Many people, including many of those who are new in their manufacturing careers or engineering or purchasing functions who deal with plastic part design and new product development, do not understand what a ‘mold’ is. What is a mold?
The first thing to understand is that an injection mold is a complex piece of machinery. The part that is produced by the mold is relatively low-cost compared to the cost of the mold that produces the part, which is something people don’t often understand. “How come the parts are $1.00 each and the mold that produces the parts is $100,000?”
This complex piece of equipment that makes your plastic components is typically made from steel or various alloys, or aluminum, depending on the material used to produce the plastic parts. Molds consist of an “A” half (the stationary side of the mold) and a “B” half (the moving side). The cavity side is the “A” half of the mold, and generally produces the outside of the part being molded; the shape is a void or an impression – as its name indicates. The “core” is the “B” half of the mold and makes the inside shape of the part. Typically these are made of heat treated materials.
The core/cavity or “inserts” are placed into a mold base which is clamped (the “A” half) or bolted into the molding machine. The “B” half is attached to the moveable side of the mold. Other components of the mold include the ejector pins which provide a means to push the part from the mold. In addition to the ejector pins, there are numerous other moveable components in a mold to ensure the part is molded to exact specifications including mechanical or hydraulic side actions and lifters in some molds to aid in to creating various features of the part such as undercuts, sprue puller, locating ring (which locates the mold in the molding machine), and the sprue – the primary channel through which the molten plastics flows into the mold and other feed channels in the mold, called runners.
There are several different types of mold including a “stripper plate mold” which aids in removing molded parts from the mold when other types of ejection can’t be used. A “three plate mold is typically used when edge gating would cause a problem with filling the part. Unscrewing molds contain “unscrewing mechanisms” when parts require threads such as bottle caps, various types of polymer bolts and screws.
There are “hot runner” or “runnerless” molds in which the parts are directly gated from the sprue and have no runner system in a multi-cavity mold. This differs from a traditional mold “cold runner” mold that has a runner system in which the molten plastic flows through the runner system from a single sprue and into the various parts. The advantage of a hot runner mold is that you do not have a runner to deal with, which either becomes scrap and must be reground and either used back into the virgin material (if that is allowable), or sold to a reprocessor, which will reprocess the material and sell it as “regrind.”
Using a hot runner mold (which must have a hot runner manifold is made specifically for the application which is typically expensive – sometimes as much as half of the cost of the mold) can be more cost effective in production. With material pricing of some of the higher end engineering thermoplastic materials, a hot runner mold is often chosen to reduce the part cost. Choosing which type of mold and gating is best for your application can be a decision that you can make with your injection molder/mold manufacturer after an evaluation of the requirements.