Plastic Expansion Screw Injection Mould

Design for plastic precision injection mould

A mould’s quality is the basis for qualified plastic products, and mould design is the foundation for high-quality mould manufacturing. Here are 5 things we need to pay attention to precision mould design.

1. Check the part drawing and confirm the mould opening direction and parting line position. Each plastic product needs to determine its mold opening direction and parting line at the beginning of mould design to minimize sliders or lifters to do their best to avoid the impact of the cosmetic surfaces caused by parting lines. After determining the mold opening direction, try your best to make the product ribs, clips, protrusions, and other related structures can be consistent with the mold opening direction. In this case, it can help to avoid core pulling, reduce joint lines, and the expanding of molding time. In the meantime, an appropriate parting line can be selected to avoid possible undercut in the mold opening direction, this can improve the part’s appearance and mould’s performance.

2. When checking the parts drawing, we make DFM to customers and give suggestions of draft angle in the part. Adjusting the draft angle properly will help to avoid potential problems like drag mark, deformation, and crack. When making mould design with the structure of a deep cavity insert, the draft angle of the outer surface should be bigger than the draft angle of the inner surface to avoid sticking on the cavity (keeping the parts on the core side), and assure the uniform product wall thickness, assure the material strength and opening time.  

3. Plastic parts' wall thickness is one of the critical factors for plastic tooling. Normally, when the wall thickness is more than 4mm, it will cause problems of big shrinkage, deformation, and welding lines in parts and need a very long cooling time in the injection moulding process. In this case, we need to think about changing the plastic part structure. Sometimes, we can add ribs to enhance the part’s strength and reduce the possibility of deformation.

4. Mould cooling system is a very fatal element we need to consider when mould design. Cooling will have a big impact of moulding cycle time and parts’ deformation risk. A good design of the cooling channel can help to shorten moulding cycle time, postpone mould life and reduce the risk of moulded part’s deformation.

5. The gate position is also very important. It affect the part’s cosmetic surface, deformation risk, injection pressure, moulding cycle time, and if customer want runner can be cut directly after moulding to save workforce cost, how chose the gate is a must consideration.

Working principle of injection molding machine

With the continuous improvement and expansion of the information technology foundation, mechanical products in various industries are gradually realizing the operation mode of intelligent production. Especially in the machinery manufacturing industry, the improvement of the performance of the product itself and the perfection of the working principle will make the product itself more market profit share in the market. In this regard, we will talk about the working principle of the injection molding machine.

The working principle of the injection molding machine is similar to that of the syringe used for injection. It is a process of injecting the plastic that has been plasticized into the closed mold cavity with the thrust of the screw (or plunger) and then getting the product after curing and shaping.

Injection molding is a cyclic process, each cycle mainly includes quantitative feeding – melting plasticizing – pressure injection – mold cooling – mold opening, and parts taking. After removing the plastic part, the mold is closed again and the next cycle is carried out.

Operation items of injection molding machine: Operation items of injection molding machine include control keyboard operation, electrical control system operation, and hydraulic system operation. The selection of injection process action, feeding action, injection pressure, injection speed, ejection type, monitoring of the temperature of each section of the cylinder, adjustment of injection pressure and back pressure, etc.

The general molding process of the screw injection molding machine is: First, granular or powdery plastic is added into the cylinder, and the plastic becomes molten by rotating the screw and heating the outer wall of the cylinder. Then, the machine moves the mold closing and injection seat forward, so that the nozzle is close to the gate of the mold. Then, pressure oil is injected into the injection cylinder, so that the screw is pushed forward so that the molten material is injected into the closed mold at a low temperature at high pressure and fast speed. After a certain amount of time and pressure holding (also known as pressure holding), cooling, so that it solidifies, the mold can be opened to take out the product (the purpose of pressure holding is to prevent the reverse flow of the molten material in the mold cavity, to replenish the material in the mold cavity, and to ensure that the product has a certain density and dimensional tolerance). The basic requirements of injection molding are plasticizing injection and molding. Plasticization is the premise to realize and guarantee the quality of the molding products, and in order to meet the molding requirements, the injection must ensure that there is enough pressure and speed. At the same time, due to the high injection pressure, the corresponding high pressure in the mold cavity (the average pressure in the mold cavity is generally between 20 and 45MPa), so there is a large enough mold closing force. It can be seen that the injection device and the mold device are the key parts of the injection molding machine.

The evaluation of plastic products mainly has three aspects, one is the appearance quality, including integrity, color, luster, and so on; Second, the accuracy between size and relative position; The third is corresponding to the use of physical properties, chemical properties, electrical properties, etc. These quality requirements are based on the use of products on different occasions, and the scale of the requirements is also different. The defects of the products mainly lie in the design, manufacturing accuracy, and wear degree of the die. But in fact, the technicians of plastic processing plants often suffer from the difficult situation of using technological means to make up for the problems caused by mold defects and little effect.

The adjustment of technology in the production process is a necessary way to improve product quality and yield. Because the injection cycle itself is very short, if the process conditions are not good, the waste will flow. When adjusting the process, it is best to change one condition at a time and observe several times. If the pressure, temperature, and time are all adjusted together, it is easy to cause confusion and misunderstanding, and the problem does not know what the reason is. There are many measures and means to adjust the process. For example, there are more than ten possible solutions to solve the problem of product dissatisfaction. Only by choosing one or two major solutions to solve the crux of the problem can we really solve the problem. In addition, attention should be paid to the dialectical relationship in the solution. For example, products appear to sag, sometimes to improve the material temperature, sometimes to reduce the material temperature; Sometimes you have to increase the amount of material, and sometimes you have to reduce the amount of material. Acknowledge the feasibility of reverse measures to solve the problem.