Plastic is one of the four engineering materials. With high molecular weight synthetic resin as the main component, plastic can be molded into a certain shape under certain conditions and keep the same shape at room temperature. For the structural design of injection molded thermoplastic parts, we make some preliminary discussions and conclude the key points of structural design of injection molding parts from the aspects of mold draft, wall thickness, ribs, support column, hole, tolerance and so on.

Wall thickness

Other shapes and dimensions, such as ribs and radius are designed based on the wall thickness.

The wall thickness of plastic products mainly depends on the requirements of plastics,

That is, whether the external force to be borne by the product is used as the external force of other parts

Support, the number of columns to be supported, and the number of protruding departments,

Selected plastic materials, weight, electrical properties, dimensional stability and assembly, etc

Depending on the requirements.

If the wall thickness is not uniform, the filling speed and cold of the plastic melt will be reduced

Uneven shrinkage will cause depression, vacuum bubble and deformation

Even cracking.

Uniform wall thickness is a major principle in the design of plastic parts.

The general thermoplastic thickness is designed in the range of 1-6mm.

The most commonly used is 2 to 3mm, and large workpieces also have more than 6mm.

When wall thickness is designed and determined, below are some pointes to be taken into consideration.

1.When meeting the strength and hardness requirements, the minimum wall thickness shall be considered as far as possible to reduce the material cost and shorten the production cycle.

2. Try to make the wall thickness of the workpiece uniform, otherwise it will cause uneven shrinkage, deformation, bubbles and depression of the workpiece.

The thicker area cools slower than thin areas, resulting in shrinkage marks, stress marks, deformation, different colors or different transparency, etc.

If the thickness cannot be uniform, it shall be designed to change gradually as far as possible, and under the proportion of no more than 3:1.

(Bad design)

(good design)

(best design)

3.Try to design ribs to change the overall thickness if thickness cannot be changed. This design will save material cost and production time.

4. Flow path has to be taken into consideration when the wall thickness is designed, Flow path, means the distance from the gate to the cavity. Generally, the flow path has a certain proportional relationship with the wall thickness. For example, the larger the wall thickness, the longer the flow path. If the ratio is too large, there will be a shortage of material on the areas with long distance from the gate.

Table 1

Recommended minimum wall thickness of thermoplastic products and recommended values for common wall thickness

Demolding Direction and Draft

For each injection molding product, the mold opening direction and parting line shall be determined during the tool design staget o ensure that the core pulling structure and the influence of parting line on the appearance are minimized.

1.For example, the demolding direction of automobile bumper is generally along the length of the vehicle. After the mold opening direction is determined, the ribs, snaps, protrusion and other structures of the product shall be designed as the same  direction as the mold opening direction as possible, so as to avoid core pulling, reduce joint lines and prolong the tool life. After the mold opening direction is determined, the appropriate parting line can be decided to improve the part appearance and performance.

2.When the part is demolded from the mold, the withdraw force and opening force should be overcome.

Demolding means that the part comes out of the tool cavity. The parts shrinks during the cooling process. Parts will  has a tightening force on the cavity. Excessive withdraw force will cause a lot of issues on the parts, like white strain, deformation, scratch mark etc on the surface. Draft is a major factor determining the opening force.  Generally plastic injection molding parts are attached to the core after cooling and shrinkage. In order to make the product wall thickness uniform and prevent the product from being attached to the cavity after mold opening, the draft should be design equally on the core and cavity.

However, under special circumstances, if the product is required to be attached to the cavity after mold opening, the draft on the areas attached to the cavity can be reduced appropriately or an appropriate amount of snaps can be added on the core.

3. When considering the draft, in principle, the larger the draft, the more conducive to demolding. However, attention must be paid to ensure the part has accurate size and the deviation which is caused from the draft are still within tolerance.

There is no a confirmed rule on how to set the draft.

5.For parts with large shrinkage and complex structure and shape, a large draft should be considered.

Below are some recommended values.

High glossy surface is about 0.125 or 0.25 draft, smooth surface is about 0.5 draft. Fine texture surface is about 1 degree draft and big texture surface is 1.5 or bigger draft.

Ribs

The strength of the workpiece does not depend entirely on the wall thickness. On the contrary, the increase of wall thickness causes internal stress on the shrinkage surface, which reduces the strength of the workpiece.

The strength of plastic workpiece is mainly stiffness. Thin wall composite structure is mostly adopted with some ribs designed to increase stiffness.

However, after the ribs are added, the joint between the ribs and the main wall will become thicker.

1. The thickness of the ribs must be less than 1 / 3 of the product wall thickness, otherwise there will be shrinkage mark on the surface.

2. The slope of one side of the ribs shall be greater than 1.5 degrees to avoid top injury.

3. The stiffener should not be designed too small or too large. If it is too small, the stiffener is easy to deform, and the material should not be filled and the mold should be stuck. If it is too large, there will be shrinkage marks on the surface of the workpiece. Generally speaking, when the thickness of the stiffener is too small, the height increases accordingly to strengthen the stiffness.