plastic melt enters the mold cavity from the nozzle of the injection molding machine through the main channel, the split channel, the cold material well, and the gate .
01: flow channel
flow channel is an important channel from the main channel to the gate, and is the flow channel of the molten plastic emitted from the nozzle of the injection molding machine. The runner should be designed to be low in resistance and prevent cooling. Typically, the runner is designed to be trapezoidal or circular.
Common flow channel shape
For multi-cavity molds, in order to obtain good dimensional accuracy, the design of the runner is very important. The typical flow channel design of a multi-cavity mold in the figure below.
multi-cavity mold flow channel
02: gate
gate system design, such as position, number, geometry and size are very important for production efficiency and dimensional accuracy. The role of gate is summarized as follows:
1. The volume and direction of the plastic melt flowing into the mold cavity
2. Before solidification, seal the melt in the mold cavity and prevent the melt from flowing back to the runner
3. It is generated due to heat caused by viscosity dissipation. It is easy to cut the runner and simplify the post-processing of the product
Category:
Non-restrictive gate is called a straight gate, as shown in the figure below. This gate-type mold is simple in design, easy to operate, easy to form and reduce shrinkage. However, this gate forming cycle becomes longer and molding defects such as cracks, warping and residual stress are prone to occur.
straight gate
Because of the cross-sectional area of this gate, the restrictive gate is designed to cure quickly. The advantages of this form of gate are as follows:
1. Due to the reduction of residual stress and deformation around the gate, the cracks, warping and deformation of the product are reduced;
2. Due to the reduction of injection pressure in the mold cavity, a larger projection area of the product is allowed;
3. Due to the shortening of gate closure time, the molding cycle is shortened;
4. Due to the elimination of post-processing, the product quality is improved.
The following six restriction gates are introduced
① Side gates
Side gates
Side gate thickness is usually 30% to 40% of the product wall thickness. At the same time, its width is about 3 times the thickness of the product wall, and the side gate can be applied to almost all plastics. The stacked gate and spoke gate are deformation designs of the side gate.
② The fan gate
Fan gate
The cross-section of the fan gate is wide and flat, effectively eliminating the defects of the gate and is often used in flat products.
③ Film gate
Film gate
The figure above is a typical film gate design, with its gate width consistent with the width of the product, but much smaller in thickness. Like a fan gate, it can effectively eliminate residual stress and deformation of the product.
④ Disc gate
Disc gate
A thin disc gate surrounds a disk or annular product to avoid the generation of weld marks. The annular gate is a deformation of the disc gate.
⑤ Needle gate is also called: point gate
Needle gate
Needle gate
Needle gate is usually located in the middle of the product and is often used in multi-point gates. Since the diameter of the gate is usually 0.8--1.2mm, a small cross-sectional area causes high flow resistance. It is recommended to use low viscosity plastics or high injection pressure to avoid underinflation. The characteristics of
pin gate are as follows:
1. The selection of gate position is relatively unrestrictive
2. The residual stress around the gate is low
3. For multi-mold cavity molds, it is easy to achieve gate balance
4. For products with large projection area, multi-point pin gate effectively eliminates warpage of the product
5. Needle gates are easy to cut, and for three-plate molds, automatic gate cutting and separation of the product is easy to achieve.
⑥ Late gate
latent gate
latent gate is shown in the figure above.Usually, the gate is located on the mold splitting surface of the mold, but only the runner is located on the mold splitting surface. The gate is usually located on the moving or static plate of the mold, and sometimes also on the mold cavity. Although it is not much different from the needle gate, one of the benefits of the latent gate is that it can even be applied to the two-plate mold. When the molded product is ejected, the gate automatically falls off.
03: gate balance
For multi-cavity molds, each mold cavity is evenly filled with plastic melt, and it is very important to obtain gate balance. As the plastic melt flows from the population to the end of the mold cavity, the polymer pressure drops accordingly. Therefore, gate balance should optimize the length, width and depth of gate.
Suitable gate balance and runner balance design can avoid forming defects during the actual molding of multi-cavity molds, such as flow marks, shrinkage, under-inflation, dimensional fluctuations and weight changes.
04: Notes
The basic factors to consider for gate positioning are: parts design, flow and product final use requirements.
should remember the following points :
1. Large parts that require multiple gates should have gates that are tight enough to reduce pressure loss. This allows the cooling of the resin flow leading edge junction to minimize the cooling, thereby providing better fusion line strength. The appropriate gate size should be selected so that the resin filling has a reasonable pressure and speed.
2. The length of the gate transition section should be kept as short as possible.
3. The impact gate will help ensure that the inflowing fluid is directly facing the cavity wall or core, thereby avoiding swirls.
4. To avoid air clamping, air should be directed to the exhaust tank from the gate resin flow.
5. The gate position should be determined so that the resin flows from the thick wall to the thin wall; minimize the fusion line; and stay away from the impact and stress areas.
6. To minimize vortex, radiation spots and gate whiteness, the gate should have a suitable angle to the runner.
7. Pouring directly onto the decorative surface will cause surface defects.
