The Basics of Mold Design 3 (Part 3): DFM Checklist

This article on the basics of mold design using SolidWorks, you will attain a comprehensive understanding of the following components: 

• Inserts
• Ejector inserts
• Guide pillar
• Support block
• Relief
• Free play inserts
• Multigate position for a long part

Inserts 

In the image shown above, the blue portion is the insert, and the region colored in red is the component.  

The insert is placed at the core side of the mold. When the mold closes, the insert is present between the core and the cavity, and the molten material is injected. Once the molten material solidifies, the ejection pins eject both the part and the insert.  

You can also notice a small hole on top of the insert in the image shown above. The hole separates the insert from the part using a screw. Thus, the desired component is obtained. 

Ejector Inserts 

Below is a diagram showing ejector inserts. 

Assume that you have a component with a large surface. In such a case, instead of providing multiple ejection pins to eject the part, you can use an ejector insert. 

In the figure above, you can see the cross-sectional view of the mold. The green part is the ejector pin, the blue part is the molded component, and the orange part that you see is the ejector insert.

Thus, insert helps in distributing the ejection force over a larger surface without the need to provide extra ejection pins.  

Ejector inserts can also be used in case where the customer does not want any ejector pin marks. Ejector inserts covering a large area of the part would help avoid any imperfections that would have been left behind if multiple smaller surface area ejector pins were used. 

Guide Pillar 

The guide pillars are used to align the core and the cavity of the mold. The core side is on the left in the picture below, and the cavity side is on the right.

In the above diagram, you can see a projection in the cavity side towards the middle. This is called the guide cone and is used to interlock the cavity and the core. 

Support Block 

The core plate thickness is always greater than the cavity plate. The reason is that there is a chance of deformation of the core plate due to the hot injected material. The pressure of injected material will be around 600 to 700 bars.

An additional support block is placed between the bottom moving plate of the mold to avoid any deformation of the core plate. The deformation problem can also be solved by increasing the thickness, but there is a limit up to which increasing the thickness is cost-effective. 

The reason is that when you increase the thickness, you would also increase the material cost of the core plate. The length of the holes that need to be milled for the ejector pins also increases. Hence, a support block is used instead.

Relief 

There are two types of cutters used for machining.

 

• The ball nose cutter
• The flat cutter

 

Ball nose cutters are used for 3D profiles and take more time. So, if a parting line is along a 3D surface, then you need to make a 3D profile that would require more time. That is one more reason to make parting lines along the same plane.  

When you use ball nose cutters for machining sharp corners, it produces a fillet at the corner. The creation of this filler makes it challenging to match the core and the cavity side. Hence, some relief is provided near sharp corners near the mating surfaces. 

Free Play Inserts 

In some cases, the rib length is very long, making it difficult to eject the rib from the mold. This is because the plastic gets stuck to the wall of the cavity. For such cases, you can make use of free play inserts.

The first diagram shows the cavity region and the part formed within it through injection molding. Now, if this rib's length is more, it would be more difficult to eject it out of the mold. In such cases, a free plat insert is added. The purple section in the diagram above is the free-play insert and is present in the cavity section.  

When the material is formed through injection molding, a screw you see at the top (in the image) drives the component downward. The free-play insert, in turn, pushes the part out of the cavity.  

Multi Gate Position for a Long Part 

If the manufactured plastic part is too long, you need to provide multiple gates. The exact length depends on the material property known as flow length. Flow length is the distance after which the molten material solidifies.   

For example, in the diagram below, assume the length of the part to be around 1500 mm with a flow length of 250 mm for a given wall thickness. 

Since the part length here is greater than the flow length, you would have to provide additional gates. The number of gates should be a minimum of the value obtained after dividing the part's length with the flow length.

Conclusion 

In the basics of mold design, we understand the hard but necessary work it takes to perfect the DFM process. We are almost finished with the DFM checklist. 

Before proceeding further, you must read about balancing in molds. This concept is explained comprehensively in the next article.  

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