What is injection moulding machine? How to select?
The injection moulding process forces molten plastic material under high pressure into a closed mould to form the part in the required shape, design and size. This requires the selection of the optimum machine suitable for the selected product to be moulded. A designer will consider all the requirements of a plastic part that needs to be injection moulded to meet the specific requirements of the end-user. Few of the most important factors to consider in selecting the right machine to mould the product are listed below:
- Selection of the right Thermoplastic material and grade from the Raw material manufacturer. Collect the Technical data sheet and Processing Parameters of the material from the Supplier of the material.
- The designer can estimate the shot weight which is the total of the part weight, Cold runner weight and the number of cavities
- The designer further has to arrive at the mould dimensions to shortlist the machine required for this particular component.
- With the help of Mould flow analysis, the estimated cycle time can be calculated. Also, the cooling time for the part can be estimated.
- Depending on the component design requirements, the barrel & screw is decided whether it has to be a Low pressure, Medium or High-pressure type. Usually, High-pressure screw & barrels are used for moulding thin wall components and components to be run in fast cycles. A low-pressure Screw & Barrel is used for making medium and thick wall components. Generally, the injection pressure must be in the range of 4-5 tons per Sq. Inch depending on the material to be used for most products. It is balanced against the clamping pressure of the machine and is calculated based on the size and shape of the part, as well as the dimension of the gate opening.
- The Plasticizing capacity of the machine is a key factor in selecting the Injection moulding machine. Plasticizing capacity is the quantity of plastic that can be converted into a homogeneous melt in the barrel per unit time. It is measured as lb/hr or Kg/hr. If the plasticizing capacity is too low with the shot size of the component to be moulded, the Injected plastic will not be completely in the molten state which will result in moulding rejections.
- The performance of an injecting unit of the moulding machine is also related to the Injection rate and Injection Speed of the machine. The Injection Rate is the volume of molten plastic that is injected into a close mould in one second and is expressed as cm3/s. The Injection Speed of a moulding machine is the speed of forward movement of the plunger or reciprocating screw which injects the molten plastic into the closed mould, this is expressed as mm/s.
- Another key factor in the moulding machine is the provision for Hold on time and Hold on Pressure in the machine controls. Hold on time – during the injection moulding process, once the required amount of plastic is injected into the cavity, it must be held there until the gate freezes off. This is a time-controlled phase during which no more plastic is injected into the mould.
- Hold Pressure – This is the pressure against the cooling plastic in the cavity of the mould when it is getting solidified. Usually, it is used for the final 5% filling of the mould cavity. This Hold Pressure will help in ensuring that the part is moulded with uniform pressure and has a very controlled shrinkage after moulding.
- Ejection Speed – The speed at which the Ejection system moves forward to push the moulded part from the core or cavity of the mould is Ejection Speed. This depends on the geometry of every single component moulded. Usually, long-drawn components are to be ejected with slow speeds. The surface on the moulded component around the ejector pin will leave a white mark on the moulded part is ejected at a high speed. It may also deform due to the high speed.
- Ejection pressure – This also plays a very important role in the ejection of the moulded part along with the Ejection speed. This pressure is set to remove the part from the mould without any deformation, stress mark on the component.
The above are a few important details to be considered in selecting the right machine for production.