Design for Manufacturing (DFM) for Injection moulding

What is Injection moulding

Injection moulding is a method of producing pre-designed plastic parts from thermoplastic polymers. Plastic in the granular form is melted and injected into the steel mould, after cooling the mould the solid part is taken out. The Injection moulding is performed in an injection moulding machine comprising of a heating barrel with a hopper mounted above it and a reciprocation screw inside. The steel mould has two halves named Core and Cavity where one of them (Commonly the Core) is attached to the ejector mechanism and the other to the injection mechanism of the injection moulding machine.

The plastic in granule form is loaded into the hopper. The rotary motion of the reciprocating screw will move the granules into the heating area of the barrel. Plastic granules heated at around 300°C are melted or plasticized. The reciprocating action of the screw will push the melted plastic at high pressure (10,000 to 16,000 psi) and inject them into the steel mould with the cavity of the desired part. The steel mould has several cooling lines for cooling the plastic inside the cavity to solidify. A mould opening & ejector mechanism is used to open the mould and simultaneously eject the solid part into the finished parts tray. The injection moulding method is generally used for high volume production.

What is Design for manufacturing (DFM) for Injection moulding parts

Design for manufacturing or DFM is a part of DFX or Design for X (the X is replaced with Manufacturing in this case). DFM is the process of designing a part or component to make it easier to manufacture. DFM basically consist of a set of industry-recommended design guidelines to increase the manufacturability of the part design. DFM will always ensure the quality of the part design. In addition to manufacturability, DFM guidelines are used to reduce the common defect that occurred during the injection moulding process as well as to ensure the strength of the plastic part. Plastic parts which do not follow these guidelines may invite unnecessary expenses incurred due to redesign and mould modification.

Why DFM is recommended in Injection moulding parts

DFM is recommended in injection moulding in order to increase manufacturability, reduce common defects and ensure strength in the injection moulded parts. It is highly recommended to design each part by keeping DFM in mind so that costs incurred by mould modification & tooling can also be reduced. Designing parts with DFM will help manufacturability of the following components:

• Injection mould gate design
• Wall thickness
• Hole design
• Rib design
• Boss design
• Undercut design
• Insert design
• Draft angle design
• Mould parting line design

Below we will go through all the above-mentioned components in brief. Part designs that do not follow DFM are often rejected by the mould manufacturers and request for design changes which takes time. DFM can also be done using a computer DFM software application where you can load your 3D design model and enter all your requirements. DFM software application analyses the model and its requirements output the list of all features in the 3D design which are not compliant with the DFM design guidelines.

Design guidelines for Injection moulding parts

Suggested Injection mould gate design:

A gate in an injection mould is the entry point into a mould cavity. There are several gate designs to satisfy the needs of different cavity designs. Commonly used gate designs throughout the industry are Fan gate, Center gate, Tab gate & Edge gate. The selection of the gate depends upon the design of the part which should be injection moulded. the gate location plays an important role in surface finish. Visible flow marks are common defects found in parts produced with inappropriate gate design. See Illustration 1.1

Suggested wall thickness:

Appropriate wall design plays an important role in ensuring the strength of an injection moulded part. Walls should be of uniform thickness as possible. A gradual transition of wall thickness is recommended if the uniform wall thickness is not possible in the design. For smaller parts, it is recommended to use a thinner wall design which will also help to reduce the plastic required to produce the part. There are recommended wall thicknesses for various plastic materials which can be seen in Table 1.1.

Material	Short sections (mm)	Small sections (mm)	Average sections (mm)	Large sections (mm)
Acetal	0.6	0.9	1.9	3.2–4.7
Acrylic	0.6	0.9	2.3	3.2–6.3
Acrylonitrile butadiene styrene	0.9	1.3	1.9	3.2–4.7
Cellulose acetate butyrate	0.6	1.3	1.9	3.2–4.7
Nylon	0.3	0.6	1.5	2.4–3.2
Polycarbonate	0.4	0.8	1.8	2.4–3.2
Polyethylene (Low-density)	0.9	1.3	1.6	2.4–3.2
Polyethylene (High-density)	0.9	1.3	1.9	3.2–4.7
Polypropylene	0.6	0.9	1.9	3.2–4.7
Polystyrene	0.8	1.3	1.6	3.2–6.3
Polyvinyl chloride (Flexible)	0.6	1.3	1.9	3.2–4.7
Polyvinyl chloride (Rigid )	0.9	1.6	2.4	3.2–4.7

Suggested Hole design:

Designing holes in the injection moulding part can adversely affect its strength. Following recommended design guidelines will reduce the adverse effects of adding holes in the design as well as the defects caused by inappropriate hole design.

