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Using Coated Carbide Inserts For Mold Making

Plastic injection mold makers face a very daunting task when it comes to picking the correct coating for carbide inserts. Just consider the number of quality companies offering carbide inserts, plus the geometries and coatings available from each company!
Add the choices for carbide insert substrates, coolants, high speed machining and the many types of tool steels used in mold making and the selection process becomes even more complicated.
This article is one of a series dealing with carbide inserts, specifically the coatings used. There are quite a few coatings in use for the machining of injection mold cores and cavities. Our discussion will focus on the milling of cores and cavities rather than mold bases.
Carbide inserts are comprised of three main features:
Substrate material
Type of coating
Insert geometry
What are substrates?
The substrate is the base material used in the carbide insert. Most thinly coated carbide inserts use tungsten carbide, grade C-2. This is commonly used for a wide range of coated inserts, and it is increasingly less important what type of carbide substrate is used because the coatings are so efficient. However, there are many grades of substrates that can be used depending on the application.
The various coatings used on carbide inserts, such as diamond coatings, are applied by chemical vapor deposition, or CVD. The diamond must stick to the substrate with a very strong adhesion in order to function well in a machining application.
Some carbide inserts have as many as seventy coats of diamond applied to the substrate! Various gases, such as hydrogen and methane are used to create a pure diamond crystal. Under pressure and heat, a chemical reaction takes place that causes the diamonds to grow on the substrate surface.
There are many, many other types of coatings, such as titanium nitride (TiN), titanium carbonitride (TiCN), titanium carbide (TiC), aluminum oxide (Al2O3), zirconium carbonitride (ZrCN), titanium aluminum nitride (TiAlN) or aluminum titanium nitride (AlTiN). Each of these coatings has unique properties designed for different applications.
Most coatings are applied using a chemical deposition process (CVD) or a physical deposition process (PVD). This enables the extremely thin layers to be deposited, up to 70 layers.
Do coatings really make a difference?
Yes, in test after test, plus practical experience, the carbide inserts with coatings out perform uncoated inserts. Increased speeds and feeds, improved surface finishes, heavier depth of cut, longer tool life, and improved wear resistance are just a few of the typical benefits.
Depending on the specific application, the savings can be substantial over uncoated carbide inserts Carbide tooling is expensive to begin with, and any savings can really add up over time. Another savings is in the machine down time used in changing or indexing inserts.
How to choose a carbide insert coating
It would be great if there were a simple, cut and dry answer to this question, but there is not one. The variables are too numerous and the choices too great to give a simple answer. Another complication is that most injection mold makers do not have the luxury of running in-house tests. There is just too much work to be done!
It is possible to experiment with different coatings though in a real-life situation without wasting precious machine time. Your carbide insert supplier should be able to give you a great deal of information that is relevant to your particular need.
Very often, they have already run tests, though your application hardly ever seems to fit the test! Nevertheless, a good supplier can help you to navigate through the confusing waters of choosing a coated carbide insert.