Injection Mold Making: Types of Molds
Mold is a piece of block used in the injection molding process to mold various plastic components. It might be the most expensive component used along with the injection molding machine due to its complex design, precise characteristics, and manufacturing methods. In other words, mold is a hollow metal block where the molten plastic gets injected to manufacture the desired shape. Usually, tooling is done by CNC machines.
Molds are fastened to the platens of the injection molding machine. They have drilled holes and channels responsible for controlling the temperature of the mold by circulating hot water. As far as the mold's working is concerned, molten material flows into molds through a sprue and fills the cavities. After a calculated time, the molten material adopts the shape of the mold and solidifies. Then the injection molding machine pushes the ejector plate of the mold to repeat the process for further manufacturing.
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Types of Molds Used in Plastic Injection Molding
Mold materials must be wear-resistant and withstand many cycles, the clamping force required to hold the material set up, and the high-pressure forces to shape the liquid plastic and achieve close tolerances. While considering the materials, molds can get divided into two categories: aluminum molds and steel molds. Molds made up of these materials have their specified applications.
Aluminum has been on the cards for mold manufacturing for the last few years. While being less expensive solutions of P20-tools, automotive industries are shifting molds to aluminum. Usually, aluminum molds manufacture low-volume tools or prototypes since molds are malleable and softer than conventional steel material. Consequently, wear and tear are more pronounced, and continuous use might cause marks on the manufactured product.
Steel molds have been a conventional approach in the field of injection molding. Nonetheless, molds made up of steel are highly suitable for high-volume production because steel has a more remarkable ability to withstand internal and external stresses and remains invincible against wear and tear for a longer run. Additionally, the steel molds are highly durable with expanded options of use even after maintenance, though they cost heavily compared to aluminum molds.
Advantages of Aluminum Molds
Here are some of the most substantial advantages of aluminum:
1. Lower Tooling Cost
If compared to the molds of steel, the tooling cost for aluminum molds is much lower. Some aluminum tooling is as more economical as half of the cost of steel tooling. Although the return on investment for steel mold and aluminum mold may vary greatly depending upon their use, aluminum mold requires a meager initial investment compared to steel.
2. Reduced Heating and Cooling Times
Aluminum has much higher dissipation rates than steel, so aluminum can instantly turn hot to cool and vice versa. Thus, the cycle time of aluminum mold is 20-30% better than steel molds. Keep in mind that an improved heat dissipation rate is not only appropriate to abstain from cooling and heating lines, but it also diminishes the intricacy of mold and maintenance costs.
3. Ease of Modification
Aluminum is suitable for low-volume production since it is relatively softer than steel and gets worn sooner than steel. However, the quality of being soft allows engineers and manufacturers to modify and optimize the design according to advanced needs.
Disadvantages of Aluminum Molds
Aluminum has some disadvantages too that might cause its replacement with steels molds:
1. Less Compatibility with Materials
Unlike steel molds, aluminum has compatibility issues with complex injection molding resins. For example, polymers with fiberglass cannot be used with aluminum molds even for a very few cycles. Moreover, aluminum molds are softer and gentler and more inclined to scratches/damages brought about by some special resins.
2. Low-Volume Production Runs
Manufacturers must keep in mind that they cannot depend on aluminum molds for high-volume production runs due to their ability to be more delicate than steel molds. Otherwise, the quality of the eventual product gets badly affected and damaged.
Advantages of Steel Molds
Steel has been the focus of injection molding owners for a long time. Here are its reasons:
1. Perfect for Detailed Features
The prominent feature of steel is its ability to be suitable for high-volume production because of its resistance against wear and tear and higher hardness and strength. At the same time, this is the reason that helps manufacturers achieve detailed features of mold. For example, steel molds tightly follow thin, non-uniform walls and rounded angles to bring a better shape to the final product.
2. Improved Durability
Though steel is more expensive than aluminum, steel molds offer more comprehensive and durable services than aluminum. In addition, steel with a better ability to sustain stresses does not fall for repairing or early retirement. Also, steel is ideal for long production runs and frequent utilization of the mold throughout the long term.
3. Higher Compatibility
Steel molds are highly suitable for complex and advanced injection molding resins since steel can stick to even hard-featured products, which is usually challenging with the aluminum product. Likewise, steel molds are compatible with resins reinforced with additives such as glass fiber.
Disadvantages of Steel Molds
Apart from its advantages, here are some of the disadvantages of steel molds:
1. Higher Tooling and Repairing Cost
Steel is highly reliable and durable, but it possesses excelling costs that are not justified compared to aluminum or its other counterparts. Given that, for making the steel mold ready, there needs a higher tooling investment. Equally, steel molds, if fall for warping or some other defects, the repairing costs are higher due to their increased hardness.
2. High-Volume Production Runs
Since steel molds are expansive in tooling and repair, they will be suitable and economical only for high-volume production runs. On the other hand, as mentioned earlier, aluminum molds are preferable for low-volume production runs.
The Process of Mold Making
The mold manufacturing for injection molding is a multi-staged process. Throughout the process, it is inevitable to consider the required dimensions of the final product. Plus, risk management to avoid mold failure and ensure its accuracy, tightness, and ventilation is also necessary.
Things to Consider about Mold Making
Mold manufacturing entails accuracy and techniques and may involve block molds, glove molds, blanket molds, poured molds, and then some. The time may differ with various strategies regarding creating easy to complex geometry. The types of molds, like single or multiple cavities, can be picked as per requirements. Additionally, ensure access to adequate materials, provisions, and tools & equipment. Diverse material alternatives, such as aluminum and steel, are available, and each has specific features, capacities, and suitability. Nevertheless, the budget of the project is also one of the key things to consider, since mold making cost highly affects the final cost of injection molding.
Primary Operations like Milling and Turning
Typically, the drawing of the final product gets prepared before mold manufacturing. This design of the final product further leads to mold making. Metal blocks of required sizes are taken and aligned on a CNC machine. Before starting the secondary operations, CNC performs primary functions like milling, turning, and heat-treating on the metal block. During the mold making process, a selection of cutting tools gets made considering the material of molds. For steel, carbide tools get widely used because they are among the hardest cutting tools.
Secondary Operations like Mold Construction and Peripheral
After completing the primary operations, the metal block is all ready to go under the secondary process, which ensures the mold-making process's success. These operations may include mold construction and mold peripheral. Mold construction provides closing and clamping of molds, mold wall thickness and centering for proper heat absorption and dissipation, and mold surfaces through grinding, polishing, sandblasting, or shot peening. Besides, mold peripheral comprises ensuring mold venting, non-permanent release agents to get applied to the mold surface for improved demolding properties, permanent release coatings for better flow properties and gradient blend, and threaded inserts such as snap inserts or magnetic holders.
Injection mold making is a highly technical and skill-required task, such as process control and troubleshooting of quality defects. It needs to eliminate loopholes that may cause potential losses throughout the process. For now, the molds are revolving around two materials: steel and aluminum. Molds made up of these materials have their specified advantages and disadvantages along with their specific costs. And choosing the mold material depends on many factors, such as the cost-efficiency and product specifications to be achieved.
But remember, even though the innovation has been around for a long time, the field of injection mold making keeps on developing, considering new technological improvements, market disruptions, and different social and economic aspects. Accordingly, the five key industry trends that shape the heading of injection mold manufacturing include moving towards sustainability, exploring various substitute materials, automation, machine learning, and advanced analytics.
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