Injection Mold, Hot Runner, Quick Mold Change

Extending the life of plastic injection molds It is well known that the molds used to create plastic parts ranging from water bottles to automobile bumpers are the most critical – and expensive – part of the injection molding process. In some cases a single mold can cost hundreds of thousands of dollars, not including the ongoing maintenance and running costs. However, despite being made of hardened steel, the very process of injecting melted resins at high temperatures and pressure into cavities over many cycles begins to wear away and erode the surface of cavities and moving components. Even the thermoplastic material itself can be abrasive or “sticky,” acting like sandpaper or leaving residue that wears down the surface. Now, with the increased utilization of even more abrasive material in the form of long glass and composite fibers, the amount of abrasion and friction within molds is increasing. Subsequently, molds are taking even more of a beating. It is for this reason that injection molders are turning to a variety of coatings to protect their investment and reduce maintenance and running costs. These coatings, applied to mold cavities as well as moving, sliding components within the mold, come in a variety of styles from carbon-based coatings, PVD & PACVD deposition to nitriding techniques that share one overriding goal: hardening the surface of the steel to protect it against 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,” explains Thomas Vermland, Global Application Manager for Oerlikon Balzers, a company that has been producing specialized PVD coatings for components and tools for more than 30 years. Reinforced glass fiber products In the automotive industry, manufacturers are placing greater emphasis on design and weight reduction. Subsequently, automotive designers are increasingly using glass fibers as reinforcements in many parts, including automobile bumpers. As a result, automobile bumpers are constructed using 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,” explains Vermland. This inevitably creates more friction (sheering effect) which can cause polymers like polyethylene or a polypropylene to become unstable during crystallization. To compensate, reinforced glass fibers are often utilized as an economical way to create a lighter, stronger part that retains some flexibility. Initially, when short glass fibers were used this did not create much of a problem because it did not affect the crystallization significantly. The slightly abrasive nature of the glass fibers also had a positive “cleaning” effect by removing residue on internal cavities caused by outgassing. However, the long glass fibers popular today are more abrasive, resulting in micro cutting within the mold’s cavities and runners due to sharp, jagged edges and a hardness of up to1200 HV. This can create significant issues, particularly along the parting line and any sharp contours within large molds. For large molds like those used for automobile bumpers and tailgates, a nitriding process can be used to significantly increase the surface hardness of the metal. Nitriding is a heat treating process that involves hydrogen, plasma and electricity that creates a case-hardened diffusion layer on the surface of a metal. Because it is not a coating, it does not affect the overall dimensions of the component. Vermland says Oerlikon Balzers’ BALINIT PRIMEFORM diffusion treatment increases injection mold surface hardness up to 1400 HV. This makes the molds significantly more robust (including versus long glass fibers) and reduces residue build-up, spalling and edge embrittlement. “We can take the entire bumper mold and place it in the machine and treat it in one operation,” says Vermland. “With the