Injection mold design is a process of many steps. Mold engineering enables designers to work on various parts of the mold in parallel, compressing time and costs.
Undercuts are features that inhibit the ejection of the part from the mold. They require in-mold mechanisms like side-action cores, jiggler pins, and lifter rails that add to the cost of the mold.
Designing for Moldability
Injection molding is cost-effective, but it’s important to design your part for the right results. The goal is to balance quality, functionality, cost, and time to market.
One of the first considerations is determining the appropriate wall thickness. Thicker sections will take longer to cool down, affecting the molded material’s strength, visual appearance and durability. The best practice is to ensure uniform wall thickness and avoid thick sections with ribs and bosses where possible.
Another concern is undercuts, recessed or overhanging surfaces that make it difficult to eject the molded product from the mold without damage. Eliminating these features will speed manufacturing time and reduce the need for tools like side-action cams or lifters. This can also significantly cut costs and help you stay within budget. It’s also recommended to add radii in sharp corners of your parts, as these weaken the material and create molded-in stress.
Designing for Parting Lines
The parting line is the point where two surfaces of a plastic product meet. It’s important to consider this line’s effect on mold and production costs when designing your product.
Ideally, part lines should not appear on the finished product. This requires careful design and machining practices. However, it’s only sometimes possible to avoid parting lines completely.
To reduce the impact of these lines, it’s recommended to use rounding for corners. Sharp edges can cause molded-in stress during cooling, and they can also lead to difficulty with ejection.
Injection molded parts with undercuts must be designed with internal mold mechanisms to help facilitate ejection. These typically include side action slides, jiggler pins, and lifter rails. It is also best to minimize undercuts in the parting surface when possible. This will make the injection molding process easier to flow smoothly and create an accurate part. It will also help to reduce maintenance issues such as flashing or abrasion of the parting surface.
Designing for Ejection
Ejection is the mechanical process that pulls or pushes a plastic part out of the mold after it has cooled. This is done by a series of ejector pins on the B side (also known as the functional side) of the mold, where the parts stay once the mold opens. Ejector pins can sometimes leave marks on the parts called witness lines. Too few ejector pins can cause these marks, too small a diameter for the pins, or positioning them in areas where they can cause damage, such as on ribs, metal inserts, and flat surfaces.
The best way to prevent witness lines is to use enough ejector pins with a large diameter and to position them so they distribute the force over a larger area. This also helps avoid breakage of the ejection pins and reduces the stress at a single point on the part during ejection.
Designing for Warping
The overall goal of the molding process is to produce quality products quickly at a competitive cost. Concurrent engineering allows multiple designers to work on different areas of a mold simultaneously, compressing product delivery times.
For example, one designer can finalize the ejection while another finishes core side details. This type of simultaneous engineering can significantly cut down the total design time of a project.
A key factor in plastic molding warp is differential shrinkage, where different areas of the molded part cool and shrink differently. This can be due to varying features, flow direction, processing temperatures, or other factors that affect how a shot of molten plastic is cooled and processed.
The best way to avoid warping is to keep wall thicknesses as consistent as possible. This will ensure that the shot of melted plastic fills the mold, cools evenly, and doesn’t experience differential shrinkage. This can also help to prevent unwanted stress in the molded part, which may result in warping.