Mold Parts
Injection molding is a common manufacturing method that allows for high volume production of plastic products. Its process produces very little unused or waste plastic which is good for the environment.
Gating and ejector pins are essential components for plastic resin to strategically enter and plastic parts to effectively be ejected from the mold. The design of these holes should be carefully considered to minimize stress and costs.
Runner System
A runner system is a series of channels that carry the molten plastic from the sprue into the mold cavities. The shape and size of the runner system can impact how quickly the molten plastic fills the mold cavities, and also the quality of the parts produced.
A well-designed runner system can improve part quality, reduce cycle time, and minimize material usage. However, if the runner system is not properly designed, it can lead to cosmetic defects such as flow marks or surface blemishes that affect part functionality and appearance.
A hot runner system is an internally heated system added mould parts to a plastic injection mold that eliminates the need for a cold sprue or cold runner, and injects molten plastic directly into the mold cavity. This allows for shorter cycle times and reduces energy costs by reducing the amount of melted plastic that needs to be cooled. It also offers more flexibility when it comes to gating and positioning of gates, including hot tip gating, valve gating, or edge gating, which helps with optimal filling, cooling, and simplifying part finishing.
Ejector Pins
Ejector pins are used on the B-side of a mold (mounted to the moving side of the injection molding machine) to help push plastic parts out when the mold opens. They also can be used on the A-side of a mold for reverse ejection. Ejector pins are usually made of stainless steel or aluminum and can be shaped to have flat ejection pads that push against the molded plastic. They should be strong enough to withstand the high injection pressure without bending.
The placement, type, and size of ejector pins depend on a variety of factors. The shape of the part is one, but factors like draft and texture of the sidewalls and depth of walls and ribs can increase the likelihood that areas of the part will cling to the mold. Resin choice can also influence pin placement or size, as some resins are stickier and require more force to release from the mold.
Increasing the diameter of the ejector pins helps reduce blemishes from resistance as the part is removed from the mold. A textured surface on the ejector pad can also make the difference between smooth or blemished ejection.
Inserts
Inserts are pre-fabricated metal components that are inserted into the molten plastic during the injection molding process. This allows designers to include features in their products that they could not achieve with a single plastic part. For example, threaded metal inserts can be used to ensure a robust fastening system in the finished product. This is especially important in applications that require heavy use and can be difficult to fasten with traditional plastics. Another advantage of using inserts is that it can reduce the weight of a final product, which is crucial for manufacturers trying to meet federal fuel efficiency standards.
When choosing the right insert, it is essential to consider its intended application and its location in the finished product. The insert should be able to withstand the extreme temperatures and pressures experienced during the injection molding process. In addition, it should be easy to load and unload in accordance with production demands. Depending on the complexity of the part mould parts and order volumes, automatic insert loading is often preferred over manual loading. This can be accomplished by either robotics or a machine operator.
Sprues
The sprue is the primary channel through which the injection mold transfers molten plastic to individual parts. Sprues are typically made of the same material as the molded part, such as polyethylene (PE), polypropylene (PP), or acrylonitrile butadiene styrene (ABS).
They also serve as a venting system and dissipate heat generated by the molten metal as it fills the mold cavity. Lastly, they help keep the mold packed to avoid surface blemishes known as sink marks during cooling.
Depending on the type of part being molded, MIM molders choose between a cold sprue or a hot sprue system. Cold sprues are straight channels that extend from the injection machine nozzle to the runner and then to each gate, while hot sprues are comparatively shallow channels milled into one or two mold halves.
The sprue’s size, location, and design can greatly impact the quality of the finished part and production efficiency of the injection molding process. The best sprue designs allow for balanced flow and provide optimal distribution of molten plastic throughout the injection mold, ensuring consistent filling and uniform cooling of each part.
Parting Lines
Parting lines are the boundaries between the two halves of a plastic mold. They can have a big impact on the appearance and functionality of the finished product. They are also a good place to check for any defects.
Typically, the parting line is designed in a plane that’s perpendicular to the direction of the injection molding machine’s mold opening. In some cases, it is stepped or curved to accommodate specific part shapes.
The location of the parting line can have a major effect on both the cost and quality of a plastic injection molded product. This is because the molding process will often leave vestiges of the mating surfaces of the two half molds on the resulting part.
A poor parting line design can result in uneven shrinkage, which will cause the parts to misalign and lead to a loss of function or durability. Fortunately, there are several ways to fix this problem. Among them, you can polish the plastic to smooth out the parting lines. You can also use a technique called “blueing” to check for proper alignment of the A and B sides of the moulds.