Two shot injection molding

Two shot injection molding is the process of ‘over-moulding’ two materials in one moulding cycle and requires a special injection moulding machine. The term “injection” refers to the volume of material injected into the mould cavity. Two shot injection molding is performed on a machine that is programmed to perform two injections in one cycle.

In the first cycle, a nozzle injects the plastic into the mould. The mold then rotates automatically and different types of plastic are injected into the mold from a second nozzle. Double shot injection molding can be used to bond one part of a rigid material to another part of a soft material. The process combines the moulding of two components in a single two-cavity mould.

It can greatly enhance the attractiveness of the product in terms of feel, colour, etc.

Insert moulding, sometimes referred to as pseudo-secondary injection moulding. The moulding of two plastic materials into two parts on different injection moulding machines. The product is removed from one set of moulds and placed in another. The second injection moulding (hence why we call the first injection an insert).

Typically, this two-colour injection moulding process usually uses 2 sets of moulds. It does not require a special two-colour injection moulding machine.

Two shot molded components are used in almost every industry. From consumer goods to construction, but are most often found in applications that require:

• Removable segments or components
• Rigid base material with soft grip
• Vibration or anechoic
• Surface description or marking
• Multi-coloured or multi-material components

The key elements that must be met in the material selection process are primarily the application properties and the bonding between the moulded materials. Not all materials will bond perfectly, but some may form very strong molecular bonds. It is recommended that the following principles are followed during the selection phase:

1：Choose the same material as CTE whenever possible.

2：Try to choose a material with the same shrinkage rate.

3：Selection of resins with similar chemical properties.

4：Avoid amorphous and crystalline bonded materials.

A chart of bonding compatibility is given below:

GO4 mold performs a flow simulation analysis before starting the manufacture of the mould. Identifies all possible defects in the process and eliminates them before the mould is manufactured. Ensures that the structural part of the design is created in the first shot, maximising savings in manufacturing costs.

1：Consider the shrinkage of both materials. In general, the shrinkage depends on the first (or innermost) injection of material.

2：The sub-gates must be able to close the corresponding runners during part of the process.

3：Consider the position of any moulded logo to prevent damage to it during any part of the process.

4：The design may allow oversizing of the component for the first injection. This ensures higher pressure and stronger adhesion during the second injection.

5：Ensure that the parameters of the injection unit provide the required pressure, flow and cooling capacity.

6：Verify that the structural strength of the first injection can withstand the injection pressure of the second injection.

1) Product function requirements. For example, power switches, mobile phone buttons, keyboard indicators, car switches, etc. Some LED requirements are transparent. For example power switch buttons, LED lights.

2) Improved feel. Some handheld products require the use of rubber in the handheld section for a more comfortable feel. Examples include walkie-talkie housings, power tool handles, spanners, screwdriver handles, toothbrush handles, thermos cups, etc.

3) Adding aesthetic appeal. Products manufactured in the two shot process look great while retaining their desired structural characteristics. In this process, load-bearing structural components can be shaped in the first step. Final touches can be added in the second step using more attractive materials.

4) Some areas of the product need to be plated. In the plastics range, only ABS and polysulfone (PSU) surfaces can be plated.If the product is to be plated partially or on one side, the plated area should be filled with ABS . Areas that do not require plating should be filled with other materials such as PC.

5) Increased efficiency. Two shot injection molding allows multiple components to be formed using a single tool. This reduces the labour required to run the part and eliminates the need to weld or join components after the forming process.

6) Reduced costs

The process does not require more complex tooling and, as the parts are manufactured in one piece, there is no need to weld or join components together. This saves time and minimises labour, resulting in savings in overall manufacturing costs.

There are 3 types of overmoulding: insert moulding, two-colour moulding and co-injection moulding. Like secondary moulding, overmoulding is a multiple injection moulding process that uses two or more different thermoplastics to produce a single end product. This process is ideal for engineers who want to build strong, functional, aesthetically pleasing parts that will not separate over time.

On the downside, the tolerances of parts made by overmoulding are usually not as good as those that can be achieved by two shot moulding. It is also important to remember that plastic compatibility requirements can limit the designer.