Mould Parts and Their Importance
Mould parts play a critical role in guiding liquid plastic into the mold for the final injection molding procedure. They are typically made from materials that can withstand high temperatures, pressures and repetitive movements. Tool steels like H-13 are common for their strength and wear resistance.
The injection molding process uses a reciprocating screw to melt the polymer granules. They are then evenly coated throughout the mould, creating a hollowed part with uniformly thick walls.
Runner System
The runner system is the conduit that brings molten plastic from the injection machine nozzle to the mold cavity. It’s important that the nozzle seat and sprue seats have the same radius or else material will leak at this point, resulting in inconsistent parts.
The main function of the runner is to allow a controlled flow of melt through a series of channels until it reaches mould parts the gate in the cavity. It’s essential to optimize the runner size in order to minimize cycle times and maintain a consistent fill pattern. This is where a runner balance analysis comes in handy.
A valve gate is a type of injection point that features a mechanical valve that closes when the injection machine injects plastic into the mold. This results in a smooth closure and eliminates the need for scrap plastic, making it ideal for highly aesthetic projects.
A hot runner system is similar to a cold runner except it uses internal heating to keep the sprue and runners hot. This allows for faster injection and better quality. This is also ideal for polymers that are sensitive to temperature changes. The sprue and runners are then cut along the mold’s parting line, cooled, solidified, and ejected with each injection cycle. They are then either discarded or recycled through regrind and used again in the process.
Gating & Ejector Pins
Ejector pins are an integral part of the injection molding process. They are a hollow, hard-surfaced pin that supports thin products and ejects them from the mold when the mold opens. They are often lubricated to reduce friction. They are typically positioned in a fixed sleeve made of a different material than the pin to protect it from galling during ejection.
Injection molding gate placement is a critical aspect of mold design. The location of a gate affects the quality of molded parts, because gates can leave blemishes on the surface of a product. Ideally, the gate should be located as close to the thick-walled areas of a part as possible to minimize internal stress and weld marks.
Injection molding gates are an essential component of any injection mold. However, there are many different gate types that can be used. When designing a new injection mold, there are several factors that need to be considered, including the size and geometry of the part, the runner system, and the gating and ejector systems. Injection molding designers also need to consider the possibility of undercuts, which are protruding elements that prevent a part from being ejected directly from the mold. When undercuts are present, a more aggressive gating strategy may be required to ensure successful ejection of the finished part.
Parting Lines
A parting line is the boundary that separates the two halves of an injection molded plastic component. It can affect both the aesthetics and the strength of a finished product. There are many factors that go into choosing the best location for a parting line. The injection molding process, the size and shape of a final product, and other considerations all play into it.
A good plastic parting line can make the difference between a beautiful finished product and one that is not as durable or functional. Ideally, the plastic parting line should be located in a spot that is easy to machine and that does not interfere with any other parts of the mold. However, this can be challenging when the design of a product calls for a parting line to run along a protruding feature like a rim or handle.
In these instances, machinists can use various CNC processes to create a surface that will be easier to machine and that will blend in with any other surfaces on the resulting plastic component. This is called a stepped parting line and it offers a solution that may not be possible with other types of parting lines.
Regardless of the type of plastic parting line used, it is important to work closely with your injection molders to ensure the best results. Industry veterans have a repertoire of strategies that they can draw upon to manage parting lines, including using simulation tools to predict potential issues before the mold is even fabricated.
Venting
Using an optimally designed venting system is the single best way to minimize flash. However, it injection moulding parts manufacturer is often overlooked and the resulting poor venting can lead to short shots (incomplete parts), burns on the plastic and carbonization of the injection mold steel.
Proper venting involves a balance of multiple factors, including the injection rate and volume, the material type and grade, shear inducers and processing conditions. A common mistake is to over-vent a mold by installing too many vents, which can result in excessive plastic on the part.
It is also important to understand the relationship between venting and clamp pressure. Some tests have shown that a high clamp pressure may not be required to prevent flashing, but only if the vents are properly located and functioning.
The location and size of the vents is key to good venting. Vents should be placed at every runner turn, at the ejector pins and on the end of the main runner to capture any cold slugs. They should not be placed in ribs or other areas that are difficult to keep clean. Static vents that are not easily accessible for cleaning (such as at the bottom of a deep rib on an insert) can become clogged and trap air, causing flash and short shots. Ideally, the vents should be dynamic or “moving” vents that are easy to clean in the press without disassembly.
