Plastic parts are everywhere in our daily life, it is cheap, excellent performance, make our life more diversified, but do you know, these plastic parts are how to design and production out? What do we have to pay attention to when designing them? However, today we are going to focus on design guide for injection molding parts.
What is injection molding and Injection Molding Process
Injection Molding, is a manufacturing method in which parts are made by injecting material into a pre-build tooling. It is a type of plastic processing. The pre-build tooling we called it injection molding.
In this process, plastic is placed into a hopper, which then heats the plastic to inject it, and it is pushed through a long chamber with a reciprocating screw. Afterwards, it is softened into a fluid state. The nozzle is located at the end of the chamber and the fluid plastic is forced through the nozzle to cool and close the mold. As the plastic cools and cures, the semi-finished product is withdrawn from the press
Forming methods of plastics
Generally speaking, there are the following molding methods for plastics: injection molding, extrusion molding, Compression Molding, foaming, blow molding, vacuum blister molding, hollow molding, machining, etc.
Due to the type and performance of plastics, use occasions, molding processes and other conditions, the structural design of plastic parts will naturally produce some special requirements and methods, involving quite a lot of content.Today, however, we will focus on the structural design of injection molded thermoplastic parts. The main points of structural design of injection molded parts are wall thickness, draft angle, Rib, hole, support pillar, hook, interference connection, tolerance and so on.
Wall thickness
It is very important to reasonably determine the wall thickness of plastic parts, other shapes and dimensions such as rib and rounded radius are referred to the wall thickness. The wall thickness of plastic products is mainly determined by the requirements of the use of plastic, i.e., the external forces that the product needs to withstand, whether or not it is used as a support for other parts, the properties of the plastic material selected, weight, electrical properties, dimensional accuracy and stability, and assembly and other requirements.
The general thermoplastic wall thickness is designed in the range of 1 to 6 mm. The most commonly used is 2 to 3 mm. large parts are also available in excess of 6 mm. Here below shows suggested wall thickness of some thermoplastics.
| Plastic material | Min Wall thickness | Small part suggest wall thickness | Middle part suggest wall thickness | Large part suggest wall thickness |
| Nylon | 0.45 | 0.76 | 1.5 | 2.4~3.2 |
| polyethylene（PE） | 0.6 | 1.25 | 1.6 | 2.4~3.2 |
| Polystyrene（PS) | 0.75 | 1.25 | 1.6 | 2.4~3.2 |
| PMMA | 0.8 | 1.5 | 2.2 | 4~6.5 |
| PVC | 1.2 | 1.6 | 1.8 | 3.2~5.8 |
| PP | 0.85 | 1.54 | 1.75 | 2.4~3.2 |
| PC | 0.95 | 1.8 | 2.3 | 3~4.5 |
| POM | 0.8 | 1.4 | 1.6 | 3.2~5.4 |
| ABS | 0.8 | 1 | 2.3 | 3~5 |
The key factors that determine the wall thickness of plastic parts include:
1) whether the structural strength of the part is sufficient. Generally speaking, the thicker the wall thickness, the better the strength of the part. But the wall thickness of the parts more than a certain range, due to shrinkage and porosity and other quality problems, increase the wall thickness of the parts but will reduce the strength of the parts.
2) parts molding can resist demoulding force. Parts are too thin, easy to deformation due to ejection.
3) Can resist the assembly of the fastening force.
4) When there is a metal insert, the strength around the insert is sufficient. General metal insert and the surrounding plastic material shrinkage is not uniform, easy to produce stress concentration, low strength.
5) parts can be uniformly dispersed by the impact force.
6) whether the strength of the hole is sufficient, the strength of the hole is easy to reduce because of the impact of melting line.
Wall thickness should be as small as possible, because the thicker wall thickness of the parts will not only increase the cost of materials, increase the weight of the parts, while prolonging the cycle of parts molding, thereby increasing production costs
Uniform wall thickness of parts
The most ideal wall thickness distribution of the parts is in any cross-section of the parts of uniform thickness. Uneven part wall thickness can cause uneven cooling and shrinkage of the part, resulting in surface shrinkage of the part, internal porosity, warpage and deformation of the part, dimensional accuracy is difficult to ensure defects.
If the part uniform wall thickness is not possible to obtain, then at least need to ensure that the part wall thickness and thin wall at a smooth transition, to avoid sharp changes in the wall thickness of the part. Rapid changes in the wall thickness of the parts affect the flow of plastic melt, easy to produce stress marks on the back of the plastic, affecting the appearance of the product; at the same time easy to lead to stress concentration, reducing the strength of the plastic parts, making it difficult for the parts to withstand the load or external impact.
