Rapid Tooling For Technology For Injection Moulding

Rapid Tooling describes the process where a Rapid Prototyping (RP) model is used as a master pattern to create a mould rapidly. The Rapid Prototyping model may also be used directly as a tool. The two halves of a tool are referred to as the ‘core’ and ‘cavity’.

Rapid Tooling (RT) first evolved in the early 90’s with the introduction of RTV silicone tooling from SLA master patterns, the mid 90’s saw the introduction of investment casting tooling, direct AIM tooling, sand casting and metal SLS tooling. By the late 90’s die casting and laminate tooling were introduced.4

The difference between Rapid Tooling and conventional tooling is:

• A time reduction of up to 1/5th is made when using Rapid Tooling.
• Rapid Tooling can cost less than 5% of the conventional tooling cost.
• Conventional tools generally have a longer life cycle.
• Larger tolerances for Rapid Tooling than for conventional tooling. [1] 
• The two types of RT method available are Direct and Indirect tooling. Direct tooling is a soft tooling method which uses an RP model directly as a tool for moulding whereas indirect tooling is where the RP model is used as a master pattern to create a mould or die.

Reasons for Rapid Tooling

Within the last 25 years market trends have changed greatly, the product life span of many products such as mobile phones has been reduced drastically with updated models being released as often as every 3 to 4 months. The variation and complexity of products available has dramatically increased with manufacturers under increased pressure to reduce the time to market for these products. Taking all of this into account it is clear to see that parts need to be produced cheaper and quicker, therefore enhancing the need for manufacturers to adopt RT methods.4

Two of the most important things that toolmakers need to consider are ‘if’ and ‘when’ to adopt RT methods. RT has many advantages over conventional tooling methods:

Speed: The majority of RT techniques offer an increase in speed compared to conventional tooling methods. A tool with ribs and bosses may take multiple operations i.e. CNC programming, CNC milling and EDM; however with RT the same tool may be done in one swift operation. [2] 

Cost effective for complex tooling: With RT methods it is possible to create complex geometries which would not difficult to produce by conventional methods.

Automation: Automation of many of the RT processes means tooling can be build 24 hours a day without any human interaction. This improves productivity, and more tools are produced without the increased amount of manpower it would take to produce the same number conventionally.

Human error: Human error can be significantly reduced by adopting RT methods and building a tool directly from the master pattern. Conventional methods may incur incorrect CNC programming or misinterpretation of CAD/technical drawings.2

Design possibilities: It is possible to integrate conformal cooling channels into complex tooling inserts when using RT methods. Tool design is not limited to designing tools which can be conventionally machined.2

Figure – Rapid Tooling Vs Conventional shows the typical time savings which can be made by employing RT techniques oppose to conventional machining techniques.

Rapid injection molding vs. conventional injection molding

Figure – Rapid Tooling Vs Conventional Tooling [3] 

Direct Tooling

Direct AIM Tools

Direct AIM (ACES Injection Moulding) is the process whereby tools are created directly on an SLA machine. The tools are initially designed using CAD software and the process involves creating a part by SLA which is basically a shell on the underside. The purpose of the shell is to leave a cavity so that each half of the mould can be filled with a backing material such as an epoxy resin, metal or ceramic. By backfilling the mould a thermal conduit is provided for the heat exchange process and it is also possible to add any cooling channels to the mould at this stage. [4] 

The surface of the moulds is finished to improve the quality of the surface. Using this method it is possible to create up to 100 parts with an accuracy of ±0.15 – 0.3mm.

Typical application for this type of tool would be for smaller parts, mainly prototype injection moulding tools, low volume wax injection tooling and low volume foundry patterns. [5] 

Advantages

• A relatively fast process – a mould can be designed and built within a 2 week period.
• Cheap process for small tools, such as mobile phone and mp3 player casing. Building large parts on an SLA machine is not cheap.

Disadvantages

• A CAD model of the tool is required as this has to be saved as an stl file in order for the SLA machine to build the 3D tool.
• Low durability – the complexity of the tool and thermoplastic material used to build the tool all affect its life cycle. Moulds produced this way can create as little as 10 parts.
• Moulds typically degrade gradually with each part that is moulded on it.

