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18 Nov 2020

The Basics of Mold Design 3 (Part 1): An Overview of Tonnage and DFM

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Skill-Lync

The Basics of Mold Design 3 (Part 1): An Overview of Tonnage and DFM

 In this part of the basics of mold design using SolidWorks, you will learn about the following: 

  • Machine tonnage
  • Design for manufacturing (DFM)

Machine Tonnage 

The pressure with which the molten material is injected in the injection molding machines is very high. In most of the products, the injection pressure is around four to five tons per square inch, and in some cases, it is much higher than that. 

Hence, to ensure that the core and cavity do not get separated due to this high injection pressure, proper clamping is provided to them.  

What Is Tonnage?

The tonnage of the machine decides the amount of clamping provided by the clamping mechanism. For example, if you have a 100-tonnage machine, it means that the clamping mechanism lays down 100 tons of clamping force.

Hence, tonnage can be defined as the amount of force exerted by the molding machine to keep the mold closed.  

Selecting the Right Tonnage

Now, it is probably easy to imagine why you cannot have a very low clamping force. If the machine's tonnage is significantly less, then the core and the cavity will be separated when the material is being injected. This will result in flashes. 

However, you also should not use machines that have a tonnage value much higher than the requirement. If the mold's clamping force is too high, it can cause defects like burns, short shots, and gloss differences. Moreover, a very high clamping force can damage the venting system, inserts, and the core and cavity block. 

The clamping force can neither be too high nor too low as either of them can lead to unwelcome issues. The clamping force should be within a specific range to ensure proper injection molding process.

Calculating Machine Tonnage 

The tonnage of a machine is calculated based on the following formula;  

Tonnage of machine (in tons) = [Projected area (sq cm) * Injection pressure (kg/sq cm)] / 1000  

where,  
Projected area = Projected surface of the entire model along the tooling direction.  
Tooling direction = It is the direction along which your model is oriented, and your core and the component will be moving.  

Let us say that the projected area is about 35 sq cm, and the injection pressure (if the material is ABS) would be approximately 500 kg/sq cm. After plugging in these values in the above equation, we get the tonnage value as 17.5 tons. So, 17.5 tons of clamping force will be required for this machine.  

However, the machines available in the industry come in specific sizes. So, a 17.5 tonnage might not be available. In that case, you would need a machine with a tonnage value close to the calculated value but not lower than that. For example, if you have the option of choosing either a 15 tonnage or a 20 tonnage machine, then you would have to go for the 20 tonnage molding machine. 

Design for Manufacturing (DFM) 

The DFM receives the plastic component for which the mold is required by the customers or the product design team. The DFM team reviews the product in terms of design, manufacturing, and molding aspects. They also check if all data is available to proceed with the tool design.  

If the DFM team thinks that there might be problems creating the mold for the given product, they convey that to the customers or the product design team.  

Below is a checklist followed by the DFM team to ensure that they have all the inputs required to design the mold. 

  • Part details
  • Parting surface
  • Gate point
  • Wall thickness
  • Cores
  • Tolerance
  • Venting
  • Ejector pads
  • Inserts
  • Ejector inserts
  • Guide pillar
  • Support block
  • Relief
  • Free play inserts
  • Multigate position
  • Gate location
  • Flow leader

Let us look at each of these DFM aspects in detail. 

Part Details 

Part Details 

Whenever the molten material in the mold solidifies, it shrinks. So, the actual part size obtained is less than the size of the mold. Hence, to build a part with correct dimensions, the size of the mold must take the shrinkage of plastic into account.

To design the mold size in such a way, you will need the exact shrinkage value for the material injected into the mold. 

Material 

The material here refers to the plastic material used for making the component. 

Number of Cavities 

The number of cavities required is asked to the customers. The DFM team can suggest it, but the final decision is that of the customers. 

Inlet Type 

There are different types of gates used in the mold design process. Depending upon what types of gates you use, there may be some marks on the final product. For this reason, the customers are informed beforehand of the effects of using specific types of gates. Based on their requirements, the gate type is selected.

Surface Demands 

The type of surface finish required is also asked the customers – whether they want the surface to have a textured finish or glossy finish.

If the manufactured part is not visible from the outside, there will be no need to polish the tools and provide a good surface finish. This is because having a good surface finish requires additional cost and time, and must be avoided where it does not necessitate.  

The type of surface finish required will also affect the draft angle given for the geometry. 

Engraving 

Things like engraving text or the company logo on the products are taken as inputs by the DFM team. 

Certain standards must be followed for the length and width of the engraving because CNC machines are used to mill them. So, based on the size of the cutters available, there are standard engraving sizes.  

Conclusion 

This part of the basics of mold design goes over machine tonnage, DFM, and part details. In the next part of this series, we will dive deeper into the DFM checklist.  

Until then, check out some of the job leading industry-relevant online engineering courses at Skill-Lync. Click here


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Navin Baskar


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