Design of molds and tooling phases for injection molding service

Design of molds and tooling phases for injection molding service makenica.com/design-of-molds-and-tooling-phases-for-injection-molding-service February 21, 2021

Injection molding service is a high-precision processing technique that injects molten plastic into a precisely crafted mold. The plastic turns cools and hardens into the required component or product. The piece is then expelled from the mold either as a final product or as a near-final product sent for secondary finishing.

The injection mold consists of two components: the mold core and the mold cavity. The space these two sections make when the mold is closed is called the part cavity.

Depending on the manufacturing requirements, "multi-cavity" molds can be built to produce several similar pieces simultaneously.

The construction of the mold and its different components (referred to as tooling) by plastic injection molding companies is a highly technical and often complicated operation, involving high precision and technological know-how in the manufacture of high-quality, narrow-scale components.

For instance, the proper steel grade must be chosen by the injection molding companies so that parts that run together do not wear out prematurely.

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Steel hardness must therefore be assessed to achieve the correct equilibrium between wear and durability. Waterlines must be well aligned to optimize cooling and eliminate warping. Tooling engineers also need to calculate the gate/runner sizing parameters for proper loading and minimum cycle times, as well as the best shut-off methods for tooling.

During the process of plastic injection molding service, the molten plastic flows into the mold cavity into channels called "runners." The flow path is regulated by the "gate" at each channel's end. The runner systems and gates must be appropriately planned to ensure equal delivery of plastic and eventual cooling.

Proper positioning of cooling channels in mold walls to disperse water is also essential for cooling to deliver a finished product with homogeneous physical properties, resulting in repeatable product measurements. Defects called "hot spots" – places of weakness that affect repeatability – can result in uneven cooling.

More advanced products of plastic injection molding service usually require more complex molding. They also have to work with features such as undercuts or threads that typically need more mold parts. Other parts/components can be added to the mold to form complex geometry; rotating devices (using mechanical racks and gears), hydraulic rotary motors, hydraulic cylinders, floating plates, and multi-form slides are just a few examples.

Key Tooling Phases of Plastic injection molding service Phase 1: Manufacturing and feasibility Design engineers, tooling engineers, material engineers, manufacturing engineers, quality engineers, and lab technicians of plastic injection molding companies work together at this initial stage. This is to determine product specifications, mold component functionality, mold materials, operational constraints, and any necessary improvements and enhancements.

The team mainly looks for any potential problems in part geometry or tolerance that could result in poor steel conditions or need special tooling features like slides, lifters, and threading/unthreading. The selected resin's chemical and physical properties are also evaluated so that proper mold steel can be chosen and the cooling of the mold can be reviewed.

The plastic injection molding companies also evaluate the mold flow to pick the best type of gate and gate locations and determine appropriate vent locations.

The manufacturing review includes confirming standard plastic design practices and the inclusion of tooling details to make the most robust design possible. The tooling specifications and tooling sources are finalized, and the source components purchased are qualified. Process failure mode effects analysis (PFMEA) is also completed.

Phase 2: Design

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Preliminary 2D and 3D design models are built to determine mold sides and sizes of steel.

Once they have been reviewed and finalized, the detailed design is completed.

Phase 3: Final Design Specifications The tool builder shall be provided with the tool design specifications for mold construction. Final adjustments and modifications shall be made in-house, with special attention to manufacturing capability and critical dimensional requirements.

Phase 4: Construction of primary and secondary tools Detailed Tool drawings are made, and construction standards are reviewed and verified.

Progress of the tool builder is closely monitored, and on-site meetings are held. The completed mold is checked against a comprehensive checklist.

Phase 5: Bring the In-House Tool for an Initial Sample A molding process is established, which is acceptable to the manufacturing department.

Processing parameters are recommended and defined. Initial sampling shall be carried out using scientific molding practices; cavity pressure transducers in the mold shall accurately determine the filling profile over time. Sample parts are qualified for this.

Phase 6: Make any corrections to the final tool Any appropriate process changes shall be made as needed. Tool construction is checked, and the procedure is detailed and documented to be used in the future with a minimum configuration time. Perfect pieces are re-sampled and submitted to the consumer. The plastic injection molding companies initiate the manufacturing process after the final approval is received from the consumer.

Steel vs. Aluminum Many molds are made of hardened or pre-hardened steel. Hardened steel (heat-treated after machining) has a higher wear tolerance than pre-hardened steel and lasts longer.

While steel molds are more costly than molds made from other materials, such as aluminum, they are more durable and support higher production rates.

Design engineers of injection molding companies must consider the hardness of steel against the brittleness of steel. Harder steel is more fragile and thus not a suitable choice for mold parts exposed to sideloading or impact, and if it flexes, it fractures. Harder steel is also required to molding glass-filled materials, which can wear out tooling prematurely; wear may also be severe on runner systems and gates.

