Every plastic part starts in a mold. Injection molds are classified into two main types, cold runner and hot runner. Each has its advantages and disadvantages. Your plastic injection molder will be able to give you the costs and benefits of using these different systems. However, by understanding the key differences of these technologies, you can have a more educated discussion on the type of mold that would best fit your project.
The cold runner mold
Cold runner molds usually consist of 2 or 3 plates that are held within the mold base. The plastic is injected into the mold via the sprue and fills the runners which lead to the parts in the cavity. In 2 plate molds, the runner system and parts are attached, and an ejection system is used to separate the pair from the mold. For those of you who assembled a model car at some point in your youth, the runners and the parts were not separated. The child assembling the model was responsible for that final part of the process. In 3 plate molds, the runner is contained on a separate plate, leaving the parts to be ejected alone. In both systems, the runner is re-cycled and reground, reducing plastic waste. However, these processes can increase cycle time.
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The hot runner mold
Hot runner molds consist of 2 plates that are heated with a manifold system. The manifold sends the melted plastic to nozzles which fill the part cavities. There are several types of hot runner systems, however, in general, they fall into two main categories; externally heated and internally heated. The externally heated systems are well suited to polymers that are sensitive to thermal variations. Internally heated systems offer better flow control. The hot runner process eliminates runners entirely, so recycling and regrind (which can only be done with virgin plastics) do not impact cycle times. A variation of this system is called an insulated runner. The insulation, rather than heat, keeps the plastic in a molten state. This system can only accommodate a few types of plastics, specifically semi-crystalline polymers which have a low thermal conductivity.
Advantages and disadvantages of each system
Cold runner systems
-Comparatively cheaper to produce and maintain
-Accommodate a wide variety of polymers, both commodity and engineered
-Color changes can be made quickly
-Fast cycle times if the systems include robotic assist in removing runners
-Cycle times are slower than hot runner systems
-Plastic waste from runners (particularly if they cannot be reground and recycled)
Hot runner systems
-Potential faster cycle times
-Eliminates runners and potential waste
-No need for robotics to remove runners
-Can accommodate larger parts
-More expensive molds to produce
-Color cannot by easily changes
-Higher maintenance costs and potential downtime
-May not be suited to certain thermally sensitive materials
Professionals in the field of plastic injection molding should be your primary resource for determining the best injection molding system for your project. Look for injection molders who are familiar with all types of plastics processing. They will be able to provide you with a cost/benefit analysis of the various systems available based on the part and the material used.