Study of NC Injection Molding

Abstract:

The purpose of this experiment is to understand the working of NC injection molding machine. The basic functions and their usage were understood. A product of poly ethane was manufactured using this machine.

Introduction:

Injection molding is a process in which a polymer is heated to a highly plastic state and forced to flow under high pressure into a mold cavity, where it solidifies. The molded part, called a molding, is then removed from the cavity. The process produces discrete components that are almost always net shape. The production cycle time is typically in the range of 10 to 30 sec, although cycles of 1 min or longer are not uncommon for large parts. Also, the mold may contain more than one cavity, so that multiple moldings are produced each cycle.

Complex and intricate shapes are possible with injection molding. The challenge in these cases is to fabricate a mold whose cavity is the same geometry as the part and that also allows for part removal. Part size can range from about 50 g (2oz) up to about 25 kg (more than 50 lb), the upper limit represented by components such as refrigerator doors and automobile bumpers. The mold determines the part shape and size and is the special tooling in injection molding. For large, complex parts, the mold can cost hundreds of thousands of dollars. For small parts, the mold can be built to contain multiple cavities, also making the mold expensive. Thus, injection molding is economical only for large production quantities. Injection molding is the most widely used molding process for thermoplastics.

Equipment:

Figure 1

An injection molding machine consists of two principal components: (1) the plastic injection unit and (2) the mold clamping unit. The injection unit is much like an extruder. It consists of a barrel that is fed from one end by a hopper containing a supply of plastic pellets. Inside the barrel is a screw whose operation surpasses that of an extruder screw in the following respect: in addition to turning for mixing and heating the polymer, it also acts as a ram that rapidly moves forward to inject molten plastic into the mold. A non-return valve mounted near the tip of the screw prevents the melt from flowing backward along the screw threads. Later in the molding cycle the ram retracts to its former position. Because of its dual action, it is called a reciprocating screw, a name that also identifies the machine type.

To summarize, the functions of the injection unit are to melt and homogenize the polymer, and then inject it into the mold cavity.

            The clamping unit is concerned with the operation of the mold. Its functions are to (1) hold the two halves of the mold in proper alignment with each other; (2) keep the mold closed during injection by applying a clamping force sufficient to resist the injection force; and (3) open and close the mold at the appropriate times in the molding cycle. The clamping unit consists of two platens, a fixed platen and a moveable platen, and a mechanism for translating the latter.

The mechanism is basically a power press that is operated by hydraulic piston or mechanical toggle devices of various types. Clamping forces of several thousand tons are available on large machines

An ejection system is needed to eject the molded part from the cavity at the end of the molding cycle. Ejector pins built into the moving half of the mold usually accomplish this function. The cavity is divided between the two mold halves in such a way that the natural shrinkage of the molding causes the part to stick to the moving half. When the mold opens, the ejector pins push the part out of the mold cavity. A cooling system is required for the mold. This consists of an external pump connected to passageways in the mold, through which water is circulated to remove heat from the hot plastic. Air must be evacuated from the mold cavity as the polymer rushes in. Much of the air passes through the small ejector pin clearances in the mold. In addition, narrow air vents are often machined into the parting surface; only about 0.03mm (0.001 in) deep and 12 to 25mm (0.5 to 1.0 in) wide, these channels permit air to escape to the outside but are too small for the viscous polymer melt to flow through.

NC Injection molding machine:

The following figure illustrates the user interface of the NC machine;

Figure 2

Figure 3

1.      The top row is called the parameter keys which are used to set up the conditions during the operation of the machine.

2.      Numeric keys are used to enter the value of any parameter.

3.      The mode keys are used to set the mode, there are four modes;

·         Manual

·         Semi-Automatic

·         Sensor

·         Time automatic

4.      Functions keys are used to operate the machine, they consist of;

·         Mold open (to open the mold)

·         Mold close (to close the mold)

·         Inject (to inject the molten polymer in the mold cavity)

·         Suck back (to suck back the molten polymer)

·         Ejector return (to return the ejector back to its initial position)

·         Ejector advance (to advance the ejector so that the product could be pushed out)

·         Nozzle advance (to move the nozzle towards the mold)

·         Nozzle return (to return the nozzle to its initial position)

·         Charge(to take the feed of the stock)

·         Auto punch

·         Lubrication (for lubrication of machine)

·         Motor on/off (to turn on or off the motor of hydraulic pump)

·         Heater on/off (to turn on or off the heaters)

Working:

    1)     The mold is closed and clamped.

    2)    A shot of melt, which has been brought to the right temperature and viscosity by heating and the mechanical working of the screw, is injected under high pressure into the mold cavity. The plastic cools and begins to solidify when it encounters the cold surface of the mold. Ram pressure is maintained to pack additional melt into the cavity to compensate for contraction during cooling.

    3)  The screw is rotated and retracted with the non-return valve open to permit fresh polymer to flow into the forward portion of the barrel. Meanwhile, the polymer in the mold has completely solidified.

    4) The mold is opened, and the part is ejected and removed.

Discussion

Following are some of the common defects found in injection molded parts;

1)      Shrinkage:

                        During cooling the molten polymer may shrink and results in the decrease of desired dimensions. This can be decreased by adding fillers.

2)      Short shots:

                 As in casting, a short shot is a molding that has solidified before completely filling the cavity. The defect can be corrected by increasing temperature and/or pressure. The defect may also result from use of a machine with insufficient shot capacity, in which case a larger machine is needed.

3)      Flashing:

             Flashing occurs when the polymer melt is squeezed into the parting surface between mold plates; it can also occur around ejection pins. The defect is usually caused by (1) vents and clearances in the mold that are too large; (2) injection pressure too high compared with clamping force; (3) melt temperature too high; or 4) excessive shot size.

4)      Sink marks and voids:

              These are defects usually related to thick molded sections. A sink mark occurs when the outer surface on the molding solidifies, but contraction of the internal material causes the skin to be depressed below its intended profile. Avoid is caused by the same basic phenomenon; however, the surface material retains its form and the shrinkage manifests itself as an internal void because of high tensile stresses on the still-molten polymer. These defects can be addressed by increasing the packing pressure after injection. A better solution is to design the part to have uniform section thicknesses and use thinner sections.

5)      Weld lines:

           Weld lines occur when polymer melts flows around a core or other convex detail in the mold cavity and meets from opposite directions; the boundary thus formed is called a weld line, and it may have mechanical properties that are inferior to those in the rest of the part. Higher melt temperatures, higher injection pressures, alternative gating locations on the part, and better venting are ways of dealing with this defect.