Radius
Sharp corners need to be avoided on the inside and outside of plastic parts. Sharp corners can impede the flow of molten plastic and easily produce cosmetic defects; at the same time, stress concentration can easily occur at sharp corners, reducing the strength of the part and making the part fail when subjected to load. Therefore, in the sharp corners of plastic parts, rounded corners should be added to make the parts smooth transition.
Avoid sharp corners on the outside of the part (except at the parting line)
to avoid the external sharp corners of the parts can not be generalized. Rounded corners at the parting surface of the part will cause the mold structure to be complicated and increase the mold cost, and at the same time, the part is easy to break the difference and affect the appearance. The design of right angle at the parting surface of the part is better, such as the figure
If a flat section of about 1.5m is added to the joint, the flanges and gates can be easily removed during the flanges and gates removal process.
Avoid sharp corners in the direction of plastic melt flow
Avoid sharp corners in the direction of plastic melt flow in plastic parts, in which the direction of the arrow is the direction of plastic melt flow. In the original design, the sharp corners tend to cause trapped air in the injection process, and the local high temperature causes plastic decomposition, and produces appearance defects on the surface of the part, while the sharp corners tend to produce internal stress; in the improved design, the sharp corners are avoided through design optimization to ensure the smooth flow of molten plastic.
Rounded corners on all sides of the plastic part model, generally the first rounded corner recommended value of R0.5
As the above requirement of avoiding sharp corners, since rounded corners are desired for external sharp corners, flow directions, and connections, it is the same as the entire plastic model needs rounded corners. So the authors suggest that all the edges of the plastic model are rounded. The rounded corners of the parting line can be removed by the mold engineer later.
Ribs
Ribs is an essential feature in the design of plastic parts, used to improve part strength, as a flow channel to assist the flow of molten plastic, and to provide guidance, positioning and support for other parts in the product. The design parameters of the ribs include the thickness, height, draft angle, root radius and the spacing between the rib and rib.
| Factor | Value | Reason |
| Rib Thickness | 0.5~0.6 Wall thickness | The thickness of the rib is too thick, easy to cause the part surface shrinkage and bring the appearance of quality problems, rib thickness is too thin, the part injection difficulties, and the strength of the part to increase the role of limited |
| Rib Height | Less 3 times wall thickness | In order to improve the strength of the part, the higher the height of the rib, the better. However, the height of the rib is too high, the part injection idle difficult, it is difficult to fill, especially when the rib increases the release slope, the top size of the rib becomes very small. The height of the rib is generally not more than three times the wall thickness of the plastic parts, that is, H ≤ 3T. |
| rib root radius | 0.25~0.5 times Wall thickness | As described in the previous section, the root of the reinforcement needs to increase the rounded corners to avoid stress concentration as well as to increase the plastic melt flow, the size of the rounded corners are generally 0.25 to 0.50 times the wall thickness of the part, that is, R = 0.25T ~ 0.50T. |
| Rib Draft Angle | 0.5 ° ~ 1.5 ° | In order to ensure that the rib can be smoothly removed from the mold, the rib needs a certain draft angle, generally 0.5 ° ~ 1.5 °, the angle is too small, the rib is difficult to release, easy to deform or scratch when the mold is removed; draft angle is too large, the top size of the rib is too small, injection difficulties, low strength. |
| Distance between rib to rib | At least 2 times Wall thickness | The distance between the rib to rib is at least 2 times the wall thickness of the plastic part, to ensure adequate cooling of the rib, that is, S ≥ 2T. |
| Rib direction | Align with material flow direction | The direction of rib should be consistent with the direction of melt flow to ensure smooth melt flow, improve injection efficiency, and avoid air trapping and other injection defects. |
| Rib Fillet | Rib design needs to comply with the principle of uniform wall thickness. Rib and rib at the connection, rib and parts of the wall connection after the addition of radius, it is easy to cause the wall thickness of the part is partially too thick. |
Example 1
Example 2
Support Pillar
Support Pillars are used in plastic parts for guiding, positioning, supporting and fixing between parts in the product. The design parameters of the pillar include the outer diameter, inner diameter, thickness, height, root fillet and release slope of the pillar.
