Which machine: single station or multi-station?
Thin wall parts with low volume requirements are best served by single station injection molding machines.
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Multi-station machines are a must for producing thick parts and meeting high volume requirements combined with the possibility of highly varied production. Multi-station means multiple molds which can produce different products at the same time.
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Single Station
Co-injection machines are derived from traditional high pressure, high tonnage injection molding machines. They are slightly reduced in tonnage, and an additional injection unit is added. This make-up explains why some of our colleagues have said that co-injection machines cost 56% to 73% more than conventional presses.
Multi Station
Co-injection, on the other hand, derives from structural foam with a completely different concept involving large platens, large injection units but a small clamp with low tonnage. Here, the make-up involves only a small increase in tonnage and the additional injection unit. The increment in the cost of the press is only in the 18-35% range.
Typical configurations:
A
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Single Station
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Multi Station
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| Injection capacity |
4 + 4 pounds
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4 + 4 pounds
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| In between tie bars |
32 inches
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32 inches
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| Clamping Force |
500 Tons
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200 Tons
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B
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Single Station
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Multi Station
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| Injection capacity |
25 + 25 pounds
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25 + 25 pounds
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| In between tie bars |
55 inches
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55 inches
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| Clamping Force |
1500 Tons
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650 Tons
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In conventional injection molding, high pressure is utilized to fill the mold cavity. In co-injection, the material is injected with low pressure, and the cavity is filled by foaming the core material which pushes the skin. Just as high pressure is not required to fill the mold cavity, the tonnage of the clamp need not be elevated. In conventional molding, molds are typically cooled to the point where sometimes material freezing can be an issue. Again, high pressure is required to make sure the material flows.
On rotary, multi-station machines, on the other hand, because cooling continues on the carousel after injection, cooling time is less critical. For this reason, molds are not normally chilled to the maximum, and the material flows smoothly into the cavity. Thus pressure and tonnage can be maintained low.
Multiple stations - 4, 5, 8, 10, etc. allow the processor to manufacture different parts or components of an item on a single machine with the same basic operating cost of a conventional molding machine. The production of these components on the same machine facilitates subsequent assembly of the final product.
This also applies to the simultaneous production of completely different items which facilitates just-in-time inventory management.
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