Laser Sintered Tooling

Tooling inserts made by sintering are initially designed using CAD software and then produced by using DMLS (Direct Metal Laser Sintering) or SLS (Selective Laser Sintering) methods.

SLS Rapid Steel

Rapid steel powder is used to directly build a tool cavity using laser sintering; the powder consists of a stainless steel particles coated in a polymer binder. The parts which are produced are called ‘green parts’ which are then put into a furnace. The furnace removes the polymer binder and infiltrates bronze into the mould to create a dense 60 (steel)/40 (bronze) part. The tooling inserts are then finished and fitted to a bolster.

Advantages

• A relatively fast process which will produce a strong metal tool.
• Conformal Cooling channels can be built into the tool.
• Possible to create complex geometries.

Disadvantages

• Finishing and polishing is required.
• Poor accuracy.
• Equipment cost is high.
• Size limitations, max 200 x 200 x 100mm

Copper Polyamide Tooling

A Copper and polyamide powder is sintered to form the tool. Only the polyamide particles in the powder are actually sintered. The advantage to this process is the tool strength and heat transfer compared with other methods. Copper provides the tool with these characteristics, allowing the tool to be used at high pressure and temperature. [6] 

This method is suitable for several hundred mouldings.5

DMLS (Direct Metal Laser Sintering)

Using a laser sintering machine, metal in the form of powder is sintered to produce a tooling insert. The two available materials are Bronze and Steel based, the bronze based material offers a higher definition of features than the steel based one.6

Laminated Tooling

Laminated tooling is very similar to the LOM (Layer Object Manufacturing) process as slices of a CAD model are replicated by layers of cut sheet metal. The steel laminations are laser cut or cut with a water jet.

Tooling inserts are initially designed using CAD software; the CAD model must represent the cavity of the tool in order to produce the mould. The slices of the cavity are cut in sheet metal which has a thickness of 1mm and then bonded, clamped or brazed together. The use of a thick laminate results in a poor surface finish so the tool must be finish machined.3

Typical application for this type of tool would be large complex tools and aerospace tooling.2

Advantages

• Efficient use of material due to layers being cut to the exact size required.
• Conformal Cooling channels can be built into the tool easily if required.
• Standard steel sheet is used, making the process relatively quick and cheap.
• Good for large tools – up to 2000 x 1000 x 500mm
• Design of parts can be easily changed by replacing a laminate layer as long as it has not been bonded.

Disadvantages

• Tools have to be finish machined to remove the step like features to obtain accuracy.
• The joints between each lamination provide the tool with a weak link.
• Part complexity is dependent upon layer thickness.

Indirect Tooling

Rigid Cast Resin Tooling

This process manufactures a tooling insert using aluminium filled epoxy resin as the tool material. A master RP model is initially manufactured and the part is set up on a split line. The resin is then cast onto the model which is within a bolster. The resin is then left to cure, a release coat is applied to the mould, the shut-off material removed and the process is repeated for the other half of the mould. When both halves of the mould have cured, the shut off material is removed and a bolster and ejector pins are added.2,4

Typical application for this type of tool would be a small sized tool, low volume RIM (Resin Injection Moulding) tools or low volume press tools. Accuracy of the tool is dependent upon each step within the process so shrinkage and deformation must be taken into account when reviewing the overall accuracy.

Advantages

• Quick to produce, 2-3 days.
• Cost is typically 40% less than with conventional tooling. [7] 
• Quick repair on tools is possible.

Disadvantages

• Flash can occur resulting in more effort required to trim mouldings.
• Difficult and slow to mould
• Fragile and easy to break.
• Repairs are not long lasting.
• Distortion is possible with larger tools due to exothermic processes.

Cast Metal Tooling

Sand Casting

A master pattern is placed in Green sand to create a mould, the pattern is removed and the cavity of the mould is filled with molten metal. The metal is left to cool and the sand mould is broken away to leave a finished casting.

Investment Casting

A master pattern is created from wax or a material which can be melted. The wax pattern is then dipped in slurry consisting of plaster of Paris, binder and silica repeatedly to create a surface on it. The mould is then heated up in an oven leaving the wax to melt away. The completed mould can then be filled with a molten metal to create the part.

