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Mold Design & Tooling for Injection Molding

Mon, 05/06/2013 - 2:10pm
Kaysun Corporation

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Injection molding is a high-precision manufacturing process that injects molten plastic into a carefully designed mold, where the plastic cools and hardens into the specified part or product. The piece is then ejected from the mold, either as the final product or as a near-final product that is sent on for secondary finishing. 

The injection mold consists of two parts: the mold core and the mold cavity. The space that these two parts create when the mold is closed is called the part cavity (the void that receives the molten plastic). Depending on production needs, “multi-cavity” molds can be designed to create multiple identical parts (as many as 100 or more) during the same run.

Designing the mold and its various components (referred to as tooling) represents a highly technical and often complex process that requires high precision and scientific know-how to produce top-quality parts with tight dimensions. For example, the proper grade of steel must be selected so components that run together do not wear out prematurely. Steel hardness must also be determined to maintain the proper balance between wear and toughness. Waterlines must be well-placed to maximize cooling and minimize warping. Tooling engineers also need to calculate gate/runner sizing specifications for proper filling and minimal cycle times, as well as determining the best shut-off methods for tooling durability over the life of the program.

During the injection molding process molten plastic flows through channels called “runners” into the mold cavity. The direction of flow is controlled by the “gate” at the end of each channel. The system of runners and gates must be carefully designed to assure even distribution of plastic and subsequent cooling. Proper placement of cooling channels in mold walls to circulate water are also essential for cooling to create a final product with homogeneous physical properties, resulting in repeatable product dimensions. Uneven cooling may result in defects called “hot spots”—areas of weakness that affect repeatability.

In general, more complex injection-molded products require more complex molds. These often must deal with features such as undercuts or threads, which typically require more mold components. There are other components that can be added to a mold to form complex geometry; rotating devices (using mechanical racks and gears), rotational hydraulic motors, hydraulic cylinders, floating plates, and multi-form slides are just some examples.

Main Stages of Tooling

Stage 1: Manufacturability and Feasibility

In this initial stage, design engineers, tooling engineers, materials engineers, manufacturing engineers, quality engineers, and lab technicians work together to determine product specifications, mold component functionality, mold materials, operational constraints, and any needed product enhancements and improvements. The team especially looks for any potential problems in part geometry or tolerance that might result in poor steel conditions or require special tooling features such as lifters, slides, and threading/unthreading. The physical and chemical properties of the selected resin are also evaluated so that the proper mold steel can be selected and mold cooling be reviewed. Mold flow evaluation is also undertaken to determine the best type of gate and gate locations, in addition to determining proper vent locations.

Manufacturability review includes confirmation of standard plastic design practices and incorporation of tooling details to create the most robust design possible. Tooling specifications and tooling sources are finalized and purchased component sources qualified. A comprehensive process failure mode effects analysis (PFMEA) is also completed.

Stage 2: Design

Preliminary 2D and 3D design models are constructed to determine mold sides and steel sizes. Once these are reviewed and approved, the detailed design is finalized.

Stage 3: Final Design Specifications

The tool builder is given the tool design specifications for mold construction. Final adjustments and modifications are done in-house, with special attention given to manufacturability and critical dimensional requirements.

Stage 4: Construction of Primary and Secondary Tools

Detailed tool drawings are completed and construction standards are reviewed and verified. The tool builder’s progress is closely monitored and on-site meetings are held. The completed mold is inspected against a comprehensive checklist.

Stage 5: Bring the Tool In-House for the Initial Sample

A molding process is established that is acceptable to the manufacturing department. Processing parameters are recommended and established. Initial sampling using scientific molding practices is carried out; cavity pressure transducers in the mold accurately determine the filling profile over time. Sample parts are qualified. 

Stage 6: Make Any Final Tool Corrections

Any needed process adjustments are made as required. Tool construction is verified and the process is detailed and documented so it can be used in the future with minimal setup time. Perfect parts are resampled and submitted to the customer. After final approval is obtained from the customer, the production process is launched.

Steel versus Aluminum

Most molds are made from hardened or pre-hardened steel. Hardened steel (heat treated after machining) has superior wear resistance compared to pre-hardened steel and lasts longer. Although steel molds are more expensive than molds made from other materials, such as aluminum, they are more durable and support a higher rate of production before they need to be replaced.

Design engineers must take into account steel hardness versus steel brittleness. Harder steel is more brittle and therefore not a good choice for mold components that are subjected to side loading or impact, because if it flexes it will crack. Harder steel is also required for molding glass-filled material, which can prematurely wear down tooling; wear can also be heavy on runner systems and gates.

