07-10
Gas assisted injection molding is a special injection molding technology developed on the basis of traditional thermoplastic injection molding technology, which is suitable for special performance req...
The main basis of mold design is the plastic product drawing, real sample, 3D modeling and specific requirements provided by the customer. The mold designer must evaluate whether the design requirements of the plastic parts are reasonable, including the wall thickness, draft angle, dimensional precision, shape and position tolerance, reinforcement rib, supporting surface, rounded corner, appearance and so on. The specific contents of mold design review are as follows:
1. The content of structural design review of plastic parts
Structural design of plastic parts
The external shape design of plastic should go through polishing, modification and other surface design, and it is necessary to consider whether the position of the Parting Line directly affects the appearance of the product
Functional structure design of plastic parts
Check the impact, wear, strength and rigidity of plastic parts under static and dynamic load.
Process structure design of plastic parts
It is mainly to consider the feasibility of molding, the deformation of plastic parts, and it is also necessary to consider whether the plastic parts can be assembled at high speed on the production line.
2. Wall Thickness uniformity of plastic parts
If the wall thickness is not uniform, the filling rate and cooling contraction of the plastic will be uneven. This results in many quality problems such as sink mark, vacuum bubbles, warping and even cracking. Determining the appropriate wall thickness is one of the main contents of product design.
3. Reinforced Ribs and Boss of plastic parts
Setting the reinforced ribs, the shape must be correct.
4. Whether the Draft Angle is reasonable
Draft Angle is directly related to the release quality of plastic products. Therefore, plastic products are required to have sufficient draft angle. The direction of the draft angle should be suitable for the parting surface when the plastic product is formed, otherwise it will affect the appearance and dimensional of the wall thickness of the product, and even the strength of some parts of the plastic products.
5. Color of plastic parts
In the case of large products, it is easy to produce uneven color. For more than two pieces of plastic parts that have color and are not allowed to have color differences, it is best to consider the same mold with multiple cavities, and do not separate the plastic parts into a single cavity mold.
6. Dimensional Precision
Check whether there are any problems with the relative dimensions of the plastic parts.
7. Whether Post-processing is required after plastic products are formed
Some plastic products need heat treatment or surface treatment after forming.
When calculating the molding size of products requiring heat treatment, the influence of heat treatment on their size should be considered.
There are many factors affecting the dimensional precision, and they are very complicated.
This makes it difficult to determine appropriate dimensional tolerances for plastic parts.
There are several main factors that affect the dimensional precision:
1. The influence of mold factors
Different mold types have different effects on the dimensional precision of plastic products, and the precision of plastic products formed by single-cavity molds is higher than that of multi-cavity molds. Each increase in the number of cavities of the mold will reduce the precision of the plastic parts by 5%.
2. The influence of plastic material factors
The properties of plastics, such as rigidity and shrinkage, will affect the dimensional precision of the product. Shrinkage rate is the most basic and important factor affecting the dimensional precision of plastic products.
3. The influence of design factors
If the geometric shape structure of the product itself is reasonable, the shrinkage and deformation are small, and the dimensional precision is higher.
4. The influence of molding process factors
Molding operating conditions, such as temperature (mold temperature), pressure (transfer pressure, injection pressure), time (injection time, cooling time) will affect the dimensional precision of the product.
1. Shrinkage of plastics
After the plastic parts are removed from the injection mold and cooled to room temperature, the size of each part of the plastic parts is reduced than the original size in the mold, this performance is called the shrinkage of the plastic, and the shrinkage depends on the type of resin, forming conditions, mold design and so on.
2. Molding shrinkage rate
The molding shrinkage caused by external factors such as heat or pressure in injection can be divided into three types: one is the molding shrinkage caused by the inherent shrinkage of the resin, the second is the molding shrinkage caused by the shape of the product, and the third is the molding shrinkage caused by the forming conditions.
Plastics with a small shrinkage rate | |||||
Plastic name | Polystyrene | Hard Polyvinyl Chloride | Polymethacrylate Methyl Ester | Polyphenyl Ether | Semi-hard polyvinyl chloride |
Shrinkage Rate (%) | 0.5-0.8 | 0.6-1.5 | 0.5-0.7 | 0.5-1 | 0.5-2 |
1. Three common classification methods of injection machines
According to the molding capacity of the injection machine, the injection machine can be divided into five types:
Injection machines are classified according to molding capacity | |||||
Types | Subminiature | Minor | Middle-sized | Large-scale | Ultra large size |
Clamping Force/kN | <160 | 160-2000 | 2000-4000 | 5000-12500 | >16000 |
Injection Capacity/cm³ | <16 | 16-630 | 800-3150 | 4000-10000 | >16000 |
According to the different plasticizing methods, there are two types of commonly used machines：plunger injection machines and screw injection machines.
The shape structure of the injection machine has an impact on the process operation, production efficiency and mold design, and there are usually three types according to its characteristics：vertical injection machine, angle injection machine, horizontal injection machine.
2. Composition of injection machine
The injection machines can generally be divided into the following parts:
Injection Mechanism.The main function of the injection mechanism is to make the solid plastic uniformly plasticized into a molten state, and inject the plastic melt into the cavity with sufficient pressure and speed.
Mold Closing Mechanism. The function of the mold closing mechanism is to ensure sufficient locking force during injection and pressure holding, so that the mold is locked and the overflow of the plastic parts is prevented.
Drive Mechanism and Electrical Control. The driving mechanism has two main functions :
To provide the required power for the injection machine to operate according to the process requirements;
To meet the requirements of the force and speed required by the moving parts during movement.
Control Mechanism. It controls the various actions of the injection machine, so that they can achieve effective control and adjustment of parameters such as time, position, pressure and speed according to pre-established procedures.
The selection of iniection mold steel should be based on the type of plastic, the shape, size, precision, quality of the plastic products to be formed, and consider the conditions and processing methods of injection, to select different types of steel, such as tempered steel, high carbon tool steel, corrosion resistant steel and so on.
The selection of mold materials should follow the principle of "meeting product requirements, giving play to the potential of materials, and combining economy and technology." In particular, the following aspects should be considered:
1. Select the ideal material with good performance.
2. Under the premise of meeting the quality of the product, the overall economy of the mold should be considered.
3. Consideration of mold life time. Under the premise of ensuring the quality of products, the life time of the mold is often the main factor to be considered in the selection of mold materials.
