Injection molding is the industry’s choice for the manufacturing of plastic products. The process makes it easy to manufacture thousands to millions of identical products. It does this in reproducible cycles with cycle times within seconds. It gets used for large and small products that get used in all industries. As the demand for plastic products has grown, so has the complexity of the designs. The plastic in pellet or granular forms passes through the barrel. They get mixed and melted by the screw then injected into the mold. The mold is an important feature in the injection molding machine. It is the part of the process where the part gets its final form and dimensions. Often one describes the molding process as melt solidifying within the mold cavity. This is true but one should also understand the complexity of the mold. Important is how the mold opens and closes and how the product gets ejected. A lot goes into the design of the mold. So much so that the cost of the mold design makes up around 40% of the mold budget. The mold must be fit for function and also for manufacture. So the design of the mold features does not only consider how they get used but also how they get made. You can save a lot of added costs by investing enough time and money at the design stage.
A good mold can reduce the amount of rejected parts per million. Cycle times, tooling, and postprocessing can also get reduced by a well-designed mold. The more complex the design of the product, the more complex the mold. This is even more complex where the product has undercuts in the design. These are holes, slits, indentations, or protrusions on the product. These make the ejection of the product a bit challenging. The most basic mold design is one where the mold opens in two halves. In such a design, the product is easier to remove. It is preferable that the design is such that it needs no post-processing after release. For products with undercuts, their ejection calls for a more complex mold design. Lifters are one of the features that make removal of parts with undercuts possible or easier. Undercuts are necessary features in some products. The conventional mold ejection mechanism makes them difficult or impossible to remove. Conventional ejection involves a simple unidirectional push of the ejector pins. Where undercuts are present this would mean damage to the product to release. In this article, we look at the design of the lifters. We discuss their purpose, and design considerations to ensure effectiveness.
How do lifters work?
There are different types of lifters. Different mold makers also have different approaches to design and fabricate the lifters. The lifter is a feature incorporated into the molding section of injection molding. The exact manner of construction and installation varies with the other machine specifications.. It also varies with the product design details. But the general mechanism of action of the lifter remains the same for every system. Regardless of the type of lifter or mold system, the goal is to aid product release. It does this by allowing more range of motion. That would otherwise not occur in conventional ejection systems. Lifters comprise the lifter body and the lifter forming parts. They can get classified as integral or non integral. The integral lifters have the body and forming parts as one unit. The nonintegral lifters combine two separate units as the body and forming parts. The integral lifters are better suited to molding smaller parts. They are simpler and preferred for ease of production and maneuvering. Nonintegral lifters work in larger parts. This way they can get designed larger thus easier to work with and maintain. The integral lifters are more common. They present superior durability and are more compact. The lifter can also vary by shape. There is the cylindrical lifter and the T shaped lifter. The cylindrical lifter is more common as it is simpler to produce. The T-shaped filter gets used more for larger products that need to get molded to high precision.
Lifters as the name imply, in principle, give a “lift” to aid the product ejection. They allow lateral movement of the product during ejection. They get incorporated into the mold close and open mechanism in an inclined manner. The lifter fits into a locating block which has a corresponding inclined hole. The block comes between the ejector plates and the lifter. When cooling is complete the mold is ready for opening. In the conventional systems, the opening of the mold causes the ejector plate to push the pins out. This pushes the product out of the mold. There are different types of ejectors such as blade and pin. But the mechanisms of release are similar. The image below shows a conventional ejector system.
Image illustrating conventional ejector systems. This is an ejector system that does not include a lifter. The product molded here has no undercuts. The ejector pins push out the product in one single motion in one straight direction.
In the lifter type ejector system it is necessary to move the part in more than one direction. Here the main action is the motion of the lifter. The ejector plates perform the conventional motion of moving upwards. This motion exerts a push in the same direction on the lifter. This moves the lifter outwards from the locating block. Because the lifter has an inclined shape, it also moves sideways at the same time. This motion corresponds to the removal of the undercut. This motion allows the secure lifting of the product out of the mold. The image below illustrates this.
