What are the Design Considerations for Insert Injection Molding

Injection Molding

Part and mold design guidelines

Before you start the insert injection molding process, you should consider the application of the part to be molded, the size, and the number of products to produce using the molds. These are the main factors that will affect the part and full mold design

Here are the design guidelines to help you get the best results with insert injection molding:

  • Part design: When designing the inserts, you should ensure that the inserts are smaller than the plastic mold. This helps to prevent marks on the finished product as larger inserts might sink into the mold during injection of the molten plastic. The insert should be a minimum of 4mm from the plastic mold and you should ensure that the space under the insert is one-sixth of the insert diameter. 
  • Use rounded corners on the mold: Round corners on the mold ensure easy flow of molten plastic during the injection molding. Sharp corners cause stress on the mold walls, which might require machining the final product. Sharp corners could also cause the shrinking of the final product during the cooling process.
  • Use optimized draft angles – Having optimized draft angles ensures easy removal of the final product from the mold. This is because as the injected plastic cools down, it shrinks and sticks to the mold. Having a low draft angle causes defects in the final product which appear as marks called abnormalities. The optimal draft angle is one or two degrees. The CAD software can help you optimize draft angles during the mold design phase. 
  • Use durable resins – A strong and durable resin ensures no cracking of the plastics surrounding the metal insert. Cracking the plastic around the insert is one of the most common problems when using a weak resin. This is because of the cooling and shrinking of the plastic as it cools. 

Material selection for both inserts and plastic resins

The insert and plastic resin materials determine how well the insert bonds with the plastic resin. When choosing these materials for mold manufacturing method, you should ensure that they bond perfectly with each other. Some of the common materials are:

  • Thermoplastics – Thermoplastics are ideal for plastic resins because of their chemical resistance. This means they change their chemical structure even under high temperatures. They are also very efficient because they can be recycled and used multiple times. Some examples of thermoplastics include polystyrene, polyethylene and nylon. 
  • Thermosets: These are similar to thermoplastics but have one major disadvantage. They can only be used once. For this reason, engineers avoid using thermosets because they are not eco-friendly, cannot be reused, and are expensive. Examples are epoxy and phenolic.
  • Elastomers: These are synthetic polymers with similar characteristics as rubber. Elastomers are very eco-friendly and engineers prefer using them because of their elasticity. They are used to manufacture various parts used in automotive and home appliances. Common examples are rubber and polyurethane. 

Dimensional accuracy and tolerances

These factors determine the dimensional accuracy of insert injection: 

  • The material used – Each material used in injection molding has their own attributes. The materials selected for this process should have suitable stability and chemical composition to achieve accurate dimensions of the mold and parts produced. 
  • Injection molding tools: The molding machine’s capabilities and precision also determine the mold’s accuracy. For example, using CNC machining ensures you achieve the highest level of accuracy because it is computer guided.
  • Mold design – The mold’s design and complexity directly affect the final product’s accuracy and appearance. A good mold design should be stiff and compact enough to hold the injected plastic and produce an accurate product
  • Parameters of the process: These refer to the exact steps used in insert injection molding. The parameters should be consistent and uniform to produce only accurate products. A documented procedure of the process parameters allows errors to be easily tracked and corrected. 

The tolerance standards refer to the acceptable variations in minor dimensions of the mold. The design, functions and applications of the products determine these tolerances. Although insert plastic injection molding is very accurate, some variations might occur. 

Surface finish and cosmetic requirements

There are different standards for finishing and polishing various products after production. The surface finish gives the final product its look and appeal to the users. All plastic products have different polishing standards categorized through the method used for the product’s appearance. These are polished and textured finishes.

The polished finishes are further subdivided to:

  • High gloss: This is the highest finish grade and falls under the SPI standards A-1, A-2, and A-3. The standard used to polish the mold depends on the type of plastic used.
  • Semi-gloss: This is in the second grade and falls under SPI B-1, B-2 and B-3. Sandpapers and various filing tools are used during polishing in this grade. 
  • Matte: This is the lowest grade finish and falls under SPI C-1, C-2, and C-3. It is less expensive and fastest to polish. Materials polished in this method are rough and used in places where they are not visible. 

The textured finishes are subdivided to

  • Bead blasting: This method uses pressurized air to spray various materials on the surface of the mold. It falls under SPI D-1, D-2, and D-3. Molds polished with this method are usually prototypes produced from rapid prototyping. 
  • Chemical etching: This method uses a film to cover the mold, which is then placed in an acid bath and the parts not covered by the acid are etched away. The result is a textured finish. The process is complex and takes two to three days to complete.
  • Mirror ED: This is a complex process used on molds that do not require hard machining. A copper electrode is shaped to the shape of the mold. The electrode is mirrored to the mold and produces a spark that burns away the unwanted parts leaving only the desired shape of the mold. This process is called mirror electro-discharge machining. (EDM)


Please enter your comment!
Please enter your name here