The Basics of Mold Design 2 (Part 3): Summing up With Drafts and Parting Lines

In the previous part of the basics of mold design using SolidWorks, we had a comprehensive discussion on molding parts like guide pillars, register rings, and more. We also looked at a snapshot of the types of molds, namely the 2-plate and 3-plate molds.  

In this part, you will read about: 

  • Draft
  • Parting lines



If you take a look at the plastic parts created using injection molding, you will notice that they include drafts. 

The draft angle is an inclination from the vertical axis of the mold or a taper provided in the molds. In the case of plastic molding, mold designers always include a small draft angle for the components being manufactured. 

The draft angle, in essence, is added to facilitate the easy removal of parts from the mold. If no draft angle is provided, then the component will scrape the edges of the mold when you try to remove it, as shown in the image below. 

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This scraping can damage the component and can result in scratches. In some cases, it may also cause the part to bend or even break.  

Zero draft angles may also result in the formation of a vacuum when the material is pulled. Thus, more force will be required to push the component out of the mold.  

The amount of draft angle will depend on the polish and surface finish of the mold tools, which, in turn, depends on customer requirements. For example, it is more difficult to remove a textured component, which is why designers will add higher draft angles for them.  

Furthermore, all components with a glossy finish are made using high polish tools. As a result, lower draft angles will be required to manufacture them.   


Draft Angles Provided in Geometries 

Draft angles generally used may vary from as low as 0.25 degrees to 2 degrees. The diagram below shows how the draft angles are provided for these components. 

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Technically speaking, you can still eject the parts without providing a draft angle. However, without this key component, ejection can cause the plastic to stick on some parts. Plus, after removing the parts from the mold, you would be able to see scratch marks on it, which is not desirable. 


Relation Between Draft and Depth 

The draft angle will also depend upon the molding depth. In the graph shown below, the depth increases as high draft angles are required. 

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A point to note here is dimensional variations do not always make it possible to give higher draft angles for deep parts. For shallow parts, the draft should be around 0.5 degrees or more. On the other hand, deep parts must get a minimum of a 0.25-degree draft if higher angles are not a feasible option.  


Drafts Based on Material 

Draft angles also depend upon the material properties. For example, since it will be more challenging to eject stiffer materials, higher draft angles are provided when working with them.  

Below are some examples of draft angles for various plastics: 

  • Polystyrene = 0.5 degrees
  • Poly acetal = 0.25 degrees
  • Polyethylene = 0.25 degrees
  • Acrylic = 0.5 degrees
  • Nylon = 0.25 degrees


Problems Due to Zero Draft Angle 

  • Ejector pin marks will be present as due to zero draft angle; more force needs to be applied by the ejector pins to eject the part, which will leave a mark on it.
  • Ejector pin breakage may occur due to the excess force applied.
  • The plastic components may undergo warpage.
  • There will be scratches formed at the core and the cavity.
  • The ejector system may also bend due to zero draft.

That was a quick rundown on drafts and draft angles. Let us now move on to the parting lines and understand their role in mold defects.  


Parting Lines 

The parting line is essentially a line that splits the mold system into two parts. The section formed on one side of the parting line will be the core, while the other section will form the cavity.

The parting lines are generally kept as straight lines in a single plane. If the parting lines are not straight, it becomes tougher to manufacture and assemble the mold.  

Here are some examples showing the best practices of parting lines. 

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A closer inspection of both diagrams reveals that the molded product at the top will have a mark along the parting line. However, the one at the bottom will bear no such mark as the parting lines do not cut through the part. So, you should always try to provide a parting line on the outer edge.

In some instances, it will not be possible to provide the outer parting lines on the outer edge. Hence, a bead can be inserted at the parting line section, as shown in the image below. 

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Introducing such beads along the parting lines will allow easier cutting of the gate system, and the defects will not be very apparent.  

In the next figure (below), you can see that the top and the bottom sections are misaligned. So, the top left bead is offset slightly to extend it over the lower bead. This way, the defect will not be very apparent, as in the first case.

Image source


This article offers a thorough understanding of the fundamentals of mold design. As a result, you can get started on designing your own molds using a computer-aided design (CAD) and computer-aided manufacturing (CAM) software like SolidWorks. This software tool is ideal for students, teachers, small businesses, and medium businesses alike.  

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