Injection Mold Design 66264n

Injection Mold Design Dr. Naresh Bhatnagar Mechanical Engineering Department Indian Institute of Technology-Delhi

Basic Structure of an Injection Mold

DESIGN PROCESS • FEEDING SYSTEM (Sprue, Runner, Gates) • COOLING SYSTEM (Channel, Coolant, Temp) • EJECTION SYSTEM (Pins, Stroke, Actuation)

Mold Types Different types of molds: (a) the cold-runner two-plate mold (b) the cold-runner three-plate mold (c) the hot-runner mold (d) the insulated hot-runner mold (e) the hot-manifold mold ( f ) the stacked mold.

Two Plate Injection Mold

Three Plate Injection Mold

Important Phases in Deg Injection Mold • • • • • • • • • •

Parting Lines Weld Lines Meld Lines Sink Marks Vent, Trapped Air, and Ejector Undercuts Blind Holes Runner Design Mold-Cooling Design Gate Design

Parting Lines

Vent, Trapped Air, and Ejector

UNDERCUT

Example of an undercut made possible by using an ejector pin

Multiple-Cavity MOLDS

Partially filled cavities in an eightcavity balanced runner mold

Types of Runner Systems Naturally Balanced

Artificially Balanced

Types of Runner Systems Cold Runners 1

1

2

2 2-Plate Mold

3 3-Plate Mold

Types of Runner Systems Hot Runners Cartridge Heater Nozzle Frozen Layer

Shut-Off Pin

Heater Gate

Melt Cavity Melt

Insulated

Internally Heated

Melt

Externally Heated

What is the Best Runner Diameter? • Allows the Mold to be Filled Quickly • Minimizes Scrap in the Runner • Delivers the Melt as Uniformly as Possible

What is the Best Runner Diameter? 800

For PS, ABS, SAN

700

G: Weight (g) S: Nominal thickness (mm) D: Reference diameter (mm)

500 400

300

5

4 4.5

3 3.5

2 2.5

100

1.5

200 S=1

G(g)

600

0 2 2.5 3 3.5 4 4.5 5 5.5 6 6.5 7 7.5 8 8.5

D’

When To Use a Hot Runner System? • Long Runner Lengths (High Pressure Loss in Cold Runner) • Reduce or Eliminate Scrap in the Runner System • Minimize Cycle Time for Thin Walled Parts

Determining the Best Gate Locations • Should Allow Part to Fill Within Target Pressure • Must Properly Place Weld Lines • Should Not Be Located in Structural Areas

Determining the Number of Gates Needed • • • •

Dependent on Flow Length Dependent on Part Thickness Dependent on Resin Being Used Should Promote a Large Process Window

Gate Effects

An example of jetting in an injection mold

Melt front progression during injection molding of the part. The part is one of the two covers for a 3.5 inch computer floppy disk. The illustration shows the melt front just before the cavity is filled. Before the availability of molding CAE, the short shot was the molding engineer’s favorite diagnostic tool.

MOLDFLOW RESULTS

Air traps

Orientation at skin

Weld lines

Fill time

MOLDFLOW RESULTS

MOLDFLOW RESULTS Temperature at flow front

Clamp force XY plot

Fill time

Air traps

FILLING SIMULATION

Mold-Cooling Design

Differing temperatures on opposite sides of the mold causes the part to be concave towards the hot side

Cooling Design • How Cooling Time Affects Cycle Time • How Wall Thickness Impacts Cooling Time • Considerations for a Good Cooling Design

Good Cooling vs. Bad Cooling

Proper Cooling

Poor Cooling

Better Part in Shorter Time

Poor Part in Longer Time

Part temperature distribution at ejection shows that initial cooling system does not provide even cooling of the part.

Improved cooling system results in more even temperature distribution.

Mold layout with cooling channels and cavity

Volumetric shrinkage distribution across the molded part at the time of ejection.

Mold wall temperature distribution at the start of the injection cycle.

Walls close to the intersection of the rib and the base structure are warmer than other areas. Shrinkage will be higher there.

Cooling line layout with manifolds. The flow rate in each cross channel is different, ranging from 4 liters per second at the channel closest to the inlet and outlet, to half that for the channel farthest from the inlet.

Serial connection of cooling system has equal flow in all legs.

Fiber Orientation

Fiber Orientation in a section of a Glassfilled polypropylene part

Tensile stress/strain behavior of a 30% carbon fiber reinforced polycarbonate parallel and perpendicular to the flow direction.

Gate Effect on Flexural Strength

Edge-Gated part performed but Center-Gated failed

Gate Positions

The effect of Gate Position on fiber orientation

Warpage

Warpage of a Tsection, end-gated part with and without fiber filler

Gate effect on Warpage The effect of

differential shrinkage on a flat fiber-filled part that is center-gated

Warpage Tendency

More shrink on the side with the hole causes warpage.

Model of the radiator end cap with a central gate

Cavity filling of the automotive radiator end cap

Warpage resulting from the fiber orientation from a central gate

Warpage from the central gate, with the reference plane changes to the nodes at one end

Warpage predicted for the center gated radiator end cap. The upper left window shows the model. The upper right widow shows the warpage referenced to a plane defined by the to be near the end of the cap. The lower right shows the shrinkage in the X-axis direction due to differential area shrinkage. The lower left window shows the shrinkage from orientation.

Center gate flow directions as the melt front es each element. These can be correlated with the fiber orientation.

Resulting flow directions and fiber orientation for a part with an end gate

Melt front temperature from a mold filling simulation, showing effects of the melt hesitating in the thin region.

Figure 10.10:

Finite element mesh of the part, automatically generated from CAD solids model.

Volumetric shrinkage distribution demonstrates potential different shrinkage that can cause stresses between different regions of the part. Red areas represent regions of high shrinkage; cooler colors represent regions of lower shrinkage.

Distribution of process-induced shrinkage of the molded part. Process-induced warpage includes the effects of both non-uniform shrinkage and uneven cooling of the cavity faces.

Pressure distribution at the end of the packing phase shows over packing near the gate. This will also result in lower volumetric shrinkage in this area. Since the time is at the end of the packing phase, pressure at these points indicate a residual pressure that time.

Fiber orientation on the skin of a molded part. The short lines follow the direction of fiber or molecular orientation on the surface of the part. This layer is oriented by shear stresses between the layers of the plastic as the cavity is filling. As the plastic touches the cold cavity wall, it is frozen with the orientation effects in place.

Comparing a mold filling simulation to an actual filling pattern in an 8-cavity mold. The error in the simulation results from the use of simplified 1D beams, which are standard with most of today’s state of the art injection molding simulation programs.

Hybrid Composites

Hybrid composite materials, incorporating both fiber and flake reinforcements, have mold shrinkage values that tend to be more isotropic than conventional fiber-reinforced polymers

SIMULATION

Mold Flow simulated model

Fill Time

Shear Stress at Wall Shear stress at wall at gate location for different types and sizes of gates Material TUFNYL S13 maximum allowable shear stress = 0.5 MPa

FILLING SIMULATION

Fiber Effects Comparison between flow and crossflow shrinkage and the effect of fiber type on shrinkage

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