INJECTION MOLDING MACHINE

INJECTION MOLDING MACHINE

The injection molding machine (IMM) is one of the most significant and rational forming methods existing for processing plastic materials. A major part in this development has been by the forward-thinking machinery industry, which has been quick to seize on innovations and incorporate them into plastic molded products. The most recent examples are the all-electric and hybrid IMMs. A major focus continues to be on finding more rational means of processing the endless new plastics that are developed and also produce more cost-efficient products. A simplified general layout for an IMM is shown in Figs.

Figure:  General layouts for an injection molding machine

CLASSIFICATION OF INJECTION MOLDING MACHINE

Injection molding machine mainly consist of two units:

1.      Injection unit.

2.      Clamping unit.

Injection unit contains hopper and plasticizing unit. Granular plastic material will be fed through hopper to the hot barrel. The screw provides for comparatively rapid heat exchange between the hot barrel and relatively colder plastic material. The plastic material is gradually melted and injected through the nozzle to the injection mold.

Functions of the injection unit:

·         Movement of the barrel to bring the nozzle in contact with sprue bush of the mold.

·         Generation and maintain of pressure between the nozzle and the sprue bush during injection of molten plastic material.

·         Rotation of screw during feed stage.

·         Arial motion of screw during the injection stage.

Injection unit are mainly classified into two types. They are:

1.      Plunger type: In plunger type injection molding machine, the raw materials are fed into the feed hopper. A measured quantity of raw material is fed into the cylinder where the plastic material is heated with the help of band heater to soften. Band heaters are placed in such a way that temperature increases gradually from hopper side to nozzle side. In this type, the function of the plunger is to push the plasticized plastic material into the closed mold that is fed from the hopper to the cylinder. Although the plunger-type machine is inherently simple, its popularity was limited due to the slow heating rate of the plastic resin.

 Fig.  Schematic diagram of injection end of a single stage plunger machine

2.         Screw type: Reciprocating, or single-stage, IMMs are a conventional type where plastic is melted using a combination of conductive heat from heater bands surrounding the barrel and frictional heating created by a rotating screw inside the barrel as shown in Fig.

In screw type injection molding machine, raw material from the hopper falls on the screw. As the screw rotates, plastic material through the helical groove of the screw comes in contact with the heated cylinder and this cold raw material is converted into a homogeneous melt. Plasticized raw material through the groove is accumulated in front of the screw and this accumulated melt pushes the screw back. With the help of the hydraulic piston, it is able to inject the molten polymer in a forward direction, as a plunger. 

Figure: In-line reciprocating screw units with hydraulic drive schematic.

In plunger machine, there is a large pressure loss in the cylinder. The pressure at the end of plunger is considerably higher than the pressure at the nozzle. In a screw machine, the ram pressure and nozzle pressure are almost the same.

Clamping unit consist of stationary platen, movable platen and tie bars. Locating ring side of the mold is clamped at the stationary platen and the ejection side of the mold is clamped at movable platen. The movable platen is moved towards stationary platen, until the two mold halves meet. Plastic material is injected from the stationary platen side through sprue bush to cavities. This unit clamps the mold halves by toggle or hydraulic system until the injected plastic part is solidified.

Clamping unit is mainly classified into two types.

Toggle clamp: Toggle, or mechanical, clamps use the mechanical advantage of a linkage to develop the force required to hold the mold closed during the plastic melt injection portion of the cycle. Figure shows the process (from top to bottom): partial injection, degassing, final injection, and ejection after the product is sufficiently solidified. Normally the linkage is designed so that slowdowns are built in. The advantage of a toggle clamp is that less hydraulic fluid is required to open and close the clamp than with a conventional hydraulic clamp. A main disadvantage is that the actual clamp tonnage is not precisely known.

The toggle clamp has extremely fast closing and opening actions. It is usually lower in cost than the straight hydraulic. The energy required to hold the developed tonnage is less, but this energy is in any case small compared to the total energy usage of the machine. With good lubrication the toggle bushings and pins last a long time. However, they must be reworked after several years of service. The toggle design will also develop higher than lockup tonnage if the clamp is overpowered by the injection end, or there is temperature buildup in the mold.

In this type, a clamping unit with a toggle mechanism is directly connected to the moving platen. A hydraulic cylinder or some mechanical force device is connected to the toggle system to exert the opening and closing force and hold the mold in closed position during injection.

 

Toggle mechanical clamp systems.

2.   Hydraulic clamp: The hydraulic clamp system uses a hydraulic cylinder and piston to develop clamp force directly. The two-platen version typically features a drive mechanism that pulls rather than pushes the moving platen. Hydraulic systems include other designs, particularly the use of a series of smaller hydraulic cylinders . Common arrangements include the three platen, two-platen, C-clamp, rotating-platen, and tie-bar machines, each providing different benefits. As an example, the two platen usually is much the shortest IMM, requires less floor space (by 20 to 40%), and weighs less than a three-platen hydraulic or toggle systems.

In this type, a hydraulic cylinder that is directly connected to the moving platen actuates a clamping unit. Direct fluid pressure is used to open and close the mold. It also provides the clamping force to hold the mold closed during injection.

Figure:  Hydraulic clamping.

Clamp Type	Advantages	Limitations
Toggle	Less expensive. Fast clamp motion. Energy efficient. Auto decelerated clamping.	Requires more maintenance. Clamping force may not be concentrated at center of platen. Difficult to adjust.
Hydraulic	Fast mold set up. Easily read clamp pressure. Low maintenance. Low platen deflection. Force concentrated at center of platen.	Requires large volume of hydraulic oil. Energy Inefficient. Must overcompensate due to compressibility of oil. Not floor space efficient.

              Table:  Advantages and Limitations of the two clamping mechanisms