Plastics and injection moulding

Injection moulding is a forming technique for producing batches of moulded parts, preferably macromolecular materials. This is a key method used when working with plastics, particularly thermoplastics, but also elastomers and thermosetting plastics. The heavily automated injection moulding process and its variability means it is possible to produce products in almost any shape and size. Injection moulding also has several other significant advantages, particularly with regard to the mass production of complicated parts.

Injection moulding and optimal tool tempering -with temperature control solutions from Tool-Temp

Advantages of injection moulding

  • No intermediate steps between raw material and finished part
  • Zero or minimal subsequent processing of the produced part is needed
  • Fully automated process
  • High precision reproducibility of moulded parts
  • Surfaces can be custom designed

When working with thermoplastics, the tool is usually cooled. Elastomers and thermosetting plastics, on the other hand, require the tool to be heated. This is essential for the injected material to harden or vulcanise. We will be happy to advise you on any of these issues.

The injection moulding process
There are 4 steps to injection moulding:

  • Step 1: Plasticizing and dosing
  • Step 2: Injecting
  • Step 3: Applying pressure and cooling off
  • Step 4: Ejecting from the mould

Plastic injection moulding - perfect production conditions with Tool-Temp

Tool temperature is among the most important factors in injection moulding. It affects the cooling off time and thus the economic viability of production. Quality of the finished part is dependent on the crystallinity and imparted stresses.  The crucial factor that influences the speed at which the heat from the screw can be removed after injection is the surface temperature of the tool. Tool tempering is necessary for heating up the tool at the start of production and for maintaining the desired tool temperature. If one area of the tool is cooled more effectively than another, then the moulded part will cool faster in that area. This can result in localised areas of increased internal stress, different shrinkage behaviour of the part and warping upon ejection. The dimensional stability of more complex parts is subsequently negatively affected.

The design of the tool form as well as the geometry and arrangement of cooling holes are the key factors for ensuring high-quality moulded parts and an even cooling of the tool surface. For larger tools, it is recommended to divide temperature control into several circuits so that refrigerants do not get excessively hot.

The type of plastic used determines the optimal tool surface temperature. To ensure reliable production, all Tool-Temp temperature control units feature electronic flow sensors. The circulation of the medium is constantly monitored and an alarm raised if there is a blockage or a sudden drop in circulation. Temperature sensors fitted into the unit regulate the medium. For precision parts, additional sensors can be fitted into the cooling water inlet and return to regulate the inlet temperature. The control unit issues an alert if the actual temperature deviates from the set temperature. To ensure constant temperature control capability, all temperature control units from Tool-Temp are fitted with a process water filter. This prevents the build up of deposits in the circuit that affect heat transfer and thus the quality of moulded parts.

  • High-quality surface finishes
    Precision temperature control at areas where flow lines would otherwise appear ensures that the flow of plastic is optimised in problematic areas. The plastic merges and internally binds together without any visual seams. We adapt our temperature control solutions to your individual injection moulding application and tool.
  • Partial impression optimisation
    Optimisation of partial impressions requires a higher tool wall temperature. This can be achieved using our highly reactive pressurised water range of Tool-Temp heating and cooling systems.
  • Improved cycle times
    Water temperature control systems with powerful cooling capacity up to 90°C and pressurised water up to 160°C allow for short cycle times and flawless quality of moulded parts.
  • Fast return on investment and simple maintenance
    We use exclusively high-quality materials and efficient batch production to guarantee temperature control solutions at an unbeatable price. All our products are developed and produced in Switzerland. Reliable supply chains are guaranteed at all times. Tool-Temp temperature control units stand out for their excellent serviceability. We guarantee that we will always provide replacement parts worldwide. This makes maintaining the equipment extremely simple and affordable.

The thermoplastic to be injected is first funneled into a rotating screw in granulate or powder form. The rotation moves the granules towards the end of the screw. The friction from the rotating conveyor screw and simultaneous heating melts the granules within the injection unit. Further along in the process, the melted plastic accumulates at the end of the screw conveyor where the outlet nozzle is located. The nozzle is closed at this point. This generates pressure on the screw.

The molten plastic is pushed under great pressure through a nozzle into the sprues on the tool and fills the cavity. The pressures that arise during this process are between 500 and 2,000 bar. These pressures help press the molten plastic through the sprue or sprue system of the tool and into the cavity.

The next steps in the injection moulding process are pressing and cooling. At between 20 and 120°C, the tool is much cooler than the molten plastic, which is at 200 to 300°C. The molten plastic therefore starts to cool down and turns to a solid at the material’s freezing point. During cooling, shrinkage occurs that can negatively affect both the dimensional stability and the texture and quality of the surface of the moulded part. This shrinkage is countered by further application of pressure onto the liquid plastic. Once this application of pressure has stopped, the nozzle on the screw cylinder is closed and dosing and plasticizing of the mass for the next part can begin. The material in the mould meanwhile continues to cool until the liquid core of the part has solidified. This is usually sufficiently solid for the part to be removed from the mould.

The ejection side of the tool is opened and pins are inserted into the tool cavity to push out the moulded part. It is almost always necessary to remove the sprue from moulded parts. This is done either in a separate work step, or automatically during ejection from the mould. Sprueless moulding requires the use of a hot channel system. Once the part has been removed from the mould, the process starts over again from the beginning.

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