Designing Parts for Injection Molding
Oct 12, 2019
Injection molding can be used to create a variety of complex plastic parts, from food packaging to functional mechanical systems.
Nonetheless, the injection molding process is a fine art, and engineers must follow certain design rules and principles when designing parts for injection molding. If a part is not designed with the mold and plastic injection process in mind, it may not perform to the best of its abilities. It may not perform at all.
This article outlines some of the most important design considerations for injection molding, from wall thicknesses to draft angles to embossed text. By following these few simple rules, your parts will come out of the mold in perfect shape, ready for their end-use.
Design for manufacturability
Whether you intend to create parts using injection molding or another process like 3D printing, it is important to follow the practice of designing for manufacturability (DFM).
At its most basic level, DFM means thinking about the manufacturing process before the very first sketch of a part. It means that, instead of designing an ideal component and then thinking about how to make it, you should factor manufacturing considerations into the design itself. If the manufacturing equipment will struggle to make a particular detail of a part, then the detail must be changed or removed at the very outset.
In this respect, injection molding is no different from other manufacturing processes. The part design must take into account the physical and material limitations of the injection molding process — how the plastic is heated, how it enters the metal mold, how it then cools down within the mold — which may result in a significantly different design to the one initially conceived.
The following sections describe various rules and guidelines for designing parts for injection molding.
One major design consideration for injection molding is the use of gates. Unlike parts designed for CNC machining or 3D printing, the metal molds used in injection molding must incorporate openings that allow the molten plastic to enter.
These openings are called gates, and can drastically affect the final appearance and function of an injection molded part.
Gates can be automatically or manually removed from a finished part, and different gate designs exist to serve different functions. Gate designs include:
- Edge gate: Suitable for flat parts, located on the edge of a part, ideal for thicker areas
- Hot tip gate: Used with hot runner molding systems, found on the top of a part, suitable for round sections
- Sprue gate: Suitable for large cylindrical parts and single-cavity molds, easy to design, leave a large scar at removal point
- Sub gate: Removed with ejector pins, can be placed in a variety of locations on a part
Gates typically leave a small scar on finished parts, and incorrect placement can cause deformation of the part. However, steps can be taken to ensure smooth operation.
• Larger parts require larger gates (or higher pressure)
• Place gates away from cores and pins to ensure the direct flow of material
• Place gates at heavy cross-sections to reduce sink
• Use two gates if demanded by part size and shape
Injection-molded parts are often hollowed out, with their walls typically no thicker than 4 mm. Choosing the appropriate wall thickness can have significant effects on part function, as well as manufacturing time and cost.
Perhaps the most important consideration for injection molding wall thickness is consistency. Thin walls cool faster than thick walls, and it is, therefore, important to ensure a uniform wall thickness across a part. Inconsistent thickness can lead to warping and deformation because the not-yet-solid thicker sections will shrink around the already-solid thinner sections.
Uniformity is key, but a uniform thin wall is better than a uniform thick wall. Thin walls require less material and less manufacturing time since they cool and solidify faster than thick walls. However, some materials are more suitable for thin walls than others: plastics like acrylic, PEEK and TPE can be molded to fine thicknesses, while those like rigid PVC and PU are better suited to thicker walls.
• Use uniform wall thicknesses
• If uniformity is impossible, ensure a gradual transition from thick to thin
• Keep walls thin to reduce time and cost — if the material allows
Draft angles are one of the most important and unique features of molded parts, and you’re unlikely to find them on parts made using different manufacturing processes.
When designing a mold for injection molding, it is important to taper the walls of the mold instead of creating straight lines at right angles. By tapering the walls of the mold, it is much easier to eject the plastic part from the mold once it has solidified.
Think of it like removing jelly from a bowl. It’s easy to remove an entire bowlful of jelly by flipping the bowl upside down, precisely because the sides of the bowl are wider at the opening than the base. By contrast, it would be difficult to remove jelly from a Pringles tube or rectangular box, since these shapes have straight walls.
• Smoother surface textures require less draft since friction is reduced
• Rougher surfaces require more draft
• 2 degrees of the draft is standard for average surface finishes
Sharp edges and corners are a rarity in injection molded parts, and this is for several important reasons. For one, it is much easier to create edges and corners with radii when using a CNC machine to create a metal mold. Creating the right angles requires much greater precision and therefore demands more time and money.
But radii also result in better-molded parts. Like draft angles, rounded edges lead to easier part removal from the metal mold, while they also improve the flow of the molten plastic in the mold, meaning no material gets forced into a corner.
Importantly, the incorporation of radii also produces stronger parts, since stress concentration is reduced.
• Round all edges and corners
• Ensure uniform wall thickness with fillets 50% wall thickness on internal corners and 150% on external corners
Injection molding can be used to create thick parts, but these parts work much better if they are hollowed out. Keeping parts hollow can reduce deformation and thus lead to stronger, more effective components.
Rather than use huge wall thicknesses, it is better to create hollow parts supported with carefully placed ribs. Hollow parts with ribs can be stronger than thick, solid parts.
• Use thin walls supported with ribs instead of thick walls
• Ribs should be no thicker than 50% of wall thickness
• Ribs should be no longer than 300% of wall thickness
Undercuts are protrusions or features of a design that prevent a part from being ejected from a regular mold. Dealing with undercuts can involve creating a multi-part mold or eliminating the undercut.
If at all possible, it is best to eliminate undercuts from a part design. Undercuts always result in a more expensive project, so they should only be used if absolutely critical.
To make parts with undercuts, a more complex mold design may be required — one that uses a side pull, an extra section of the mold that is moved in a different direction. This extra mold component increases the cost of the operation.
Alternatively, it may be possible to modify the design to accommodate the undercut. If all material below the undercut can be removed — perhaps resulting in a cavity elsewhere in the part — a side pull may not be necessary. This is called a shutoff.
Another option is simply forcing the undercut over the mold as the part is ejected, a technique known as a bumpoff. This is only possible with relatively flexible plastics and when the lead angle of the undercut is between 30 and 45 degrees.
• Avoid undercuts whenever possible
• Explore the possibility of shutoffs and bumpoffs for critical undercuts
• Utilize more expensive side pull mold if other options not feasible
Many injection molded parts are individual components that will form part of a larger system. For this reason, many molded parts require threads for screws and other fasteners.
In general, it is best to avoid designing threads directly on parts, since threads are a kind of undercut and therefore run into the problems associated with those features. (Plastic bottles are made with molded threads, but they use bumpoffs for ejection from the mold.)
Instead of incorporating threads on parts, standalone bosses can be used. Bosses are typically cylindrical and can be used as a mounting fixture. Metal inserts can be used to create a threaded hole within the boss.
• Avoid directly molding threads if possible
• Bosses should be made to same specifications as ribs
• Bosses should be cored to their base
• The outer diameter of the boss should be 200% that of the insert
Adding embossed text to injection molded parts is relatively simple, and maybe preferable to stickers for reasons of longevity. This may be crucial for textual information such as safety warnings.
It is easier to engrave text on a metal mold than embossing it, so it is, therefore, easier to achieve embossed text on the plastic part itself. (Because the part is an inverse of the mold.)
For clarity and to avoid deformation, simple fonts without serifs should be used.
• The text should be embossed at 0.5 mm height
• Fonts should have a size of at least 20 points
• Fonts should have even thickness without serifs
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