Design Tips for Injection Moulding

This article sums up the best practices for designing injection moulding to help you get quality polymer parts at a reasonable cost.

The injection moulding process is widely used in large volume production as it produces comparatively low scrap production and has high repeatability. The versatility of the injection moulding process demands much broader design considerations. Most of the design considerations will be made on the mould after setting out the product requirements. 

Some of the factors that affect the injection moulding design include: how the part will be used (singular product or for assembly), its dimensional and mechanical requirements, and its ability to withstand elements such as chemicals or pressure. Some vital tips to consider when designing for injection moulding are explored below.

1. Carefully choose materials suitable for your design

Different injection mouding materials offer varying properties. For example, some injection moulding materials provide more dimensional stability than others. Similarly, some bond better with adhesives than others. Material design considers the following: temperature, pressure, biological and chemical interactions. 

Thermoplastic resins can be broadly classified into amorphous and semi-crystalline.  While semi-crystalline thermoplastics offer better chemical and electrical resistance, their amorphous counterparts are much more dimensionally stable and more resistant to impact. Material selection can affect the required tolerance level or certain features, like wall thickness.

	Semi-crystalline resins	Amorphous resins
Advantages	• Excellent for bearing, wear and structural applications • Good chemical and electrical resistance • Lower coefficient of friction	• Bond well with adhesives • High dimensional stability • Good impact resistance
Disadvantages	• Difficult to bond with adhesives • Average impact resistance	• Low resistance to fatigue and stress cracking

2. Take into account the part tolerance

Tolerances are affected by the shrinkage that occurs during the cooling process. Amorphous materials like PLA generally have tighter tolerances than semi-crystaline materials like PEEK. 

Tight tolerances make production more expensive, but they may be necessary for your part to fit or function properly, especially if it is used in an assembly.

We recommend contacting your supplier at the design stage to discuss the tolerance standards that they use.

For example, DIN16901 contains a general tolerance table as a reference for different materials. If your supplier uses this standard and you need tighter tolerances or other standards, they will ask you to provide 2D drawings.

3. Choose the right wall thickness

There are a few key points to consider to ensure you choose the right wall thickness for your injection moulding design:

• Thinner walls shorten the cycle time and lower the cost of your part. For lots of applications a wall thickness of 1.5-2.5 mm is sufficient, but you can also refer to recommended wall thicknesses for different materials
• Unlike CNC machined parts, plastic injection moulded parts benefit from a consistent wall thickness. If a part is thicker in one section than another, a sink mark will appear at that location.
• Non-uniform wall thicknesses also lead to warping, as these walls cool and shrink at different rates. If you require a non-uniform thickness, the change in thickness should not exceed 15% of the nominal wall thickness and should always have a smooth or tapered transition to achieve a high quality part.

The following are the recommended wall thicknesses for different materials:

Material	Recommended wall thickness
ABS	1.143 mm – 3.556 mm
Acetal	0.762 mm – 3.048 mm
Acrylic (PMMA)	0.635 mm – 12.7 mm
Liquid Crystal Polymer	0.762 mm – 3.048 mm
Long-Fiber Reinforced Plastics	1.905 mm – 27.94 mm
Nylon	0.762 mm – 2.921 mm
PC (Polycarbonate)	1.016 mm – 3.81 mm
Polyester	0.635 mm – 3.175 mm
Polyethylene (PE)	0.762 mm – 5.08 mm
Polyphenylene Sulfide (PSU)	0.508 mm – 4.572 mm
Polypropylene (PP)	0.889 mm – 3.81 mm
Polystyrene (PS)	0.889 mm – 3.81 mm
Polyurethane	2.032 mm – 19.05 mm

4. Add draft angles to your design

Many material removal processes such as CNC machining can produce vertical walls. However, creating a part’s design for injection moulding with vertical walls will cause the part to get stuck, particularly at the core, as the part contracts on cooling.

If too much force is applied to eject the part, the risk of damaging the ejector pins and even the mould becomes very high. Design the walls of parts with a slight slant to avoid this problem. This slanting is called a draft. 

Due to the high complexity, it creates in designing, the draft is usually added at the final stages of the part design. Different surfaces require varying drafts. Textured surfaces require the most draft. Some common surfaces found in injection moulding and their minimum draft angles are as follows.

