An Overview of the Use of V-Tin® Low Temperature Titanium Nitride on Plastic Injection Mold Tooling
Provided by: The Staff of Vergason Technology and Erie Industrial
Updated: January, 2002
This article was developed to provide our customers and prospective customers details about the PVD coating process for Titanium Nitride on mold applications as it is performed at Vergason Technology. Very little specific information is available in general literature. Much of the application information has been developed over the past several years by working closely with the industry.
This article is not intended to be highly technical. It is a compilation of our experience and an effort to answer a number of questions that we are asked repeatedly. We hope that by reading the article you will gain insight into the effective use of TiN on mold components and that you may be able to determine if this technology can be of assistance in your application. We are always available to answer any questions.
The History and The Problems
In the early 1980s the vacuum coating industry began to search for new markets for the wear resistant coating titanium nitride (TiN). The coating had demonstrated excellent abrasion resistance, could be easily applied to various tool steels and the rich gold color helped to identify areas that were experiencing wear. One of the new target markets identified included the plastic injection tooling industry.
Plastic injection molds seemed to be ideal candidates for the relatively new physical vapor deposition (PVD) coating technology. It was a very large market with significant growth potential. The high value and production rates required of the molds demanded long life. Early tests indicated that improvements could be obtained in wear resistance, release properties, flow characteristics and other critical aspects through the application of titanium nitride. Cores, cavities, inserts, slides, feed screws, nozzles and tips became as important for the coating service facilities as cutting tools. And then ..everything stopped. The coating industry had missed on very important point. Most of the tool steels used in the molding industry could not tolerate the typical process temperature of PVD coatings.
Most PVD processes developed during the 1980s required application temperatures of 450·C (850·-900·F). At these temperatures tool steels like S-7, A-2, O-1, D-2, and to some degree H-13 began to change dramatically. The steel began to soften, move, grow and change in many ways that would make the mold or component unusable. Inserts would not fit back into cavities and ejector pins would not move to eject the parts due to growth in the tool steel. Finally, the changes in the coated mold resulted in the production of under sized parts. Needless to say, the situation was a disaster for mold makers, molders and the coating service industry. If the coating could not be applied to the mold without changing the physical properties of the tool steel, then the advantages provided by the coating could not be realized.
In the late 1980s, Vergason Technology Inc. (VTI®) developed an innovation in PVD coating technology, low temperature TiN trademarked today as V-TiN®. By refining the core process of cathodic are deposition, we discovered a method to reliably apply titanium nitride at 200·-230·C (400·-450·F). One of the key capabilities of the arc process allows extremely high ionization levels of the titanium source material. While this in itself might not sound significant, it is a critical element to applying low temperature coatings.
This high ionization level as compared to other PVD methods enhances the plasma reaction with the nitrogen gas. In addition, this factor is important in providing a large plasma zone with what is generally considered a high ion flux. Simply stated complex shapes can be coated effectively and uniformly with a wear resistant coating less than .0001 in thickness. The uniform thin film modifies the surface of the mold to provide the desired properties of the TiN without significantly changing the size of the component and will not affect the properties of the tool steel. Finally, the current state of the technology allows the safe removal of the TiN from virtually any tool steel commonly used in the molding industry. We can now apply the coating, remove it and return the customers part to them in the same condition as received. This can facilitate changes, repairs or other requirements presented by our customers. These qualities are once again expanding the use of titanium nitride in the molding industry.
What are the Benefits of Titanium Nitride in Molding Applications?
Wear Resistance is the first benefit that strikes our customers. The hardness of TiN and the low coefficient of friction are properties that enhance the life and durability of the mold and its components. Lets be specific by discussing applications that relate to problems experienced by molders. In the following tow cases the material molded was a polycarbonate alloy resin.
