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Engineering Guidelines to Designing Plastic Parts for Injection Molding

What is injection molding?

Injection molding is the technique where molten plastic is injected into a metal mold. The mold is composed of two halves, the “A” side and “B” side. The halves are separated and allow the plastic component to be removed once it has solidified, thus creating plastic parts.

What should we keep in mind when designing for injection molding?

Draft Angles

Draft angles allow for removal of the plastic from the mold. Without draft angles, the part would offer significant resistance due to friction during removal. Draft angles should be present on the inside and the outside of the part.

Green illustration shows correct draft angle and red shows a lack of draft angles

The deeper the part, the larger the draft angle. A simple rule of thumb is to have a 1 degree draft angle per inch. Not having enough draft angle may result in scrapes along the sides of the part and/or large ejector pin marks(more on this later).

Radiused Edges

Having radiused edges and corners (both inside and outside) of a part is a multipurpose feature. This allows for better removal during part ejection, — in conjunction with drafted sides — and better material flow (more on this later). Most importantly, however, is that it will not only prevent excessive expenses when creating the mold, but it will prevent cracks from forming due to stress concentration.

Example of radiused corners on the design of a plastic part

Keep in mind that radiused corners should maintain same wall thickness, which means that if inner r=½ thickness then outer R=3*½ thickness.

Red illustration shows a lack of radiused corner and green shows proper radius on a part

Undercuts

Undercuts are items that interfere with the removal of either half of the mold. Undercuts can appear just about anywhere in the design. These are just as unacceptable, if not worse than a lack of a draft angle on the part. However, some undercuts are necessary and/or unavoidable. In those instances, necessary undercuts are produced by sliding/moving parts in the mold.

Illustration shows acceptable undercut due to sliding portion of the mold

Keep in mind that creating undercuts is more costly when producing the mold and should be kept to a minimum.

Solid vs Shell

As the plastic cools in the mold, it also shrinks, which is a common characteristic for most materials.

Making a component a shell versus a solid helps reduce the amount of shrinkage or warpage that happens during the cooling process. It also helps lowers the cost of material needed to be used in that part. There are numerous ways to shell a design, one example is shown on the sphere below.

Example of a solid design (left) prone to shrinkage and warpage vs. proper shelled design (right)

Sink Marks

Sink marks are as they sound, a spot or segment of the plastic surface that appears and/or feels as if it has sank into the part. These marks are caused by a number of items:

  • inconsistent wall thickness (may also lead to voids);
  • bad radius in corners;
  • thick support ribs (to be discussed later);
  • sharp corners;
  • not shelling parts out.

Support Ribs/ Gussets

Red illustration shows lack of support ribs and green illustration shows correct addition of support ribs

Support ribs/gussets are used to give a product’s walls additional support. They are used mainly in two situations: (1) where the part has 90-degree angled walls that meet, and (2) where a part may be too long or large and the wall thickness leaves the part flimsy or weak. Support ribs tend to work best in the direction in which they are needed, such as running the length of a long section.

Red illustration shows incorrect direction of support ribs and green illustration shows proper direction of support ribs

When designing support ribs, it is important to consider draft angles and base thickness at no more than ⅔ the thickness of the wall it is attached to.

Connecting Parts Together

Products are generally composed of two or more parts connected together. These parts can be connected with mounting bosses, snap hooks, screws, etc. A common way of uniting two parts is using screws through mounting bosses. Having lips/grooves where the parts touch is a common way to align parts together.

Top square shapes show lip/groove and bottom two show mounting bosses

When planning on how parts will connect, keep in mind all the previous tips throughout this presentation, such as draft angles, shelled parts, support ribs/gussets, etc.

Part Lines

Part lines are where the two halves of the mold meet. This generally creates a physical line on a part that is both visible and noticeable to the touch. These lines, however, can be hidden or minimized when placed along edges of the part. When designing a part, always keep in mind its part lines.

Example of a part line on a screwdriver
Illustration shows where part lines occur in the mold

Ejector Pin Locations

Ejector pins are what allow the part to be removed from the mold. These pins literally push the part out after the material has been injected into the mold and set. However, while pushing parts out, these pins leave marks on the part. These marks are generally not removable, so location is key to keep in mind when designing the part.

Arrows point to ejector pin marks on a battery holder plastic enclosure
Illustration shows where ejector pin marks happen due to ejector pins (grey) pushing pieces out

Gate Locations and Material Flow

Along with ejector pin locations and part lines, it is also important to know where to have the gate locations. Gates are where the molten plastic enters the cavity of the part in the mold. These gates, once the part cools, leave a mark/ indication of where the gate was, even when attempted to be removed by a post process. Gate location is sometimes determined by:

  • where it will be less noticeable;
  • where it will not interfere with the rest of the part;
  • how the plastic material will flow evenly through the part;
  • or a combination of all these.
Although harder to see, gate locations should be strategically placed due to its prominence as shown in the picture
Gate location is shown in this illustration where it is separated from the plastic piece

Material and Thickness

Depending on (1) how the product works, (2) the environment it will be in and (3) the preference of the designer, a material should be selected. These materials change how the part feels, looks and operates. Some are flexible, some are rigid, some are strong, some are brittle. The type of material chosen will often have a significant effect on the design of the part. Some support ribs may need to be removed or added, walls may need to be thicker or thinner, etc.

Wikipedia article shows sample picture of support ribs on a plastic part to strengthen the design

Recommended Wall Thickness

Conclusion

All the concepts presented in the previous slides make up just some of the good practices an engineer has to keep in mind when designing parts to be mass manufactured by means of injection molding.

These practices are also known as DFM (Design For Manufacturability) and should be used as a checklist constantly throughout the design and redesign of products.

Remember, keeping these practices in mind will ensure long-term savings in manufacturing costs and time for customers.

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