Why My Design can't turn out to be the product I envisioned? How to bridge Design & Manufacturing?


This month we cover the topic “design for manufacturing” – DFX is particularly relevant in plastic part design as it will impact part quality as well as costs (both tools & parts). Enjoy the content.


1.0 Introduction:


Product design is driven by trends, new materials, visual aspects and costs. The problem is that industrial designers, product designers etc. often lack manufacturing & process experience as they rely on 3D software tools to create models that look good but are difficult to manufacture.

We illustrate problems in the case of plastic parts as they play an important role in most of today’s products, be it a car, a coffee machine or a laser printer.

Similar examples could be tabled for die-casting, investment casting and to a lesser degree for stamped and CNC machined parts.

To be at the tail end of product designs (= manufacturing) is often not enviable – it will require resourcefulness, flexibility to find ways to make parts via process, material and tool adjustments.



2.0 Interplay Part Design – Tool Design:

Plastic part manufacturers and customers know that the successful production of high- quality parts depends on optimum design of parts and tools.

Here we highlight the key drivers that influence the results:


a) Design for injection molding:


The culprit for problems with molding parts is often the neglect of basic part design rules. Here we summarize a few essential guidelines:


· Uniform wall thickness

The fundamental issue with tick & thin wall sections is that they cool at different rates. Temperature differences associated with differential shrinkages can result in distortions, deformations/warpage. If uniform wall thicknesses are not possible, ensure that the material flow is from thick to thin. Better is to keep a uniform wall thickness and add ribs to provide strength.


· Rib design

Ribs should be ~ 70% of the wall thickness – spacing and height must be considered carefully. If ribs are too thick, sink marks will be visible on the opposite side of the rib.


· Boss design

Bosses are used for fixing plates, printed circuit board assemblies etc.; bosses on ribs generally use two 90deg gussets; free standing bosses four gussets and corner bosses three. Gusset thickness is approx.. 70% of the part wall thickness. Use of sufficient draft angles will help to reduce part ejection forces.


· Part corner design

Sharp corners in product design violate part manufacturing criteria. The price for gaining extra interior volume, achieving better mating between 2 components or for aesthetic reasons is disproportionate with the molding disadvantages (stress concentration, lack of torsional stiffness, restriction of heat flow (inside  outside of the tool). Guidelines for corner designs (radii, chamfers) can be found in many text books.


· Part draft angles

Draft angles on a part are needed to assist the ejection of parts from the mold. Product designers frequently avoid the incorporation of draft angles in their designs as they alter part appearance and impact internal part volumes. Adding sufficient draft angles is important, in particular in parts with surface texturing and fiber glass filled materials. Draft angles generally range between 1.5 – 5 degrees.


· Avoid undercuts

It may not be possible to avoid undercuts but the designer should be mindful if the added complexity undercuts cause in the tool design (sliders, lifters, …). Such mold components not only increase tool costs but impact tool life, reliability and tool maintenance.


b) Tool design for manufacturing:


Good tool design is equally important to achieving optimal part molding results. It is far outside this article to cover details of good tool design.

In all our customer tool projects, we outline the basic tool concept in our quotation and refine details once the project is implemented. During the DFM process details of the tool layout are addressed:


  • Gate design

  • Venting

  • Cooling system design

  • Ejection system design

  • Shrinkage & warpage considerations


Mold type, mold cavities, tool material materials and runner types are selected based on part size & complexity, tool life, molding process and material, surface finish requirements etc.


3.0 Product Design and Manufacturability Cases


In this section we show you examples that cover early project phases as well as issues that needed to be solved once we had molded parts in our hands …


3.1 Moldflow analysis and part design optimization


Figure 3.1 shows the assembly of a frame and cover. The example highlights risk points and how they have been addressed


Issues:

There are 4 risk points in the frame and cover that will impact function and aesthetics


a) Cover:

Risk(1): The 2 side ribs of the cover are prone to warpage which will affect the part assembly and function

Fix: We design separate cooling circuits for the ribs such that we can improve the warpage by cooling temperature adjustment


b) Frame:

Risk(2): The design of the side walls have a high chance of deformation

Fix: We design separate cooling circuits for the ribs such that we can improve the warpage by cooling temperature adjustment


Risk(3): Part walls are visible and flow/weld lines are not allowed

Fix: Using Moldflow analysis allows us to optimize the position and size of the gate to move weld lines into the part corners


Risk(4): There is a high risk that the part will stick to the cavity side

Fix: We use moving inserts to release the ribs first before opening the tool


3.2 Tool cost reduction through smart part re-design


Figure 3.2 shows a filter housing part and is an example of how tool costs can be reduced by “design for injection molding”


Issue: The geometry of the grill ribs required costly angles sliders


Fix: By redesigning the shape and structure of the grill ribs, sliders could be eliminated without compromising the function of the part


3.3 Fix of poor part sealing problem after initial mold trials


Figure 3.3 shows the housing of a wireless leak detector; we illustrate how we overcame a detector sealing problem in a 2k housing design


Issues: The sealing of 2 housing parts failed to pass IP65 testing conditions.

Parts & materials are:

  • The lower housing (ABS) is a 2k part with a TPE sealing strip

  • The upper housing and battery cover are in ABS

  • There are gaps in the TPE sealing strip in the vicinity of 2 connectors


Fix: Change the geometry of the lower/upper housing parts in the area of the connectors to get continuous sealing and hence an improvement of the gasket function


3.4 Solve part warpage problem by changing the molding material


As suppliers of injection molding tools, our project managers focus on the tool design aspects and generally take part data from our customers as provided. Even if a tool has been designed well and the correct material shrinkage has been used, there can be surprises one you mold the parts.


Figure 3.4 shows an adapter (approx.. 200mm long) where the molded part was bent like a banana!


Issue: Significant part warpage occurred (up to 7.5mm) over the length of 200mm rendering the part useless.


We embarked upon a series of experiments to find a pragmatic way to solve the problem.


a) Experiment 1: Adjusting molding parameters

  • Holding pressure adjustments: no effect

  • Cooling time adjustments: no effect

  • Cooling temperature adjustments: no effect


b) Experiment 2: Simulation of wall thickness change


Simulation of top wall thickness reduction by placing thin copper sheets into the mold

  • 0.5mm reduction: 25% improvement

  • 1.0mm reduction: small negative warpage


c) Experiment 3: Manufacture part with different molding materials

  • ABS: 90% improvement (warpage < 1mm)

  • PP (higher hardness): 60% improvement but still NG

  • PP+30GF: 95% improvement (warpage acceptable ~ 0.5mm)



4.0 Conclusions:


We have been involved in the production of > 500 injection molding tools and understand the complexities and finesses of manufacturing high quality tools.

Our project managers are experts – they can judge good tool design but also know the art of optimizing molding conditions to produce excellent parts.


However, if the part/product design is poor – no matter what you do, you may never be able to produce a good part. It all starts with design! Good design not only impacts the process results (part function, visual aspects) but cost (tool cost, fill time, cooling time, tool life, etc.).


Design for manufacturing is maximizing part and tool design for the benefit of long-lasting tools capable of producing excellent parts.


We play an important role in the interplay between these 2 elements. We use the expertise and knowledge to iron out problems, correct mistakes and find ways to reduce tool complexity and cost.


The simpler a tool – the lower the chance of problems in production.



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