Whether you will use injection molding or other processes (such as 3D printing) to design parts, it is important to conduct design for manufacturability (DFM) analysis in the early stage.
At the most basic level, DFM means considering the manufacturing process before drawing the first sketch of the part. This means that instead of designing an ideal component first and then considering how to manufacture it, it is better to incorporate manufacturing considerations into the design itself. If the manufacturing process/equipment is difficult to make the specific structure of the part, the detail must be changed or removed at the beginning.
Different from CNC machining or 3D printing designed parts，metal molds used in injection molding must contain openings that allow molten plastic to enter.These openings are called gates, which can greatly affect the final appearance and functional dimensions of the injection molding part. The gate can be automatically or manually detached from the finished part.
The commonly used gate design methods as follow:
Side Gate
The side gate flows in from the side of the product. After taking out the molded product, it is always necessary to cut the gate with pliers, etc. (requires secondary processing). Although the mold structure is simple, the initial cost and operating cost need to be considered due to the need for secondary processing.
Tunnel Gate
Like the side gate, the tunnel gate flows in from the side of the product, and the inlet point is at a certain angle with the gate inflow direction, so that the gate can be automatically cut off when the mold is opened and closed or the product is ejected. The advantage is that the gate can be automatically cut off, but there are also disadvantages such as the gate entrance is often relatively small, and the molding conditions are limited, residues will be generated when the gate is cut, and poor molding will occur. (It can also be designed in the ejector pin position, and then cut after molding, without affecting the appearance)
Direct Gate
Suitable for large cylindrical parts and single cavity molds, easy to design. It is directly connected from the spool to the molded product without passing through the runner. Since the holding pressure is easily transmitted to the product during molding, it is suitable for large molded products and thicker molded products. The gate marks are large, and the gate needs to be cut after finishing. As far as possible, design the gate location on the secondary appearance surface of the product.
Point Gate
The point gate also has a structure that does not need to cut the gate twice. This is not a 2-plate mold structure like a sub gate, but a 3-plate mold structure. The gate cannot be made too large, because the product part and the runner part are torn when the mold is opened and closed. Although it has the advantage that the gate does not need to be cut off, due to the problem of gate fracture and smaller gates, it may have problems such as excessive gate remaining during the molding process, depending on the material.
Tip
Larger parts require larger gates (or higher molding holding pressure).
Place the gate away from corners and thinner places to ensure the fluidity of the material.
Design the gate in a larger cross-section to reduce sink marks.
If the part size and shape require it, two or more gate designs will be used.
In order to smoothly release the molded product from the mold, it is necessary to design the draft angle.
The draft angle refers to the inclination in the direction in which the molded product is released from the mold (the direction in which the mold is opened). At the moment the mold is opened, there will be a gap between the cavity and core. Because of the gap, the molded product can be taken out smoothly.
Please imagine what the pudding looks like. The pudding box is tilted. If the box is vertical, the pudding will hardly fall off. In order to smoothly detach from the mold, it is necessary to provide an inclined surface on the side surface.
The recommended draft angle under the following conditions：
Vertical plane----------------------------0.5°
General structure surface---------------1.0° Section(impact surface)-----------------2.0° Fine etching surface---------------------3.0°
Coarse etching surface------------------5.0°+
The dividing surface of the mold is also called PL.
There are slight steps on the dividing surface, so it looks like the product has lines. Many molding methods such as injection molding and blow molding use split molds. Therefore, many plastic products have parting lines somewhere in the product.
In actual product design, when the molding product is taken out of the mold, it is a shape that cannot be released only in the mold opening direction, and the shape that cannot be simply divided into two and taken out is called an undercut. For example, the shape shown in the figure is an undercut.
To manufacture products with undercuts, a more complicated mold design may be required-as shown in the slide mold design, the slide part of the mold moves in different directions.
Due to the special structure of the mold, the undercut processing will increase the cost of the mold.
Another basic rule when designing plastic parts is to ensure the uniformity of the thickness and make the surface as thin as possible when the design permits.
The uniform thickness helps the material flow in the injection mold and reduces the risk of sink marks, molding stress and different shrinkage rates.
In addition, when the overall thickness of the product is designed to be the thinnest and consistent as possible, cost can be saved.
The reason is that thin-walled products will cool faster, which means shorter production cycles, and more products will be produced per hour.
In addition, the product is designed to have a uniform thickness.
This makes the cavity easier to fill. If the thickness is not uniform, the thinner part will cool first, and then as the thicker part cools and shrinks, it will create stress near the boundary between the two.
Because the thinner part is already solidified, it will not shrink. As the thicker part shrinks, it will cause warping or deformation of the thin surface, and if severe, cracks may occur.
For non-uniform thickness joints, the thickness change should not exceed 20% of the thin side.
ABS 1.1mm--3.5mm
PA 0.7mm--2.9mm
PC 1.0mm--3.1mm
PE 0.7mm--5.0mm
PP 0.6mm--3.8mm
PS 0.8mm--3.8mm
POM 0.7mm--3.0mm
PPS 0.5mm--4.5mm
PMMA 0.6mm--3.8mm
LCP 0.7mm--3.0mm
The above note lists the general recommended thicknesses for various plastics. The thickness required in the actual design depends on the shape of the product and the molding method. Although the minimum thickness is effective for products with small shapes, the flow distance of plastics is long for products with large shapes, so the thickness needs to be designed to be thicker. Our recommend thickness is 1.0 – 3.0 mm.
Convex design will be used when assembling parts with self-tapping screws and insert nuts. Since the roots are often thicker, it is necessary to pay attention to the occurrence of sink marks and voids. If the overall height of the boss increases, the wall thickness at the base will increase due to the draft, so the design should be kept as low as possible.
In addition, when designing a separate convex, use ribs/reinforcing ribs to connect to the inner wall when necessary to ensure structural stability.
The outer diameter of the convex is preferably 2.5 times the diameter of the screw or nut.
Protrusions should be completely avoided on parts with polished surfaces, as they usually cause dents etc, on the surface.
The thickness of the bottom of the reinforcing rib used to fix the convex should not exceed 60% of the thickness of the adjacent wall.
The draft angle of the inner diameter and outer diameter of the convex should be 3°.
Sharp corners should be avoided in the design of plastic parts.
They are the main causes of component failures, stress concentrations, poor plastic flow patterns, and increased injection mold abrasion.
When we use CNC machining to make molds, it is much easier to machine corners with radii.
Designing for a right angle requires higher accuracy, and therefore requires more time and cost.