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Resins flow better in TiN-coated molds.

Resins Flow Better In TiN-Coated Molds

Titanium nitride (TiN) coatings have been used on injection molds for several years, during which they have demonstrated wear and corrosion protection, as well as inherent lubricity (low coefficient of friction), which aids mold release (see PT, Feb. '85, p. 29). Now, an additional benefit is reported by a supplier of TiN coatings, whose customers have been observing better flow of a variety of thermoplastics in molds coated with TiN.

To verify this, tests were undertaken by the European partner of Balzers Tool Coating, Inc., North Tonawanda, N.Y. Coating tool walls with TiN increased spiral flow in a test mold for all materials studied. Resins tested included amorphous and semi-crystalline thermoplastics, liquid-crystal polymer (LCP), and thermoplastic blends. Some of the materials tested were glass-filled nylon 66, acetal, ABS, polyethylene, nylon/LCP blends, and PPO alloys.


A test mold was built with four square-spiral channels feeding from a single sprue (Fig. 1); two of these measuring channels were coated with TiN, two were left uncoated. Each measuring channel had a max. flow length of 400 mm; the cross-sections measured 6 mm wide and either 2 mm or 1 mm deep. Melt was injected into the coated and uncoated channels simultaneously. The measuring channels had a roughness value (Ra) of 0.1 micron.

To compare the development of internal mold pressure in the coated and uncoated channles, each of the four spiral-flow cavities was fitted with four piezoelectric pressure sensors placed at intervals along the flow path. The sensors were calibrated after installation to ensure an exact measurement. Heating and cooling ducts were placed symmetrically with respect to the melt-flow channels, so that temperature effects would not distort the flow comparisons.

All tests were conducted on a Netstal 115/60-1 injection machine with MP-Sycap controls. After the machine had come to thermal equilibrium, melt was injected simultaneously into the coated and uncoated cavities. Because of the material-dependent maximum ratio of flow path to wall thickness, the 2-mm deep measuring channels were used most frequently. Development of internal mold pressure, melt and mold temperatures, machine parameters, and flow-path lengths in the coated and uncoated channels were all measured.

Infrared fiber-optic measurements of the mold-surface temperature were recorded on videotape, and a computer interpreted the images with a resolution of 0.1[degree] C. This, as well as temperature-indicating paint, showed that the TiN coating, only 2-4 microns thick, has no effect on the mold-surface temperature.

The viscosities of the various melts were also measured in a capillary rheometer using TiN-coated and uncoated slot capillaries (50 X 10 X 0.5 mm) held at the same temperature as the melt. The shear-rate range was between 10 and 10,000 [sec.sup.-1].


Results of the experiments indicate that coating the tool wall with TiN alters the development of pressure inside. Higher pressure was sensed at each point along the TiN-coated spiral-flow path, indicating less pressure loss to friction. Figure 2 shows the development of pressure in the coated and uncoated measuring channels. The higher internal mold pressure all the way along the coated channels results in increased flow-path length for all materials studied.

Difference in flow length depends on both the material used and the injection rate. Large differences between coated and uncoated cavities are seen primarily with glass-fiber-filled and liquid-crystal thermoplastics, and also with blends containing liquid-crystal polymers. While the maximum flow-path difference found with glass-filled and liquid-crystal thermoplastics is 40-60 mm, the difference between the coated and uncoated cavities for ordinary thermoplastics is usually less than 10 mm (Fig. 3).

Figure 4 shows the difference in flow length for nylon 66 with 25% glass. For 50% glass-filled nylon, jet filling produces a rougher appearance in the material from the uncoated cavity, while the flow front is smoother in the coated cavity (Fig. 5).

In the capillary rheometer, the studies show lower apparent viscosity for the TiN-coated than for uncoated slot capillaries (Fig. 6).



Balzers' tests indicate that TiN coating of injection molds can have a beneficial effect on mold filling. Better transmission of pressure means that the required pressure is lower. Thus, it's conceivable that problems such as sink marks and poor mold filling in cases of long flows through thin wall sections can be remedied with TiN coating. However, company sources recommend that molding conditions with coated cavities should probably be modified from those used previously with uncoated molds. for glass-filled thermoplastics in particular, there can be sizable changes in rheology. (CIRCLE 41)
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Title Annotation:titanium nitride
Author:Fallon, Michael
Publication:Plastics Technology
Date:Jun 1, 1990
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