Plastic exists in nearly every portion of our everyday lives. Considering how much we have come to depend on plastic, it is surprising to consider just how recently this extremely useful material was developed. The first man-made commercial plastic was developed in 1861, by Alexander Parkes in Britain. This initial prototype was rather brittle and extremely flammable. Seven years later, an improved version was designed by John Wesley Hyatt; he also designed the first injection molding machine.
Plastic injection molding (or moulding, for the Brits) has come a long way since it first came into vogue during World War II. In today’s molds, there are two common types: cold runner molds and hot runner molds. While both designs are able to get the job done, they each have their share of limitations and capabilities.
Cold Runner Molds
These molds typically come in two different versions. The first type is a two-plate system: the molten plastic is injected and follows small tunnels called runners to the main cavity. When the piece has cooled, the entire structure is ejected – including the runners. Manufacturers will either leave the runners on, to connect smaller parts, or use robotic assistance to remove them.
The second type of cold runner mold is a three-plate system. A very similar process is followed, except that the runners will be contained in the extra plate upon ejection. Cold runners have a relatively low initial cost, and are economical to maintain. They are compatible with a wide variety of polymers (both commodity and engineered) and the runners can usually be re-ground and recycled.
Hot Runner Molds
This system is typically more expensive than cold runner molds, but has many unique advantages. Hot runner molds are essentially two plates heated with a manifold system. Hot runners eliminate leftover scraps by injecting the molten plastic directly into the cavity via small nozzles.
There are many variations of hot runner molds, but the two most popular are the external heat and internal heat models. Hot runners with external heat are ideal for certain polymers that are sensitive to fluctuations of temperature, while the internal heat versions have better flow control.
Cold vs. Hot
With so many different types of plastic injection molds on the market, it can be difficult to decide which one is appropriate for your project’s needs. There are a few pros and cons to keep in mind when attempting to make a decision:
- Hot runner molds typically have faster cycle times
- Color changes are easier to accomplish in cold runners
- Leftover plastic is not a concern with hot runner molds
- Cold runner molds may render certain plastics un-recyclable
- Maintenance on hot runners is typically longer and costs more
Whichever direction you decide to go, it is highly recommended to ask a professional for their expert opinion. Seek out a person who has experience with a wide variety of materials and completely understands your project’s unique needs. While both versions have their attractions and issues, ultimately you must choose the one that will best suit your needs.