How to choose an injection moulding machine?

News source:Haishi plastic machine Time:2015-02-02
Generally speaking, customers who have been in the injection moulding industry for many years are most likely capable of making their own judgement and choosing the right injection moulding machine for their production. However, in some cases, the customer may need the assistance of the manufacturer to decide which size of injection moulding machine to use, or the customer may only have a sample or an idea of the product and then ask the manufacturer whether the machine can produce it or which model is more suitable.
  In addition, some special products may require special devices such as accumulators, closed circuit, injection compression, etc. to produce them more efficiently. As such, deciding on the right injection moulding machine for production is an extremely important issue. The following information is provided for the reader's reference.
  Important factors that usually influence the selection of an injection moulding machine include the mould, the product, the plastic, the moulding requirements, etc. Therefore, the following information must be collected or available before making a selection.
  ◆ Mould size (width, height, thickness), weight, special design, etc.
  ◆ The type and quantity of plastic to be used (single raw material or multiple plastics).
  ◆ The external dimensions (length, width, height, thickness), weight, etc. of the finished injection moulded product.
      ◆ Forming requirements, such as quality conditions, production speed, etc.
  After obtaining the above information, the following steps can be followed to select the right injection machine.
  1. Choose the right type: The product and the plastic determine the type and series of machine.
  As there are so many different types of injection moulding machines, it is important to determine at the outset which type of injection moulding machine or series of machines should be used to produce the product, for example, whether it is a general thermoplastic or Bakelite or PET material, a single colour, a two-colour, a multi-colour, a sandwich or a mixed colour. In addition, certain products require high stability (closed circuit), high precision, ultra-high injection speed, high injection pressure or fast production (multi-loop) and other conditions, the right series must also be selected for production.
  2、Placement : The mould size is used to determine whether the machine's "large column internal distance", "mould thickness", "mould minimum size" and "mould tray size "To confirm whether the mould can fit or not.
  The width and height of the mould should be less than, or at least one side should be less than, the internal distance of the large column.
  ◆ The width and height of the mould should preferably be within the size of the mould tray.
  ◆ The thickness of the mould should be between the mould thickness of the injection moulding machine.
  ◆ The width and height of the mould must be in line with the minimum mould size recommended for the machine, too small is not acceptable.
  3. Accessibility: The mould and the finished product should determine whether the "mould opening stroke" and "mould holding stroke" are sufficient to allow the finished product to be taken out.
  The opening stroke must be at least twice the height of the finished product in the opening/closing direction, and must include the length of the sprue.
  ◆ The mould holding stroke must be sufficient to lift the finished product out.
  4、Lockable: The product and the plastic determine the "clamping force" tonnage.
  When the raw material is injected into the mould cavity at high pressure, a clamping force is generated, so the clamping unit of the injection moulding machine must provide sufficient "clamping force" to keep the mould from opening. The clamping force requirement is calculated as follows.
  ◆ Find the projected area of the finished product in the direction of the opening and closing of the mould from the external dimensions of the finished product.
  ◆ The clamping force = the projected area of the finished product in the direction of the opening and closing of the mould (cm2) x the number of cavities x the pressure in the mould (kg/cm2).
  ◆ The in-mould pressure varies with the raw material, generally 350-400kg/cm2.
  ◆ The clamping force of the machine should be greater than the clamping force, and for the sake of safety, the clamping force of the machine should be more than 1.17 times of the clamping force.
  The following steps must then be taken to confirm which injection unit has the required screw diameter.
  5. Full shot: The weight of the finished product and the number of cavities are used to determine the required "shot volume" and to select the appropriate "screw diameter".
  The weight of the finished product should be calculated taking into account the number of cavities (how many cavities in a mould).
  For the sake of stability, the injection volume should be more than 1.35 times the weight of the finished product, i.e. the weight of the finished product should be within 75% of the injection volume.
  6、Good injection: the plastic is judged by the "screw compression ratio" and "injection pressure" and other conditions.
  Vertical injection moulding machine some engineering plastics need higher injection pressure and suitable screw compression ratio design, in order to have a better molding effect, so in order to make the finished product shot better, in the choice of screw also need to consider the demand for injection pressure and compression ratio issues.
  In general, the smaller diameter of the screw can provide higher injection pressure.
  7、Fast shooting: and "injection speed" confirmation.
  Some products require a high injection rate and speed in order to be stable, such as ultra-thin products. In this case, it may be necessary to check whether the injection rate and speed of the machine are sufficient, and whether it needs to be equipped with a pressure accumulator, closed circuit control and other devices. Generally speaking, a screw that can provide a higher injection pressure will usually have a lower injection speed under the same conditions, and conversely, a screw that can provide a lower injection pressure will usually have a higher injection speed. Therefore, when choosing a screw diameter, the choice between injection volume, injection pressure and injection rate (injection speed) must be cross-checked.
  Alternatively, a multi-loop design can be used to shorten the forming time by synchronising the compounding action.
  After the above steps, you can in principle decide on the right injection moulding machine for your needs, but there are some special issues that may need to be considered, including
  ◆ The issue of size matching.
  In some special cases, the customer's mould or product may have a small mould size but a large injection volume, or a large mould size but a small injection volume, in which case the standard specifications set by the manufacturer may not be able to meet the customer's needs, and the so-called "size matching" must be carried out, i.e. "big wall, small injection" or "small wall, big injection". "or "small wall, big shot". By "big wall, small shot" we mean that the original standard clamping unit is matched with a smaller injection screw, while "small wall, large shot" means that the original standard clamping unit is matched with a larger injection screw. Of course, there may be several levels of difference between clamping and injection.
  The concept of fast or high-speed machines.
  In practice, more and more customers are asking for so-called "high speed machines" or "fast machines". Generally speaking, apart from the needs of the product itself, the aim is to shorten the moulding cycle, increase the output per unit time, thus reducing production costs and improving competitiveness. Usually, there are several ways to achieve these objectives.
  ◆ Faster injection speed: increase the motor motor and pump, or add accumulator (preferably with closed circuit control).
  ◆ Faster filling speed: increase the motor motor and pump, or change the filling oil pressure motor to a smaller one, so that the screw speed is faster.
  ◆ Multi-loop system: adopt double-loop or triple-loop design to synchronise compound action and shorten the forming time.
  ◆ Increase the mould water circuit to enhance the cooling efficiency of the mould.
  However, there is no such thing as a free lunch. While upgrading or modifying the performance of a machine can increase productivity, it often increases investment costs and running costs.
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