Many facets affect a company's ability to achieve high injection molding production numbers while maintaining low scrap and minimal unplanned down time. One of the largest factors is the machine itself. There is much debate over which OEM produces the best press, but at the end of the day it is the planning and research conducted prior to purchasing a machine that will make the biggest difference. This article will delve into what needs to be taken into consideration before purchasing a press.
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New or used? Many larger companies will purchase only new equipment and replace it every five to seven years with another new press. The strategy is to avoid unplanned maintenance at all costs. This system can be quite effective if the company is producing premium parts that generate high margins. For a smaller molding operation, however, the cost of new machinery may put this approach out of reach. If used machines are the best option, it is highly recommended that presses be no more than 10 years old. Also, the buyer should make every effort to get a sense of how the seller dealt with preventive maintenance.
Press controller. The controller is a huge consideration when choosing a press. If your processing requirements are complex, the controller will need to have adequate functionality to accomplish the processing capabilities needed for minimal scrap and high productivity. Keba and Mosaic controllers are among my favorites, but there are several other controllers that can rival their dependability and technical applications.
Hydraulic, electric, or hybrid? Which of these three systems is the most reliable is debatable, but the fact is that each one has its pros and cons. Hydraulic presses continue to be a solid contender, but they can be messy and require greater maintenance. Electric presses are cleaner and need less maintenance but they can sometimes struggle with higher pressure process applications. Hybrids are just a combination of hydraulic and electric systems and, thus, combine the pros of both systems. It is recommended that a company first determine the type of molding operation it intends to use. High-tolerance molding is best served by electric presses, which hold more exact tolerances and are best suited for medical, aerospace, and high-tech automotive applications. Hydraulic presses, on the other hand, are more cost efficient.
Screw and barrel. When planning a machine purchase, the importance of screw and barrel evaluations can't be overstated. Understanding what molds you will be running in a press will define the screw throughput requirements. Materials that spend too much time in a barrel are prone to degradation and splay. Shot sizes that are too large for barrel throughput will inhibit your ability to run some molds in a press. Call on the expertise of your materials and screw suppliers to ensure that screw and barrel functionality will meet your needs.
Clamping system. Clamp requirements are a crucial decision when buying a press. What are your mold thickness requirements? What is the size of your largest mold, and will you be able to mount it to the platen? What is your maximum tonnage requirement? These measurements and setpoints are important to the press design needed to produce parts in your facility.
Auxiliary functions: Any extra bells and whistles you want on the machine should be the final consideration. How many cores do you expect to need? Will the hot runner be built into the main controller? Will mold temperature be a main controller function? Will you be using valve gates, and will they be part of the main controller function?
Every plastic injection press in your plant must adequately achieve productivity and final yields after scrap to make your operation successful. Take time to determine all press requirements prior to making the final decision to purchase. This ensures an end result of high profitability, which is key to any company's success.
Got a problem with this, that, or the other thing? You might find answers in some previous "Troubleshooter" columns:
How to Identify and Correct Shear-Related Splay Defects
The Role of Moisture in Injection Molding Splay Defects
Preventing Flash in Injection Molded Parts
How to Prevent Common Failure Modes in Injection Molding
Building a Validated Plastic Injection Molding Process
Fixing Color Defects in Injection Molded Parts
Key Steps for a Stable Injection Molding Process
Fundamentals of an Injection Molding Plant or Cell Startup
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About the author
Garrett MacKenzie is the owner/editor of plastic411.com and a consultant/trainer in plastic injection molding. He has provided process-engineering expertise to many top companies, including Glock, Honda, Johnson Controls, and Rubbermaid. MacKenzie also owns Plastic411 Services, which provides maintenance and training support to Yanfeng Automotive Interior Systems, IAC, Flex-N-Gate, and other top automotive suppliers. He was inducted into the Plastic Pioneers Association (PPA) in , where he serves on the Education Committee evaluating applications from college students seeking PPA scholarships. You can reach him via at [ protected].
#2 is all about drying your PEEK polymer. PEEK granules are supplied nominally dry but may absorb moisture from the atmosphere. In our datasheets and injection moulding guide, we provide some guidance on drying. In short, we suggest drying the pellets on trays in circulating ovens for 2'3 hours at 150-160'C or, alternatively, overnight drying ' please refer to our guidance for further information. The aim is/should be to reduce the residual moisture content to <0.02%. Dehumidifying or desiccant dryers may also be used and must be capable of maintaining a dew point or saturation temperature of -40'C.
#3 is a big requirement ' keep it clean! Any contamination should be avoided when processing PEEK. As you will appreciate, most other polymers will degrade at PEEK's processing temperatures and therefore it is imperative to start with clean drying ovens, hoppers, injection moulders and any other containers used to hold runners or parts. Pull screws and properly clean and brush the barrel and screw before starting to mould PEEK. Check for hang ups in hoppers.
Regrind can be a big source of contamination and we suggest that you use dedicated equipment for the regrinding of PEEK materials. Any contaminants during the injection moulding process will show up as black specs in your final component.
Moving along the injection moulding process we come to #4 ' PEEK likes big gates. Gate size is dependent on the part volume, number of cavities and component geometry. PEEK is a semi-crystalline polymer and exhibits much higher shrinkage compared to an amorphous thermoplastic. Moreover, cooling from a much higher processing temperature to ambient causes more shrinkage compared to lower temperature processing polymers. Using the biggest gates possible will enable that you fully pack out your component. At Victrex, we have a general rule called the 2/3rds rule of the maximum section thickness of your component. Depending on the grade of PEEK that you'll be moulding this might be a minimum of 1mm for unfilled PEEK and 2mm for compounds. Gate designs such as tab, side or fan gates are most commonly used. Submarine or tunnel gates can be considered for very thin walls or small parts but tend to lead to freeze off during moulding of bigger parts.
#5 is critical ' good temperature control on your mould. As discussed in a previous blog post by John Grasmeder, Chief Scientist at Victrex, on Polymer Crystallinity, the mould temperature is critical to obtain a semi-crystalline component that can fully utilise PEEK's unique set of properties. Let's look at some components that are not moulded correctly.
The extremes shown in a simple tensile bar.
Moulded component showing crystalline and amorphous areas.
The brown sections are the amorphous PEEK and the beige coloured sections are semi-crystalline PEEK. To mould PEEK and its compounds successfully one needs to be able to maintain mould temperatures between 170-200'C (to avoid quench-cooling into the amorphous state), depending on the grade being moulded. It is important to note that this should not only be the set temperature on the mould, but indeed the surface temperature on the mould. This can be achieved using electrical heaters for smaller parts and oil heaters with isolated flow lines for larger shot sizes or tools with large/deep cores. Consider using insulation boards between the mould and the machine platen as well as packing the tool with thermal insulation for uniform temperature distribution across the mould.
In summary, there are many other factors that play a role in successful PEEK moulding, but the five outlined above, are a good introduction to what is important when starting to mould PEEK.
Victrex´s technical service teams across the globe are available to support customers when injection moulding questions come up. You can also download our Injection Moulding guide and datasheets from our Victrex.com site or contact us through the various channels that are available on our website.
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