3D CAD Design: Opening the Door to Plastic Molding
Injection molding of plastics is one of the most cost effective processes for manufacture of parts in volume. While mold costs can be significant, amortization over many parts can make the overall cost of injection molding highly competitive with other manufacturing processes. The wide range of available polymers multiplied by the huge array of specific blends offer a tremendous range of physical, thermal, electrical, and chemical properties. Engineering plastics, classified by mechanical properties such as stiffness, toughness, and low creep, increasingly replace metals on a cost and performance evaluation.
Designing for injection molded plastics requires planning. Too often parts will be presented to a molder or tool designer late in the product development process only to be confronted with feasibility issues. If that happens the developer faces decisions to rework part designs or to face higher tooling and part costs. Leaving design for manufacturing and assembly (DMFA) considerations until late in the development program is a common mistake the misses out on optimization and disrupts the transition to manufacturing.
Planning begins in preliminary design. Some will argue that consideration for manufacturing early in the program will inhibit creativity; the reality is that it does not if perspectives are kept in balance. In fact design committee often err in committing to a design that later reveals feasibility and cost issues. While designers and engineers need to be free to brainstorm potential solutions, taking time to evaluate for manufacturing options is vital to assume a successful program.
Design concept modeling is a vital step in preliminary product design. Form and function can be evaluated in blocked-out quick CAD studies. Drawings produced from 3D CAD model studies can help evaluate the size, cost, and architecture of a proposed design. Multiple preliminary model studies are a good use of time because many factors can be evaluated after a few hours of work. Such studies can include component packaging, part break out, and overall size and weight. Concept parts can be submitted for preliminary price quotations. DMFA (design for manufacturing and assembly) has become a hot buzz word in product development the truth is that consumer products manufacturers have been doing DMFA for decades as a means of competitive survival.
Injection molding is particularly advantageous for assemblies wherein components can be mounted using ribs and bosses inside the shell of the parts allowing to easy assembly most commonly using screws, push nuts, snap latches, or heat staking. Components are commonly captured between two shells. Consideration for assembly procedure in part design is critical to reducing cost and boosting assembly line yields. In many industries, cost competitiveness is key factor in market success. Secondary assembly operations can include sonic insertion of threaded fasteners and plastic welding operations such as thermal welding, ultra-sonic welding, spin welding, vibration and laser welding.
Consumer product industries were the first to focus on aesthetics for competitive advantage wherein one product would out-sell another primarily because of form and function. The field of Industrial Design sprung up wherein artistic individuals entered into product design realm bring their drawing, rendering, model sculpting skills and aesthetic sensibilities into the product development process. Early pioneer Raymond Loewy in the 1930s came from the fashion industry and proved to be a fastidious designer with great attention of detail and construction. Solid modeling CAD systems offer powerful 3D (three dimension) surface modeling capabilities that can satisfy high expectations for appearance in the field of Industrial Design. Surface modeling provides the tools to capture complex surface geometries for seamless data transfer to machine tooling operations for injection molding. CAD data is captured electronically and interpreted by CAM (computer-aided-machine) operations. CAM software programs define specific cutter tool paths for efficient and accurate cutting of mold cores and cavities. CAD/CAM processes can capture virtually any surface configuration that a designer envisions. CAM data is used for CNC (computer-numerical-control), EDM (electro-discharge-machining or spark erosion) and wire EDM cutting methods.
Once a design direction is selected and parts are to be finalized the CAD designer has many decisions to make. The first decision is to establish the exact orientation of the part with respect to the direction of draw with is the axis in which the core and cavity will open and close. In the molding process, two blocks of steel, a core and a cavity, need to mate at a parting surface wherein a void is left between them. When the mold is shot, melted plastic is forced under high pressure into the void to form a part. As the plastic cools it solidifies in the form inside the mold and is then ejected from the mold when the core and cavity separate to complete the molding cycle. Mold cycles can range from a few seconds for small parts to over one minute for larger parts. Plastics shrink as that cool. For this reason, the mold is made larger than the part will be at room temperature. CAD designers apply tiny gaps called "reveals" at the parting line to avoid visually apparent mismatches between mating parts.
Once the parting lines and wall thickness of the shell is determined, the next step for the CAD designer is to populate the core side (side not seen when assembled) with ribs and bosses which are used to capture and assemble the product. Drafts and rounds are applied to all surfaces. Drafts assure that the part will easily release from the mold and not lock on to the core or cavity. Rounds break sharp edges on the cores and cavities that provides for easy flow of the melted plastic.
More complicated molds feature special mechanical "actions" that include cam actuated movements that release mold steel out of the path of ejection. Side action devices can be timed to release mold steel prior to the ejection of the part from the core. Mold designers are limited only by their tool building imagination regarding features and functions that they can incorporate in a mold. Commonly snap latches are designed in that eliminate the need for additional fasteners. Often the extra expense of a complicated tool is recaptured in elimination of additional parts or secondary operations. Molds can be designed with stepped parting lines and non-planar part lines to capture more complex parting line geometry.
Two analysis tools are available to optimize part design. One is called mold-flow that estimates the path that the melted plastic will take in filling the part. This is important as plastic often flows around parts of the mold and rejoins which can leave "knit lines." Knit lines are tiny visible line where the plastic may be weaker. This occurs because the leading boundary of the melted plastic cools faster and therefore may not weld completely. FEA (finite element analysis) is a tool that allows the designer to evaluate high stress points in a part and helps the designer to optimize design for maximum part strength.
Part appearance is especially important in product presentation. A product must look professional to be taken seriously in the market. Many considerations are available to part designers principally color and texture. Melted plastic takes on the surface of the mold. If the mold is polished or textured, the part with have the identical look. Colors can be matched very precisely in polymers commonly used as "appearance" parts whereas engineered plastics often have fillers such as glass that do not have cosmetic appeal. Secondary operations are available to add graphics to surface. Sublimation printing is one method; molded in graphics can be seen in "wood grained" instrument panel for automobiles. Metallic finished can be applied by thermal sprays, vacuum metallization, and vapor deposition.
Polymers fall along a continuum ranging from amorphous to crystalline wherein engineering polymers are characteristically more crystalline and more dimensionally stable. Engineering polymers are more commonly are used for structural parts or for industrial products. Low cost polymers such as polypropylene (PP) and polystyrene (PS) are use where price per pound is important and strength requirement are lower. Acrylonitrile Butadiene Styrene, (ABS) and polycarbonate (PC), and polyamide (PA) or nylon materials are commonly used in higher quality products where strength and surface appearance quality are more important. Engineered plastics include high strength materials such as, polyetheretherketone (PEEK), polyphenylene sulfide (PPS), polyphenylsulfone (PPSU) and liquid crystal polymer (LCP).
Final documentation for the part includes notes concerning material and finish and other specification relating to gating and inspection. Inspection dimensions and acceptable tolerance need to be shown on the accompanying drawing. When a part is finalized for tooling, the mold builder will create a mold design layout and submit it for approval by the client. Consideration is given for placement of ejectors on the core side (which will leave witness marks) and gates which can be either in a cosmetically unacceptable place of located in a place hidden in the assembly. Gating can be achieved through an ejector pin in the core side of the mold. For higher volume requirements, molds can have multiple cavities to produce more shots per cycle. Multi-cavity molds can be family molds wherein cavities can produce different parts per shot in matched sets.
In summary, design for injection molding requires the expertise of a professional CAD designer who can bring the full range of design considerations into the product development process.