A plastic injection mold is one of the most important tooling for injection molding, whose quality directly determines the quality of the final product. In the meantime, the mold cost takes up a great proportion in the production cost of an injection molder. If the injection mold life is short and the precision is not well kept, not only will the product quality be affected, but also huge waste will be caused, including material waste and labor waste during plastic injection molding. Therefore, increasing the service life of injection mold has a significant impact on reducing the cost of plastic products, as well as improving tooling productivity and company competitiveness.
During the plastic injection molding process, some plastics will decompose and thus release some corrosive gases under high temperature and high pressure conditions, which causes corrosion and damage to the mold surface. When the mold is damaged, or the shape, dimensional accuracy and surface finish of the plastic part are not up to standard due to the excessive wear of the mold, severe flashing will occur, and if the mold is beyond repair, mold failure will be inevitable. The number of molding cycles or the total number of parts molded before the mold fails is referred to as the service life of a mold. Usually, an injection mold has a service life of more than 300,000 cycles.
Factors that affects the service life of injection molds, and the basic ways to improve the service life
Different injection temperatures and pressures are required for different raw plastic materials, while different working conditions have different effects on the service life of a mold, and different plastic parts also cause different level of tear and wear to the mold. Therefore, under the prerequisite of meeting the product performance and quality requirements, it is usually necessary to select a raw plastic material with great processability to produce the molded product. This is beneficial to both the product molding process and the improvement of the mold service life.
Speaking of mold structural design, the cavity and the core are the main molding parts to shape the plastic products – the cavity shapes the exterior surface of a plastic part, and the core shapes its inner surface. Cavities and cores of different structural designs have different levels of strength and rigidity, thereby different levels of convenience with regard to the repair and replacement of wear parts. Therefore, from the perspective of the service life of a mold, the structural form that is relatively good in strength and rigidity, while being easy to repair is able to extend the service life of a mold.
In addition to the molding parts, other structural parts in a mold should have sufficient strength and rigidity to withstand the mold opening & clamping forces, injection pressure and thermal stress during operation, so as to avoid excessive deformation.
The structure and precision of the guiding mechanism have a direct influence on the clamping of the core and the cavity, which affects the accuracy of the plastic part and the service life of the mold. Therefore, it is necessary to select an appropriate guide form and guide accuracy. Common injection molds mainly rely on the guide pin mechanism to ensure the matching precision. In high-precision injection molds, in order to ensure the matching precision of the mold core and the mold cavity, the tapered precision locating mechanism, or the cylindrical pin locating mechanism and the guide pin can be adopted.
During mold structural design, attention should also be paid to keeping the thermal balance of the mold. The appropriate design of the gating system, the temperature control system and the venting system is able to reduce the hot tearing tendency of the mold, thus improving its service life.
The mold material and the heat-treated injection mold cavity are usually more complicated, with relatively higher requirements for precision and surface roughness. The quality of the mold material influences the quality and service life of the mold. The selection of the mold material must meet the customer’s requirements for the product quality, taking the cost of the material and the strength during the preset period into consideration. During material selection, certain working conditions must be met, such as wear resistance, toughness, thermal resistance, cold / heat fatigue resistance and corrosion resistance.
Whether the heat treatment process is appropriate or not also greatly affects the service life of the mold. The factors affecting the quality of heat treatment include heating rate, quenching temperature, quenching cooling rate and tempering temperature. The heat treatment process should be reasonably carried out during the mold making process, with process conditions strictly controlled.
During the mold making process, the method and accuracy adopted for mold machining and surface finishing have a direct influence on the quality and service life of the mold. If the mold is not appropriately cut, sharp corners or excessively small radius of the fillet will be caused, which may lead to severe stress concentration during mold operation.
In order to ensure the precision of each part in a mold, high-precision machining methods, such as EDM, wire cutting and CNC should be appropriately selected in the mold making process, in a bid to improve the mold quality and service life.
Whether the mold is appropriately used and maintained is also a major factor affecting the service life. Whether the mold is appropriately set up and commissioned, whether the parameters of the injection molding machine are set in conformity with the design requirements during production, and whether the mold is regularly maintained as planned can all improve the service life of the mold to a certain extent.
Conclusion: The factors that influence the service life of the plastic injection mold are multi-faceted, so comprehensive measures should be taken to improve the service life of them. In the design, manufacturing and application process of the molds, it will help improve the mold quality and service life if you can select the appropriate raw plastic material, reasonably design the mold structure, select the appropriate mold material / heat treatment process / mold machining method, and properly use and maintain the mold.