Meaning
Technical specifications for high performance moulds frequently require the application of thin film layers to enhance the hardness and lubricity of the steel. This process, known as physical vapor deposition coating, involves the condensation of a vaporized material onto the tool surface within a vacuum chamber. It creates a highly dense and well adhered layer that can significantly improve the wear resistance and release properties of the mould.
Common materials used in this process include titanium nitride, chromium nitride and diamond like carbon. The resulting film is typically only a few microns thick, so it does not alter the critical dimensions of the tool.
Deposition Process
The coating is applied by bombarding a target material with an electric arc or a high energy beam, causing it to evaporate and deposit onto the tool. Physical vapor deposition coating takes place at relatively low temperatures, which prevents the tool steel from losing its hardness or undergoing dimensional changes. This is a major advantage over other coating methods that require higher heat.
The process can be precisely controlled to achieve a uniform thickness across complex geometries and deep cavities. However, the line of sight nature of the deposition means that the tool must be carefully oriented or rotated to ensure full coverage. This technology is widely used to protect the sharp edges of the gate and the delicate surfaces of the cavity.
The resulting layer is extremely hard and provides a barrier against both mechanical and chemical attack.
Hardness Improvement
Adding a hard ceramic or carbon based layer to the tool steel significantly increases its resistance to the abrasive flow of reinforced polymers. While the base steel provides the structural strength, the physical vapor deposition coating provides the surface properties needed for high volume production. It can reach hardness levels that are several times higher than that of the hardest tool steels.
This hardness prevents the fillers in the resin from scratching or eroding the surface, preserving the intended finish of the parts. It also reduces the likelihood of adhesive wear by lowering the surface energy of the tool. This means that the polymer is less likely to stick to the metal, which reduces the ejection force and the risk of part damage.
The durability of the coating is a key factor in the long term performance of the mould.
Tool Performance
The use of these advanced coatings leads to more stable and efficient moulding processes with fewer quality issues. A mould treated with a physical vapor deposition coating will typically require less frequent cleaning and maintenance, which reduces the total cost of ownership. The improved release properties also allow for faster cycle times and a lower scrap rate.
When the coating eventually wears away, it can often be stripped and reapplied, further extending the life of the tool. This makes it a cost effective solution for high precision and high volume manufacturing. The data from metrology tools can be used to monitor the state of the coating and schedule the recoating process before the underlying steel is damaged.
Consistent performance is necessary for meeting the high standards of the plastics industry.
