How to realize thin wall injection molding with traditional equipment?

In the plastic injection molding process, the wall thickness of the part is a very critical parameter. Thin-walled injection molding parts have many advantages, such as reducing the weight of parts, production scale, material expenditure and molding cycle. However, it is not cost-effective to use expensive high-speed injection molding machines to manufacture thin-walled products.
Whether the traditional injection molding machine is competent or not, let's analyze this problem:
First, we need to understand what thin-wall injection molding is. In general, thin-walled injection molding refers to an injection molding part with a surface area of 50 square centimeters and a wall thickness of 1 mm. This level of injection molding can be called thin-walled injection molding.
However, the traditional injection molding machine can not meet the requirements of thin-wall injection molding. Take a traditional injection molding machine for making parts with 3mm wall thickness as an example: when the front part of the melted thermoplastic flows through the mold cavity, it will contact with the core or the inner wall of the mold cavity with lower temperature and form a solidified thin skin. This kind of prematurely solidified skin accounts for about 20% of the total wall thickness.
Inside this layer of skin, the molten material injected is still flowing forward. Obviously, if the wall thickness of the part is reduced to the extent of thin wall, its cooling rate will also be accelerated, resulting in an increase in the proportion of the solidified skin to the whole wall thickness, that is, its subsequent molten core flowing into the mold cavity will be reduced. On the contrary, the time interval for condensation of parts is shortening. This all adds difficulty to the continuous flow of materials, which makes it more difficult for the parts to achieve the requirements of filling before condensation.
In order to overcome the filling difficulties of thin-wall injection molding, special design or modification of the injection molding machine is usually required, such as the use of multi-channel injection ports, the application of injection pressure up to 241Pa and the injection speed of 1000MM/S. However, these practices will cost considerable money.
So, can we control some process parameters on the traditional unmodified standard injection molding machine to meet the requirements of thin-wall injection molding?
The answer is yes. It was reported that someone had done this experiment on a traditional injection molding machine with a maximum clamping force of 90 hectares and a maximum injection volume of 170g: a mold with a fan-shaped injection insert, an injection port and a cavity was placed on this machine. The length to thickness ratio of the insert is 140:1, and the cavity thickness is 1mm. The plastics used are Lexan SP7602 and Magnum 9015.
The weight of the product part is the only variable output value. Under the same mold cavity condition, the change of the part weight is obviously closely related to the degree of the molten material filling in the mold cavity during the injection molding process. It is said that the reliability of the results of the analysis of the weight change of the parts can be as high as 95%. Therefore, this experiment is based on the relationship between the process parameters and the weight of the parts. For this reason, five pressure and temperature converters are specially installed in the mold cavity. A data control system tracks the pressure and temperature curve in the cavity.
The experiment adopts a half fractional factor design to study the nozzle temperature, mold temperature, cooling time, injection speed and whitening holding pressure. It is said that these five parameters can affect the weight of parts. In order to establish these parameters to determine their impact on the weight of parts, different combinations of high and low values are used for injection molding.
PC and ABS were tested. The experimental conditions are: respective melting temperature, standard mold temperature and part weight, standard part tension strength and maximum allowable injection speed. In addition, the relative viscosity of the two materials can also be established at different shear rates.
The experimental results are as follows:
1. When the melting temperature of ABS material rises from 260 ℃ to 280 ℃, the weight of its parts will increase from 6.6 g to 7.4 g, that is, 12%.
2. For PC materials, when the melting temperature is raised from 290 ℃ to 300 ℃, the weight of parts increases from 7.3 g to 8.9 g, which is 22%
3. When the mold temperature rises from 80 ℃ to 90 ℃, the weight of parts made of PC and ABS increases, but PC is more sensitive. The weight of parts made of the latter can increase from 8.4 g to 8.8 g, an increase of 4.8%.
4. Changes in melting temperature and mold temperature will lead to changes in the tensile strength of parts. However, the increase of melting temperature will reduce the strength, while the increase of mold temperature will increase the strength.
5. Shortening the cooling time and increasing the injection speed will increase the weight of PC parts, while ABS materials are not affected by these two parameters.
Result analysis:
For PC materials, melting temperature, mold temperature, cooling time and injection speed are the key parameters that affect the weight of parts; For ABS, the parameters that affect the weight of its parts are only melting temperature and mold temperature.
The increase of melting temperature will make the material have higher heat energy and lower viscosity, which will make the molten material flow more easily, form a longer streamer length and fill the cavity more smoothly. However, if the melting temperature is too high, the material will deteriorate and degrade. Therefore, this parameter can only be used to ensure the filling of the cavity within the upper limit allowed by the material.
The increase of mold temperature will reduce the condensation layer of the material in the mold cavity and make the molten material flow more easily in the mold cavity, so as to obtain greater part weight and better surface quality.
Shorter cooling time can make the molten material stay in the container shorter and reduce the possibility of degradation. It is believed that reducing wall thickness by 50% will lead to a 4-fold reduction in cooling time. In addition, the cooling time constitutes about 70% of the forming cycle, and its reduction means the improvement of production efficiency.
The injection volume of the machine should be as high as possible. This also helps to reduce the residence time of molten materials in the container. Increasing the injection speed will also reduce the relative viscosity of the molten material, which is the result of the influence of pseudoplastic when the shear becomes thinner. At the same time, this shear heating only occurs in less than a second, which is insignificant for causing obvious degradation.
The increase of injection speed will reduce the viscosity of PC material and increase the weight of parts, but it is much less than that of parts with higher melting temperature. However, because it can also make the material more resistant to degradation, it is advisable to improve the injection speed.
The change of injection speed has almost no effect on ABS material, which is because its relative viscosity has not significantly decreased at this time.
By changing some process parameters under the condition of traditional injection molding machine, the weight of parts has been increased.
This result actually reflects the increase of the ability of plastic to fill the 1mm cavity in the molten state, which is to improve the ability of thin-wall forming.
According to the experimental results, it is also possible to process thin-walled parts on traditional injection molding machines. During operation, the injection speed can be adjusted to the maximum allowable upper limit. On this basis, the two parameters can be improved as much as possible according to the maximum melting temperature limit and the maximum mold temperature standard recommended by the material. This is the main countermeasure to achieve high-quality thin-wall injection molding with low-cost selection on traditional injection molding machines.