Wash out is a phenomenon exhibited when molding abrasive an non-abrasive materials. In one application a mold produced 3.5-inch computer disk cases and the gate was located opposite the primary textured surface of the part. Prior to coating, the molder found it necessary
In a similar application, a major camera manufacturer was concerned with maintaining the uniformity of the texture between the front half and the back half of the camera that were produced in two different cavities The gate locations were expected to cause wash out on the decorative surfaces. Identical molds were operated in side by side tests for six months, one coated and one uncoated. The coated mold operated with less down time, required little maintenance, cleaned easier and produced more parts. The customer estimated that the increased number of parts produced in the coated mold would result in additional revenue to the company in excess of $100,000, as compared to the uncoated mold.
Abrasive wear is a common problem with filled materials. One of the early applications successfully coated by VTI involved a six-cavity mold producing small 40 percent glass filled nylon gears for an automotive assembly. Gate erosion, wear along the sprue and loss of part dimension due to wear in the cavities occurred when 4 to 5 million parts were produced. The mold then required major rework. The V-TiN coating extended the life of the mold to over 14 million parts produced. The removal of the gold coating was obvious. The mold was removed, maintenance performed, qualified, coated and returned to full production in just over one week.
An application that is more difficult involves thermoset materials that are often mineral filled. While we do not expect to see the same dramatic results in mold life when compared to thermoplastic resins, increases in component life of 50 to 100% are common. Mineral filled materials often act like grinding compounds abrading core pins, cavities and inserts. They can also be highly corrosive and the resin will attempt to bond to the tool steel as the cross-linking of the polymer takes place. Many of our customers will fabricate inserts and details from 400 series stainless materials to provide the corrosion resistance. The V-TiN will then enhance the wear resistance resulting in a more predictable molding sequence. Again, the color difference between the base material and the coating provides a clear indication when the mold requires service.
Release characteristics have been improved by the application of titanium nitride. This has been most evident in an application in which wear is not a critical factor, molding polyethylene. A customer molding travel mugs with a very slight draft was having difficulty ejecting the parts from the mold. The mold surface was extremely smooth and the combination of surface area and stickiness of the material made production difficult. TiN was applied to the core, the customer polished the coated surface with a fine diamond paste following the coating process and the release problems were eliminated. The molds have been in service for over a year without further attention and the customer estimated that the mold operates at 2-3% faster following the coating process.
Corrosion Protection is a property of titanium nitride that is often overstated. Yes, it is correct to say that TiN is corrosion resistant. However, all TiN coatings are thin film ceramic coatings and they do have a structure. Rarely, if ever, can a two-micron thick ceramic coating be considered pore free and therefore a corrosion barrier. In the case of coatings applied with the cathodic arc process, small particles of pure titanium are uniformly dispersed throughout the coating. While our process is designed to minimize these particles, they do exist with the result that there is a path that can allow corrosion of the base material.
In most mold application this characteristic has little affect on the performance of the coating. However, there are exceptions. In the case of molding PVC one of the by-products is hydrochloric acid. This is extremely aggressive and will attack the base material at the interface of the coating. The best results are achieved when a corrosion resistant base material like stainless steel is selected for the mold.
Alternatively, when tolerances will allow the additional thickness, a barrier coating of medium phosphorus electroless nickel can be applied. Electroless nickel is the best choice since it provides a uniform build up and continuos layer without voids. Electrolytic plating processes tend to build up thicker coatings on sharp edges making assembly difficult.
This is also effective for polymers with high outgassing rates due to chemistry of the resin or the addition of additives like flame retardant. Always keep in mind that the coating modifies the surface properties of the mold material, it does not change the core properties of the material. If a mold material will not survive without a coating, the coating will only delay the corrosive effects of the molding process.
Tool Steel Requirements for TiN
Can TiN allow the use of lower grade tool steels for molds? This is a question that is asked repeatedly. There is not a clear answer. In general, the best surfaces for applying coatings have hardness in excess of Rc45. The most common mold steels selected for coating include:
Tool steels are usually selected for their machinability, stability, wear resistance and corrosion properties. With the exception of H-13, all of the above materials are tempered for their optimum properties at less than 250·C (500·F). It is always recommended that the tool steel is selected based on the best choice for the application without regard to the coating. Then, the coating will enhance the capability of the mold. We do not recommend the use of a coating to permit a selection of lower grade mold materials for any application.
