In general,most customers who work In injection molding industry for many years
have the ability to determine and select appropriate injection molding machine to produce.
However, in some cases,customers may need manufacturer’s assistance in order to decide which injection molding machine to use.Even customers may only product samples or ideas, then asked whether the machine manufacturers can produce, or what kind of model is more suitable.Even some customers only have product samples or ideas, then ask manufacturers whether the machine can produce or which model is more suitable.
In addition,some special products may require special device such as pressure accumulator, closed-loop, injection compression and so on can be more efficient production.
Thus, it is an extremely important issue that how to determine the appropriate injection molding machine to produce.
The important factors that generally affect the choice of injection machine including mold , products, plastic, molding requirements and so on.Therefore, before you make the selection must collect or have the following information:
◆ mold size (width, height, thickness), weight, special design;
◆ Use type and quantity of plastics(single feedstock or more plastics) ;
◆ injection products external dimensions (length, width, height, thickness) and weight;
◆ molding requirements, such as condition of quality , processing speed.
After obtaining the above information, you can follow these steps to select the appropriate injection molding machine:
1.select the right type:products and series determined by plastic
Because injection molding machine has a lot of species so judge accurately to choose which machine or which series of injection molding machine to produce.For example, general thermoplastic plastic or Bakelite raw materials or raw materials such as PET,monochrome, color, multi-color, laminated or mixed color and so on.
In addition, some products require high stability (closed-loop), high-precision, high rate of fire, high injection pressure or rapid production (multi-loop) and other conditions,so you must select the appropriate series to produce.
2.can contain: judged by the size of the mold to decide”distance between tie bars”, “mold thickness”, “mold minimum size” and “mold plate size” of machine is whether appropriate to contain the mold.
◆ mold width and height must be less than or at least one side is less than the distance between tie bars;
◆ Mold width and height is preferably within the size range of the template;
◆ mold thickness required be less than injection molding machine thick;
◆ mold width and height must meet the proposed minimum size of the mold of the injection molding machine , nor too small.
3.can take out: Judge by the mold and finished product to decide “opening stroke” and “ejector stroke” is whether enough to take out the finished product.
◆ Opening stroke at least be greater than finished product in the switching mode height direction for more than two times , and it contains sprue length;
◆ Ejector stroke should push finished product out.
4.can fix:Determined by the product and the plastic to decide “clamping force” tonnes.When the raw material at high pressure into the mold cavity will produce a power of hold mode,so injection clamping unit of molding machine must provides enough “clamping force” to make the mold will not be softened. The requirements of clamping force calculation are as follows:
◆ Determined by the size of the physical dimension to calculate finished product in the projected area of the switch mode direction;
◆ hold power= finished product in the projected area of the switch mode direction (cm2) × number of mold cavity × pressure in mold (kg / cm2);
◆ Pressure in mold with different raw materials, general raw materials takes 350 ~ 400kg / cm2;
◆ Machine clamping force must be greater than hold power, and to be safe, the machine clamping force generally must be greater than hold power more than 1.17 times.
So far it has initially decided specifications of clamping unit, and generally sure machine tonnage, then the following steps must be carried out to confirm which screw diameter of emitted unit is more suitable.
5.Shot and be filled:Judge by the finished product weight and number of mold cavity to decide the required “shot size” and choose the appropriate “screw diameter”.
◆ calculate the finished product weight to need the number of cavity (a mold with how many cavities)
◆ For the purpose of stability, injection amount is more than the weight of the finished product 1.35 times ,that is the weight of the finished product is within 75% of required injection amount.
6.shoot well:Judge by the plastic to decide”screw compression ratio” and “injection pressure” and other conditions.
Some plastics require high injection pressure and a suitable screw compression ratio design to have better molding effect.Therefore, in order to make the finished product better shot, when choose the screw need to consider the demand of injection pressure and the problem of compression ratio.In general, the smaller the screw diameter can provide higher injection pressure.
7.shoot fast: confirm “injection speed”.
8.Some finished product requires high rate of injection speed to molding steadily, such as ultra-thin class finished.in this case, you may need to confirm whether the rate of ejection rate and the rate of fire is enough,whether need to be with the pressure accumulator, closed-loop control and other devices. In general, under the same conditions,the screw which can provide high pressure of injection with low rate of fire.On the contrary, the screw which can provide low pressure of injection with high rate of fire.
Therefore, when you choose screw diameter, injection amount, injection pressure and injection rate (injection speed), you should cross consider and trade-offs.
In addition, multi-loop design can also be used to synchronize the compound action to short the molding time.
After the above steps,the injection molding machines in principle meet the needs but there are some special problems may also need to be considered, comprising:
◆Configuration problem: In some special cases,the customers’ mold or product may need large injection amount with small volume,or small injection amount with large volume.In this case, the manufacturers preset standard may not be able to meet customer demand,taking “size distribution”, that is “great wall of small shot” or “little wall big shot”.The “big wall small shot” refers to the original standard clamping unit with a small injection screw.On the contrary, the “small wall big shot” that is based on the original standard clamping units with larger injection screw. Of course,in the match may differ several levels in clamping and injection.
◆ fast or high-speed machine concept:In practice, more and more customers will require to buy the “high-speed machine” or “fast machine.” In general, the purpose in addition to the demand for the product itself, the other mostly shorten the molding cycle, improve the yield per unit of time, thereby reducing production costs and improve competitiveness. Typically, to achieve the above object, there are several approaches:
◆ injection speed: Increase the electrical motor and pump, or increase the pressure accumulator (best is increasing closed-loop control);
◆ speed up feed speed: Increase the electrical motor and pump or shrink feed hydraulic motor to accelerate the screw speed ;
◆ multi-loop system: Three-loop or double-loop design.Do composite operation meanwhile to shorten the molding time;
◆ increase mold waterways, improve the cooling efficiency of the mold.