EQUIPMENT SELECTION FOR MULTISHOT
For the first-time multishot molder, equipment selection is often a daunting task. Should the molder invest in a new or used two-shot press? Would it be best to use an existing press with open capacity and convert it to a two-shot machine? If this is the molder’s first multi-component project, the projected production hours and product life cycle should be the foundation of the decision. If a project will command more than 5000 press hours in a single year and anticipated product life is three years or more, a dedicated two-shot press should be considered.
A dedicated two-shot press, where all second-shot equipment is permanently integrated, does have a few potential downsides. The lead times for two-shot molding machines are much longer than for standard presses—usually much longer than mold-build lead times.
If the part life turns out to not meet projections and volumes dry up, you have a white elephant on your floor waiting for a project using a similar mold size and matching first- and second-shot volumes. If any part of the machine goes down, you have no options. One significant failure—for example, in either the injection unit or rotary platen—and you are at the mercy of the press OEM’s service team and the availability of replacement parts.
If portable injection units are used on a standard press, you can temporarily replace a faulty secondary unit or, if the primary unit malfunctions, you can move the secondary unit and platen to another press in your facility.
In our theoretical case study, we are building a single-cavity two-shot mold, which means that production volumes are relatively low. In this scenario, most molders are likely to purchase the appropriate ancillary equipment to convert an existing standard injection machine into a two-shot press. That means a rotary platen if it is a common-core mold. The thickness of the turntable will eat up daylight in the press: Is there enough real estate in the machine for both the rotary platen and the open mold? Do you have to get an end-of-arm robotic fixture in there to get the part out?
Obviously, a standard machine will also require a secondary injection unit. A small (hand-held size) part with a second-shot trim feature may only need a few grams of resin, so a mold-mounted portable injection unit will do. A large automotive or appliance part may require over a pound of second-shot resin calling for either a platen-mounted injection unit or a floor-mounted, horizontally positioned injection system.
Now the molder must consider the real-estate availability on the molding floor. A large vertically mounted unit can extend over 5 ft above the press, while a horizontal unit and its support frame can be the equivalent of attaching an 80-ton press to the base machine.
Regardless of the application, each system is portable and can be moved to other machines. The servo or hydraulic injection unit requires its own power supply, operator interface, and controller and is integrated into the base machine via standard SPI robotic or Euromap 67 interface. The secondary system may require an independent support system for cores and valve gates, along with a connection for a rotary-platen interface and other required operations (like a vision system, for example).
There are now quite a few sources available for secondary injection units, and a thorough understanding of your project and resource capabilities by the equipment provider is required. Which materials are you planning to use? What is your projected cycle time? This will help determine the capacity of the secondary barrel. If you are planning short cycle times, you’ll want an appropriate amount of plasticated second-shot material available. If you are molding large parts with long cycle times, you will need to avoid excess resin cooking in the barrel.
Which portable injection unit technology is best—servo or hydraulic? Electric systems are touted for their energy efficiencies and precision. Hydraulic units are smaller and lighter and are generally easier for a molder’s in-house maintenance team to maintain and repair. The answer depends on your intended immediate and long-term use and the molding machinery your personnel is familiar with.
The big question for molders is: Are you losing opportunities because you do not have multishot capabilities in-house? The entry price for new equipment and the learning curve associated with any new manufacturing technology may have caused you to hold back.
Consider, though, that this really isn’t new anymore. The technique is absolutely on solid ground and has been embraced by many leaders in our industry. Multi-component molding continues to evolve and many brand owners are designing new products that take advantage of it. Once you have a program or two under your belt, it is likely more opportunities will come your way.
The article comes from China injection mold manufacturer - Mold Best Assurance Company Limited, website is www.mbamoldanddesign.com
