INJECTION MOULDING MACHINES

2.0 INJECTION MOULDING MACHINES

2.1 Introduction:

Moulding machine performs the job of plasticizing and injecting the thermoplastic materials into a mould that is held between clamping plates. Mould clamping in most machines is of two types, mechanical (toggle) and hydraulic. Injection machine capacities range from 10kg/n (SP15) to over 317kg/n (SP 800 D). An injection machine may be horizontal or vertical. Means used for plasticizing are plunger injection, pre-plasticizer two stage plunger, pre-plasticizer two stage screw, and reciprocating screw. The reciprocating screw machines are widely used.

2.2 Types of Moulding Machines

a) Plunger Injection Cylinder

Plunger injection cylinder consists of a barrel fitted with electrical resistance heater bands, in which is fixed a spreader or torpedo. The interior of the barrel and the surface of the 'spreader is usually nitrided and chrome plated, thus providing wear and corrosion resistance. The spreader is also heated electrically in some machine designs. The barrel heater bands are generally arranged in zones to achieve the required temperature across the cylinder. The plunger forces the plastic granules fed from the hopper, through the narrow passageway between barrel and the torpedo. As the granules move forward towards the nozzle they are melted by the heat obtained from the barrel and torpedo. The granules are then fed to the cylinder, either volumetrically or by a weigh feeder, which deposits a precisely weighed charge into the cylinder. The weighed mass equals the weight of the shot which consists of the combined weight of product plus the sprue and runner system. Packing or under fill of the mould must be avoided to ensure best dimensional control of the part, best physical properties, and lowest stress level in the product and consistent product weight.

b) Two Stage plunger type Cylinder:

The shooting cylinder is identical with the plunger injection cylinder. The purpose of the pre-plasticizer cylinder is to feed melted plastic materials into the shooting cylinder. The shooting cylinder is fitted with a nozzle and a plunger. The plunger is provided with a stroke control, which may be mechanical or electrical such as a limit switch which closes the hydraulic valve controlling the shooting plunger piston. In this way, precise shot weight can be obtained which is the prime advantage of the system. Other advantages are lower injection pressure that can be used, greater shot weight and lower cylinder temperature. In operation, the pre-plasticizing cylinder forwards melted plastics through the open valve, between it and the shooting cylinder. The shooting plunger which is forward at this point, is forced backward as the cylinder is filled. It moves back until it hits the stop, and the valve closes. When the shooting cylinder plunger is made to go forward by hydraulic pressure on the piston, the plastic mass flows through the nozzle in to the mould.

c) Pre-Plasticizer Two stageScrew Injection Cylinder:

This system is identical to the two stage plunger type pre plasticizer except that a screw extruder is substituted for the plunger type injection cylinder. Improved homogeneity of the plastic mass delivered by the screw to the shooting cylinder is the prime advantage of the system. Other advantage is the same as the plunger type.

The main disadvantage with both systems is the degradation of heat sensitive materials, which is the result of multiple flow paths and possible hang - up areas.

d) Reciprocating Screw Injection Cylinder:

Almost all injection machines today use the reciprocating screw injection cylinder. The thermoplastics resin flows from the hopper into the cylinder. As the screw rotates, the granules are conveyed forward and melted. The melt flows from the last flight of screw through the non - return valve. As the mass collects ahead of the screw, it exerts a pressure on the end of the screw and forces it backward. Back pressure on the material can be regulated by adjustment of a relief valve in the hydraulic injection cylinder circuit.

Suitable back pressure adjustment for the material being moulded is very important to obtain ultimate homogeneity of the plastics being moulded. Special shut off nozzles is required when high back pressure is generated. The screw is fitted with a stroke limiting device (limit switch) which stops screw rotation and feed from the screw when the required plasticized shot volume is created ahead of the screw.

During injection, the hydraulic ram moves the screw forward. The non - return valve closes as the movement starts, preventing back flow into the screw. The screw thus acquires the characteristics of a plunger and forces the plastics mass through the nozzle into the mould.

The reciprocating screw machine has many advantages over the other plasticizing methods. The primary advantage is the uniformity of plasticity obtained from the screw which produces high quality mouldings at lower temperatures and apparent pressures. A broader range of materials can be moulded including heat - sensitive materials such as rigid polyvinyl chloride (PVC). Colour changes and material changes can be accomplished rapidly with small material loss while changing

2.3 Injection Machine Basic Arrangements:

A modern injection moulding machine consists of the following basic parts

1. A feed hopper for holding and feeding the plastic material.
2. A reciprocating screw and a barrel, where the plastic material is heated and bought to the stage of inspection.
3. An arrangement for applying pressure to the screw to force the plastic material into the mould.
4. A fixed and moving platen on which the mould is mounted. The moving platen will have ejecting arrangements.
5. Moulding clamping arrangement either by toggle or direct hydraulic lock,  which holds the mould halves together during the injection.
6. A system of control for operating, plasticizing, injection and mould closing mechanism in the correct sequence and with variable arrangements for regulating during the process cycle.

2.4 The Screw

The screw of an injection moulding machine is commonly divided into three sections or zones

1. The feed section
2. The compression section
3. The melting section

The feed section transports the plastic material]: from the hopper to the heated portion of the barrel. The plastic material which is in powder or granules form is compressed to a homogeneous melt in the compression section. The final mixing and heating of the material in to a homogeneous melt is carried out in the melting zone.

The drive for the rotation of the screw during feed cycle is obtained from a hydraulic motor or a variable speed electric motor. check valves are used on screw to avoid the material being coming back along the screw during injection stroke.

2.5       Nozzle

The nozzle is connected to the end of the barrel, through which the softened material passes in to the mould

Standard Nozzle:

Nozzle is connected to the end of the barrel, through which the softened material passes in to the mould.  General purpose or Standard nozzles are made to varying diameters to accommodate the cylinder bores and sprue diameter and in varying lengths dependent on the heating requirements. Standard nozzles are made with straight holes restricted at the outlet end and tapered as required to match the cylinder bore at the inlet end. To facilitate the melt to be injected at the optimum temperature, band heaters or internal cartridge or heating cells are installed in the nozzle.

Reverse Taper Nozzle:

1. It is also known as Melt Valve nozzle.
2. It is used for moulding materials which show a very low melt viscosity.
3. It is used to freeze off the melt or prevent drooling between the shot.
4. This nozzle can be used for Cellulose and ABS for better flow.
5. ease of cleaning and improved sprue breaks with out long strings.
6. For corrosive material like PVC, stainless steel nozzles are used.
7. Chrome plated nozzles are not much used because of plating peeling off.

Drooling:

It is the leakage of plastic material through the nozzle in between shots

Non-Return Valve

2.6       Machine Specification:

Injection moulding machines are commonly classified according to the clamping force that they can generate and the amount of material that they can inject.

Injection capacity or Shot capacity:

It is the maximum volume of material that can be injected by the screw during one cycle of operation

Plasticizing capacity:

It is the amount of material that can be processed by the machine per hour. It is expressed in kg/h

Injection Pressure:

It is the maximum pressure by which the material is injected through the nozzle. It is given in kg/cm²

Injection rate or Injection Velocity:

It is the maximum rate at which the screw can inject or shoot material from the barrel during one shot.

Clamping Force:

It is the maximum force that the clamping system can exert on the mould or it is the maximum force by which the mould halves can be closed together. It is given in ton or kilo Newton

Maximum Daylight:
It is the maximum distance that the machine platen can be separated from each other and it can be obtained by adding the maximum mould thickness to the maximum opening stroke