• For “near-vertical” requirements: 0.5°
• Most common situations: 1 ~ 2°
• All shutoff surfaces: 3°
• Faces with light textures: 1 ~ 3°
• Faces with medium textures: 5°+

5. Add ribs and gussets to certain parts

Certain parts require ribs. Ribs and gussets give additional strength to parts and help to eliminate cosmetic defects like warping, sink and voids. These features are essential for structural components. Therefore, it is preferable to add them to parts rather than increase the thickness of parts to increase strength. 

However, if not properly designed, this can lead to shrinkage. Shrinkage happens when the cooling rate of certain parts is much faster than others, resulting in the permanent bending of some sections. The warping can be effectively reduced by keeping the rib thickness between 50 – 60% of that of the wall it is attached to.

6. Add radii and fillet to part design

Applying radii to parts, when possible, eliminates sharp corners, which improves the flow of material and the part’s structural integrity. Sharp corners cause weakness in the part as the molten material is made to flow through the corner or into the corner. The only places where sharp corners are unavoidable are the parting surfaces or shut-off surfaces. 

Radii and fillets also aid in a part ejection as rounded corners are less likely to get stuck during ejection than sharp corners. Furthermore, sharp corners are also not structurally advisable as they lead to stress points that can fail. Radii help to smoothen out the stress on the corners. 

Also, including sharp corners in your part will exponentially increase the cost of production as this would require the mould to feature sharp corners that can only be achieved using very expensive manufacturing techniques.

Add internal radii at least 0.5 times the thickness of the adjacent wall and external radii 1.5 times the size.

7. Avoid undercuts and provide slots where possible

Snap fits are obtainable through undercuts. The straight-pull mould, which consists of two halves, and is the most straightforward design, is not suitable to manufacture parts with undercut features. This is due to the difficulty in machining such a mould with CNC and the tendency of the material to get stuck on ejection.

Undercuts are usually created using side cores. However, side cores significantly increase tooling costs. Luckily there are some design tips to achieve the function of an undercut without using side cores. One way of doing this is by introducing a slot instead. 

This is also referred to as a pass-through core. Another way is to adjust or move the parting line of the part. When doing this, also adjust the draft angle accordingly. Moving the parting lines is most suitable for undercuts that are on the outside of the part.

You can also use stripping undercuts, also referred to as bump offs. However, only use this feature when the part is flexible enough to deform and expand during ejection from the mould.

Also, give enough clearance: bump-offs must have a lead angle of 30° to 45° for effective ejection. All these alternatives to expensive side cores require significant redesigning of the part. When the redesign of a part is not possible due to the possibility that it may affect the functionality of the part, then you have to employ sliding side-actions and cores to deal with undercuts.

These features slide in as the mould closes and slide out as it opens. The side cores must move perpendicularly and have appropriate draft angles.

8. Attach bosses to side walls or ribs

Bosses are cylindrical standoffs moulded into a plastic part to accept an insert, self-tapping screw, or pin for assembling or mounting parts.

The outer diameter (OD) of the boss should be 2.5 times the diameter of the screw diameter for self-tapping applications.

Bosses shouldn’t be freestanding. Always attach bosses to a side wall or to the floor with ribs or gussets. Their thickness should not exceed 60% of the overall part thickness to minimize visible sink marks on the outside of the part.

For example, a part with an outer wall of 3 mm should have internal ribs that are no more than 1.7 mm thick.

9. Gating: Highlight visually important surfaces on your part where there must not be any marks

In order to properly design and manufacture your part using injection moulding, it is important for the manufacturer to understand from the outset what your requirements are in terms of its appearance.  

One key point for the tool maker to consider is the gate location. Gates are entry sections through which the molten material enters the mould. The tool maker has to choose the type of the gates and position them strategically to minimize potential quality issues. 

Gates also leave gate vestige or a visual indication that the part was gated—even if it is subtle.

That’s why we recommend letting your supplier know about any aesthetic and functional requirements and defining where not to gate.

Design your parts for injection moulding and source them with Eco Rapid

At Eco Rapid, we offer injection moulding services with over 30 materials, such as plastics, synthetic and silicone rubber, and elastomers. Simply contact us and get a free quote.

Design Tips for Sheet Metal Bending

Various kinds of operation processes for the sheet metal fabrication from eco rapid

Bending is a form of deformation, one of three primary processes in sheet metal fabrication; the other two being cutting and joining. Bending is done by holding the workpiece in position using clamps or dies and strategically applying force on an area of a workpiece. The force applied must exceed the yield strength of the material to cause the plastic deformation of the part. This process results in a v-shape, u-shape, or channel shape over an axis, creating a new part geometry. Bending changes the shape but the volume of the workpiece remains the same.

