Plastic parts are needed everywhere. Injection molding is the most popularly manufacturing method for producing these plastic parts. The process is a quick, economical way to mass produce a number of parts, ranging from high-precision engineering components to cosmetic product enclosures.
There are several factors that may affect the quality of the final part and the repeatability of the process. To take full take advantage of the process, it is important that your design follows a set of design guidelines. In this article, we cover the considerations of the injection moulding process, best design practices, and design tips to keep costs to a minimum.
Rapid injection moulding is a low-to-mid volume plastic part production process. After the part’s design is finished, a mould is created and machined with precision to form the part’s features. The mould is installed into an injection mould press, and molten plastic is injected into the mould tool under pressure. The part is then cooled, ejected, and the process repeats.
Injection Moulding Process Considerations
Wall Thickness
Use a uniform or constant wall thickness throughout the part (if possible) and avoid overly thick walls to prevent parts from warping or sinking. Designing wall thickness can be tricky and differ depending on material choice. Generally, staying within a 1.2–3.0mm thickness is safe for most materials.
Recommended wall thickness for common resins available at HLH Rapid:
Common materials | Recommended wall thickness (mm) |
ABS | 1.2 – 3.5 |
Acrylic (PMMA) | 0.7 – 3.8 |
PC/ABS | 1.2 – 3.5 |
Polycarbonate (PC) | 1.0 – 4.0 |
PEEK | 1.0 – 3.0 |
Polyethylene (PE) | 0.8 – 3.0 |
POM | 0.8 – 3.0 |
Polypropylene (PP) | 0.8 – 3.8 |
Nylon (PA) | 0.8 – 2.9 |
Ribs
Use ribs for strength. Injection moulded parts with poorly designed ribs are most prone to sinking. Rib thickness should be equal to 0.5 times the nominal wall thickness, and the rib height should be limited to less than three times its thickness.
Draft Angles
Parts with vertical walls (and no draft angle) are most prone to drag marks. A minimum draft angle of 2° is recommended. A good rule of thumb is to increase the draft angle by one degree for every 25 mm. For example, add a draft angle of 3 degrees to a feature that is 75 mm tall.
Threads and Bosses
For best results, avoid adding threads directly on your injection moulded part. Threaded inserts can be overmoulded into the part.
Corners
To avoid flaws on injection moulded parts, avoid sharp corners and round all edges. When designing internal edges, experts recommend using a radius of at least 0.5 times the wall thickness.
Text and Logos
Because an injection molded part is essentially the inverse of its machined metal tooling, the rules on engraving/embossing are reversed. For injection moulding, choose embossed over engraved text, as it’s easier to CNC machine such detail into the mould tool. We recommend a minimum height of 0.5mm to ensure readability. Use a font with uniform thickness and a minimum font size of 20 points for better results.
Walls Thickness | Stay within 1.2 – 3.0mm |
Rib Thickness | No more than 0.5x of wall thickness |
Rib Height | No more than 3x of rib thickness |
Draft Angle | Minimum of 2° |
Internal Edges | At least 0.5x of wall thickness |
Embossed Details | Minimum of 0.5mm |
When using injection moulding, you have to be prepared to bear the high initial investment associated with making the hard tools. While it is a given that tooling will be costly, there are several ways you can optimize your injection mould design to save on costs: