Basis and determination procedure for parting lines (PL) in injection molds.

In injection mold design, the parting line (hereafter PL) is an important factor that directly affects product quality, formability and mold fabrication costs. This article explains the basic role of PLs and the concept of determining the optimum PL.

What is a parting line (PL)?

The parting line is the boundary line that separates the fixed side (cavi-side of the mold) and the movable side (core-side) in an injection mold. It appears as a line on the surface of the molded product and therefore affects appearance quality. The PL is also the starting point for determining the mold structure and plays a very important role in the design process.

Basic role of the PL

Clarifies the direction in which the mold opens and closes (die-cutting direction).　

The PL is basically positioned perpendicular to the direction of the mold opening (usually the Z-axis direction). The appropriate PL setting facilitates smooth removal of the molded product.

Criteria for determining the presence or absence of undercuts.

1. 

   Undercut refers to a shape that does not pull out with respect to the die-cutting direction; the PL is directly related to minimising undercut, as the extent of undercut depends on the position of the PL.

   
   

   Influence on the appearance quality of the molded product

   
   

   Incorrectly positioned PLs can result in lines appearing on prominent external surfaces of the product, which can spoil the design The PLs are generally placed in a position that is difficult to see or on an inconspicuous surface.

   
   

   Basic procedure for determining the parting line.

   
   

   1. determining the die-cutting direction (mold release direction)

   
   

   To set the PL, it is first necessary to define the molding direction of the molded part. Usually, the Z-axis direction is used, but the most suitable direction should be selected according to the shape and external surface.

   
   

   2. Checking undercuts

   
   

   With the die-cutting direction assumed, undercut areas are checked - this can be done quickly using the automatic detection function of CAD software, for example.

   
   

   3. Setting and evaluation of PL candidates

   
   

   Several PL candidate lines are set and each is evaluated from the following perspectives:

   
   

   - Minimising the amount of undercut

   Whether the PL is in an inconspicuous position

   ∙ Whether a sliding mechanism or lifter is required

   -How difficult is the machining and polishing process?

   Whether the mold structure can be kept simple.

   
   

   4. determination of the optimum PL

   
   

   Based on the evaluation results, the line with the best balance between formability, appearance and cost efficiency is determined as the final PL.

   
   

   Key points and points to note in practice

   
   

   Consideration for appearance

   

   For appearance-oriented products (e.g. automobile interior parts, household appliance covers), PL must be installed where it cannot be seen. As both design and formability are required, it is important to envisage PLs from the product design stage.

   
   

   Flatness of split surfaces　

   
   

   The more complex the dividing surface, the more difficult it is to process the mould, leading to mould clamping faults and burrs. Ideally, the dividing surface should be as flat and simple as possible.

   
   

   Consistency with ejection structure

   

   The PL and ejector (ejector) structure work in tandem to improve the efficiency of moulded product ejection. In many cases, the PL is moved closer to the core side to facilitate ejector design.

   
   

   Recent design support technologies

   
   

   In recent years, advances in 3D CAD and CAE tools have enabled functions that automatically suggest PL candidates and visualise undercuts. This is making it possible to select PLs rationally, without having to rely on the experience of the designer.

   
   

   Conclusion.

   
   

   Parting line setting is one of the most important initial steps in injection mould design. High-quality, efficient moulds can be achieved by selecting the optimum position from the perspectives of minimising undercuts, moulded product appearance and simplifying the mould structure.

   In particular, as the demand for visual quality is increasing for recent products, incorporating visual evaluation and CAE verification from the PL design stage directly leads to the prevention of mould problems. Be sure to make rational PL decisions by utilising tools as well as experience.