A mold is constructed using a series of components including various plates, pins, bushings, pillars, ejector systems, and many other items used for many purposes. Figure 1 shows some of the basic items and where they are located in the mold.
Figure 1 shows the mold in the closed position. When the mold is open it separates between the A and B plates. This plane is called the parting line because it designates where the mold parts or separates. Figure 1-5 shows the mold in this separated stage.
We refer to each half of the separated mold by whether it contains the A plate or the B plate. Therefore, the half of the mold containing the A plate is called the A half, and the half containing the B plate is called the B half. We also use the term live half for the B half because it usually contains the moving section known as the ejector system. Normally, the A half does not contain any moving sections, so it is referred to as the dead half.
The Injection Half of the Mold
The injection half of the mold involves the sprue bushing, A and B plates, runners and gates, and cavity .
The Sprue Bushing
In most cases, the A half contains the sprue bushing. The term sprue was borrowed from the metal die-casting industry. The sprue bushing in an injection mold is the component that allows molten plastic to enter the mold and begin its travel to the cavity image.
The molten plastic leaves the injection press and enters the mold through the sprue bushing. The sprue bushing directs the material through the runner system, past the gate, and into the cavity. The sprue bushing is the interface between the injection mold and the injection machine. It is made from hardened tool steel, highly polished to minimize sticking, and is a replaceable component of the mold.
Typically, the critical dimensions of a sprue bushing are determined by the viscosity of the molding material, the volume of material traveling through the runner system, and the thickness of the mold plates of the A half of the mold. In addition, the diameter of the orifice of the nozzle is a factor.
The A and B Plates
The mold components that form the molded part are the A and B plates. To take advantage of the ejector system (usually located in the B half of the mold) we mold the plastic product in the B plate whenever possible. However, there are some situations in which the plastic product design requires some of the cavity image to be placed in the A plate, such as in a three-plate mold . Figure 2 shows the A and B plates from a standard two-plate mold used to make triangular plastic safety signs. The mold is a two-cavity mold, meaning two pieces are molded at the same time.
Runners and Gates
After the molten plastic leaves the injection molding machine and enters the mold through the sprue bushing, it must be directed to the cavity image and allowed to enter the cavity image to form the finished part. This is accomplished by using a runner system and gates,. The runner system is a pathway machined into the face of the mold base. Usually this is accomplished by use of a ball end mill, but it also can be created using electrical discharge machining (EDM).
The gate is located at the end of the runner system and is considered an opening in the mold, like a window. It is designed and constructed to allow molten plastic to enter the cavity at proper velocity and volume needed to fill the cavity quickly, but in a controlled condition. The thickness of the gate has a major function in determining the overall cycle of the injection molding process and must be kept as thin as possible while allowing proper flow.
While it is theoretically possible to fill any size or shape cavity image with only a single gate, sometimes it is necessary to utilize the flow profile and characteristics of multiple gate systems.
The shape of the final molded product is determined by the shape of the cavity image that is machined into the A and B plates (as well as the characteristics of the specific material being molded). This image is machined into the plates using common machining methods such as milling, grinding, lathe-working (turning), and stoning/polishing. But, the most common method of cavity image machining today is EDM (also known as EDNC or electrical discharge numerical control). This method utilizes electrical current to erode metal from the cavity plates. It can be controlled to the point of requiring no further machining if the cavity image does not need a high-polish finish.
The Ejector Half of the Mold
The ejector half of the mold involves the ejector housing, support plate, support pillars, ejector pins, return pins, sprue pullers, and ejector system guide pins and bushings.
The Ejector Housing
The ejector housing is a U-shaped metal box, formed as a forging in a solid, one- piece unit. This is done to attain maximum strength and minimum deflection during molding. To cut corners and save money, some moldmakers attempt to fabricate the housing from bolted plates. This is an unsafe practice because the side rails of the box will distort under injection and clamp pressure and the box may crack. In addition, the distortion causes excessive wear on the ejector pins and the holes in which the pins ride. It is questionable whether or not any money is actually saved by this fabrication process, certainly when the potential danger and damage is considered.
The ejector housing (commonly called the ejector box) is used to contain the ejector system. This system consists of ejector pins, return pins, sprue-puller pins, and possibly many springs, bushings, and guide pins. Guided ejector systems utilize guide on the ejector system components. Although this practice adds slightly to the overall cost of the mold, it does minimize repair costs that occur due to ejector system wear.
Because the ejector housing is U-shaped, there is a great amount of unsupported area directly under the B plate of the mold. This open area allows the B plate to distort by bowing or bending under injection pressure. The amount of distortion is about 0.010 in. (0.254 mm) or more. This is enough to cause a lot of flash on the parting line and will quickly result in a weakened B plate. To keep this from happening, a support plate is mounted directly behind the B plate. The use of two plates (the B plate and the support plate) creates a much greater resistance to distortion than increasing the thickness of the B plate.
In addition to the support plate, support pillars are also used to help reduce (or eliminate) the B plate distortion columns that are placed as props between the ejector housing and the support plate of the mold.
Ejector Pins, Return Pins, and Sprue Pullers
Ejector pins are steel “nails” with a head, body, and flat face. These pins are used to push the finished, molded product out of the mold at the end of the injection molding cycle. Ejector pins are also used to eject the surface-style runner system. These pins are called ejector pins only if they contact the plastic surface of the molded product or runner.
Where there is an ejector pin, there is going to be an impression showing on the molded part. This impression will take the form of either a depression in the plastic or a raised pad of excess material.
Return pins are used for pushing the ejector system back into its proper place when the mold closes in preparation for the next molding cycle. These pins differ from ejector pins in that they do not contact the surface of the molded part, but rather contact the steel surface of the A plate as the mold closes. This contact forces the return pins to push back the ejector system, thus returning it to the proper location.
Sprue puller pins are more elaborate ejector pins. Their primary purpose is to assist in releasing the sprue from the sprue bushing when the mold first opens after the injection process is complete. The sprue puller pin usually incorporates an undercut of some type on its end. This undercut can take the shape of a groove machined into the pin diameter, a Z-shaped notch cut into the diameter, or even a ring groove cut into the hole in which the pin rides. In all cases, the undercut traps material from the sprue. When material in the undercut hardens, it causes the sprue to be pulled from the sprue bushing as the mold opens up. After the mold opens all the way, the ejector system activates and the sprue puller acts as an ejector pin to push the sprue undercut off the B half of the mold.
Ejector System Guide Pins and Bushings
Because the mold is usually operating in a horizontal plane, gravity tends to pull the ejector system downward. This action will cause undue wear on the diameters of all the pins as well as the holes in which they ride. This will cause an elliptical hole to form in the mold and flash will fill it. One way to minimize the effects of gravity is to use guided ejector systems. These systems utilize guide pins and bushings that are designed to overcome the gravity effects and keep the ejector system operating on a true horizontal plane at all times.