Published in： 2021-04-27 17:27:47, published via： cyanbat | clicks: 0 | Information Sources：
The runner and gate system for metal injection molding is responsible for guiding the molten metal from the nozzle of the molding machine into the cavity of the tool. The gate structure and its location will affect the cycle time, processing cost, delivery cycle, the location of the certification mark, and many common potential defects of injection molded parts.
The following are the basics of door design that every product developer should know. Obtaining this information will help you make informed decisions when negotiating with suppliers to manufacture injection molds.
1. Importance of door position
The position of the door directly affects the quality of the finished product, so please consider the following general rules:
· The gate should be far away from pins, iron cores and other internal obstacles. Otherwise, this may cause welding marks as molten metal flows around it and is modified on the other side
· The gate should be placed as close as possible to the thick-walled area to ensure complete packaging
· If the build volume of the entire part is not uniformly filled, incorrect gate position will cause the part to deform
· Gates is an area of high pressure. Try to find the locations on the finished product that are affected by the stress marks or the potential degradation of the metal in these areas
· Gates need to be located in an area where precipitation is easy, whether it is manual or automatic. Please note that some metals are susceptible to high shear forces, so they can only be cooled manually
· Thin-walled parts may require runners or additional gates to provide sufficient metal in the shortest cycle time
2. Door design will affect the quality of parts
All gates will press molten metal into the cavity under pressure, thereby accelerating and heating the metal. This has a variety of effects that may require changes to the door design strategy. The most common defects include:
If the gate is too small, the pressure drop across the gate will cause spray, that is, spray into the cavity instead of flowing smoothly. The jet causes wave-like distortions called "worms." This may require reducing the pressure, expanding the door or both.
The high jet velocity through the door generates heat through friction. Excessive heat at this time may cause the resin to decay by breaking molecular bonds.
However, reducing the injection speed in order to avoid degradation may lead to other defects, such as poor mechanical strength at weak weld lines. And slower cycle time means fewer parts per hour, increasing processing costs.
Therefore, a given volume of resin can be separated on multiple individual gates. This helps to dissipate pressure and avoid overheating, but it also creates multiple flow fronts that can cause welds and gas traps where these fronts meet in the cavity.
3. The difference between hot runner system and cold runner system
The hot gate/runner system uses an electric heating cylinder to transport the preheated metal into the mold cavity. The runner is embedded in the mold in the form of a runner plate and a plurality of hot runner droplets.
Typical hot runner system for injection molds
· Precise temperature control to obtain a better appearance of the finished product
Eliminate waste in multi-cavity tools
Reduce cycle time
More complicated and expensive
The cold runner system is cheaper, but the length of the runner represents the metal that is wasted in each cycle, which in turn increases the cost. Moving the witness mark to a more favorable position should be balanced with the increase in material consumption due to long-distance runners.
4. Advantages and disadvantages of the most common door types
There are gates of different sizes and shapes to consider, and each has a compromise. Here are the most common:
Edge doors are the most commonly used. They are processed into the mold at the parting line and fill the cavity from the side of the part.
Edge doors are usually used for flat, thin p.arts
· Low design and manufacturing costs
· If necessary, easy to enlarge without removing the mold from the machine
The position of the witness sign may not be ideal
When the metal flows from a single gate, it will produce obvious welding lines when it flows around obstacles in the mold
High injection pressure and speed at the small gate will cause metal degradation
Direct or sprue gate
Direct access to the tool-fixed or built-in gate on the A-side gate.
The gate is inserted into the fixed half of the injection mold.
The simplest type of gate
Able to inject a large amount of metal quickly
Suitable for round or cylindrical parts where concentricity is important
· Leave a large witness mark, and may leave a dent on the other side
Must manually cool down
The runner guides the metal to the edge of the cavity at the parting line, but the gate falls below the parting line and tunnels upward to fill the part from below. This design is only suitable for double-plate mold structure.
The submarine gate fills the cavity from below the parting line.
Move the witness mark to a more ideal position
Excess exhaust gas can be pushed out of the mold from below
Automatically degrade when the mold is opened
· Manufacturing is more complicated, expensive and time-consuming
Another type of edge gate, the fan maintains the same thickness, but expands the range to increase the volume of a larger area. Recommended for polycarbonate metal
The door helps eliminate injection pressure.
Applicable to increase the flow of thin-edged parts
Reduce the injection pressure of a given volume
A large label-like witness mark that must be processed later may be left