties. The process uses nitrogen hydrogen and other gases. Voltage is used to ionize the gases that create a plasma. Various pressures are used, depending on part geometry. The process can be accomplished in cold wall vessels that use only plasma energy to bring the workpieces to nitriding temperatures or hot wall vessels that use heaters similar to a vacuum furnace that heat the work pieces to a set point before creating the plasma for nitriding. The power supplies can be direct current or newer pulse supplies. Pulse plasma supplies minimize micro-arcing during the process and generally give better results for surface finishes.
Most pre-hard steels in the 28 to 40Rc range are ideal candidates for this process. This hardness range is excellent for ma-chining, but may not be hard enough on the surface for extended mold life. The results from ion nitriding can be measured by processing a coupon of like material and then measuring the results in a metallurgical pre-pared sample. Since ion nitriding is a case-hardening technique samples are measured from the surface to typical case depths of up to .025" deep. Most steels are hardened to .005" to .012" deep and are measured effectively to 50Rc. The hardness is determined by the alloying elements contained in the steel. Typical nitriding elements are Chromium, Molybdenum, Vanadium and Aluminum.
Mold materials that are quenched and tempered steels like 4140, P-20, H-13 and 420 Stainless can be nitrided to various depths. The 15-5 and 17-4 steels can be treated by many of the processes. Nitraloy 135 is used for barrels and screws as is the powder metal materials where high wear applications require hard surfaces.
Nitriding and thin film metal coatings provide a low coefficient of friction to ease part release from injection molds and components. They also provide excellent corrosion resistance characteristics. Certain resins like PVC form corrosive gasses during the molding process and these coatings can protect tooling longer.
In time and with practice, each polisher will develop his or her own tricks and short cuts.
Proven step-by-step instructions for polishing aluminum and useful and accurate pointers for those applications where a diamond finish is not required.
Mold cleaning is just one part of a systematic and comprehensive approach to mold maintenance. Here’s one moldmaker’s strategy for injection mold cleaning and an explanation of where and why different techniques are employed.