Monday 30 April 2018

Plastics Part Design: 10 “Holy” Design Rules For Injection Moulded Products

In today’s post I would like to give you 10 design rules which can be used as a checklist or little helper when finishing up your part design for injection moulded products.

Let’s start by listing all the 10 at once and then present detail information for each rule [1, 2, 3]:

  1. Wall thickness as thin as possible
  2. Continuous wall thickness to prevent accumulation of mass
  3. Corners and edges with radius
  4. Ribs designed for moulding: 40-60% of wall thickness for ribs
  5. Avoid plane and even surfaces
  6. Use draft angels
  7. Avoid undercut sections
  8. No more accurate machining as necessary
  9. Check for possibilities of function integration
  10. Past performance of design can be guarantee of future results

1. Wall thickness as thin as possible: wall thickness has influence on the following factors:
  • The allover part mass and therefore the material costs
  • Cycle time
  • Surface quality, warpage and shrinkage cavities
  • Flow length
  • Tolerances
  • Stiffness build-up (use ribs)
  • Orientation of molecules and additives such as glass fibers
Furthermore, designers seek for thin wall thicknesses since the latter enlarges the cooling time to the square (cooling time = wall thickness^2/ thermal conductivity). It has also the side benefit of saving material costs. Tolerances play an important role in terms of shrinkage and warpage. They are defined for plastic parts in several standards. One standard used in central Europe is DIN 16901. Here, you can find tolerances <1% for normal injection moulding, <0.6% for technical moulding and <0.3 for precision moulding. For achieving your production tolerances, the absolute value of your shrinkage is not as important as the shrinkage spread/deviation resulting through the process over time. Increasing the tolerances will not lead to a better part, it will most probably lead to a lower overall yield.

2. Continuous wall thickness to prevent accumulation of mass:
Different wall thickness influences mainly:
  • Creation of porosity
  • Sink marks
  • Internal stresses (crystallization, shrinkage, polymerization) and warpage
When designing parts, it is important to consider that thick areas will take longer to cool down, especially when these are located far from the injection location and connected over thin wall areas. As a result, full freezing of the part does not happen in the packing phase and porosity and sink marks will be present in the final part. In the figure below, an example of how to connect the so called “eyes” to the main part.
3. Corners and edges with radius
In case you have a wall thicknesses change (thicker to thinner; thinner to thicker) or have corners, it is necessary to apply a radius for lowering the stresses.
The graph below shows the optimal radii as a function of wall thickness.

4. Ribs designed for moulding: 40-60% of wall thickness for ribs
There are several ways to increase the overall stiffness of your part:
  • Increase the wall thickness: the cross sec­tional moment of inertia I = b*h^3/ 12 shows that increasing the wall thickness will increase the stiffness of your part to the power of three. It is more effective than changing the material. However, cooling time will increase significantly too. Therefore, the optimum needs to be found.
  • Material selection: changing to a material with high modulus.
  • Beads: usually with a factor of 1.8 times more effective than a rib in the same dimensions.
  • Ribs: they are just getting effective when they are 5-10 times higher than the wall thickness and should be made with a thickness of 40-60% of the original wall. Injection moulding location and filling direction (molecular orientation) affects how ribs will perform in a later stage.

5. Avoid plane and even surfaces
The moulding of plane and even surfaces is one of the most challenging processes since the part tends to have sink marks and buckles. The reason for that can be a heterogeneous cooling process after moulding, or shrinkage due to variable wall thickness. Ineffective deployment of the holding pressure may also lead to buckling. It is difficult to replicate buckling since it is an unstable phenomenon.

6. Use draft angels
The cooled down polymer melt shrinks onto the mould cores which will be the inside of the final part. Draft angels are needed for the removal of the frozen part form the mould cores. A guidance on draft angels can be found below.

7. Avoid undercut sections

In nowadays injection tools is difficult to avoid undercut sections due to increased parts complexity. Polymers can withstand a forced ejection as long as the resulting stresses are within the yield stress of the polymers. As such the part can recover the deformation. Other ways of solving undercuts associated issues are:
  • Movable sliders or collapsible cores
  • Melting cores
  • Utilization of split clamping design and subsequent laser weld of the resulting parts after moulding
For avoiding undercuts, creative design solutions are available. One of them is to use snap-fits.
8. No more accurate machining as necessary
When tolerances are set to high, quality will not automatically increase. Most of the times the opposite is the case: total yield of parts decreases. Therefore, parts need to have only the necessary tolerance to still fulfill their function.
9. Check for possibilities of function integration
Function integration for having off-tool parts are challenging for part designers and tool makers, since overall tool costs will increase. However, it allows keeping a competitive edge in high wage countries. Well known examples for function integration are:
  • Metallic inserts like screw sockets and plugs
  • Hinge joints: off-tool monkey
  • Detachable screw joints
  • Multi-component moulding
  • Film hinges
10. Past performance of design can be guarantee of future results

On the stock market you get always the advice that past performance is not an indication for future performance. In case of plastics design this is not the case. Looking back at successful launched plastic products you can learn from them. Incorporation of proven design helps launch your part solution in a faster way.

Thanks for reading and happy designing!
Herwig Juster

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[1] Keuerleber and Eyerer: Konstruieren und Gestalten mit Kuntstoffen, 2007
[2] Eric Larson – Thermoplastic Material Selection