Missing from the above list is a common mold material, P-20. It is very machinable and relatively low cost tool steel usually heat-treated with a hardness of Rc32-34. It is commonly used for large molds and inserts on applications where molded part dimensions or finish requirements have fairly wide tolerances. In many of these applications some wear can be allowed without affecting part quality.
This tool steel can be easily coated and the life of the mold will be enhance. However, P-20 can also be ion nitrided to improve the wear resistance at a lower cost and this is a more common practice. The decision as to which process to use should be based on the surface property desired. TiN coating will improve the release properties and wear resistance. If wear is the only requirement, consider ion nitriding through you heat-treated. The only time that you might have a problem is when weld repair is required on an ion nitrided surface. Cracks can be induced and are difficult to fix. TiN is a conductive ceramic and can be successfully welded in the event of a crash.
Where should TiN not be Considered?
Applications where the use of TiN should not be considered fall into two categories. The first application related to resins that are not compatible with the coated mold. Acrylic polymers are a member of this family. Our experience indicates that the acrylic materials seem to have an affinity for the coating surface. Release properties are poor and there is no advantage to applying TiN in this situation.
A second category of difficult applications includes molds that have been previously coated by another type of process. These other coatings often prevent good adhesion of the titanium nitride. Included are chrome plating and a variety of specialty coatings sold for wear and release on mold components. They are marketed as; Nickel/Teflon, Chrome /Teflon (available under a variety of trade names), solid film lubricants and other types of hard coatings.
In the case of chrome plating, the adhesion of the titanium nitride seems to vary dramatically from plating source to plating source. There are times when the adhesion is outstanding and other times when adhesion is marginal at best. The only difference that we can identify is the control, or lack of it, that various platers maintain over the concentration and contaminants in their plating tanks. It is a variable that we need to inform our customer about in advance.
The other coatings that are applied for wear or release properties act as a barrier to the application of titanium nitride. They can outgas as a result of the heat applied to the part and also outgas due to the fact that the application of V-TiN is performed in a vacuum chamber and many of the materials have low vapor pressures. Rarely can these other coatings be effectively stripped and we always caution our customers that previously coated mold details are not good candidates for TiN or any other PVD coating.
Difficult applications include achieving adhesion to highly polished surfaces. These can be coated effectively with wonderful results. In one application a core pin that produced a PET bottle was leaving scratches on the inside of the part as it was removed. This produced a cosmetically unacceptable product. The customer had been using a dry film lubricant to eliminate the problem. This coating needed to be replaced every 4-6 weeks. Following the application of TiN the core has remained in production for over one year without attention.
Great care must be taken in the preparation of the components with highly polished surfaces. The selection of polishing materials that are wax and silicone free are critical to achieving good adhesion in the coating process. Coatings on this type of surface should be applied thinner to reduce the stress between the coating and the base material. Finally, the highest safe temperature for the base material should be used in the coating process. This will assist in baking out the residual materials embedded in the pores of the metal as a result of the polishing.
Our Customers Role
Our customers have an active role in preparing the mold for coating and preparing the mold for operation. No one knows his or her mold better than the customer does. Critical dimensions, assembly procedures, tolerances and finish requirements remain the domain of the mold maker and the molder. In addition, due to the high value of the molds or staff is instructed not to use abrasive techniques on any mold component for fear of damaging a surface that could affect the performance of the mold. As a result, our customers are involved by performing the following operations:
Providing complete information on the mold components is critical to providing a high quality coating. Our customers always provide material type, hardness and any heat treating data that is available to enable the VTI staff to select the best coating temperature for the application. The additional information of resin to be molded, operating temperature of the mold and any critical tolerances also allows us to be an effective consultant in making coating recommendations.