Types of Sheet Metal Bending

There are various methods for sheet metal bending. They are:

Air bending: This makes use of two dies; the upper die (also known as the punch) and the bottom die. The bottom die has a V-shaped opening. The punch forces the sheet metal into the bottom die. Air bending is not as precise as other methods.

Bottoming: In this method, the sheet metal is pressed onto the surface of the die by the punch. The metal then takes up the final angle, same as that of the die. For sheets about 3mm thick, the optimum width for the v-die opening is 6x the material thickness and about 12x the material thickness for 12mm thick sheets.

Coining: This is similar to air bending. However, the force used is usually 5 to 30 times the force for air bending. This gives a much higher precision.

Folding: Clamping beams are used to hold the longer side of the metal. The beam is free to rise and bend the sheet around a bend profile. Both negative and positive bend angles are possible.

Wiping: The longer side of the sheet is clamped, and a tool moves up and down, bending the metal around the bend profile. Wiping is relatively faster than folding but has a higher tendency of producing scratches or damaging the sheet.

Rotary bending: The top die is made of a cylinder that is free to rotate. The final shape for the bend is cut into it, and a matching bottom die. As the rolls contact the sheet, it rotates. The process bends the sheet.

Joggle bending: This is an offset bending. The two opposite bends are less than 90 degrees each. A neutral web separates the opposite bends.

Design Tips for Bending

To ensure a hitch-free bend and to avoid deformation, the following 10 tips are vital when designing.

1. Part thickness

Parts must have a uniform wall thickness throughout. Eco Rapid is capable of manufacturing bent sheet metal parts up to 6.35mm in thickness, but this tolerance mainly depends on the geometry

2. Hole and slot clearance

The distance of holes from the bend should be at least 2.5x the material thickness. Slots require more clearance. Slots should be placed at a distance of at least 4x the material thickness from the bend edges. This is because holes and slots are likely to deform when placed near a bend. Also, to avoid a bulging effect, place these features at a distance of at least 2x the material thickness from part edges.

3. Bend radius

The radius of bends must be a minimum of 1x the material thickness to prevent parts from fracturing or distorting. Also, the Bend radius should be kept consistent to minimize cost.

All bends in the same plane should be designed in one direction to prevent part reorientation. This saves both money and time.

Large, thick parts should not have small bends due to the high tendency to become inaccurate. As a rule of thumb, the inside bend radius should at least equal to the material thickness.

4. Curls

The outside radius of curls must be at least twice the material thickness.

In addition, the distance of holes from curls should be at least equal to the curl radius plus the material thickness. Other bends should be placed away from the curl at a distance of at least 6x the material thickness plus the curl radius.

5. Clearance for countersink

Countersinks on sheet metal parts are usually produced with hand tools. They must not be deeper than 0.6x the material thickness. This means that the maximum depth of a countersink in a 10 mm thick material should be 6 mm.

Furthermore, countersinks must have a minimum distance of 3x the material thickness from a bend, 4x from an edge, and 8x from each other.

6. Hems

Hems are folds created at the edge of parts to create a safe, rounded edge. There are three hem designs with different design rules.

For open hems, the minimum inside diameter should be at least equal to the material thickness, as larger diameters will cause loss of circularity. To ensure a perfect bend, the return length should be 4x the material thickness.

Teardrop hems should also have a minimum inside diameter that is equal to the material thickness. The opening should be a minimum of ¼x the material thickness, while the run length should be at least 4x the material thickness following the radius.

7. Chamfered sides

Chamfers on flanges must leave enough room for bends to avoid deformed parts.

8. Bends next to each other

Successive bends should be avoided except when absolutely necessary. A common problem for successive bends is the difficulty to fit the already bent parts on the die. However, when unavoidable, the intermediate part should be longer than the flanges.

9. Clearance for notches and tabs

The notch to bend distance should be at least 3x the material thickness plus the radius of the bend. Tabs, on the other hand, must be 1 mm or the material thickness away from each other, whichever value is greatest.

10. Relief cuts

Relief cuts are essential in avoiding bulging and tearing at bends. The width of the relief cuts should at least be equal to the material thickness, and the length should be longer than the radius of the bend.

Calculating Required Bending Force

Different factors are involved in creating the right bend in a workpiece. These include:

• Bending strength of the material
• Degree of bending
• Thickness of the workpiece
• Bend angle
• Internal radius
• Vee die opening
• Minimum internal edge

The chart below can be used to calculate the bending force required to V bend mild steel S235 of different thicknesses, in different shapes, at an angle of 90°. Mild steel S235 has a bending strength of 42 kg/mm². The variable parameters are as follows.

