Generally speaking, customers who have been engaged in injection molding industry for many years are mostly able to judge and choose appropriate 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 products; ◆ Molding requirements, such as quality conditions, production speed, etc.
Suitable injection molding machine:

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 injection speed, high injection pressure or fast production (multi-loop) and other conditions, and appropriate series must also be selected for production.

◆ Width and height of mold must be smaller than or at least one side is smaller 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 cannot be too small.

◆ 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.

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:
◆ Calculate projected area of finished product in direction of mold opening and closing from finished product's appearance dimensions;
◆ Mold support force = projected area of finished product in direction of mold opening and closing (cm2) * number of mold cavities * mold pressure (kg/cm2);
◆ Mold pressure varies with raw material, and general raw material takes 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.

 

◆ Number of mold cavities (how many cavities in one mold) must be considered when calculating weight of finished product;
◆ For stability, injection volume must be more than 1.35 times weight of finished product, that is, weight of finished product must be within 75% of injection volume.

Some engineering plastics require higher injection pressure and appropriate screw compression ratio design to achieve better molding effects. Therefore, in order to make finished product better, injection pressure requirements and compression ratio issues must also be considered when selecting screw. Generally speaking, a screw with a smaller diameter can provide a higher injection pressure.

Some finished products require high injection rate and high injection speed 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, screws that can provide higher injection pressure usually have lower injection speeds, and conversely, screws that can provide lower injection pressure usually have higher injection speeds. Therefore, when selecting screw diameter, injection volume, injection pressure and injection rate (injection speed) need to be cross-considered and traded off.
In addition, multi-circuit design can also be used to shorten molding time with synchronous compound actions. After above steps, in principle, injection molding machine that meets needs can be determined, but there are some special issues that may also need to be considered, including:
◆ 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 and small shot" or "small wall and large shot". So-called "big wall and small shot" refers to original standard clamping unit with a smaller injection screw, and vice versa, "small wall and big shot" means original standard clamping unit with a larger injection screw. Of course, clamping and injection may differ by several levels in matching.
◆ Concept of fast machine or high-speed machine:
In actual application, more and more customers will ask to buy so-called "high-speed machine" or "fast machine". Generally speaking, in addition to needs of product itself, most of 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:
◆ Speed up injection speed: increase motor and pump, or add an accumulator (preferably add closed-loop control);
◆ Speed up feeding speed: increase motor and pump, or reduce feeding hydraulic motor to speed up screw speed;
◆ Multi-circuit system: adopt a dual-circuit or triple-circuit design to perform compound actions simultaneously to shorten molding time;
◆ Increase mold water channel to improve cooling efficiency of mold.
However, "there is no free lunch in the world". Improving and modifying machine performance can certainly increase production efficiency, but it often also increases investment costs and operating costs. Therefore, benefit evaluation before investment needs to be carefully weighed to generate the highest benefits with the most suitable model.