Can the applications of TiN improve the performance of aluminum to make it a suitable mold material? The answer to this question demonstrates the limitations of the coating process. First the generally soft nature of aluminum alloys as compared to tool steels provides extremely poor support under the coating. Remember that the selection of mold material requires that the mold must be capable of surviving effectively in the application without coating.
In applications such as prototype molds or where release properties are important, such as vacuum forming molds, an intermediate layer of electroless nickel between the TiN and the aluminum will provide a suitable base to promote adhesion and good coating structure. Keep in mind that the coating is extremely hard as compared to the base material. It is in compressive stress and once it is removed due to deflection or wear, the area of coating removal will erode preferentially at a much higher rate. The TiN coating will extend the life of a prototype mold often by two times. However, it will not transform aluminum tools into production tools for high volume applications.
The titanium nitride coating process is performed in a vacuum chamber. The process is rather straightforward. Parts are cleaned in a series of ultrasonically agitated solutions, followed by rinsing, and vacuum drying. The parts are placed in a vacuum chamber, heated to the desired coating temperature, and titanium is evaporated from a solid source of high purity material in the presence of nitrogen gas. The result is a uniform deposition of titanium nitride is deposited on all exposed surfaces.
The process cycle time will range from two to seven hours depending on the mass of material in the chamber and the desired coating thickness. VTI has optimized the thickness for mold applications to range from 1-2 microns (less than .0001) which falls within most of the tolerance limits for molding components. After coating most details and inserts can be reassembled without difficult. One customer compared the assembly time of a V-TiN coated mold versus a mold coated with nickel/Teflon. The savings were twelve hours. In their case, a two cavity mold with 125 precision inserts would typically require 20 hours assembly time and a great deal of hand stoning for the wet plated components. This was reduced to eight hours by changing to TiN.
The size of molds that can be coated, cost of the process and turn around times are related to the size of the vacuum chamber and the amount of mass of tool steel in the mold. The following guidelines are generally applicable:
The ultimate question is how much does the process cost. On a relative basis it is typically more expensive than wet plating processes, usually with better wear resistance, more uniformity and better mold performance. A cavity block of 100 cubic inches can range from $200-$250, small inserts from $2-$5 each and cores 2 diameter x 12 long $65-$80. A complete mold cavity of 200 cubic inches can range in cost from $600 to $1800 depending on the complexity, number of inserts, cores and slides. All applications are quoted separately with quantity discounts with consideration for the amount of work involved in each application. Quotes can be provided based on the size and quantity of components, a review of prints or based on a previous order for a similar product.
Vergason Technology has taken the time develop the V-TiN coatings so that it can be applied to the critical surfaces of the plastic injection mold. It is applied at temperatures that will not affect the properties of the tool steels including hardness and size. Weve taken the time to understand the needs of the plastic injection molding industry. We recognize the value of the components we are coating and have developed handling procedures that will insure the safe coating of the molds. We will even make recommendations on shipping and packing methods to enable the parts to make the hazardous journey with common carriers. This positions our capabilities and knowledge for this type of application among the leader in the PVD industry.
Taking this thought one step further, VTI has removed the risk for the process engineer to use PVD coatings to improve mold performance. We can demonstrate this by answering the following questions with and unequivocal yes:
1.Can we coat a mold component with V-TiN without affecting the hardness and only adding the thickness of the coating to the size? YES!
2.Can we remove the coating without damage to the tool steel? YES!
3.Once the coating is removed with the customer receive the part back in the same condition and hardness as given to VTI except without the coating? YES!
These are questions that all contract-coating services must be able to answer positively when applying any coating to a mold component. The technology exists today. Customers need the assurance that these high value components will meet their performance requirements and that they can be restored to their original condition when requested.
Please contract us if you have any questions. We hope that this overview provides new insight into the PVD coating process for Titanium Nitride. The VTI® tradename for our low temperature process is V-TiN. Ask for it by name.
Contact VTI or Erie Industrial sales staff today to find out how your injection molding process can run more efficiently and cost effectively.
Phone (814) 452-3231 Fax (814) 456-7176