Rapid Tool Design Study Guide

Rapid Tooling Design Training Objective After watching the program and reviewing this printed material, the viewer will understand the principles of rapid tooling design and processes. • • • •

Design aspects are detailed Indirect and direct processes are explored Mold creation is demonstrated Cooling and cycle times are examined

Rapid Tooling Basics Rapid tooling broadly describes any mold-making process that can create tools quickly and with minimum direct labor. The rapid tooling concept includes tool-making approaches that apply additive, subtractive, and pattern-based processes. All can be applied in a variety of applications from injection molding to casting and sheet metal stamping operations. Rapid tooling can be deployed in two forms: • •

Indirect tooling Direct tooling

Indirect tooling incorporates pattern-based methods where the tool is cast from a rapid prototype that depicts the part to be molded. Direct tooling is a method where the mold, inserts, or other components are made directly by a rapid prototyping machine. Rapid prototyping, which is also called additive fabrication, additive manufacturing, and freeform fabrication, grows physical objects directly from 3D CAD data by bonding layers of material. The process is particularly suited to prototype mold creation. The additive nature of the process allows intricate details and complex shapes to be manufactured with little effort and no impact on time or cost. Also, much of the process is completed without direct labor or a machine operator in attendance. There are numerous rapid prototyping technologies used today including: • • • •

Stereolithography Laser sintering Fused deposition modeling Various ‘3D’ printing processes

Although most rapid prototyping technologies can be used for indirect, pattern-based rapid tooling, the bulk of the direct tooling work uses a metal-based technology such as direct-metal laser-sintering, selective laser melting or laser cusing. Indirect Rapid Tooling Processes Indirect rapid tooling processes produce molds by forming them from a pattern. A common example of this technique is RTV, or room temperature vulcanized rubber molding. Typically, the pattern used to produce this type of indirect rapid tooling is constructed using a rapid prototyping machine. Following the pattern’s construction, it is postprocessed to prepare it for mold creation. Once post-processed, development of the first mold half typically begins by securing the pattern in oil-based clay, or other building materials, and constructing the parting line. Fundamentals of Tool Design Study Guide, DV08PUB3

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Rapid Tooling Design Silicone RTV molding material is then poured around the pattern and allowed to cure. Typically, it is necessary to apply a vacuum to either the RTV molding material before pouring, or the tooling assembly after pouring, to remove air bubbles from the molding material. Once the silicone has cured, the second mold half is prepared and poured. A gate for injection is added along with venting to allow air in the mold to escape. Once the second mold half cures, the pattern is removed and the mold is prepared for use. RTV tooling can be used to mold small to medium quantities of parts out of a large variety of urethane, epoxy, or other polymer materials. RTV tooling can typically be used to mold many parts before replacement tooling becomes necessary. The tool life depends on the casting material, accuracy, finish requirements, and the complexity of the part geometry. Dozens of simple parts can be produced from a single silicone rubber mold, but 10-20 is typical if the parts are more complex. Direct Tooling Processes Direct rapid tooling typically uses metal-based rapid prototyping technologies to generate molds and mold elements. These processes are still emerging and developing as new technologies, materials, and system features are introduced. The principle advantage of direct tooling is its time and cost-saving aspects, often up to 66% over other mold-making methods. Each element of the tool is evaluated separately to determine the best construction process. Rapid tooling can be combined with machined metal cavities and purchased mold bases. This digression from traditional machining and manufacturing processes promotes the use of rapid prototyping-based tooling when elements of a mold are intricate and highly detailed. The most common metal-based rapid prototyping methods are: • •

Direct metal laser sintering (DMLS) Selective laser melting (SLM)

These additive processes produce tools made of stainless steel, cobalt, chromium, maraging steel, titanium, and alloy blends. Additionally, some companies have found success using stereolithography to create composite-plastic mold inserts using resins such as the ceramic filled nanotool to produce small, short-run molds for injection molding. The basic, four-step process for constructing injection molds with rapid tooling techniques is the same as that when machining a mold: 1. 2. 3. 4.

