Omar Khalid
Injection mold construction is a process used to create plastic parts. It involves injecting molten plastic into a mold, which consists of two halves: the core and the cavity. The core, also known as the male part, forms the internal shape of the molding, while the cavity, or female part, forms the external shape. The core is an integral part of the mold, and its placement is critical to the success of the injection molding process. Cores are typically made from steel or polymer, with steel being stronger but more expensive. The choice of core material can impact the cost and quality of the final product. Fusible core injection molding, or lost core injection molding, is a specialized process used for creating internal cavities or undercuts that are not possible with demoldable cores. This process is often used for automotive, aerospace, plumbing, and other parts.
| Characteristics | Values |
|---|---|
| Definition | The male part that forms the internal shape of molding |
| Other Names | B-side, sand core |
| Composition | Steel, fusible alloy, soluble plastic, polymer, metal |
| Types | Inner sand core, outer core, sand filling core |
| Process | Plastic is injected into the mold, cooled, and ejected |
| Advantages | Can be molded in traditional injection molding machines, cost-effective for small production runs |
| Disadvantages | Prone to cracking, breaking, warping, distortion, melting, collapse, difficulty in removing core material |
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What You'll Learn
Fusible core injection molding
Injection mold construction is the process of creating molds that are used to perform injection molding operations using an injection molding machine. The core is the male part of the mold that forms the internal shape of the molding. Fusible core injection molding, also known as lost core injection molding, is a specialized plastic injection molding process used to mold internal cavities or undercuts that are not possible to mold with demoldable cores.
The term "fusible core injection molding" specifically refers to the use of a fusible alloy as the core material. When the core material is made from a soluble plastic, the process is known as soluble core injection molding. The fusible core process is often used for automotive parts, such as intake manifolds and brake housings, as well as aerospace parts, plumbing parts, bicycle wheels, and footwear. The most common molding materials are glass-filled nylon 6 and nylon 66, but other materials such as unfilled nylons, polyphenylene sulfide, and glass-filled polypropylene are also used.
The fusible core injection molding process consists of three essential steps. First, a core made of a low-melting-point metal is poured into the shape of the specified cavity for the molded component. This core is then inserted into the injection mold, and plastic is injected. Finally, the molded component and core are demolded and immersed in a heated bath to melt out the core. The bath temperature is higher than the core alloy's melting point but not enough to damage the injected part. Induction heating reduces the melt-out time to a few minutes.
One disadvantage of fusible core injection molding is that induction heating does not completely remove all of the core material, so it must be finished in a hot bath or brushed out. Additionally, induction coils must be custom-built for each molding, and induction heating systems cannot be used with moldings that have brass or steel inserts as they can be destroyed or oxidized. The process also requires a large space to house the necessary equipment, including injection molding machines, casting machines, and melt-out equipment.
Despite these disadvantages, the fusible core process is advantageous for creating complex shapes that cannot be easily achieved with traditional injection molding. By modifying the equipment, small molded parts like valves or pump housings can be manufactured using fusible core injection molding.
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Core and cavity placement
When determining the placement of the core and cavity, it is important to understand that the part must stay in the half of the mould that contains the ejector system. This is because, when the mould opens, the part will release from the A-side and stay in the B-side, where it can be pushed off from the core by the ejector system. If the mould design is reversed, the part will stick to the A-side, where there are no ejector pins.
To aid in determining the placement of the core and cavity, CAD software can be used to simulate shrinkage, temperatures across a cycle, and how evenly the core and cavities are filled through a runner system. This can help to decide where to place the core and cavity and save time and money. It is also important to consider the draft angle of the mould, which is the angle of the walls to the mould's vertical axis. This angle is important to take into account shrinkage, allow for easy ejection, and prevent any damage during removal.
Other considerations for core and cavity placement include the need for a smooth ejection process, with rounded corners and hollow sections making moulding easier. The core and cavity should also be polished along the material flow direction to ensure a good surface finish.
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Core shift
It is critical to determine the placement of the core and cavity in the mould to prevent core shift and other issues. For instance, in the case of a plastic drinking glass, the outside of the glass (cavity) should be formed in the A-side of the mould, with the inside of the glass (core) formed in the B-side, which contains the ejector system. This ensures that the glass releases from the A-side and can be ejected from the B-side without sticking.
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Core materials
Cores are generally made from steel, with grades including P20, En 30B, S7, H13, and 420SS used. Steel cores are strong and solid, but they can distort. Metal cores are also used, though they are only 10% as strong as steel. Polymer cores are a more affordable option for small production runs, but they are not as recyclable as metal alloys. Fusible core injection moulding uses a fusible alloy as the core material, while the process is known as soluble core injection moulding when the core is made from a soluble plastic. The most common moulding materials are glass-filled nylon 6 and nylon 66, with other materials including unfilled nylons, polyphenylene sulfide, glass-filled polypropylene, and rigid thermoplastic urethane.
Cores are susceptible to the same issues that can affect the performance of the moulded parts. For example, during the injection moulding process, core shift can occur if the core moves from its intended position, leading to alignment issues in the final object. Cores can also experience warping, cracking, or breaking due to high temperatures and repeated pressure cycles. Insufficient cooling or incorrect heat transmission can also cause issues, impacting the cycle duration and part quality.
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Core and cavity design
Injection mold construction is the process of creating molds used to perform injection moulding operations. The mold is the most important part of the injection moulding process, as the success of the product depends on it. The core and cavity of the mold must be designed correctly for the part to come out perfectly.
The core is the male part of the mold, forming the internal shape of the molding. The cavity is the female part, forming the external shape. The core and cavity are designed as two halves of the mold, which are aligned to ensure quality. The core half is also known as the B-side, and the cavity half as the A-side. The B-side contains the ejector system, which pushes the final product out from one side. The A-side is attached to the fixed side of the press, and the B-side to the moving clamp side.
When designing the mold, it is critical to determine how the part will be placed in the mold. The part must stay in the half of the mold that contains the ejector system. For example, when moulding a drinking glass, the outside of the glass is formed in the cavity (A-side), and the inside is formed by the core (B-side). As the plastic cools, the part shrinks away from the A-side and onto the B-side. When the mold opens, the glass releases from the A-side and stays in the B-side, where it can be pushed off the core by the ejector system. If the mold design were reversed, the glass would stick to the A-side, where there is no ejector system, causing a serious problem.
To ensure quality, the core and cavity are aligned using guide pillars and bushes. Usually, four guide pillars and bushes are used, with three pillars of one diameter and one of a different diameter, forcing the plates into a single configuration. The core, cavity, runner and sprue should have a good surface finish and be polished along the material flow direction. The gate should be placed so that the component is filled from the thicker to the thinner section. The mold filling process is important to ensure the plastic flows and cools well, and the final part can be easily removed.
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Frequently asked questions
A plastic injection molding core, also known as the B-side, is the male part of the mold that forms the internal shape of the molding.
The core is where the plastic part is injected. It creates hollow portions in the plastic component.
Cores are usually made of steel. The grade of steel used will affect the plastic tooling cost.
Cores are susceptible to cracking, breaking, abrasion, erosion, and fatigue. They can also experience issues with cooling, such as insufficient cooling or incorrect heat transmission. During the injection molding process, a phenomenon known as core shift can occur if the core moves from its intended position, leading to alignment issues in the final product.
The core and cavity are the two halves of a mold. The cavity is the female part that forms the external shape of the molding. Together, they are used to produce plastic parts through injection molding machines.