Injection molding companies often use aluminum for tooling due to its rapid cooling characteristics. It will also mitigate the time taken to create the mold because it is cheaper to machine than steel, allowing quicker turnaround and output cycles.

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However, since it is softer than steel, hardened aluminum is harder to solder, challenging to maintain, and wears faster—making it more ideal for prototypes and short runs.

Depending on the application and mold size, hybrid molds may often be created mainly steel but use aluminum in low-wear areas for heat transfer.

Aluminum is not a suitable option for complicated components or rough, glass-filled plastics due to premature wear. Copper alloys are often used as an aluminum substitute when quick heat dissipation is needed.

Both aluminum and steel molds can be coated with special materials to increase wear resistance and minimize friction significantly when molding fiberglass reinforced plastics, allowing the tooling to last longer. Popular coatings are nickel-boron and nickel-Teflon (0.0002 to 0.0004 inches in thickness).

Critical components for mold design in injection molding service Gates Gates are the openings at the runner ends that direct the molten plastic flow to the mold cavity. Gates differ in size and shape based on the nature of the component and the resin's material. Design engineers must consider a variety of considerations for deciding gate types and positions to achieve maximum flow, filling pressure, cooling time, and dimensions/tolerance.

It is necessary to place gates where they will not affect the component's output or appearance (flow marks, shrinkage, warping).

Draft One feature of the mold design that cannot be underestimated is the simple removal of the finished product from the mold without compromising the component's surface. This is achieved by applying a shape angle, or taper, to the walls of the mold.

The amount or degree of angle of draft depends on various factors, including the design of the component, the material, the depth of the mold cavity, the surface finish, the texture, and the amount of shrinkage.

Usually, an angle of just a few degrees is added to the mold's sidewalls, which provides enough room for the component to be removed quickly when the mold is opened.

Generally, the larger the cavity, the more draft is expected. Draft angles usually range from around 1 to 5 degrees.

Finishing and texture The cooling of the mold and the cooling of the part is essential for deciding the surface finish. E.g., the smooth surface finish of a 50-percent glass-filled resin depends on careful temperature control. The surface must be resin-rich with fiberglass slightly deeper in the component, which requires a hotter mold, ensuring that it takes about ten percent longer to cool.

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The molds can also be built to add a texture or pattern to the mold surface—which can help remove assembly steps by making a business logo in plastic, for example. Texture may also have improved product function, such as enhanced grip or decreased friction wear.

Forms of textures include matt, gloss, graphics, grains, logos, and geometric shapes.

Depending on the form, depth, and position of the texture, the configuration can be changed to facilitate the ejection of the component as decided during the mold design process.

Manufacturing and Lifecycle Expenses The key purpose of mold design and tooling in plastic injection molding service is to produce a high-manufacturability product. This high-quality process is quick and reliable, long-lasting, easy to operate and maintain, and satisfies all consumer demands at the lowest possible cost. The fulfillment of these expectations relies on designing the right tooling solution for each customer's needs.

To do this, tooling decisions must be taken by the injection molding companies at the earliest point of construction. The tool-maker must be engaged as soon as possible to have a practical machining viewpoint on the specification of the component, the tolerances requested, the tool's design, the materials are chosen, and the related costs.

Taking this step forward is the only way to reduce unnecessary time and rework, bringing substantial costs to the tooling budget. The design of the component and the instrument's design are mutually related and should thus be performed simultaneously wherever possible.

Customers are still worried about prices for a good cause. After all, tool-making is one of the highest costs in the manufacturing process of injection molding service. Properly planning, installing, and using equipment for each component involves a highly trained team of engineers and technicians using the latest in advanced construction and production technology.

However, labor costs can be minimized when working closely with a competent, reliable tooling team that takes reasonable choices on material quality and design trade-offs early in the design process.

To save money upfront, several businesses are purchasing tools according to price, searching for the lowest bid. There is typically a not-so-good explanation behind low-ball machining/tooling bids, including bad efficiency, poor repeatability, inadequate tooling, improper tools, low operating abilities, and waste/rework.

Other businesses attempting to beat the deadline can easily pick the tool provider, assuming that "things will turn out right." However, lack of due diligence typically leads to oversights or corner cuts that take far longer to straighten out. While rushing could get the first shots done quickly, the chances are that the final submission would not be faster.

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The easiest way to get full benefit for your tooling budget is to accept life cycle expenses and no upfront costs. Quality and repeatability is the ultimate objective.

This is accomplished by working with an experienced injection molder who requires time to thoroughly consider the desires of the consumer, the objectives of the product, and the manufacturing demands and configuration of the best available mold/tooling kit to satisfy those needs.

The upfront cost of quality tooling can be higher relative to cheaper vendors or offshore suppliers, but a quicker return on higher quality, fewer failures, higher throughput, longer-lasting equipment, and a greater return on the tooling investment would eventually lead to higher customer satisfaction and loyalty.

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