MOULD DESIGN:
The Quality of the Product depends on ( in Injection Moulding)
1. Moulding Temperature
2. Pressure
3. Speed of Injection
4. Method of Cooling
5. Flow behaviour of the Polymer
6. Design of Mould
7. Quality of the Mould
8. Performance of the Moulding Machine
Product, Product drawing &
Production specification
Production Volume (requirement in time)
1. Estimated Moulding Cycle (Cycle time)
2. Is the Drawing clear ?
3. Projection
4. 1st angle, 3rd angle
5. Tolerances
6. Notes on Drawing Draft angle, etc.
7. End use requirement
8. Moulding Material
9. Shrinkage
Factors to be considered during Design of Injection Moulds for Plastics
1. Optimum production requirements/time
2. Number of Cavities and Layout of Cavities
3. Selection of Moulding Machine
4. Layout of Cavities
5. Type of Mould
6. Design of Feed system
7. Temperature control (Cooling system)
8. Design of Ejection
9.Venting System
10. Shrinkage of the Plastics Material
11. Mould Material
Number of Cavities in a Mould
Factors that controls the Number of Cavities in a Mould are:
1. On the Machine Side:
Shot capacity
Plasticising capacity
Clamping force
Injection Pressure
Size of the Platens
2. On the Component Side:
Projected area of the Moulding (Clamping force)
Configuration of the Moulding (Injection pressure)
Wall thickness of the Moulding (Pressure)
Height of the moulding (pressure)
Size of the Moulding (Mould size, Platen size & Flow length)
3. On the Material Side
Injection Pressure
Injection Temperature
Mould Temperature
Shot Capacity
Density of the Material
Bulk Factor of the Material
Plasticising Capacity
Percentage Volume expansion of the Material at the
Moulding Temperature
(Crystalline and Amorphous Materials)
Total thermal heat content of the Material at the
Moulding Temperature
Specific heat of the Material
Latent heat of fusion of the Material
Temperature difference between the Moulding
Temperature and Room Temperature.
Shot Capacity: Maximum weight of the material that may be injected per shot.
1. For Plunger Machine
2. For Screw Type Machine
Shot Capacity = Swept Volume (cm3) x Density of plastic at Moulding temperature and pressure.
= Swept Volume (cm3) x d x c
Where:
d = Density of plastic at normal temperature
c = Correction for percent volume expansion of the plastic at moulding temperature.
Value of c
0.85 for Crystalline materials
0.93 for Amorphous materials
Plasticising Rate: Maximum material that the machine can bring per hour to the moulding temperature.
Plasticising rate Plasticising rate QA (Cal/gm)
With material B = with material A x ---------------------
QB (Cal/gm)
Where Q = total thermal capacity of the material at moulding temperature
Q = (Specific heat x Temperature rise) + Latent heat of fusion.
Clamping Force: Force available on the machine platen to prevent the mould from opening during injection.
Clamping force controls the maximum projected area of the moulding.
Clamping force Required (kgf) = Projected area of the moulding(cm2) X 1/3 to 1/2 of the injection pressure (kgf/cm2)
INJECTION PRESSURE:
Injection pressure inside the mould will depends on:
1. The flow characteristics of the material
2. Thickness of the moulding
3. Cavity depth
4. Projected area of the moulding
5. Flow length
6. Type, size and finishing of the feed system.
SELECTION OF INJECTION MOULDING MACHINE BASED ON NUMBER OF CAVITIES
OR
DETERMINATION OF NUMBER OF CAVITIES BASED ON ACTUAL MACHINE CAPACITY
Based on:
(a) Shot Capacity of the Machine
0.85 Ms Rw
Ns = -------------------------
Cw
Where:
Ns = Number of Impressions
Ms = Shot Capacity of the Machine (gm)
Cw = Weight of the Product (gm)
Rw = Weight of Feed system (gm)
(b) Plasticising capacity of the machine
(0.85 Mp x Tc) Rw
Np = ---------------------------------------------------------
Cw
Where:
Np= Number of cavities
Mp= Plasticising capacity of the machine (gm/hr)
Tc= Overall cycle time (hr)
(c) Clamping Force
Mc RA Ip
Nc=------------------------------------------------
CA x Ip
Where:
Nc= Number of Cavities
Mc= Machine clamping force (tons)
RA= Projected area of feed system (cm2)
CA= Projected area of the moulding (cm2)
Ip= Injection pressure (Design) (ton/cm2)
4.Layout of Cavities:
For efficient and Economical Layout the following points to be considered during Layout of the Cavities.
