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How to design injection molding system

update:29-11-2019
abstract:

Injection mold is a tool for producing plastic products […]

Injection mold is a tool for producing plastic products; it is also a tool that gives plastic products a complete structure and precise dimensions. Mainly used in the industrial field, the injection system of the injection mold is also called the runner system. It is a group of feeding channels that direct the plastic melt from the nozzle of the injection machine to the cavity. It is usually composed of the main channel, the runner, the gate and the cold Material cavity composition. It is directly related to the molding quality and production efficiency of plastic products.
Mainstream Road
It is a passage in the mold that connects the injection nozzle of the injection molding machine to the runner or cavity. The top of the main flow channel is concave so as to engage with the nozzle. The diameter of the inlet of the main flow channel should be slightly larger than the diameter of the nozzle (O.8mm) to avoid overflow and prevent blockage of the two due to inaccurate connection. The diameter of the inlet depends on the size of the product, generally 4-8mm. The diameter of the main flow channel should be enlarged inward at an angle of 3 ° to 5 ° in order to facilitate the release of the flow path excess.
Gate
The gate is also called the feed port, which is the narrow opening between the runner and the cavity, and it is also the shortest and thinest part. Its role is to use the shrinking flow surface to accelerate the plastic. The high shear rate can make the plastic fluid (good due to the shear thinning characteristics of the plastic); the heating effect of viscous heating can also increase the material temperature and reduce the viscosity. effect. ?
After the molding is completed, the gate is first solidified and sealed, which has the function of preventing plastics from flowing back and preventing the cavity pressure from falling too fast, which will cause the molded product to shrink and sag. After molding, it is easy to cut off to separate the runner system and plastic parts.
Cold cavity
It is a cavity provided at the end of the main flow channel, which is used to capture the cold material generated between the two injections at the end of the nozzle, thereby preventing the clogging of the runner or the gate. If the cold material is once mixed into the cavity, internal stress will be easily generated in the manufactured product. The cold cavity has a diameter of about 8-10mm and a depth of 6mm. In order to facilitate demolding, the bottom is often borne by the demolding rod. The top of the demolding rod should be designed as a zigzag hook or a recessed groove, so that the mainstream can be pulled out smoothly during demolding.
Shunt
It is the channel connecting the main flow channel and each cavity in the multi-slot mold. In order to fill the cavity with the molten material at the same speed, the arrangement of the runners on the mold should be symmetrically and equally spaced. The shape and size of the cross section of the shunt have an impact on the flow of the plastic melt, the release of the product, and the ease of mold manufacturing. If the flow is equal, the resistance of the flow channel with a circular cross section is the smallest. However, the cylindrical flow channel is smaller than the surface, which is not good for the cooling of the shunts. Moreover, this shunt must be set on two mold halves, which is labor-intensive and easy to align. Therefore, a trapezoidal or semi-circular cross-section runner is often used, and is opened on a half mold with a release rod. The runner surface must be polished to reduce the flow resistance and provide a faster filling speed. The size of the runner depends on the type of plastic, the size and thickness of the product. For most thermoplastics, the cross-section width of the diverter channel does not exceed 8m, the largest can reach 10-12m, and the smallest 2-3m. Under the premise of meeting the requirements, the cross-sectional area should be reduced as much as possible, so as not to increase the shunts and extend the cooling time.
Basic principles of gating system design
1. Consideration of Cavity Layout
1) Try to use Balances Layout;
2) Mold cavity layout and gate opening strive to be symmetrical to prevent uneven load on the mold to produce eccentric load, and the problem of mold flashing;
3) The cavity arrangement is as compact as possible to reduce the mold size.
2. Flow guidance considerations
1) can smoothly guide the molten plastic to fill the cavity, no eddy current, and can smoothly exhaust;
2) Try to avoid the front of the plastic melt from impacting the core and the metal insert with a smaller diameter to prevent the core from shifting or deforming.
3. Consideration of heat loss and pressure drop
1) The smaller the heat loss and pressure drop, the better;
2) The process should be short;
3) The cross-sectional area of ​​the runner must be large enough;
4) Try to avoid bending of the flow channel and sudden change of flow direction (changing the direction with an arc angle);
5) The surface roughness should be low when the runner is processed;
6) Multi-point pouring can reduce the pressure drop and required injection pressure, but there will be problems with sutures.
4. Consideration of flow balance
1) During Multi-Cavity filling, the flow channel should be balanced, and as far as possible, plastic should be filled at the same time to ensure the quality consistency of the molded products in each cavity;
2) Naturally-Balanced Layout should be adopted for the runners as far as possible;
3) When the natural balance cannot be achieved, the artificial balance method is used to balance the runners.
5. Waste considerations
Under the premise that the filling can be smoothly performed without affecting the flow and pressure loss, the volume (length or cross-sectional area) of the flow channel is reduced to reduce the waste of the flow channel and the recovery cost.
6. Cold material considerations
Design appropriate cold slug wells and overflow grooves on the runner system to supplement the colder plastic wavefront in the initial stage of filling to prevent cold materials from directly entering the cavity and affect the filling quality.
7. Exhaust considerations
The plastic should be guided to fill the cavity smoothly, and the air in the cavity can escape smoothly, so as to avoid the problem of scorching and encapsulation.
8.Consideration of the quality of molded products
1) Avoid problems such as short shots, burrs, encapsulation, sutures, flow marks, jets, residual stress, warping deformation, mold core shift, etc .;
2) When the flow channel system has a long process or Multiple Gating, the problem of warpage and deformation of the finished product due to unbalanced flow, insufficient holding pressure or uneven shrinkage should be prevented;
3) The appearance of the product is good, and the trimming gate is easy to remove, and the gate mark does not damage the appearance and application of the plastic part.
9. Production efficiency considerations
Minimize the required post-processing, shorten the forming cycle and improve production efficiency.
10. Consideration of ejection points
It is necessary to consider an appropriate ejection position to avoid demoulding of the molded product.
11. Considerations for using plastic
For plastics with higher viscosity or shorter L / t, avoid the use of runners that are too long or too small.
A high-quality mold requires not only good processing equipment and skilled mold manufacturing workers, but also a very important factor is a good mold design, especially for complex molds, the quality of the mold design accounts for 80% of the mold quality %the above. An excellent mold design is: on the premise of meeting customer requirements, making processing costs low, processing difficult, and processing time short. To do this, we must not only fully digest the customer's requirements, but also have an understanding of the injection molding machine, mold structure, processing technology, and processing capabilities of the injection mold factory itself.

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