• S (mm) – Thickness of the workpiece
• V (mm) – Vee die opening
• B (mm) – Minimum internal edge
• Ri (mm) – Internal radius

Conclusion

At Eco Rapid, we offer high-precision, fast, and quality sheet metal bending and fabrication services for the creation of parts out of sheet metal such as aluminium, steel, copper alloys, and many others. Using automated bending techniques, we guarantee high precision and quality of ready parts. Simply contact us and get a free quote.

Infographic: Design Rules for 3D Printing Technology

3D printing offers a unique combination of design freedom and ease of reproduction

In the space of a few hours. You can come up with your won unique 3D model and turn it into a physical object. All without the use of expensive molds and heavy-duty machining tools.
However, not all 3D models translate well into 3D printed objects. Some may be harder to pull off than the others. If you’re designing your own models for 3D printing, there are certain factors that you will need to consider. To help you plan ahead, there are some of the best tips from 3D printing and modeling experts.

1. Reduce Supports By Following The 45-degree Rule.
   Overhanging features aren’t really huge issues when it comes to 3D printing. Because you can easily remedy the problem by adding support structures. However many 3D printing professionals consider the addition of support as a last resort. Not only do they consume a substantial amount of filament. But the process of removing them may damage the finished print or result in uneven surfaces.
   If you are designing your own model. Then you can make a couple of deliberate decisions to reduce the need for supports. The 45-degree rule is one that’s easy to follow and remember. Any incline that goes beyond 45-degrees will need a support structure to support its weight. If necessary, you can also add a chamfer, which is a wider incline split into 45-degree segments. Even pushing the 45-degree boundary would be pushing the limitations of the strength of the filament material. So we suggest keeping all inclines close to only around 30 degrees.
2. Get Clearances Right
   If you’re looking for create mechanical fitting parts. This step is essential, no machine is perfectly precise. And 3D printers tend to be less precise than other manufacturing technologies.
   This means that parts you design must have proper clearances or gaps between each other to guarantee a fit. For instance, if you have a mount that is intended to hold a bearing. The hole you design for the bearing should be slightly larger than the bearing itself. That way, even if the print has few small artifacts on its surface, the bearing will still fit.
   Find the clearances you need for your printer, and don’t forget to include them in your designs!
3. Mind Overhangs
   In 3D printing, objects are created from the ground up, usually in a layer-by-layer process. This means that features hanging in mid-air don’t fare too well. Becoming deformed or separating from the rest of the model.
   The solution supports structures, they prop up overhanging areas to retain their intended shape. Different 3D printing technologies have different support requirements. SLA printing for instance, almost always demands supports. While in SLS the plastic powder used for printing also doubles as the support material. In FDM, the most common technology supports are required based on the model’s geometry. Which has a significant effect on printing and post-processing time. To avoid the hassle, keep an eye overhanging areas when designing models for this technology. A good rule of thumb is to not to exceed 45-degrees when possible. It’s an extra bit of work, but minding overhangs will save you loads of time in post-processing your print.
4. Orient Based On Resolution And Strength
   3D prints made using FDM technology naturally come out with visible layer lines. This is an inherent consequence of the reliance of FDM printers on relatively wide nozzles. However, the resolution along the z-axis can somewhat be controlled by setting the thickness of these layers. Resolution on the x-axis and y-axis, However, is determined by the size of the nozzle. This is something you may need to consider if your model has very fine details. If you want precise details in your model. Then it would be best to have those details oriented along the z-axis. While it’s true that models can be rotated any axis in the slicer software. You still need to keep in mind that resolution is not equal among all three axes. If you are designing a part that is meant to bear a significant load. You may also need to consider the strength limitations of FDM printing. Basically, the layer lines are the print’s weakest points. Aany stress parallel to them can cause them to get pulled apart from each other. For best results. It would be best to design your model so that any stress is applied perpendicular to the layer lines.
5. Split The Model Into Multiple Parts
   If you are designing a model for printing on a desktop 3D printer. Then the limitation on build size is something you may need to consider. After all, your 3D printer may not be large enough to create that action figure or prop you’re designing.
   Splitting the model into multiple parts can also be advantageous. If you want to avoid having too many support structures on your print. For instance, splitting the model into two or three parts. And orienting them in different directions may mean each part would have fewer overhanging features. This is worth the effort just to avoid the hassle and extra filament that support structures demand.
