injection molding machine

1-3.
For years so-called product innovation was the only rich source of new developments, such as reducing the number of molded product components by making them able to perform a variety of functions or by taking full use of material’s attributes. In recent years, however, process innovation has also been moving into the forefront (Fig. 1-16). The latter includes all the means that help tighten up the manufacturing process, reorganizing and optimizing it. All activity is targeted for the most efficient application of production materials, a principle which must run right through the entire process from plastic materials to the finished product (Fig. 1-15 and Chap. 4). 28
2 Injection Molding Machines
INJECT ION (OR PLASTICIZING) UNIT
29
Fig. 2-1 General layout for an injection molding machine.
designer should also review Chap. 15 regarding micro injection molding. The type and size of IMM to be used are dependent on the molded product dimensions and volume, which determine the processing ) requirements and the shot size (Chap. 4 , as well as the required pressure and material behavior (Chap. 6). Examples of product dimensions that directly influence the size of the machine required include all part dimensions; the number of parts to be molded in a single cycle; the mold runner system needed to produce required number of parts; the mold width, length, stack height (if stacking is
used), and opening distance; and the ejector rod spacing. This information will determine the preliminary requirements for the IMM. Reciprocating (Single-Stage) Screw Machines 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 (Figs. 2-2 and 2-3). The screw
2
Injection Molding Machines
Introduction
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. 2-1 and
30
2 Injection Molding Machines
2 Znjection Molding Machines moves back to allow melted plastic to accumulate ahead of it, then moves forward, injecting all the melt into the mold in a single stage. The accumulation of melt at the screw tip forces the screw towards the rear of the machine until enough melt is collected for a shot. The back pressure required on the screw during this plasticating action is low, and when the shot size is produced, the screw stops rotating. With the mold halves closed, the nonrotating screw acts as a plunger and rams the melt into the cavity or cavities, using controlled injection pressure and rate of travel. After injection of the melt is complete, the screw rotates to prepare the next shot. The advantages of the reciprocating screw IMM over the two-stage IMM include the following: (1)reduced residence time, (2) self-cleaning screw action, and (3) responsive injection control. These advantages are key to processing heatsensitive plastics. Figure 2-4 describes a simplified sequence of operations for a reciprocating screw ma-
EXTRUDER SCREW
FLOW VALVE
I Iபைடு நூலகம்
HYDRAULIC INJECTION CYLINDERS
I
JR AIR LIFT FOR SCREW REMOVAL
I
PULL-IN CYLINDER
HEATING BANDS
Fig. 2-2 In-line reciprocating screw unit with hydraulic drive schematic.
Even though modern IMM with all its ingenious microprocessor control technology is in principle suited to perform flexible tasks, it nevertheless takes a whole series of peripheral auxiliary equipment to guarantee the necessary degree of flexibility. Examples include (1) raw material supply systems; (2) mold transport facilities; (3) mold preheating banks; (4) mold-changing devices, including rapid clamping and coupling equipment; ( 5 ) plasticizer-cylinder-changing devices; (6) molded-product handling equipment, particularly robots with interchangeable arms allowing adaptation to various types of production; and (7) transport systems for finished products and handling equipment to pass molded products on to subsequent production stages. There are different types and capacities of IMMs to meet different product and costproduction requirements. The types are principally horizontal single clamping units with reciprocating and two-stage plasticators. They range in injection capacity (shot size) from less than an ounce to at least 400 oz (usually from 4 to 100 oz) and in clamp tonnage up to at least 10,000 tons (usual from 50 to 600 tons). Other factors when specifying an IMM include clamp stroke, clamping speed, maximum daylight, clearances between tie rods, plasticating capacity, injection pressure, injection speed, and SO on, as reviewed in this chapter and Chap. 4. The