Rubber Plaster Casting

A master RP pattern is created and shut off, silicone is cast in the shape of the tool. Liquid plaster slurry is poured around the silicone, once cured the silicone is removed. Molten metal is then poured into the plaster mould. [8] 

Advantages

• Solid metal tools are produced.
• Conformal cooling is possible.
• One master can allow multiple tools.
• Steel tools can be made but with increased difficulty.

Disadvantages

• Tools may need to be finish machined and polished.
• Difficult to hold tolerances.

http://www.crptechnology.com/sito/images/stories/ElementiFissiHome/rapid-casting.jpg

Figure – Investment Casting, RP model on left. [9] 

Metal Spray Tooling

This method is used to produce soft tooling inserts. A master pattern is produced and shut-off; a thin shell of 1-2mm of zinc is sprayed over the pattern, this shell is then removed and backed up with an epoxy resin or ceramic to make the mould more rigid. This is then repeated for the other half of the tool. The surface of the metal shell is usually polished and even sealed.

Electric Arc Spraying

In this process two conductive metal wires are melted by means of an electric arc. The metal melts, and the molten material is atomised by a gas and propelled on to the surface of the pattern. The molten particles on the pattern rapidly solidify to form the metal coating of the shell. [10] 

High Velocity Oxygen Fuel

Metal powder particles are injected into a high velocity jet. The jet is formed by oxygen and fuel combusting and heating and accelerating the molten metal towards the surface of the pattern. Metal coatings produced this way are strong and very dense allowing a thicker coating to be applied to the pattern compared to electric arc spraying. [11] 

Advantages

• High quality surface finish.
• Relatively quick.
• Fine detail such as graining can be achieved.
• Conformal cooling is possible.
• Large scale tools can be produced.

Disadvantages

• Line of sight limitations – impossible to spray undercuts or narrow slots.
• Surface is porous so may need to be sealed to reduce infiltration.
• Any repairs and modifications are very difficult to undertake.
• Special equipment and operating environment is required.

Figure – H.V.O.F process [12] 

Electroformed Nickel Tooling

Nickel Shell Tooling

This method involves a nickel surface being created on an RP model. A master RP pattern is produced and shut-off, the part is then put in an electroplating bath to form a nickel shell on the surface. Once plated, the part is removed from the bath; the nickel shell is removed and backed up with a thermally conductive ceramic material. Cooling channels, typically made from copper can be built into the mould at this time. [13] 

Typical application for this type of tool would be large production vacuum forming tools and composite forming tooling for the aerospace industry.

Advantages

• Detail from the master model is picked up almost perfectly.
• Nickel provides a smooth surface which is dense and hard.
• Low thermal stress compared to metal spray techniques.

Disadvantages

• Slow process which can take up to 6 weeks to produce a 6mm shell.
• Line of sight limitations

Nickel Vapour Deposition (NVD)

This method converts Nickel Carbonyl gas (NiCO4) into a solid Nickel shell. A master pattern is created from aluminium or steel, and placed into a special chamber which heats the pattern up to 110-180oC. Nickel Carbonyl gas is passed over the pattern, and nickel is deposited onto the pattern to create a metal shell. The pattern is then removed from the chamber; the shell is backed up and removed from the pattern. This process is then repeated for the other half of the mould.

Advantages

• Extremely fast, 0.25mm/hr (20 times faster than electroforming). [14] 
• A more uniform wall thickness than electroforming.8
• No line of sight limitations.
• Conformal heating and cooling is possible.

Disadvantages

• A dangerous process which can be explosive.
• The master pattern must be heated evenly.

Indirect Sintered Tooling

3D Keltool Process

Keltool is the name given to the “powder metal sintering process which involves the infiltration of a fused metal part with copper alloy”. [15] 

An RTV mould is created from an SLA master pattern. When the pattern is de-moulded, slurry consisting of A6 tool steel and tungsten carbide is poured into the RTV mould. Once cured this mould is infiltrated with copper and sintered to cure the mould and increase its strength. The completed tool can be machined and has a hardness similar to A6 tool steel.9

Using this process it is possible to create a tooling insert, from master pattern to the finished product in under two weeks. Tool life expectancy can be anything between 100,000 to 10,000,000 shots dependent upon material being moulded.9

Typical application for this type of tool would be small tooling inserts.