Because of its rapid cooling characteristics, aluminum is sometimes used for tooling. It can also reduce the time required for building the mold because it is easier to machine than steel, providing faster turnaround and production cycles. However, because it is softer than steel, even hardened aluminum is harder to weld, difficult to maintain, and wears more rapidly—making it most suitable for prototypes and short runs. Depending on the product and mold design, hybrid molds can sometimes be built that are mostly steel but use aluminum in low-wear areas to transfer heat.

Aluminum is not a good choice for complex parts or harder, glass-filled plastics because of premature wear. Copper alloys are sometimes used as an aluminum replacement when rapid heat dissipation is required.

Both steel and aluminum molds can be coated with special materials to improve wear resistance and reduce friction, especially when molding fiberglass-reinforced plastics, making tooling last longer. Common coatings are nickel-boron and nickel-teflon the (0.0002 to 0.0004 inches in thickness).

Key Components of Mold Design

Gates

Gates are the openings at the end of the runners that direct the flow of molten plastic into the mold cavity. Gates vary in size and shape depending on the part design and resin material. Design engineers must take into account a number of factors to determine gate types and locations to achieve optimum flow, fill pressure, cooling time, and dimensions/tolerance. It is important to locate gates where they won’t impact part performance or appearance (flow marks, shrinkage, warping).

Draft

One aspect of mold design that cannot be overlooked is the easy removal of the final product from the mold, with no damage to the surface of the part. This is accomplished by applying a draft angle, or taper, to the walls of the mold. The amount or degree of draft angle depends on several factors, including design of the part, material, depth of the mold cavity, surface finish, texture, and amount of shrinkage. Typically an angle of only a few degrees is applied to the side walls of the mold and creates enough space that the part can be easily removed when the mold is opened. Generally, the deeper the cavity, the more draft is required. Draft angles typically vary from about 1 to 5 degrees.

Finish and texture

Mold cooling and part cooling are critical for determining surface finish. For example, a smooth surface finish on a 50-percent glass-filled resin depends on proper temperature control. The surface must be resin-rich with the fiber glass slightly deeper in the part, which requires a hotter mold—this also means it takes about ten percent longer to cool.

Molds can also be designed to apply a texture or pattern to the mold surface—this can actually eliminate assembly steps by creating the company logo in the plastic, for example. Texture can also provide better product function, such as enhanced grip or reduced wear from friction. Types of textures include matte, gloss, graphics, grains, logos, and geometric patterns. Depending on the type, depth, and location of texture, draft may need to be adjusted to facilitate part ejection, which is determined during the mold design process.

Manufacturability and Lifecycle Costs

The main goal of mold design and tooling is to create a product with high manufacturability—a high-quality process that is simple and efficient, long-lasting, easy to operate and maintain, and that meets all customer specifications at the lowest possible cost. Fulfilling these expectations depends on designing the best tooling option for each customer’s needs.

To accomplish this, tooling decisions must be made in the earliest design stages. The tool-maker must be involved as early as possible to provide a realistic machining perspective on product design, requested tolerances, tool design, selected materials, and associated costs. Taking this step up front is the best way to eliminate wasted effort and rework, which adds significant cost to the tooling budget. Part design and tool design are dependent on each other and thus should be done concurrently whenever possible.

For good reason, customers are always concerned about cost. After all, tool-making is one of the highest expenses in the production process. Properly designing, building, and using tooling for each part requires a highly skilled team of engineers and technicians utilizing the latest in sophisticated design and manufacturing technologies. Labor cost can be optimized, however, by working closely with an experienced, efficient tooling team that makes wise decisions on material selection and design tradeoffs, early in the design process.

In an effort to save costs up front, some companies shop tooling according to price, looking for the lowest bid. There is usually a not-so-good reason behind lowball machining/tooling bids, including poor quality, poor repeatability, inferior tooling, improper materials, low operational skills, and waste/rework.

Other companies trying to beat a deadline may select a tool vendor quickly, hoping “things turn out right.” Typically, however, lack of due diligence leads to oversights or cut corners that take much longer to straighten out. Although rushing might get the first shots completed quickly, chances are the final submittal won’t be any faster.

The best way to get maximum value for your tooling budget is to consider lifecycle costs, not up-front costs. The ultimate goal is quality and repeatability. This is achieved by working with an experienced injection molder that takes the time to completely understand the customer’s needs, goals for the product, and production expectations and designs the best possible mold/tooling package to meet those needs. Up-front costs for quality tooling may be higher compared to cheaper vendors or offshore suppliers, but the payback come quickly in higher quality, fewer defects, greater throughput, longer-lasting equipment, and over better return on your tooling investment—leading, ultimately, to higher customer satisfaction and loyalty.

 

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