Plastic Prototyping : Various Options

Injection molding tool development requires longer lead-time and high investment. For initial testing and validation, engineers demands plastic parts in shorter lead times. This demand can be fulfilled with various plastic prototyping techniques. 

Following Points are considered while selecting prototyping technique :-

  1. Product Functional Requirements
  2. Aesthetic Requirements
  3. Cost
  4. Quantity Required e.t.c

Plastic Prototyping Techniques

  1. Additive Manufacturing / 3D Printing
  2. CNC Machining
  3. Vacuum Casting
  4. Rapid Tooling

1. Additive Manufacturing / 3D Printing

MakerBot-Replicator-Mini

Part are created by successive layers of material. Each of these layers can be seen as a thinly sliced horizontal cross-section of the final part.

Following 3D printing technologies are available with their own strength and weakness. 

  1. FDM (Fused Deposition Modelling)
  2. SLA (Stereolithographiy)
  3. DLP (Digital Light Processing)
  4. (SLS) Selective Laser Sintering 
  5. Material Jetting (PolyJet and MultiJet Printing)
  6. Metal Printing (Selective Laser Melting & Electron Beam Melting)

 

1.1 Material Options

Various grades similar to commercial grade plastic are radially available. With the technology advancement even aluminum can be 3D printed.

1.2 Engineering Tolerances : 

Depending on 3D printer and material used tolerance in the range of 50 to 150 microns can be achieved.

1.3 Cost  and Lead time: 

Cost and lead time for 3D printed prototypes is less compared to CNC machined prototypes. 

1.4 Part Quality : 

Very good quality prototypes can be manufactured with 3D printing. But the cost will be higher.

1.5 Ideal Quantities

1 to 20 nos

1.6 Advantages Of 3D printed Prototypes :

  • Metal parts can be also 3D printed.
  • Parts with deep undercuts can be printed as well.
  • Specialized tooling is not required.
  • Fast turnaround.

`1.7 Limitations of 3D printed Prototypes

  • Moderate manufacturing Cost and accuracy.
  • Critical features are very difficult to add.
  • Material options are limited.

2. CNC Machining

CNC Machining

CNC machining is a subtractive manufacturing process. In this process, material is removed from solid block with a variety of precision cutting tools to achieve final product.

Post processing operations such as buffing, sand blasting, plating, painting or screen printing are required to achieve good aesthetic finish.

2.1 Material Options :

ABS, PC, ABS+PC, HDPE, Nylon and Hard PVC etc. sheets are available for plastic part prototyping.

2.2 Engineering Tolerances : 

General tolerances ” ISO-2768 m” is followed for cnc machined plastic prototypes.

2.3 Part Quality : 

CNC machined prototypes are functional and identical to injection molded parts. Various validation tests can be performed on CNC machined plastic prototypes.

2.4 Cost : 

CNC prototypes manufacturing cost is very high. Therefore they are recommended for small quantities.

2.5 Ideal Quantities

1 to 20 nos

2.6 Advantages Of CNC Prototypes :

  • Highly accurate and repeatable
  • Various material options are available
  • Customized tooling is not required
  • Fast turnaround

2.7 Limitations of CNC Prototypes

  • High manufacturing Cost
  • Critical features are added manually

3. Vacuum Casting / Silicon Rubber Mold

What is Vacuum Casting ?

Polyurethane vacuum casting is used for making high quality and low volumes prototypes using silicone molds. Master piece (generally 3D printed part) is required for the manufacturing of Silicone Mould.

Silicone mold is filled with two part polyurethane resin inside vacuum chamber and then processed in a heat chamber at controlled temperature.

3.1 Material Options :

Materials equivalent to commercial injection molding plastics are also available.

3.2 Engineering Tolerances : 

Tight tolerance are very difficult to achieve with vacuum casting parts . Depending on feature complexity tolerance in the range of 100 to 150 microns can be achieved.

3.3 Part Quality : 

Silicon molded plastic prototypes are functionally and aesthetically identical to injection molded parts. Very few finish operations are required on vacuum casting parts.

3.4 Cost : 

Unit part cost with silicon molding is less than CNC machining if total required quantities are more than 10. Therefore vacuum casting is recommended for high quantity requirements.

3.5 Ideal Quantities

10 to 100 nos

3.6 Advantages Of Vacuum Casting :

  • Low cost and lead time for silicon mold manufacturing
  • Various material options are available
  • Fast turnaround.
  • Manufactured parts are similar to injection molded parts
  • Optically Clear prototypes can be manufactured
  • Overmolding and insert molding is possible

3.7 Limitations of Vacuum Casting

  • Manufactured part accuracy is not good
  • Slow Process

4. Rapid Tooling

If the required part quantities are moderate such as 100 to 2000 units and aesthetics are not very important. Quick turn injection tools can be a good choice.

Cost and Lead Time : Cost for rapid tools is higher compared to other prototyping process. Lead time for rapid tooling is approximate 2 to 3 weeks. 

Part Quality : Plastic prototypes manufactured with rapid tooling are identical to injection molded parts. Rapid tools are used to injection mold the part geometry.

Aesthetics : Part aesthetics are not as good as injection molding parts.

Following are the various technologies used for rapid tooling :

  1. 3D Printed Plastic injection molding tools.
  2. 3D printed aluminum injection molding tool. 
  3. Manual injection molding tool.

Conclusion

Properties 3D Printing CNC Machining Vacuum Casting Rapid Tooling
Tooling Cost NA NA Very Low High
Unit Part Cost Moderate Very High High Low
Form and Fit Moderate Good Good (not better than CNC) Excellent
Functional Moderate Good Good Excellent
Aesthetics Moderate Good Good Excellent
Lead Time Low (hours to days) Moderate (3 to 7 days) High (min 7 days) Very high
Quantities 1 to 50 1 to 10 10 to 100 100 to 2000
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