10 Aug 2016

Rex Plastics Product Development

Plastic Part Design Fundamentals

Ejection and Surface Finish



When it comes to successful plastic part design, ejection and surface finish considerations are critical factors to consider.  The most common way to remove a part from the mold is through ejector pins. They apply a force to eject a part from the mold, and in some cases can leave marks, sometimes called ‘witness lines’.

At Rex Plastics, we make a point to design and position ejector pins that minimize their effect on your parts, moving any undesired witness lines to a location where they may not be an issue.

Pins are located in the B-side mold half, the side in which the part will stay when the mold opens. Once the mold is opened, the pins extend into the mold cavity, push the part out, and then retract, allowing the mold to close and be refilled.  The image below shows an example of what the witness lines may look like after pins push the part out of the mold:

injection molding ejector pin lines

Let us know if there are critical surfaces in the plastic part design where witness lines left by the mold ejection could potentially be an issue. At Rex Plastics, we can propose an ejection layout for approval before spending money to build the mold.


Types of Ejection

While pins are most commonly used for ejecting a part from a mold, they are not the only option. The table below shows a short list of other commonly used ejector systems:

Pins Round
Blades Rectangular
Sleeves Tubular
Stripper Plate Moving Plate

Pins are common on most parts however they are not ideal for every situation. Ejector blades are rectangular in shape and can be utilized on thin walls that cannot support a circular pin.

Sleeves are ideal for bosses. They provide a 360 degree bearing surface around the boss which facilitates easy part removal. Stripper plates provide ejection around the entire perimeter of the part. Its goal is to “strip” the part off of the core.


Surface Finish

When it comes to plastic part design and surface finish, there are two main considerations to keep in mind – polishing and texturing.

Polishing:  Polishing is a manual process that removes texture, machining marks, etc. on the mold surface and provides a uniform finish. Polishing can be very time consuming, especially if there are deep ribs in the mold. The picture at the bottom of this post shows a variety of SPI finishes.

Texturing:  There are several ways to texture a mold surface such as bead blasting, EDM, or etching. A surface with heavy texture will require increased draft angles in order to release from the mold.

SPI Surface Finishes

Below, you’ll find some standard mold finish call-outs (priced low-to-high). While these standard SPI call-outs are very common, a wide variety of textures are available. Some textures that can be applied include:

  • Natural\Exotic
  • Micro Surface Finishes
  • Multi-Gloss Patterns
  • Graphics
  • Leather Grains/Hides
  • Woodgrain, Slate and Cobblestone
  • Geometric and Linens
  • Images or Logos incorporated into the pattern

SPI Surface Finish Examples

You will find a few examples of various SPI Surface Finishes in the image, below:

SPI surface finish examples

More Fundamentals of Plastic Part Design

Learn More

Are you an inventor interested in developing a plastic product?  Learn about our Prototyping and 3D Printing Services here.

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