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Design Guide: Injection Moulding

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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.

How to design parts for plastic injection moulding

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.

 

Injection Moulding Process

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

  • Warping: When certain sections cool and shrink at different rates in different directions, it can result in permanently bents in parts due to internal stresses.
  • Sink Marks: When a section shrinks faster on the outside than on the inside, it can cause the material to collapse in on itself, resulting in sink marks.
  • Drag Marks: When the plastic is injected into the mould, it flow along the walls of the mould cavity. The plastic then cools and solidifies. During ejection, the walls of the part will slide and scrape against the mould, which can result to drag marks.

 

Injection Moulding Design Guidelines

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.

 

Designing rib thickness and height

 

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.

 

Designing draft angles in injection moulding designs

 

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.

 

Designing round internal edges for injection moulding designs

 

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.

 

Design embossed text and logos

 

 

Summary of Best Design Practices

 

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

 

 

Cost Reduction Tips

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:

 

  • Eliminate Undercuts: Avoid undercuts where possible, as they’ll add to the complexity and cost of the tool.
  • Choose Embossed Over Engraved: It’s easier to CNC machine text into the mould tool, as it involves the removal of less material which overall translates to less cost.
  • Simplify As Much As Possible: Unless it adds value to the function or quality of your design, avoid overly complex features, as they often drive costs up unnecessarily.

 

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