Tool design Component building Tool assembly Injection molding

Tool design involves determining what components are to be rapid tooled or machined. This includes the design of the tool inserts and the pockets in which they will seat in the mold core, cavity, or base. Other design factors include draft angles, gating, venting, and reinforcement ribs. Component building includes the simultaneous machining of mold elements while rapid prototyping of the mold inserts is taking place. Once rapid prototyping is complete, the inserts are post-processed. For nanotool inserts, post-processing includes parts washing, support removal and ultraviolet curing. For the DMLS and SLM processes, this includes removal of supports and any excess metal powder. While the build platform may be cut away from the metal insert, it is often included as an integral mold element.

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Rapid Tooling Design The rapid prototyping layering process results in stair-stepping when insert surfaces are not vertical or horizontal. Such steps may be removed by machining or polishing. However, this may not be possible and may make the molded part difficult to eject or affect the part’s appearance. Stock added to the insert’s design must be finished to size before assembly. This stock can be removed by grinding, milling, or electrical discharge machining (EDM). Undersized holes can be reamed to size. It is critical to avoid excessive secondary operations for the purpose of holding desired tolerances and improving surface finish. If a significant amount of machining is required to deliver the surface finish and accuracy needed for the injection mold, the time for machine set up and operation will negate the time advantage that rapid tooling offers. Tool assembly involves mounting the inserts into the pockets that have been machined to accept them. All other components are added to complete the injection mold. The mold is then mounted in the machine for injection molding. Because nanotool inserts do not have the mechanical properties of metal inserts, shot size and pressure application are critical in filling the mold. Additionally nanotool material acts as an insulator requiring oversized cooling channels. As heat retention is also a problem, cycle times must be increased. For most thermoplastics, an increase of 30 to 60 seconds is adequate. DMLS and SLM metal inserts don’t require any special changes to the injection molding machine. The strength of the metal inserts eliminates the need to ease into shot size and pressure and, their cooling rate is similar to that of the other mold components, allowing consistent cooling. Since complexity is not a concern when additively constructing a mold and its inserts, rapid tooling can accommodate cooling channels that twist, bend, and contort to follow the contours of the mold’s cavity. This circulates coolant across the entire mold cavity or in the hottest spots of the mold. The conformal cooling channels, when designed effectively, have been reported to decrease cycle times by as much as 40 percent when compare to the straight-line, gun-drilled channels of conventionally made injection molds.

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Rapid Tooling Design Review Questions 1.

Indirect tooling is: a. similar to conventional injection molding b. template based c. pattern based d. produced using stereolithography

2.

A direct rapid tooling method well suited for small, short-run mold inserts is: a. RTV b. stereolithography c. direct metal laser sintering d. selective laser melting

3.

RTV refers to: a. a rapid prototyping machine b. high temperature silicone rubber molding c. rubber molding d. high pressure injection molding

4.

With indirect tooling, tool life is not dependent on: a. part geometry b. the casting material c. finish requirements d. the pattern material

5.

Direct tooling often results in time and cost savings of up to: a. 66% b. 64% c. 62% d. 60%

6.

A a. b. c. d.

7.

The electrical discharge machining process can be used to: a. size reamed holes b. generate large mold pockets c. polish mold halves d. remove excess stock from the inserts

8.

Nanotool inserts can act as: a. insulators b. electrical conductors c. cooling devices d. vulcanizing agents

9.

Conformal cooling refers to: a. indirect tooling cooling channels b. contoured cooling channels c. cooling fluid selection d. oversized cooling channels

nanotool is a ceramic filled type of: polyester epoxy urethane resin

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Rapid Tooling Design Answers 1. 2. 3. 4. 5. 6. 7. 8. 9.

c b c d a d d a b

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