1. Plastic Injection Molding
Nachiket Kulkarni
12/08/2016

2. Introduction Various optical technologies have plastic components
Injection molding can achieve mass production
Producing components with machining process may be expensive
Economical
Plastic is an alternative to glass
Limited by physical properties

3. Principles of Injection Molding
Injection Molding Machine
Pellets in the hopper passed through heated barrel
Pellets are heated to plasticity
Injected into the mold and cooled
Mold Design
Part Model
The mold is initially designed on CAD
Optical parameters considered

4. Mold Design Shrinkage Optical/ Surface Equation Optical Insert Design
Plastic shrinks when it cools
Mold design accommodate shrinkage
Depends on material, temperature, etc.
Optical/ Surface Equation
Surfaces like freeform don’t follow correct equation
Shrinkage should be applied
Customer should apply tolerance before providing to mold maker
Optical Insert Design
Determines how to manufacture and polish
Reduces assembly, labor cost, size and weight
Split Lines
Cope and Drag
Separated by a part line
Needed to design runner and gating

5. Principles of Injection Molding
Mold flow
Flow of hot fluid must be smooth
Depends on wall thickness, viscosity, gating system, etc.
If not taken care of, removal of material is tough.
Optical Insert
Determined by shape, aperture, etc. of the optic
Adjustability feature
Tolerances to achieve optical alignment
Thermal Conditions
Factors to be considered
Hot runner or cold runner
Heat level of mold
Cooling by natural convection or external means, etc.
Thermal properties of plastic to be considered
CTE of metal and plastic considered

6. Principles of Injection Molding
Gates and Runners
Regulate the fluid flow in the cavity
Runner is a passageway from sprue to gate
Small runner under fills the cavity
Larger increase cooling time
Gates connect runner and cavity
Permits enough flow to fill cavity
Compensates shrinkage
Controls turbulence
Ejection
Pressurized transfer of polymer
Types
Pin type, sleeve type, air ejection, blade ejection
Dependent on the cross section
Sleeve type for cylindrical core, stripper plate for larger components, etc.
Venting
Provision for gases to escape from the cavity
Good venting, better the mold, less stress in the part
Insufficient venting causes scratches and other defects

7. General Part design
Optical mold needs both optical and mechanical specification
Geometric Shape
Cavity for the part is same as mask to the face
Best geometric shape based on the application
Shrinkage and draft tolerance
Optical Surface and Control Parameter
Once geometry is chosen, front and back curves with their construction points are defined
These points are used to design the mold
While fabricating in an NC machine, these construction points are used to define the movement of the tool

8. General Part design CAD files Optical Correction Raytracing
Customer provides 3d CAD model, 2d control drawing, equations if needed with SAG table
2D control drawing has optical mold requirement like engraving, callout
CAD design should include thermal effects, shrinkage, possible choices of gates, ejector pin location, etc.
Optical Correction
Wall thickness is even
Optical mold, sometime doesn’t meet such requirement; manufacturing errors
Cross section must be optimized
Avoid trapped gases and voids
Raytracing
Optical performance confirmed by ray trace
Not all optimized results are feasible to manufacture

9. Materials Material is chosen with help of manufacturer
Design changed based on material
Commonly used
Polycarbonate: needs sufficient venting, hot mold
Polyurethane: Perfect parting line, cold mold
Polystyrene, acrylic, zeonex, etc.
Selection: Based on optical clarity, impact, haze, refractive index, transmission and viscosity
Additive and Coating
Anti-fog, anti-scratch, antistatic, mirror, etc.
Ionized air to remove static charge
Gold coating for 98%reflection
Silica-Organic liquid for anti-scratch

10. Optical Mold Special Tooling
Optical geometry and special tooling design
Geometry determines manufacturing processes for making mold
If simple, turning, lapping, grinding and polishing
Asphere needs different set of operations
Freeform needs diamond turning, hand polishing
Designs for special tool must be done during mold design
Finishing
Abrasive lapping and polishing
Sophisticated finishing needs diamond turn
Alignment
Two halves must align
To retain alignment when injected, locking mechanism is made

11. Inspection Steel vs Plastic Testing Determine if steel is in place
All components like inserts, finish inspected
Everything is correct means molded product might come out perfectly
Inspect the part
Testing
Testing criteria are resolution, power, scratch/dig
Contact methods: CMM
Non Contact method: Laser scan
Polarisers test for stresses
Interferometer measure form and figure

12. References
David Borque, “Manufacturing Plastic Injection Mold”, Novel Optical Systems Design and Optimization XI, (2008)
Ulrich Greis, G. Kirchhof, Agfa-Gevaert AG, “Injection Molding of Plastic Optics”, Optical Surface Technology, 0381(1983)