Injection Molding & Design Considerations

Injection molding is a forming process using molds. Plastic materials are heated and melted, and then pushed (injected) into the mold where they are cooled to form the desired shape (that of the mold).

With injection molding, parts with diverse geometries, including those with complex shapes, can be quickly and continuously manufactured in large volumes. Injection molding is used to manufacture products in a wide range of industries and encompasses commodity items as well as precise engineering components.

Injection Molding Process

Typically, injection molding begins with resin pellets (granules) that get poured into a hopper, the entry point for the material. The pellets are then heated and melted inside the cylinder in preparation for injecting them into the mold. The material is then forced through the nozzle of the injection unit before being delivered through a channel in the mold called a sprue, then through the runners/gate into the mold cavity. After the material cools and hardens, the mold opens, and the molded part is ejected from the mold. The sprue and runner are then trimmed from the part.

Injection Molding Machine

The structure of an injection molding machine consists of an injection unit that sends the melted materials into the mold and a clamping unit that operates the mold.

There are different types of injection molding machines. That is, machines operated by servo motors, those operated by hydraulic motors, and hybrid machines that use both servo and a hydraulic motor.

After an engineer designs a product, molds are made by a mold-maker (or toolmaker) from metal, usually either steel or aluminum, through precision machining to form the features of the desired part.

Parts created for injection molding must be designed carefully to facilitate the molding process; the material used for the part, the part’s desired shape and features, the mold material, and the properties of the molding machine must all be considered during the design phase. The versatility of injection molding process is facilitated by this breadth of design considerations and possibilities.

Basic Design Guidelines for Injection Molding

  • Material considerations: Choose material that meets the functional requirements of the part and have good processing characteristics. Also consider the material’s availability [i.e., in stock or short lead time and MOQ (minimum order quantity)] is low.
  • Wall thickness: The part wall thickness should be as uniform as possible. Uneven wall thickness causes uneven flow, leading to molding defects. Also, keep the wall thickness within “recommended wall thickness” for the resin used to create the part.
  • Include Ribs: A plastic part that has been designed within the recommended range may not be strong enough to support itself/ functional requirements; ribs are added to reinforce the part.
  • Rib/Wall Thickness Ratio: Always maintain recommended rib-to-wall thickness ratio so that the thickness at the rib area is not excessive (this can cause sink). Typically, the rib needs to be 0.6 times the wall thickness. Also ensure you have the radius at the junction of the rib/wall to eliminate stress and facilitate flow.
  • Draft: Draft is the taper amount for the vertical walls of the plastic part. Without draft, the part may not eject properly or may sustain damage during ejection. Typically, 1 degree draft is recommended; textured surfaces may need more draft depending on texture depth.
  • Adjust Part Design to eliminate a complex parting line, reduce side actions that are required to release under-cut features from the mold, and eliminate features that require thin steel areas in the mold.
  • Radius: Add radius on all the sharp corners, as these can generate stress.
  • Over-Tolerancing: The injection molding process has limitations. Be aware of the process limitations when specifying the dimensional and geometry tolerance. Refer to SPI standards, guidelines for tolerance, based on the part’s material and size.

 

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