Image illustrating how the lifter works
So here we see that the lifter must fit well with the locating block. But it must also have enough clearance to move up and down. The distance the lifter moves upwards and to the side are key. This corresponds with the location of the undercut. The sideways motion is in the same direction towards which the lifter inclines. The motion of the lifter is also regulated by the kiss-off. The point at which the ejector plate meets the core plate. The distance moved by the ejector plate from start to kiss off gets referred to as the ejector stroke. The vertical distance moved by the lifter is the lifter stroke. The horizontal distance relates to the ejector stroke and lifter stroke. All these must also correspond with the length of the undercut. The lifter tilt angle gets calculated using trigonometry.
Tangent of Lift angle = Lifter horizontal stroke / Ejector plate vertical stroke
Lifter stroke = Length of undercut + smallest safety margin
The smallest safety margin added to the lifter stroke is 3mm or larger. This allowance ensures that the lift completely clears the undercut. This before the forward pushes out of the mold. This prevents damage to the part during ejection. These parameters are important in the design of the mold system. The motion of the parts during ejection happens fast and must be seamless. The moving parts must be well lubricated to avoid wear.
Considerations in Lifter Design
- The following are some considerations when designing lifters for injection molding systems.
- This must have at least 3 degrees shut off. The shut-offs aid the designs of holes, slits, and other undercut features. Well designed shut-offs to avoid leakage of plastics, hence flash.
- The travel angle of the lifter should not be more than 11 degrees.
- Except for plane surface profiles. There needs to be a 5mm flat surface on the side of the mold where the melt flows from. The head must also be larger than 1mm. This holds the lifter in place allowing it to resist the injection pressure.
- Apply chamfer to the corners of the mold core head. This can be a chamfer of C0.2.
- A 1 to 3mm distance should exist between the inclined lifter and the product.
- Ensure good drafting to account for the greater amount of abrasion. This occurs more with lifter molds compared to the simple ejector systems
- When the components move during part ejection, this causes drag which leads to wear. To avoid this allow between 0.003 and 0.005 inches pad on the lifter. These limits drag.
- The spacer block is higher for mold base with lifters. This is because it has a longer travel distance than conventional ejector systems.
- Whenever the lifter moves, there is always a chance that the parts will remain on the lifter. What is ideal is for the part to fall free after the lifter pushes it. S this must not get allowed to happen. To address this one approach is to incorporate ejector pins into the mold system. Alternative features known as gripper get used. Guide posts can also get molded in as a feature. In such a case, the ejector pin must key in a firm way. These options need careful consideration to ensure the product function is not affected.
Material of Fabrication
The lifter goes in and out of the locking block. It also exerts a force on the product. Considering that it is a long thin structure. It must get made from a strong abrasion-resistant material. Hardened steel 4507 with a hardness of around HRC 50-52 is a common choice with mold engineers. The base gets made of steel 738. The wear block gets made from bronze. The rest of the mold gets made of the steel grade suited to the type of plastic getting injection molded. The lifter type mold does incur added cost to the mold. This is the cost of design complexity and the added cost of materials.
Alternatives to lifters
The mold design engineer will analyze the process and part design. This will help to establish which features work best. The choice of feature for undercut release depends on different factors. These include tooling, capabilities, cost and the part design itself. Where lifters don’t get used other options include sliders or inserts. Sliders serve the same purpose as the lifter except uses a different mechanism. Inserts allow you to use a simpler mold ejection mechanism. This while still maintaining the undercut. With inserts, the product gets ejected with the inserts. The insert then gets removed in a separate process. This is a desirable option where the part design allows for it.
Conclusions
Undercuts add complexity to the mold design. In particular the ejection of the product from the mold. The lifter makes the ejection of products with undercuts possible. The lifter has an inclination angle. This facilitates the combination of vertical and horizontal motion. This gives the range of motion for successful ejection. The articles review the mechanism of action of the lifter. It also explores the material and design requirements. Www.cavitymold.com uses the latest in mold design and manufacturing technology. We design and produce high-performance molds. We design for companies across the world. These range from the early start-up stage to well-established manufacturers. Contact us today to discuss your injection molding project.
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