Metal Injection Molding Wall Thickness
In order to avoid distortion, an injection molding wall thickness has to be uniform. This is an aspect designed for in the MIM process, as un-uniform wall thicknesses can also lead to variations in shrinkage during the sintering step of the MIM process, in turn causing difficulty in dimensional control.
Cornering wall thickness, the following methods are used to ensure strong and uniform injection molding wall thicknesses:
Not only is coring used as a way as to ensure uniform wall thicknesses, but it can also aid in reducing both material costs as well as the time needed for processing. In some parts, coring is relatively easy – it is done by adding holes, which are formed by pins that protrude into the mold cavity.
Through holes are easier to mold when compared to blind holes, as the core pin can be supported at both ends.
Blind holes which are formed by pins (which are supported at only one end, may be off center caused by the deflection of the pin as the feedstock flows into the mold cavity. For this reason, the depth of a blind hole is in general limited to about twice the diameter of the core pin.
With holes that are perpendicular to each other, problems such as closing-off and binding-off in the mold can occur. A redesign such as designing one hole to be in a ‘D’ shape, will result in a higher strength, a better tooling function and minimize the occurrence of flashing.
Reinforcing ribs is an effective way of improving strength and rigidity in thin wall injection molding processes. Note that the width of such a rib should not exceed not exceed the thickness of the wall it is joined to. The principles of uniform injection molding wall thickness will be maintained wherever and whenever possible.
The Benefits of Reinforcing Ribs:
- Increase part strength.
- Prevent distortion during processing.
- Improve the flow of material / feedstock.
Note that by adding reinforcing ribs, the following might occur:
- Stress concentrations
- Sink marks
While different wall thicknesses cannot always be avoided in some MIM parts, it is important to apply a gradual transition from one thickness to another, helping to reduce poor surface characteristics and stress concentrations.
Designing Metal Injection Molded Parts
The following aspects, along with wall thicknesses, plays an important role in the design of MIM parts:
Generally speaking, gates should allow for the flow of feedstock from the thicker sections to the thinner sections as it flows into the mold cavity. The flow path should impose on the wall of the mold cavity or a pin.
If the flow path of feedstock from thinner to thicker sections is used, the following general problems may occur:
- Stress Concentrations
- Flow lines (in the surface of the part)
- Sink marks
In order to ensure reproducibility, the gates and runners to each cavity must be identical (in size and location). This will ensure that each mold cavity is identically filled with the same amount of feedstock and at a balanced fill rate.
Note that gates do leave a mark on the injection molded part, and must thus be carefully placed (in the design phase) in regard to both the function of the part as well as its appearance.
*Gate: The opening through which the feedstock enters the mold cavity.
Either a slight taper or drafts are needed in order to eject the part from the mold cavity. This is designed into the mold used in the process.
In cases where drafts are required, and angle of between 0.5° to 2° is generally sufficient. Knock-out ejector pins are used in order to remove the part from the mold. A good design of these pins are vital as to minimize the flash marking of the part.
In the design phase, generous radii or fillets are also considered as it will help with the ejection of the part from the mold, as well as improve the flow of the feedstock during the metal injection molding process.
Here, both interior and exterior corners should have radii that are as large as possible. These radii are typically no less than 0.4 mm (+- 0.016 inches) to 0.8 mm (+- 0.031 inches)p>
In cases where it is required, both internal and external threads can be molded into the part itself. This helps to eliminate the need for mechanical thread forming processes.
Tapping is usually the most cost-effective option, but internal threads can also be molded by using automatic unscrewing devices.
Mold Parting Lines
Formed by the opposing faces of the mold, mold parting lines (in mold with a normal construction) is transferred as witness marks or lines on the surface of the metal injected molded part.
Undercuts, be it external or internal, is in often cases needed for the function of the part itself. Undercuts can lead to an increase in the cost of tooling. This increase in tooling cost does however depend on the location of the undercut as well as the type of undercut.
Often specified on metal injection molded parts for ‘o’ ring seating, can be formed by making use of a split cavity mold. As seen with threaded components, here there will be two parting lines, 180° apart, on the surface of the undercut.
These are formed by making use of collapsible cores. Note however that most parts made using MIM are small, making the use of collapsible cores not possible.
Mold fill design is crucial as how the mold is filled with feedstock will influence the final part. An uniform injection molding wall thickness and surface finish, to name but a few examples, will be influenced by how the feedstock fills the mold.
New computer software and simulations aid designers in, for example, moving weld lines from high stress positions, to uncritical positions. Gates, filling and mold design can all be done by using advanced computer aided systems.
Features Achievable by Metal Injection Molding
The following desirable features can be achieved by using the MIM process:
- A low mass / weight (under 100 grams)
- One Flat surface for support during the sintering step of the process
- A wall thickness of less than 10 mm
- Small aspect ratio geometries
- A gradual section thickness change
- Assemblies in one piece
Design Features Allowed by Metal Injection Molding
The following design features are allowed by the MIM process:
- A part number or identification on the die
- D shaped holes
- Keyed holes
- Holes at angles to one another
- Stiffening / Reinforcing Ribs
- Knurled surfaces
- Internal threads
- External threads
- Square, hexagonal, flat bottom and blind holes
Design Restriction in Metal Injection Molding
The following features should be avoided in MIM:
- Inside closed cavities
- Thin wall injection molding when walls are thinner than 0.1 mm
- Holes that has a smaller diameter than 0.1 mm
- Very sharp corners and edges
- No undercuts on internal bores
- Long parts without drafts or tapers (to allow for part ejection)
ChinaSavvy is, in general standards, capable of delivering an injection molding wall thickness that range between 0.020 inches and 0.040 inches. Note however that we do specialize in thin wall injection molding and you can contact us for more in-depth information surrounding wall thicknesses of MIM parts.
Back to Main Page: Metal Injection Molding
Further Suggested Reading:
- Materials used in Injection Moulding
- Metal Injection Molding Tolerances
- Metal Injection Molding Surface Finish
- The Metal Injection Molding Process
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