Molds used to make plastic items ranging from water bottles to automotive bumpers are widely recognized as the most crucial – and expensive – component of the injection molding process. In certain situations, a single mold might cost hundreds of thousands of dollars, without counting ongoing maintenance and operating expenses.
Despite being built of hardened steel, the operation of injecting molten resins at high temperatures and pressure into cavities over many cycles wears and erodes the surface of cavities and moving components. Even the thermoplastic material itself can be abrasive or “sticky,” resembling sandpaper or leaving residue that damages the surface.
The amount of abrasion and friction within molds is growing as increasingly abrasive materials, such as long glass and composite fibers, are used. As a result, molds are being hit even harder. Injection molding service providers understand the importance of protecting these high-value assets, especially considering the significant initial investment and ongoing operational costs.
Importance of Mold Coatings
For this reason, injection molders are resorting to a variety of coatings to safeguard their investment while also lowering maintenance and operating costs. These coatings, which are applied to mold cavities as well as moving, sliding components within the mold, come in a variety of styles, ranging from carbon-based coatings, PVD and PACVD deposition, and nitriding techniques, all with the same overarching goal: hardening the steel surface to protect it from all manner of abuse. These coatings are applied for the simple fact that steel alone is not hard enough to protect these expensive molds and ensure optimal uptime and productivity.
Reinforced Glass Fiber Products
In the automotive business, manufacturers are putting more focus on design and weight reduction. As a result, automotive designers are increasingly employing glass fibers as reinforcements in a variety of components, including car bumpers. As a result, automotive bumpers are made with less material than before.
Inside the mold there is more pressure and higher temperatures being used to inject the plastic by a factor 2-4 times higher than in the past. This inevitably leads to increased friction (sheering effect), which can cause polymers such as polyethylene or polypropylene to become unstable during crystallization. To compensate, reinforced glass fibers are frequently used as an inexpensive approach to make a lighter, stronger element that preserves some flexibility.
Initially, using short glass fibers did not pose a serious difficulty because it had little effect on crystallization. The glass fibers’ slightly abrasive character also had a helpful “cleaning” effect, eliminating dirt from interior cavities created by outgassing.
However, the long glass fibers used nowadays are more abrasive, causing microcutting within the mold’s cavities and runners due to sharp, jagged edges and hardnesses of up to 1200 HV. This can cause major problems, especially around the parting line and any sharp shapes in big molds.
A nitriding procedure can be used on large molds, such as those used for vehicle bumpers and tailgates, to dramatically increase metal surface hardness. Nitriding is a heat treatment method that uses hydrogen, plasma, and electricity to form a case-hardened diffusion layer on the surface of a metal. Because it is not a coating, it has no effect on the component’s overall size.
This increases the molds’ strength and decreases residue buildup, spalling, and edge embrittlement. We can take the entire bumper mold and place it in the machine and treat it in one operation. This is the substitute for hard chrome in any large mold.
Diffusion treatment is ideal for enhancing vehicle plastic parts like bumpers, headlights, and interior pieces with mirror-polished surfaces, reducing the need for repolishing and improving wear resistance.
Enhancing Mold Durability
PVD coatings are used on injection mold components to increase wear resistance and durability. These coatings also provide lubricity, aiding plastic flow and reducing the need for internal lubricants or release agents. They minimize maintenance and downtime, improving overall production efficiency and reducing costs.
Choosing the right coating during mold design is crucial for optimizing performance, extending mold life, and reducing maintenance and ownership costs.
Conclusion
Injection molding relies on expensive molds subjected to extreme conditions that wear and erode. Molders are turning to advanced coatings such as PVD and nitriding to protect these critical assets and optimize productivity. For maximum mold performance and life in production, these coatings harden mold surfaces, reducing wear and maintenance costs.