   There are a couple of different ways that you can split apart a model. You can simply cut them in certain sections. Which means you’ll need to glue the parts together once they are all printed. You can also design snap-fit or press-fit connections, which have the advantage of being non-permanent. The second option can be useful for large props or prototypes. That you’ll need to take apart for transportation and put together again, such as those used in conventions or exhibits.
6. Consider 3D printer Tolerances Based On Nozzle Size
   There’s only so much detail that a 3D printer can reproduce, especially given the size of its nozzle. This holds true for any axis. Moreover, plastic filament naturally expands as it cools. This phenomenon is something you’ll need to consider when managing your expectations of how detailed a model can be when printed. A good rule of thumb to follow is that the effective size of a filament when it is extruded and cooled is around 1,2 times the diameter of the nozzle. For a standard 0.4-millimeter nozzle, this translates to 0.48 millimeters. This means that the features of your model need to be at least 0.48 millimeters or larger on each aixs.
   Tolerance refers to the distance between two nearby features that is just large enough for them to not fuse with each other. Again, the effect of thermal expansion is something you’ll need to consider when designing for tolerances. The problem is that there isn’t a single tolerance value that applies to all filament material and nozzle sizes. Your best bet would be to print this test template to check for the acceptable tolerance for your setup.
7. Consider The Material
   If you know the 3D printing technology you’ll be using, You’ll also know the materials available to you. Like printers, different materials have their own unique properties. When designing you model, account for there properties.
   In FDM printing, for instance, ABS plastic is prone to warping, so your design’s base should be large enough to stay attached to the build plate. Flexibles don’t do too well with details, so maybe omit small features. These will all decrease chances of print failure.
   Additionally, keep manufacturer specifications in mind. The same material produced by different manufacturers can behave differently, so always refer to the printing instructions and profiles provided with the material.
   Take material considerations into account for a smooth printing experience.
8. Avoid Warping By Removing Sharp Corners
   Add mouse ears to your model Warping in one of the biggest problems that a 3D printing professional can encounter. This is especially true when printing with high-temperature filaments, such as ABS or Nylon. Solving the warping issue takes a monumental effort, from tweaking the temperature settings to painstakingly applying adhesives to the print bed. Fortunately, you can take steps in the design process to stave off the warping problem.
   The most common manifestation of warping is when the corners of the base layers of the print lift off the print bed. The corners are especially prone, as these are the point where the thermal stress generated by thermal contraction accumulate.
   One of the smartest ways to avoid warping is to avoid this accumulation of thermal stress by designing rounded corners. This results in a more even distribution of thermal stress not only on the base layer but also for the rest of the print.
   While using rounded corners is not an assurance that you will no longer run into a warping problem, it should help avoid the issue from manifesting.
9. Add Mouse Ears To Your Design
   Use “mouse ears”, helper disks and cones designed into you model to help it print without the use of computer generated supports.
   Mouse ears are basically small disks located on the corners of the base layer of the model. The idea is to increase the surface contact between the corners of the model and print bed in bid to prevent these corner from lifting off. Some slicer software platforms offer the option to add mouse ears to models before printing, although not many of them do. In the case of the letter, mouse ears will have to be added to the original 3D model. The use of mouse ears allows you to retain the sharp edges in your model. However, you’ll have to live with the fact that your finished print will have mouse ears on it base. It would be a good idea to integrate mouse ears into the overall aesthetic of your design. Unlike support structures, mouse ears are practically impossible to remove without ruining the rest of your print.
10. Watch File Quality
    Before 3D printing, your design must be converted to a 3D printable file. During this process, there are few key items to note:
    Ensure that your converted file is of sufficient quality. The above image demonstrates the importance of this: a higher-quality file will be larger, but will be more geometrically accurate. This is especially significant when you have small features, which are sometimes completely omitted when file quality is low.
    Check that the file is “watertight”. This means that is has no holes in it that could confuse your slicing software. A powerful free tool for this is MeshMixer from Autodesk.
    Scale your model before exporting. This will prevent detail loss caused by scaling up your model after the fact. This is like how a low-resolution image looks fine when it is small, but becomes pixelated when you make it larger. Models exported before being scaled may seem fine when left alone, but can totally fall apart when you try to scale them later.
    Any design, However impressive, can be laid low by file errors. Watch for file quality to get the best results!

If you are interested in to know more technology about 3D printing, feel free to reach out with us at eco@eco-rp.com or 8613712611558