How to choose the right PET injection molding machine
PET plastics are generally used as raw materials for plastic bottled beverages. The advantages are: low viscosity, good fluidity, and large specific heat capacity.
Disadvantages are: easy to stick, high molding temperature, narrow temperature adjustment range, poor processability, etc.
According to the characteristics of PET plastics, we choose PET injection molding machine from the following aspects:
1, the screw. The length-to-diameter ratio of the screw is designed to be a special PET screw. The length-to-diameter ratio of the general plastic screw is about 18-22. The length-to-diameter ratio of the PET screw is generally 1:24 or more. The larger the aspect ratio, the better the plasticizing ability.
2, the model space. It is determined by the mold size whether the large column inner distance, the mold thickness, the minimum mold size and the template size of the machine are suitable to confirm whether the mold is placed down.
The width and height of the mold need to be less than or at least one side smaller than the inner distance of the large column;
The width and height of the mold are preferably within the size of the template;
The thickness of the mold needs to be between the mold thickness of the injection molding machine;
The width and height of the mold must meet the minimum mold size recommended by the PET injection molding machine.
3. Take out the product. It is determined from the mold and the finished product whether the mold opening stroke and the mold stroke are sufficient for the finished product to be taken out.
The mold opening stroke must be at least twice the height of the finished product in the direction of the switch mold;
The mold stroke and the support force need to be sufficient to push the finished product out.
4, clamping force. The product and plastics determine the tonnage of machine clamping force.
When the raw material is injected into the cavity at a high pressure, a force of the struts is generated, so the clamping unit of the injection molding machine must provide sufficient clamping force so that the mold is not opened. The clamping force requirement is calculated as follows:
Calculating the projected area of the finished product in the direction of the switch die from the appearance size of the finished product;
The modulus of the die = the projected area of the finished product in the direction of the switch die (cm2) × the number of cavity points × the pressure inside the die (kg/cm2);
The machine clamping force needs to be greater than the die force, and for the sake of safety, the machine clamping force is usually greater than 1.17 times the force of the die.
Initially determine the specifications of the clamping unit and roughly determine the tonnage of the PET machine. Then the following steps must be performed to confirm which of the injection unit has a larger screw diameter.
5, shot full. The required "shot" is determined by the weight of the finished product and the number of cavities and the appropriate "screw diameter" is selected.
Calculate the weight of the finished product, taking into account the number of cavities (one cavity and one cavity);
For the sake of stability, the shot amount needs to be 1.35 times or more of the weight of the finished product, that is, the finished product weight needs to be within 75% of the shot amount.
6, shooting well. Conditions such as "screw compression ratio" and "injection pressure" are determined by PET plastic.
PET plastics need better plasticizing ability and suitable screw compression ratio design, so that they have better molding effect. Therefore, in order to make the finished product shoot better, the plasticizing demand and compression ratio should also be considered when selecting the screw.
7. Increase the storage motor. PET plastics are easy to stick, high molding temperature, etc., require a large screw torque, and often choose a two-stage or two-stage storage motor larger than the ordinary machine to meet the needs of the machine.
8. Increase the power system. The storage motor increases the screw speed, and increases the power supply system while increasing the storage motor to ensure more stable operation and shorten product cycle time.
There are some special issues that may have to be considered again:
In some special cases, the customer's mold or product may have a small mold size but a large amount of required volume, or a large mold volume but a small amount of shot required. Under such conditions, the standard specifications preset by the manufacturer may not be available. In line with the needs of customers, it is necessary to carry out the so-called "small size", that is, "big wall small shot" or "small wall big shot". The so-called "big wall small shot" refers to the original standard clamping unit with a smaller injection screw. Conversely, the "small wall large shot" is the original standard clamping unit with a larger injection screw. Of course, in the collocation, there may be several differences between the clamping and the shooting.
In addition, the mold water path needs to be considered to improve the cooling efficiency of the mold. The PET molds are equipped with special PET water separators.