MACHINE SELECTION

MACHINE SELECTION

Important parameters involved in selecting an injection-molding machine to mold a quality part depend on

a.      Proper Screw and Barrel Selection: Screw performs various functions in injection

Molding process. It mixes the material to the proper molten state, conveys material through the barrel, compresses the material to the maximum density and forces the material into the mold. Barrel is robust cylinder, which may be nitrated. Since the pressure used in molding can be as high as 250Mpa, it is important that the walls be thick enough to withstand this pressure. Two important considerations while designing the screw are L/D ratio and Compression ratio. L/D ratio of the screw is defined as flight length over the diameter. Typical values ranges from 16 to 25. The greater the ratio the greater the mixing. Compression ratio of the screw is defined as the ratio of the volume of the material in the metering section over the volume of material of the feed section. Typical ranges are 2 to 3.5. This parameter shows how much the material is compressing.

Figure:  A typical extruder screw for injection molding.

b.      Proper Nozzle Tip Type: The reverse taper nozzle tip is recommended as it minimizes material drool that is encountered while molding crystalline material. For easy removal of sprue, sprue-bushing diameter is taken as 0.5mm to 1mm larger than the nozzle tip orifice diameter.

Figure:  General Purpose-Nozzle.

c.       Type of Check Valve: Proper designed check valve ensures a consistent shot size. Maintaining it to a proper working condition is critical to ensure quality and consistent moldings. Inconsistent shot size, dimensional inconsistency in parts, sink marks due to lack of back pressure, potential to degrade material, possible override of barrel temperature settings, etc. are several issues that may result if the check valve is not functioning properly. 

Figure:  Check Valve.

d.      Clamp Requirements: It is important to have adequate clamp force to maintain a fully closed tool during injection process. Insufficient clamp pressure results in flash at the parting line. 

2.5.2  SPECIFICATION OF INJECTION MOLDING MACHINE

Following table shows the specifications of Special purpose (SP30) Injection Molding Machine

MODEL NO		SP30
Tonnage	tones	30
Screw diameter	mm	30
Injection pressure, max	Kg/cm²	1800
Shot weight, max	gms	63
Injection rate, max	cc/sec	144
Screw stroke	mm	100
Plasticising rate, max	gms/sec	6.1
Screw speed	rpm	250
Mold opening stroke	mm	370
Min. mold height	mm
Max. mold height	mm
Max. Daylight	mm
Distance between tie bars	mm	280X280
Ejection force	kgf	2.4
Ejection stroke	mm	65
Register ring diameter	mm	120
Nozzle radius	mm	10
Nozzle diameter	mm	3.5

Table: Specification of injection Molding Machine