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Plastics Design Tricks Collection

No plastics design theory, just plenty of illustrated examples of the most common plastics design features achievable with injection molding. Where designed for plastic injection becomes real.

This collection is intended to grow over time
-suggest your ideas, comments & pictures-

Plastic Clips

Where plastic injection shines and is very hard to replace by another technic: big flexibility, small additional footprint to the cover; obtained with only a simple two parts mold design. rib for end-stop, marking for the user, and many more.

Plastic Reinforcement

Usually by adding a metal part, either

  • by indeed simply adding a part at final assembly time (ex: a simple nut placed in a hex cavity at assembly time)
  • by overmolding, usually a metal part (needs tight tolérances at least on one dimension, the one used to maintain in the mold)
  • assembling metal over plastic by a tight fit or a spring effect

Plastic Latch

Made in two parts.

Nice cinematic to be operated by hand but would not detach with pull force once closed. Typically clearances on-axis & material flexibility are necessary for such application.

An additional no-cost plastic-included function is the ability to block it with a screw, or in other cases with a lock.

Note the protrusion variant on the transparent cover to have one version that can not be dismounted on either side (in this case to retain the plastic cover from the lamp, when hanging on the ceiling)

Fragile Wall to be punched by user

Permits some flexibility in sizes of counterpart or common parts to be used in different variants or designs … at the cost of not visually perfect edges. Note the reinforcement rib around the surface to be punched, so as the channels to feed the inner surface.

This wall is not particularly easy to take out: wall thickness could have been thinner than that and easier to punch, but recess would likely have been more visible on the other side.

Plastic Ribs

The typical plastics classic feature is hardly obtainable by any other technology than injection (or Mazac, aluminum…). Its thickness should ideally be a third of the main wall thickness to avoid recess on the other side.

Living Hinges

One of the smartest design features possible with plastic, but also the one requiring the best cooperation with the mold + process team, as the exact design depends more than any other on the material choice and mold design options.

As long as the product is not too big, it permits avoiding additional parts & molds, so as assembly operations.

Initially used for low-cost packaging (food, sauce, liquids …) to connect the lid to the main box, it can be used also for product housing as long as the hinge is somehow hidden or on a back surface (example in the gallery below)

Points of interest for product design

  • injection point/gate location is critical, as some material will have to flow perpendicular across the thin hinge section. In case of several gates, mind not to join the flows at the hinge
  • If several hinges are required, the injection point/gate will typically be placed in the center of the two, to avoid the flow to cross both hinges one after the other (which would decrease pressure & injection speed)
  • hinge thickness typically from 0.2 to 0.4mm, with connecting radius & chamfers
  • in some cases, manipulating the hinge just after injection is required (can be part of the ejection system)
  • for long-lasting products, the design should incorporate some angle limiters pins, or stoppers to control the start & end positions 
  • the material of choice is PP
  • some new prototyping materials permit to obtain living hinges as in production processes

typically made out of plastic injection, it can also be made in other materials (leather, wood …) so as obtained with post-processing (milling, stamping…)

… although some of these techniques will impact the durability of the hinge

Parting Lines

Parting lines are not exactly a feature but a consequence of mold designs being in two -or more- parts and other recess or kiss-off surfaces.

Being smart in that area is where one makes mold cost savings. On the other hand, parting lines is usually where you first see the quality of the mold (enough thickness & rigidity, quality of the steel) and the wearing and maintenance over time.

Locating Pins

In any kind of shape, form & size, either to locate bottom+top plastic parts together or to fix some components

Overmolding – Bi-materials

Either mix different properties (ex: soft+hard) or have different colors or replace metal (lighter or cheaper objective).

Because of the additional mold required, or an advanced mold with several cavities or sides, plus the placing robot placing the first part into the mold (usually), this technic is mostly for high runner parts.

Cable Guides & Cable Fixtures

Mostly used in home appliances, with or without screws depending on the required pull force to withstand.

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