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Rosen M.,Corex Design Group Inc.
Plastics Engineering | Year: 2014

The interactions between part design, tool design, processing, and materials result in endless opportunities for unforeseen problems which can come up during a sampling. Over 25 years of experience in plastics consulting and injection-molding troubleshooting have revealed methodologies that have helped me diagnose the causes of molding problems and develop effective solutions for countless types of plastics parts. Understanding the behavior of the material is key to understanding molding problems. With an almost infinite number of different plastics materials, often important design mistakes are made due to designers and molders being unfamiliar with the molding characteristics of the material. Source


Rosen M.,Corex Design Group Inc.
Plastics Engineering | Year: 2014

Some troubleshooting examples of actual injection-molded parts which show solutions to problems of parts having issues with long cycle times, weak weld lines, and insufficient part strength are discussed. A large industrial part, around two feet long with a nominal wall thickness of 0.25' was being molded with general purpose ABS. The mold was designed with a cold runner feeding four small tunnel gates. The fill time was set slow, at 8 seconds. This indicated possible issues with cosmetics and/or burning at vents. For ABS, the suggested gate thickness should be 50-70 % of the wall thickness. The nozzle ID should be around 90 % the diameter of the top of the 0.30' diameter sprue. The other issue was the long cycle times due to the thicker walls of the part and the use of a small amount of blowing agent. In this case, the problems of too weak a part, a long cycle time, and surface sink were solved by switching to a new material technology and making a few simple steel-safe changes to the mold. Source


Rosen M.,Corex Design Group Inc.
Plastics Engineering | Year: 2014

Weyerhaeuser thrive™ composites, a nearly lignin-free, cellulose-filled polypropylene (PP), was found to offer advantages in terms of molding cycle times, part weight, and more. As with many bio-filled plastics, this material needs to be run at lower temperatures to prevent browning of the organic fiber. the results of 0.12-inch thick spiral melt-flow testing, AstM D3123, showed that the material flows fairly well, as long as adequate pressure is applied. Mold-filling analysis is recommended for all new molds to verify fill pressures and to optimize gating. The molding tests showed that thrive composites require short packing/hold and cooling times. Parts designed for materials such as pure PP or HDPE can be downgauged with thrive composites due to higher part rigidity. And both small and large parts can be molded; to minimize cycle time. the results of this testing showed that thrive composites composed of cellulose fiber-reinforced PPs have a structural engineering-level stiffness equivalent to 20% glass-filled PP. Source


Rosen M.,Corex Design Group Inc.
Plastics Engineering | Year: 2013

Through lessons and examples, it has been shown how management needs to understand the complexity of plastics engineering, material selection, analysis, and prototyping. it's important to build a technical team of advisors consisting of internal team members, vendors, and unbiased outside experts. Management needs to appropriate the required time and funding for these critical stages of a project, since mistakes at this early stage can result in costly fixes and project delays. Source


Rosen M.,Corex Design Group Inc.
Plastics Engineering | Year: 2015

Understanding the important role of good venting is key for the molding of quality injection-molded parts and for maximizing profits. There is the compressed air itself being pushed forward by the melt stream. In addition, there are also potentially non-aqueous volatiles which are given off as the material is heated to high temperatures during molding. Today's plastics materials are increasingly complicated in their formulations, which can include additives such as lubricants, plasticizers, flame retardants, fillers, antioxidants, UV stabilizers, anti-microbial additives, and coupling agents, to just name a few. Tests have also shown that high-pressure gasses in the mold, due to poor venting, can result in flashing even at high clamp tonnages beyond those needed for the calculated melt pressure. The theory is that the high pressure gasses can leak into the parting line of the mold, resulting in much higher projected surface area of the high gas pressures. Source

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