| Factor | Value | Reason |
| Pillar Outer diameter | 2 times inter diameter | |
| Pillar thickness | 0.6 times wall thickness | To avoid surface shrinkage and porosity of the part, the thickness of the pillar should not exceed 0.6 times the wall thickness of the part. |
| Pillar Height | H≤5 times wall thickness | Pillar is too high, the existence of the release slope will make the top size small, resulting in parts injection difficulties; if the top size, and will cause the bottom of the pillar is too thick, resulting in parts surface shrinkage and pores. Therefore, the height of the pillar generally does not exceed 5 times the wall thickness of the part, that is, h ≤ 5T |
| Pillar Root Fillet | 0.25~0.5 times wall thickness | As mentioned in the previous section, in order to avoid stress concentration in the parts and make the plastic melt flow smoothly, the root angle of the pillar is 0.25-0.50 times the wall thickness of the parts, that is, R = 0.25T ~ 0.50T. |
| Pillar Root Thickness | 0.7times wall thickness | To avoid the appearance of surface shrinkage defects, the root thickness of the pillar can be designed to be no greater than 0.7 times the wall thickness of the part, that is, t ≤ 0.7T. |
| Pillar Draft angle | 0.25 °~0.5 ° | Generally speaking, the release slope of the inner diameter of the pillar is 0.25°, and the release slope of the outer diameter is 0.50°. However, it is also possible to use a sleeve in the mold to release the strut without the release slope, but the cost of the mold is slightly higher. |
Ensuring the connection between the pillar and the part wall
To avoid isolated pillar design, the pillar is connected to the part wall by reinforcement to increase the strength of the pillar and make the plastic melt flow more smoothly.
Additional ribs around the pillar
When the pillar is far from the gate, flow marks are easily exist on the pillar, and the flow marks reduce the strength of the pillar. When the pillar is a self-tapping screw pillar, the pillar often breaks under radial force due to insufficient strength, and this is also true for the pillar with fixed metal inserts. Therefore, it is necessary to add ribs around the individual pillar to increase the strength of the pillar, and to add a certain rounded corner at the connection between the rib and the pillar.
The design of the pillar needs to comply with the principle of uniform wall thickness
Avoid the pillar too close to the part wall. When the pillar is too close to the wall of the part, it is easy to cause the local wall thickness is too thick, resulting in the surface of the part shrinkage and air bubbles. The design of the pillar should comply with the principle of uniform wall thickness
Screw boss standard design
If the pillar structure is used for self-tapping screw fixing, then the pillar is a typical standard feature design.
suggest that the pre-formed holes for self-tapping screws in plastic boss need to be designed as standard, with as little independent “innovation” as possible to minimize errors.
Hole Design
Holes, grooves and slots in plastic parts are formed through the cores in the mold. The core is the raised part of the mold, and the size of the core affects the life of the mold and the quality of the part. In the injection process of the parts, too high cores are subjected to high plastic melt impact, which can easily cause the position of the core to move, resulting in large errors in the size of the holes and grooves, or under long-term impact, the cores are prone to break and reduce the service life. Therefore, the design of holes, slots and pits in plastic parts must ensure the right size of the core to ensure the life of the mold and improve the quality of the part.
The common holes in plastic parts can be roughly divided into three types: blind-hole, through holes and counter-bore
| Blind hole | d≤5mm | Pillar height≤2d |
| d≥5mm | Pillar Height ≤3d | |
| through-hole | d≤5mm | Pillar height≤4d |
| d≥5mm | Pillar Height ≤6d |
If the hole is too deep, you can use the counter-bore method instead of molding
Avoid too thin bottom wall of blind hole
The thickness of the bottom of the blind hole should be at least 0.2 times greater than the diameter of the blind hole, as shown in “a”. If the bottom is too thin, the strength of the through-hole is low, and the back side easy have cosmetic defects. then consider using the method shown in Figure b to enhance the strength of the blind hole.
Date Code
For plastic parts, date code and “material identification” must be added for parts over 20g, as follows
Plastic Crater
To avoid shrinkage of parts surface; and also we can not remove the rib or support pillar design, so we can add crater at base of boss. to avoid the shrinkage on surface.Just like when we squeeze the hose, the water jets farther and faster.
Draft Angle
What is draft angle?
Plastic material from the molten state into a solid state will produce a certain amount of size shrinkage, so the parts around the mold core shrinkage and wrap tight. In order to facilitate the smooth release of plastic parts from the mold, to prevent scratching the surface of the parts when releasing the mold, the surface of the parts parallel to the direction of release should generally have a reasonable draft angle.
- If there is no special demand for the part, the draft angle is generally taken as 1°~2°.
- For the plastic parts with large shrinkage rate, a larger draft angle should be selected.
- Parts with high dimensional accuracy should be selected with a smaller draft angle.
- Core side of the mold draft angle is generally less than the cavity side, in order to facilitate the release of parts.
- Plastic parts with thicker wall thickness, molding shrinkage increases, so the draft angle should take a larger value.
- Bite surface and complex surface draft angle should take a larger value, the size of the bite to determine the size of the draft angle.
- For glass fiber reinforced plastics, the draft angle should take a larger value.
- The size and direction of the draft angle should not affect the function of the product.