INJECTION MOLDING DESIGN GUIDELINES

  INJECTION MOLDING DESIGN GUIDELINES

            The important guidelines required while designing the mold are

 

Section thickness: Parts should be designed with a minimum wall thickness consistent with part function and mold filling considerations. The thinner the wall, the faster the part cools, and the cycle times are short, resulting in the lowest possible part costs. Also, thinner parts weight is less, which results in smaller amounts of the plastic used per part, thereby resulting in lower part costs. Use uniform wall thickness throughout the part. This is, because, the thin section first solidifies, and the thick section is still not fully solidified. As the thick section cools, it shrinks and the material for the shrinkage comes only from the unsolidified areas, which are connected, to the already solidified thin section. This builds stresses near the boundary of the thin section to thick section that leads to warping or twisting. Also uniform walled parts are easier to fill in the mold cavity, since the molten plastic does not face varying restrictions as it fills. Due to design limitations, when uniform walls are not possible, then the change in section should be as gradual as possible.

Recommended Radii: Sharp corners greatly increase the stress concentration. This high amount of stress concentration can often lead to failure of plastic parts. In addition to reducing stresses, fillet radii provide streamlined flow paths for the molten plastic resulting in easier fills. Use generous radius at all corners. Typically, the inside radius is 0.5 x material thickness and the outside radius is 1.5 x material thickness. A bigger radius should be used if part design will allow it.

Draft angles: Design parts to facilitate easy withdrawal from the mold by providing draft (taper) in the direction of mold opening or closing. Draft angles as low as 0.25° per side is required for highly polished surfaces and as high as 1.5° per side is common. The amount of draft angle depends on the depth of the part in the mold, part design, complexity and its required end use function.

Draft (A) in mm for various draft angles (B) as a function of molding depth (C).

Ribs: Ribs are effective design features that add strength and often facilitate flow during filling. However, proper design is important as ribs sometimes cause sink marks or aesthetic irregularities. Ribs are also used to improve part stiffness in bending. This avoids the use of thick section to achieve the same, thereby saving on part weight, material costs, and cycle time costs. Rib base thickness should not exceed 50% of the adjoining wall thickness. In addition, ribs should include proper draft and a base radius of at least 0.5mm.

INJECTION MOLDING PROCESS

INJECTION MOLDING PROCESS

                  

         Injection molding is a process of introducing the molten thermo-plastic material into a cavity or impression of the desired shape under pressure and allowing the molten material to solidify. No chemical reaction occurs during the molding process. (Any reaction that occurs would be a degradation reaction, which should be avoided). It is one of the most common and versatile operations for mass production of complex plastics parts with excellent dimensional tolerance. It requires minimal or no finishing or assembly operations. The molds are generally made out of Tool Steels and are provided with channels for cooling, heating and venting for efficient work of tool. When intricate shapes with external and internal undercuts are required, molds incorporate devices such as Side cores and Slide cores become necessary. Finally troubleshooting is carried out to the tool so as reduce the scrap and improve the part quality

 Advantages of Injection Molding Process:

 Parts with good surface finish can be produced at high production rates.

 For intricate shape and small parts, the process is economic way to fabricate.

 Parts can be molded with metallic or non-metallic inserts.

 Close dimensional tolerances can be maintained.

 Relatively low labor cost per unit is required.

 Minimum scrap as runner, gate and sprue can be reground and used.

Limitations of Injection Molding Process:

  High tool cost.

  Expensive molding machine.

  High maintenance cost.

  High power consumption.

Initial Settings to be done to the Machine:

          Set the barrel temperature, injection pressure, screw speed and shot weight.

          Set the injection time, cooling time, etc.,

          Clamp the mold halves in the machine platens.

          Set the ejection stroke.

          Adjust day-light.

          Trouble shooting

INJECTION MOULDING DESIGN CONSIDERATIONS

INJECTION MOULDING DESIGN CONSIDERATIONS 

In the design of injection mold, the following machine specifications are to be considered.

1. Locking or clamping force:

          This is the maximum force with which the two die halves are kept closed to resist the injection pressure. The clamping force required to keep the mold closed during injection must exceed the force given by the product of the opening pressure in the cavity and the total projected area of all impressions and runners. Thin sections need a high injection pressure to fill the impression completely and therefore require more clamping force.

2. Injection pressure: 

          This is the pressure under which the molten material is forced inside the cavity (up to 1800 kg/cm²). This pressure is set in the machine according to the plastic material to be molded, component thickness, etc.,

3. Shot weight: 

         The shot weight is the weight of the component including the feed system. Calculated shot weight should be less than the machine shot weight.

4. Distance between Tie bars:

          It is the minimum distance between the edges of the tie bars that guides the moving platens. This measurement limits the size of molds that can be placed between the tie-bars and into the molding machine. Tie bars are used in molding machine to hold the platens. It also guides the moving platen during closing and opening of the mold.

5. Minimum and Maximum Mold Height: 

          It is the minimum and maximum distance that can be adjusted between the stationary and moving platens. The mold shut height should always lie in between the maximum and minimum mold height.

6. Ejection force: 

          The force required to eject the component after cooling of the plastic material in the impression. It is about 3-5% of the locking force.

7. Maximum Ejection Stroke: 
          This is the maximum distance through which the component can be ejected during the open condition. The ejection stroke to be adjusted on the machine depends on the component depth.

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