Advantages

• Good for complex mould geometry.
• Extremely fast process.

Disadvantages

• Size limitations – 6″ in all directions.
• Difficult to machine detailed designs.
• Figure – 3D Keltool parts [16] 

Tool Considerations

When designing a tool, a number of considerations must be taken into account:

Wall Thickness.

Sliding Cores.

Size and location of runners and ejector pins.

Gate design.

Size and number of cooling channels if required.

Split line position.

Shrinkage

Wall Thickness

It is possible to create walls with various thicknesses. A wall with an uneven thickness can cause problems for the tool designer, as thicker walls cool much slower than thin walls therefore resulting in greater shrinkage at the thicker sections. A uniform wall thickness will minimise any defects caused by uneven cooling. Shrinkage will also occur at wall intersections (tees). [17] 

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Sliding Cores

Sliding cores allow undercuts to be made; sometimes it may be possible to relocate the split line to reduce the number required. Sometimes it may be a case of re-designing a feature in order to reduce tooling costs. Any additional cores will just increase the overall cost and complexity of the tooling insert. Figure – Redesigning a feature18 shows a hinge feature which has been redesigned to eliminate the requirement for the sliding core shown on the left.

Figure – Redesigning a feature [18] 

Ejection Methods

Ejector pins are placed in the cavity or core of the mould and push the solidified moulding out of the mould. This is the most common method of ejection, the ejector pins are carried in an ejector plate which is in the mould. These pins should be positioned at points with good strength to avoid any lasting damage to the part.5

Other methods of ejection may use plates or some method of gas or air ejection to ease the part out of the moulding.

Gate Design

A Gate is the opening in the mould where the resin will enter from. The design and placement of gates is an extremely important factor to consider. Resin is injected into the mould at pressures of up to 20,000 psi. The immense pressure can cause gas to be forced into the liquid resin, which when cooled results in bubbles being formed in the solidified moulding. To eliminate this problem it may be necessary to add vents within the mould to allow air to be displaced as the resin is injected. [19] 

Gates should be positioned at the thicker areas of the part; the thinner areas will lose heat quicker causing the resin to cure before it reaches the thicker areas.

Knit lines occur when the flow of resin is split by a core in the mould. Where the resin rejoins there may be a slight defect due to cooling and the two edges not fully merging together to create a smooth blend. This will result in a visible line which may affect aesthetics or structure of the part. A more structured gate placement may improve the resin flow and eliminate any knit lines.17

Conformal Cooling

Cooling channels for Moulds are traditionally drilled in a secondary machining operation. These cooling channels are only able to follow straight lines, if a complex cooling channel is required, the mould is split into segments and channels milled into each segment. The segments are then welded back together so the channels align producing a cooling channel which is not straight. [20] 

Conformal cooling channels follow the shape of the mould and allow temperature to be distributed uniformly in the moulded material. This method is only available when using RT methods to create a mould.

Conformal cooling can save money when thermal management is extremely difficult via traditional tooling methods. Recent studies have shown a 30-60% reduction in cycle times compared to conventional methods. [21] 

Figure – Conventional Vs Conformal Cooling18 shows the same mould with traditional drilled channels on the left and conformal cooling channels on the right. The conformal cooling channels follow the curves of the mould closely.

Figure – Conventional Vs Conformal Cooling18

Split Line

The split line is the line at which the two halves of the mould meet. In some cases the tooling may not be precise allowing the mould halves to open and close without any precision. The high pressure injection process will cause resin to creep into any gaps between the mould halves; this material is referred to as flash.

Strategic positioning of the split line is necessary to improve part quality and to facilitate with ejection.

