Generally speaking, customers who have been engaged in injection molding industry for many years are mostly able to judge and choose right injection molding machine for production. However, in some cases, customers may need assistance of manufacturers to decide which specification of injection molding machine to use. Customers may even only have samples or ideas of products, then ask manufacturer whether machine can be produced, or which model is more suitable.

 

In addition, some special products may require special devices such as accumulators, closed loops, injection compression, etc. to be produced more efficiently. It can be seen that how to decide on right injection molding machine for production is an extremely important issue.
Important factors that usually affect selection of injection molding machines include molds, products, plastics, molding requirements, etc. Therefore, following information must be collected or available before making a selection:
◆ Mold size (width, height, thickness), weight, special design, etc.;
◆ Type and quantity of plastics used (single raw material or multiple plastics);
◆ Appearance size (length, width, height, thickness), weight, etc. of injection molded product;
◆ Molding requirements, such as quality conditions, production speed, etc.

Model and series are determined by product and plastic
Since there are many types of injection molding machines, it is necessary to correctly determine which injection molding machine or series should be used to produce this product, such as general thermoplastic plastics or bakelite raw materials or PET raw materials, single color, two colors, multiple colors, sandwich or mixed colors, etc. In addition, some products require high stability (closed loop), high precision, ultra-high shooting speed, high shooting pressure or fast production (multi-loop) and other conditions, and appropriate series must also be selected for production.

Mold size determines whether "large column inner distance", "mold thickness", "minimum mold size" and "mold plate size" of machine are appropriate to confirm whether mold can be placed.
Width and height of mold must be less than or at least one side is less than large column inner distance;
Width and height of mold should preferably be within mold plate size range;
Thickness of mold must be between mold thickness of injection molding machine;
Width and height of mold must meet minimum mold size recommended by injection molding machine, and it is not acceptable if it is too small.

Mold and finished product determine whether "mold opening stroke" and "mold ejection stroke" are sufficient to take out finished product.
Mold opening stroke must be at least twice height of finished product in direction of opening and closing mold, and must include length of vertical sprue;
Mold ejection stroke must be sufficient to eject finished product.

Tonnage of "locking force" is determined by product and plastic.
When raw material is injected into mold cavity at high pressure, a force to support mold will be generated. Therefore, clamping unit of injection molding machine must provide sufficient "clamping force" to prevent mold from being stretched open. Calculation of clamping force requirement is as follows:
Projected area of finished product in direction of opening and closing mold is calculated from appearance size of finished product;
Mold support force = projected area of finished product in direction of opening and closing mold (cm2) × number of mold cavities × mold pressure (kg/cm2);
Mold pressure varies with raw material, and general raw material is 350~400kg/cm2;
Machine clamping force must be greater than mold support force, and for sake of safety, machine clamping force is usually greater than 1.17 times mold support force.
So far, specifications of clamping unit have been preliminarily determined, and tonnage of machine model has been roughly determined. Then following steps must be performed to confirm which injection unit's screw diameter is more in line with requirements.

 

Required "injection volume" is determined by weight of finished product and number of mold cavities, and appropriate "screw diameter" is selected.
Number of mold cavities (how many cavities in one mold) should be considered when calculating weight of finished product.
For sake of stability, injection volume should be more than 1.35 times weight of finished product, that is, weight of finished product should be within 75% of injection volume.

Plastic determines conditions such as "screw compression ratio" and "injection pressure".
Some engineering plastics require higher injection pressure and appropriate screw compression ratio design to achieve better molding effect. Therefore, in order to make finished product better, injection pressure demand and compression ratio should also be considered when selecting screw.
Generally speaking, a screw with a smaller diameter can provide a higher injection pressure.

That is, confirmation of "injection speed".
Some finished products require high injection rate and fast injection to form stably, such as ultra-thin finished products. In this case, it may be necessary to confirm whether injection rate and injection speed of machine are sufficient, and whether it needs to be equipped with accumulators, closed-loop control and other devices. Generally speaking, under same conditions, screw that can provide higher injection pressure usually has a lower injection speed, and conversely, screw that can provide lower injection pressure usually has a higher injection speed. Therefore, when selecting screw diameter, injection volume, injection pressure and injection rate (injection speed) need to be cross-considered and traded off.
In addition, a multi-circuit design can also be used to shorten molding time with synchronous compound actions.
Some special issues may also need to be considered:
Size matching issues:
In some special cases, customer's mold or product may have a small mold volume but a large injection volume, or a large mold volume but a small injection volume. In this case, standard specifications pre-set by manufacturer may not meet customer's needs, and so-called "size matching" must be carried out, that is, "large wall small injection" or "small wall large injection". So-called "large wall small injection" refers to original standard clamping unit with a smaller injection screw, and vice versa, "small wall large injection" is original standard clamping unit with a larger injection screw. Of course, clamping and injection may also differ by several levels in matching.
Concept of fast or high-speed machines:
In actual use, more and more customers will ask to buy so-called "high-speed machines" or "fast machines". Generally speaking, in addition to needs of product itself, most of the other purposes are to shorten molding cycle, increase output per unit time, thus reduce production costs and improve competitiveness. Usually, there are several ways to achieve above goals:
Injection speed is increased: increase motor and pump, or add an accumulator (preferably add closed-loop control);
Feeding speed is increased: increase motor and pump, or reduce feeding hydraulic motor to increase screw speed;
Multi-circuit system: adopt a dual-circuit or triple-circuit design to perform compound actions simultaneously and shorten molding time;
Increase mold water channel to improve cooling efficiency of mold.