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Injection Mold Design Basics

Injection molded parts ready for quality check

The injection molding process allows manufacturers to rapidly create parts. The process involves using a mold into which a melted material, called a melt, is poured. The design of that mold is critical to the part’s visual and structural integrity. Mistakes in the design of a mold can lead to an array of defects, including warping, sink marks, and flow lines.

This article aims to help manufacturers avoid the poor mold design choices that lead to compromised parts by sharing some key injection mold design tips.

Tip No. 1 – Maintain Uniform Wall Thickness for Molded Parts

Managing the injection molded part’s wall thickness is key to quality design. A lack of uniformity in wall thickness can cause the melt to warp as it cools into a solid shape. This results in sink marks that have a negative visual impact on the part.

Injection molded part with uniform wall thickness

Maintaining consistency in wall thickness helps to avoid the creation of sink marks. Of course, it’s not always possible to have the same wall thickness throughout the whole part. If a wall needs to be thicker, it should be no more than 15% thicker than the nominal wall used for the part. Additionally, wall thickness varies depending on the material used to make the part. The following are recommended wall thicknesses for the most common injection molding materials:

· ABS: 1.143 mm – 3.556 mm

· Acetal: 0.762 mm – 3.048 mm

· Acrylic: 0.635 mm – 12.7 mm

· Liquid Crystal Polymer: 0.762 mm – 3.048 mm

· Long-Fiber Reinforced Plastics: 1.905 mm – 27.94 mm

· Nylon: 0.762 mm – 2.921 mm

· Polycarbonate: 1.016 mm – 3.81 mm

· Polyester: 0.635 mm – 3.175 mm

· Polyethylene: 0.762 mm – 5.08 mm

· Polyphenylene Sulfide: 0.508 mm – 4.572 mm

· Polypropylene: 0.889 mm – 3.81 mm

· Polystyrene: 0.889 mm – 3.81 mm

· Polyurethane: 2.032 mm – 19.05 mm

Tip No. 2 – Follow the Guidelines for Avoiding Sink Marks

Beyond maintaining consistent wall thickness, manufacturers can prevent sink marks by ensuring the proper placement of ribs, gates, and screw bosses. There are three guidelines to follow:

· Avoid placing any ribs, gates, or screw bosses on the rear side of cosmetic surfaces;

· Rib bases should be no more than 60% of the wall’s thickness;

· A rib’s height should be no more than three times the wall’s thickness.

Tip No. 3 – Save CAD Design Files in an Appropriate Format

Injection mold design tips aren’t limited to the physical manufacturing of the mold. Even something as simple as the computer-aided design (CAD) file format a manufacturer uses when designing the mold can affect its quality.

Injection molding tool in CAD

For example, let’s assume a manufacturer creates a CAD design and saves it in the .STL file format. While this may be fine for 3D-printed molds intended for temporary use, this file format has a problem. It reveals the mold’s surface as a series of triangles linked together to create polygonal shapes. As a result, this file format isn’t ideal if a mold or part has precise curves. This triangular representation can also cause issues when defining wall thickness.

In this scenario, using a STEP file created by a CAD program like Inventor or SOLIDWORKS is the better option due to this file type’s increased precision. The point is that even a well-designed mold may be subject to unintentional flaws if an inappropriate file type is used for the design.

Tip No. 4 – Try to Eliminate Undercuts

The term undercut refers to any part feature that prevents straight ejection of a part at the mold’s parting line. Using undercuts in a mold increases its complexity, leading to a higher possibility of part defects.

Injection molded part design avoiding undercuts

Ideally, the mold designer should eliminate the use of undercuts. However, that isn’t always possible, especially if the part requires a pick-out, side action, or sliding shutoff. In these cases, the manufacturer may be able to mitigate undercut-related issues by using pass-through cores or by adjusting the mold’s draft angles and parting line to facilitate easier ejection.

Eliminating undercuts is also one of the most important injection mold design tips for manufacturers that wish to reduce costs. Using undercuts creates higher tooling costs because the mold requires more pieces.

Tip No. 5 – Add Draft Angles for Injection Molded Parts

Creating parts that have vertical walls can cause problems with injection molding. The part may get stuck as it contracts upon cooling. This leads to the manufacturer having to apply more force to eject the part, which can damage both the mold and the machine’s ejector pins.

Using draft angles can solve this problem. Draft angles allow manufacturers to design the walls of a part with a slight slant, facilitating easier ejection in the process.

Draft angles are usually added at the end of part design. Typically, the angle is two degrees for most parts. However, any walls with near-vertical requirements should have a 0.5-degree angle. Shutoff surfaces and faces with light textures need three-degree angles, while any face with a medium texture usually requires a draft angle of five degrees or more.

Tip No. 6 – Arrange Polymer Flow from Thick to Thin Sections

A manufacturer may need to design injection molded parts with thicker sections, which enhance the strength and structure of the part. In these cases, maintaining uniform wall thickness isn’t possible. The problem this creates is that the melt loses pressure and temperature as it flows through the mold. Improper design can lead to the melt flowing through a thin section and into a thick one, resulting in incomplete filling.

Injection molding flow simulation

One of the simplest injection mold design tips to overcome this problem is to position gates at the thicker sections of the part design. This ensures the melt fills the thick sections before flowing into the thin ones, creating a stronger part.

Tip No. 7 – Don’t Forget About Ejection

Applying inappropriate ejection force can damage the mold, part, and machinery used in the injection molding process. Manufacturers must balance ejection force over the part’s surface area, taking mass and thickness into account in the process. This prevents the part from breaking or warping during ejection.

Furthermore, the manufacturer may have to account for the need to clear plastic from the gate in the event of a short shot. This issue can be remedied by placing stripper plates or extra ejector pins in the area surrounding the gate.

Tip No. 8 – Attach Bosses to Side Walls and Ribs

A boss is a cylindrical feature that is molded into a part. Its typical function is to receive a pin or screw. Manufacturers may use bosses for parts that come in a collection of parts that require assembly.

Injection molded part with Bosses to Side Walls and Ribs

Bosses shouldn’t be freestanding. It’s good practice to attach bosses to ribs or side walls, which ensures the part’s structural integrity. A freestanding boss could snap or break easily because it doesn’t have appropriate support.

Design with Confidence

injection molding tool assembly

With these injection mold design tips, manufacturers have some guidance on how to design a mold that produces reliable and high-quality parts. Details are critical when creating a mold. Even small design flaws can lead to visual defects and issues that affect a part’s structural integrity.


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