22 shows the same part but with the split line (red line) at different locations. On the image on the left, the walls of the part are in the bottom half and are slanting to allow the part to be ejected. This leaves the wall at the base being much thicker. If conventional methods of tooling are used, the deep narrow cut may have to be made wider to allow the machine tool full operation resulting in an even thicker wall.19

On the image on the right the top half of the tool is the core which forms the walls. This results in walls with a uniform thickness. If conventional methods are used, tooling is made easier as larger sized cutting tools can be used.19

http://www.protomold.com/designtips/2006/2006-05_designtips/images/fig1.jpg

Figure – Split line at different locations. [22] 

Shrinkage

The majority of tooling methods involve a change of phase. A material is transformed from a liquid to a solid or solid to a liquid and back to a solid. In each case, the phase change results in a decrease in volume therefore results in shrinkage. [23] 

All of the tooling processes involve some level of volumetric shrinkage therefore some sort of shrinkage compensation is usually given for each process. It is usually a case of measuring the linear shrinkage for a given material in a particular process and then applying a shrinkage compensation factor to any other part dimensions produced this way. A part is intentionally built oversized so that when shrinkage occurs, the part will be the correct size.20

In principle this sounds great but in practice it is not so easy to achieve precise dimensions through shrinkage compensation.

Case Studies

Thermoplastic composite (GMT) forming tooling using thermal spraying Zinc.5

The aim of this project was to find a way to reduce the time taken to produce tooling by evaluating a different method using thermally sprayed zinc, backed with ceramic.

The GMT floor pan assembly required 5 parts: Main floor, 2 cross beams, battery box and lid.

The master pattern was machined and thermal sprayed with a 2mm layer of zinc. The shell was then put in a steel bolster and copper cooling channels were added. The zinc shell was then backed with a chemically bonded ceramic. The die was then ready for moulding. Moulding trials took place with a 1000 tonne press.

It took 8 weeks to produce using a metal spray tooling technique oppose to the 16 weeks it would of taken using traditional machined tools. The total cost was £80,000 – a saving of £170,000.5

Feasibility study of arc spray welding onto a master RP model. [24] 

The model used for this project was a handheld phone. Overall dimensions of the model were 100 x 50 x 20mm. An ABS RP master was fabricated and put into a bolster, and then arc sprayed to create a shell of 1.5mm thickness.

Aluminium epoxy was used to back the shell; this took 24 hours to cure. The process was then repeated for the other half of the mould. The surface of the shell was polished to improve surface finishing then it was ready for injection moulding.

Tool development cost

ITEM

COST ($)

RP Master

200

Bolster

500

Sand Blasting

100

MMA resin system

500

Arc metal spraying

800

Sprue bushing

200

Reinforcement block

50

PVA

50

Labour ($20/h)

20 x 89hrs = 1780

TOTAL = $4180

Estimated cost of the tool was $4180, a traditional tooling shop quote was between $10,000 -$15,000 for the same tool. An approximate time and cost saving of 50% was achieved, the tool was also completed in less than 2 weeks.

Kodak reduces tooling costs.20

A project being run at Kodak needed 25 different plastic injection moulded geometries. By using rapid tooling as a method of bridge tooling they reduced lead times by up to 85% compared with CNC/EDM generated tools. By using a composite aluminium filled epoxy they were able to create tooling inserts capable of moulding in excess of 1000 parts. in some case product development cycles were cut by a year. 20

By employing RT methods, Kodak are typically saving about 25% in tooling cost compared with traditional methods. They are able to: Test, iterate, retest and proof multiple designs far more rapidly. Form, fit and function can be tested with true prototypes which have been injection moulded with the desired end use material.20

Conclusions

Rapid Tooling is a growing area which still has room for improvement and development. In the future, reducing the cost of tooling will play an important role in enabling smaller runs of parts to be made as well as allowing more product customisation for niche markets. Developments in Rapid Tooling will mean product development can be initiated closer to market entry time meaning manufacturers can gather more up to date market trends before the product is manufactured.

From the research conducted and case studies viewed it is clear to see that time and cost savings can be made and productivity increased when employing Rapid Tooling techniques. The production time of tooling inserts can be shortened by a near fully automatic procedure from start to finish.

Rapid Tooling is not however cheap, cost of the RP machine and other machinery such as Arc welding equipment and resins has to be taken into account. The future development of SLA resins and further improvements in Rapid Prototyping machines will only aid in the development of Rapid Tooling.

Rapid Tooling still has a lot to offer, this is just the beginning; future improvements in CAD software will allow the whole process to become far more efficient.