1. Optimum disposition of Cavities
2. Minimum Runner length
3. Balanced Layout
Optimum disposition of Cavities
1. Reduce the Mould size
2. Reduce the Mould cost.
Minimum Runner length Reduce the pressure drop
1. Fill all the cavities with required pressure and temperature.
Balanced Layout
1. Attain uniform clamping
2. Prevent local flashing of the mould.
LAYOUT OF CAVITIES
Mould size comparison
Area of Plate A = (a√2 + 2b ) 2
Area of Plate B = ( a + 2b ) 2
A>B= (a√2 + 2b ) 2 ─ ( a + 2b ) 2
= a2 + 1.656ab
5.Type of Injection Moulds
Classification of Injection Moulds based on:
1. Main Design features
2. Manner of Operation
These includes:
1. Type of Gating and means of De-gating Type of Ejection
2. Presence or Absence of undercuts
3. The manner in which the product is released from the mould.
Types of Injection Moulds are:
1. Standard moulds (Two or Three plate moulds)
2. Split cavity moulds
3. Stripper plate moulds
4. Stack moulds and
5. Hot Runner moulds
6. Design of Feed system
Feed system connects
The flow of the material from nozzle to the cavity
Feed system contains
1. Sprue alone (Direct sprue gate) or
2. Sprue, runner and gate in multi-impression moulds
Feed system is not proper
1. Difficult to get the product with optimum quality
MULTIPLE CAVITY WITH EDGE GATE:
SINGLE CAVITY DIRECT SPRUE GATE:
Runner Design
The designer should keep in mind the following points during design of runner.
1. Shape and Cross section of the runner
2. Size of the runner
3. Layout of the runner
Shape and Cross section:
Various shapes and cross sections are:
1. Circular
2. Semi- circular
3. Trapezoidal
4. Modified Trapezoidal
5. Hexagonal
6. Square
7. Rectangular
Runeer efficiency: is defining as the ratio of cross-section area to the periphery of runner. 1) Cross-sectional area. 2) Periphery of runner.
Cross Sectional Area
Runner Efficiency = ------------------------------------------
Periphery of the runner
Choice of Runner
For simple two plate tool with flat parting surface
Fully round or Hexagonal runner (Increased mould cost is relatively small)
For moulds with complex parting surfaces
semi-circular runner
For multi-plate mould
Trapezoidal or Modified Trapezoidal runner.
Gate design: Gate is a small channel that connects the cavity with the runner.
A small gate is desired so that: The gate freezes soon after the cavity filled. The injection plunger can be withdrawn immediately without forming voids due to suck-back. It is easy for degating. Small witness mark remains on the product. Better filling of cavities in multi-impression mould. More packing of material due to shrinkage effect is minimised..
Gate Location:
The location of the gate is decided based on:
1. Appearance
2. Function of the part (end use)
The area near the gate is highly stressed due to:
1. Frictional heat generated at the gate
2. The high velocity of the melt.
Gate location affects the molecular orientation of the resin during injection process which produces:
1. Directional variation in structural properties
2. Gate area should be away from the load bearing surfaces.
GATE SIZE:
The size of the Gate should be designed to permit optimum flow of material.
Optimum Size depends on:
1. The flow characteristics of the material
2. The wall section of the moulding
3. The volume or weight of the moulding
4. The moulding temperature
5. The mould temperature
Gate Area = h x w
Where: h = depth of gate
w = width of gate
n . √A
Gate width W = ----------------------------
30
Where : n = material constant (o.6-0.9)
A = surface area of cavity (mm2)
Depth of gate h = n . t
Where: n = material constant ;
t = wall thickness of the moulding (mm)
Land length L = 0.5 – 1.5 mm.
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