Injection mold is a tool for producing plastic products; It is also a tool that endows plastic products with complete structure and precise dimensions. Mainly used in the industrial field, the pouring system of injection molds, also known as the runner system, is a set of feeding channels that guide plastic melt from the injection machine nozzle to the mold cavity. It usually consists of a main channel, a diversion channel, a gate, and a cold material cavity. It is directly related to the molding quality and production efficiency of plastic products.
Mainstream Way
It is a passage in the mold that connects the injection nozzle of the injection molding machine to the diversion channel or cavity. The top of the main channel is concave to facilitate connection with the nozzle. The diameter of the main channel inlet should be slightly larger than the nozzle diameter (0.8mm) to avoid overflow and prevent blockage caused by inaccurate connection between the two. The imported diameter depends on the size of the product, generally ranging from 4-8mm. The diameter of the main channel should be enlarged inward at an angle of 3 ° to 5 ° to facilitate the removal of excess material from the channel.
Gate gate
The sprue, also known as the feed inlet, is a narrow passage between the runner and the mold cavity, and is also the shortest and thinnest part. Its function is to use a tight flow surface to accelerate the plastic, and high shear rates can improve the flowability of the plastic (due to its shear thinning properties); The heating effect of viscous heating also has the effect of increasing material temperature and reducing viscosity.
After the molding is completed, the gate first solidifies and seals, which prevents plastic from flowing back and prevents the mold cavity pressure from dropping too quickly, resulting in shrinkage and depression of the molded product. After forming, it is convenient to cut off to separate the flow channel system and plastic parts.
Cold material hole
It is a cavity located at the end of the main channel, used to capture the cold material generated between two injections at the nozzle end, thereby preventing blockage of the diversion channel or gate. If cold material is mixed into the mold cavity, internal stress is easily generated in the produced product. The diameter of the cold material hole is about 8-10mm and the depth is 6mm. In order to facilitate demolding, the bottom is often supported by a demolding rod. The top of the demolding rod should be designed in a zigzag hook shape or with a sunken groove, so that the main channel debris can be smoothly pulled out during demolding.
Diversion channel
It is a channel connecting the main channel and various cavities in a multi slot mold. In order to fill each cavity with molten material at an equal speed, the arrangement of distribution channels on the mold should be symmetrical and equidistant. The shape and size of the diversion channel section have an impact on the flow of plastic melt, product demolding, and the difficulty of mold manufacturing. If the flow rate is equal, the resistance of the circular cross-section channel is the smallest. However, due to the small surface area of the cylindrical flow channel, it is not conducive to cooling the debris in the diversion channel, and this diversion channel must be opened on the two halves of the mold, which is both labor-intensive and easy to align. Therefore, trapezoidal or semi-circular cross-section diversion channels are often used and set up on half of the mold with demolding rods. The surface of the flow channel must be polished to reduce flow resistance and provide faster filling speed. The size of the flow channel depends on the type of plastic, the size and thickness of the product. For most thermoplastic materials, the cross-sectional width of the diversion channel does not exceed 8m, with larger ones reaching 10-12m and smaller ones 2-3m. On the premise of meeting the needs, the cross-sectional area should be minimized as much as possible to avoid increasing the amount of debris in the diversion channel and prolonging the cooling time.
Basic principles of pouring system design
1. Consideration of cavity layout
1) Try to use a Balance Layout as much as possible;
2) The layout of the mold cavity and the opening of the sprue should be symmetrical to prevent uneven force on the mold, which may cause uneven loading and lead to mold overflow;
3) The layout of the mold cavity should be as compact as possible to reduce the size of the mold.
2. Consideration of flow guidance
1) Can smoothly guide molten plastic to fill the mold cavity without generating eddy currents, and can smoothly exhaust;
2) Try to avoid direct impact of plastic melt adhesive on cores and metal inserts with smaller diameters to prevent core shift or deformation.
3. Consideration of heat dissipation 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 flow channel should be large enough;
4) Try to avoid channel bends and sudden changes in flow direction (changing direction with a circular arc angle);
5) The surface roughness should be low during channel processing;
6) Multi point pouring can reduce pressure drop and required injection pressure, but there may be suture issues.
4. Consideration of Flow Balance
1) When filling the first mock examination with multiple cavities, the flow channel should be balanced to make the plastic fill each mold cavity at the same time as far as possible, so as to ensure the quality consistency of products formed in each mold cavity;
2) The diversion channel should adopt a naturally balanced layout as much as possible;
3) When natural balance is not possible, artificial balance method is used to balance the flow channel.
5. Consideration of waste materials
On the premise of smooth filling without affecting flow and pressure loss, reduce the volume (length or cross-sectional area) of the channel to reduce the generation of channel waste and recycling costs.
6. Consideration of cold materials
Design appropriate Cold Slug Wells and overflow chutes on the flow channel system to collect and fill the initially cooler plastic wavefront, preventing cold materials from directly entering the mold cavity and affecting the filling quality.
7. Consideration of exhaust
The plastic should be smoothly guided to fill the mold cavity, and the air inside the cavity should be allowed to escape smoothly to avoid the problem of encapsulation burning.
8. Consideration of formed product quality
1) Avoid issues such as short shots, burrs, encapsulation, stitching, flow marks, jetting, residual stress, warping deformation, and mold core displacement;
2) When the flow channel system has a long process or multiple gating, the problem of finished product warping and deformation caused by flow imbalance, insufficient pressure holding, or uneven shrinkage should be prevented;
3) The product has a good appearance and is easy to remove and repair the gate. The gate mark does not damage the appearance and application of the plastic part.
9. Consideration of production efficiency
Minimize the required post-processing as much as possible to shorten the forming cycle and improve production efficiency.
10. Considerations for Top Out Points
It is necessary to consider the appropriate ejection position to avoid deformation of the formed product during demolding.
11. Considerations for using plastic
For plastics with high viscosity or short L/t ratio, avoid using channels that are too long or too small in size.
A high-quality mold not only requires good processing equipment and skilled mold manufacturing workers, but also a very important factor is good mold design, especially for complex molds, the quality of mold design accounts for more than 80% of the mold quality. An excellent mold design is to achieve low processing costs, low processing difficulty, and short processing time while meeting customer requirements. To achieve this, it is not only necessary to fully digest the customer's requirements, but also to have an understanding of the injection molding machine, mold structure, processing technology, and the processing capabilities of the injection mold factory itself.