浇口是浇注系统中物料流动通道的重要组成部分。除主浇道外，浇道大部分为浇注系统中截面更小的部位，其值一般仅为引水渠道截面的3% ~ 9%。针对小门成形过程中由于流动阻力增大而引起的压降，可采用增大注射压力的方法在一定范围内进行补偿。一般来说，采用小门注射成型，具有以下优点。
Gate is an important part of material flow channel in gating system. Except for the main sprue, most of the sprue is the part with the smallest section in the pouring system, and its value is generally only 3% - 9% of the section of the diversion channel. In view of the pressure drop caused by the increase of flow resistance, the method of increasing injection pressure can be used to compensate in a certain range. Generally speaking, small door injection molding has the following advantages.
1. There is a big pressure difference between the front and the back of the small door at both ends, which can effectively improve the shear rate of the melt, generate a large shear heat, resulting in the decrease of the apparent viscosity of the melt, and increase the fluidity, which is conducive to mold filling. The characteristics of the small gate have great advantages for thin-walled products or fine grain products, as well as for plastic molding products, such as polyethylene (PE), polypropylene (PP), polystyrene (PS) and other products with viscosity sensitive to shear rate.
2. Because of the small size and fast freezing speed of the gate, some products can be frozen. The products inside do not need treatment. As long as the external curing layer can demold products have enough strength and rigidity, so as to shorten the molding cycle and improve the production efficiency.
3. In the process of injection molding, the stage of maintaining pressure and filling generally lasts until the melt at the gate freezes, otherwise the melt in the cavity will return to the outside of the cavity. If the gate size is large and the compression and filling time is long, the orientation and flow deformation of macromolecules may be increased, resulting in a large filling stress of the product, especially near the gate, resulting in warpage and deformation of the product. If a small gate is used, the adjustable volume of the small gate passes the mold test or the mold repair, so the melting keeps the pressure during the time when the gate freezes, so as to correctly control the filling time and avoid the above phenomena.
4. If using a larger door shaped product, the product surface quality requirements are higher, usually need appropriate tools or machine tool products to remove scars after treatment, especially when the door is too large, must saw the door solidification, reduce and other methods to delete. However, this can be avoided by using small doors. For example, during disassembly, the small gate coagulant can be removed automatically by hand or special mold structure. In addition, the scar behind the small door is smaller, and generally it is not necessary or only needs to be slightly trimmed. Therefore, the use of small splash is not only conducive to the pouring system of the product, but also conducive to the finishing of the product.
5. If the small gate is used in the multi cavity unbalanced pouring system, the flow resistance of the gate to the plastic melt will be much greater than that of the multi-channel melt. Therefore, it is possible for the melt to fill the channel and establish sufficient pressure before each cavity is almost filled at the same time. Therefore, the small gate can balance the feed speed of each cavity in the multi-mode cavity, which is conducive to the balance of the gating system.
At the same time, it should be noted that although the small gate has the above advantages, too small gate will cause greater flow resistance, resulting in longer filling time. Therefore, for some high viscosity or shear rate plastic melts, the apparent viscosity has little effect (such as polycarboxylate, polysulfone, etc.), so it is not suitable to use small door modeling. In addition, when forming large-scale products, we should also pay attention to expanding the gate area, sometimes even need to expand the height of the gate section, in order to approach the maximum thickness of the product, in order to improve the fluidity of the melt. In addition to the above, due to the large wall thickness, the product shrinkage is also large, generally need to have enough time to make up, so in this case, the gate area can not be designed too small.
The above is the detailed introduction of Jinan mold to the small and medium gate of injection system. For more information, please click http://www.jnhjgs.cn