• The minimum spacing between the two adjacent holes or the spacing between the edge and the hole should be the diameter of the hole itself.
• Minimum distance between a hole and edge of the part should be three or more times the hole diameter.
• Through hole is preferred than a blind hole.
• The depth of the blind hole should not be more than 2 times of the diameter. The depth should be 1 times the diameter which is less than 1.50mm.
• For blind holes with higher depth steps are introduced which reduces the diameter with depth.
• Overlapping and offset mould-cavity projections are preferred instead of core pins for producing holes parallel to the die-parting line (perpendicular to the mould-movement direction).
• It is recommended to design holes with minimal sliders which are perpendicular to the mould opening direction.

Suggested Rib design:

The ribs are vertical wall features with certain thickness and height primarily used to increase the strength of parts that are large. Adding ribs to a design eliminates the requirement of thicker walls to increase the strength of the plastic part.

• The ribs should have thickness of 40% to 60% of the base wall thickness on which the ribs are designed. this will avoid sink marks.
• Draft angle of the ribs should be 0.5 to 1.5 degree on each sides.
• Recommended radius at the base of the ribs should be 25% to 40% of the wall thickness.
• Maximum height if the rib should be 2.5 to 3 times of the base wall thickness.

Suggested Bosses design:

Bosses are extruded pad features that act as a mounting surface, locations guide & feature to secure screws during assembling the plastic parts. It is recommended to insert a boss feature at the corners of the design of the part to help the mould flow during injection moulding. Design guidelines are similar to that of ribs.

• Boses should have thickness of 40% to 60% of the base wall thickness (similar to ribs).
• Draft angle of the bosses should be 0.5 to 1.5 degree.
• Recommended radius at the base of the boss should be 25% to 40% of the wall thickness.
• Maximum height of the boss should be 2.5 to 3 times of the base wall thickness.

Suggested Undercut design:

Undercuts are features that cause interference in the smooth ejection of the part from the mould. Sliding cores or split moulds are used in the mould to produce parts with an undercut.

• Design of shallow undercuts are recommended to avoid sliders but should expect stripping of parts during ejection from the mould.
• While designing move or change the parting line to the undercut location if design permits.

Suggested Inserts design:

Inserts are prefabricated parts that are over moulded with the injection moulded part. The final product should have the insert permanently encapsulated within the injection moulded part. A boss or blind hole with depth is designed in the part for encapsulation of the insert.

• The Boss diameter at the top should be 2 times greater than or equal to the diameter of the insert.
• The depth of the hole for inserts should be greater than 2 times of insert diameter.
• A course knurl is recommended on the insert for proper encapsulation with plastic part.

Suggested Draft angle design:

Plastic parts designed for injection moulding must have a draft angle. Draft angle is the angle that is given for the vertical walls and features which are parallel to the mould opening direction. Adding draft angle helps easy ejection of the finished plastic parts from the mould. If the surface finish of the part has a rough texture then it is recommended to use the higher draft angle to prevent part striping off from the mould during ejection.

• A minimum of 0.5 degrees on all vertical faces is highly recommended.
• In general cases, 1 degree of draft angle is advised for every inch of depth.
• For part surfaces having rough texture a draft angle of 3 to 5 degrees or more should be given depending upon the surface texture roughness.

Suggested Mould parting line design:

The parting line is the surface where the two halves of the mould contact. Selection of the parting line location is important for the parts aesthetic or cosmetic appearance. Common defects caused by the parting line are excess plastic material at the parting line location called flash.

• Parting lines are recommended to be at the edge where the part has a sharp edge.
• The parting line should be straight in general.
• Clever selection of parting lines for the part while designing can reduce the movable sliders and reduce the mould cost.

Conclusion

The above guidelines only scratched the surface of DFM for injection moulding. Understanding and practising DFM in the injection mould part design will save costs incurred during the mould tooling, plastic material usage, rework and redesign. These guidelines will also ensure the manufacturability, strength and rigidness of the part by eliminating weak areas found in the finished part. It is highly recommended to practice DFM guidelines during the initial concept development of the part to produce the best product functionally and aesthetically.