Mezzo logo FirstMold

Guida completa alla progettazione strutturale dei prodotti in plastica

Condividi questo articolo:
prodotto in plastica design strutturale immagine in evidenza

The most fundamental part of plastic products is actually the structural design of the plastic product. Many people say that the structural design of plastic products is difficult, but what exactly makes it difficult?

We have previously learned about the process of developing plastic products. From the cumbersome process, it is not difficult to see that excellent plastic product design must not only adapt to changing demands and control costs but also ensure good production efficiency. Additionally, due to the design’s complexity, reliability, and accuracy, the product structural design’s workload is substantial. It requires designers to possess superb technical and engineering abilities, keen market insight, and attention to emerging technologies and constantly changing demands.

Leaving aside changing demands and keen market insight, the structural design of the product itself also requires consideration of many aspects. The following guide clearly points out these aspects:

01. Wall Thickness

For large components, the wall thickness in structural design generally ranges from 2.4-3.2mm, while for smaller components, it is around 1.0mm, with the specific dimensions adhering to product design requirements. The wall thickness should be as uniform as possible. In special circumstances, certain areas can be slightly thicker or thinner, but a gradual transition is necessary (it’s recommended to have the transition size be more than 5 times the product wall thickness) to prevent stress marks on the surface of the molded injection product.

Factors Affecting Wall Thickness Selection

a) The plastic material used. Different materials have different tassi di ritiro and fluidity during stampaggio a iniezione, which affects the recommended wall thickness. (For shell thickness, empirical data can be approximately the largest dimension of the machine / 100mm.)

b) The external forces endured. The greater the force, the thicker the wall should be. In special cases, metal parts should be used, or strength checks should be conducted.

Recommended wall thickness values for products include:

Plastic MaterialsMinimum wall thicknessRecommended wall thickness for small partsRecommended wall thickness for medium-sized partsRecommended wall thickness for large parts
Nylon0.450.761.52.4~3.2
PE0.61.251.62.4~3.2
PS0.751.251.63.2~5.4
PMMA0.81.52.24~6.5
PVC1.21.61.83.2~5.8
PP0.851.541.752.4~3.2
PC0.951.82.33~4.5
POM0.81.41.63.2~5.4
ABS0.812.33.2~6

c) Safety regulations. For example, pressure resistance requirements (the thicker the wall, the greater the pressure resistance), flammability requirements, etc.

02. Reinforcing Ribs

Most plastic product’s structural design have reinforcing ribs because ribs can increase strength without adding to the overall wall thickness, which is especially useful for large components and stressed parts, and can also prevent product deformation. The thickness of the reinforcing ribs is typically 0.5-0.75 times that of the overall wall thickness (it is recommended to be less than 0.6 times); if it’s greater than 0.75 times, the product is prone to shrinkage.

Two different types of shrinkage
Two different types of shrinkage

For plastic parts with high appearance requirements (glossy surface), the bottom thickness of the reinforcing ribs on the back is recommended to be B≤0.5T. If confident in mold design and subsequent process adjustments, B>0.56T can be designed, but it is recommended not to exceed 0.7T, as it will be difficult to adjust later. It’s important to note that the thickness of the reinforcing ribs for different plastic materials does not necessarily follow B≤0.5T.

Design Reference For Thickness Of Reinforcing Ribs (Relative To Base Wall Thickness)
I materialiMinimum Sink MarkSlight Sink Mark
PC50%66%
ABS40%60%
PC+ABS50%66%
PA30%40%
Reinforced PA33%50%
PP30%40%
Reinforced PP33%50%

Aspects Of Reinforcement Design That Need Attention

1. When multiple reinforcing ribs intersect and connect, care should be taken to prevent local accumulation of material and avoid shrink marks on the back, with the following design method recommended.

Comparison of design types for reinforcement crossings in structural design
Comparison of design types for reinforcement crossings in structural design

2. When connecting reinforcing ribs to the outer wall, try to keep the ribs perpendicular to the outer wall.

Reinforcing ribs remain perpendicular to the outer wall

3. If space allows, avoid designing reinforcing ribs or bosses on steep slopes, and take measures to prevent shrinkage if unavoidable.

Anti-sink mark treatment
Anti-sink mark treatment

4. If the thickness of the reinforcing ribs is not proportional to the main wall thickness and the parameters and location cannot be changed, consider altering the external appearance to reduce the visibility of shrink marks (this method is difficult to control and should be used cautiously).

Reduced visibility of sink marks
Reduced visibility of sink marks

03. Draft Angle

Plastic products require a angolo di sformo in structural design, except for those with shallow heights (such as a flat plate) or special requirements (but when side walls are large and without a draft angle, a row position is needed). The draft angle usually ranges from 0.5-5 degrees, typically around 2 degrees, but this varies based on product size, height, and shape, with the principle of ensuring smooth demolding without affecting functionality.

The draft angle for the mold cavity should generally be 0.5 degrees greater than that of the mold core to ensure the product remains in the mold core when opening. Generally, areas like shut-offs, inserti, and kiss-offs need a draft angle.

The table below recommends draft angles for different materials:

I materialiDraft Angle
Mold CoreMold Cavity
ABS35’~1°40’~1°20′
PS30’~1°35’~1°30′
PC30’~50′35’~1°
PP25’~50′30’~1°
PE20’~45′25’~45′
PMMA30’~1°35’~1°30′
POM30’~1°35’~1°30′
PA20’~40′25’~40′
HPVC50’~1°45′50’~2°
SPV25’~50′30’~1°
CP20’~45′25’~45′
RibsGenerally 0.5°, Minimize 0.25°

Aspects Of Draft Angle Selection That Need Attention

1. Choose a smaller draft angle, such as 0.5° for plastic parts with glossy surfaces and high precision requirements with a low shrinkage rate.

2. For taller and larger specifications, a smaller draft angle should be chosen based on specific calculations.

3. Choose a larger angle for plastic parts with a high shrinkage rate.

4. For plastic parts with thicker walls, which cause the mold to close more tightly, a larger standard value for the draft angle should be chosen.

5. The draft angle for fully transparent parts should be increased to prevent scratches. Generally, for PS materials, the draft angle should not be less than 2.5°~3°, and for ABS and PC materials, it should not be less than 1.5°~2°.

6. For plastic parts with textures or sandblasting treatments, the draft angle should be between 2° to 5° depending on the depth of the texture. The deeper the texture, the larger the draft angle should be.

04. R Corner

Except for areas where special requirements specify sharp edges, plastic products usually have rounded corners in structural design to reduce stress concentration, facilitate plastic flow, and ease demolding.

1. If there are no special requirements for the product design, the transition radius (R) is determined by the adjacent material thickness (t), with the internal corner radius typically ranging from 0.50 to 1.50 times the material thickness, but the minimum radius must not be less than 0.30mm.

Good and bad corner design
Good and bad corner structural design

2. When designing rounded corners on the internal and external surfaces of the product, maintain uniform wall thickness.

When designing r corners, the wall thickness should be kept uniform
When designing r corners, the wall thickness should be kept uniform

3. In the structural design of plastic product, especially avoid rounded corners on the mold parting surface unless specifically required. Rounded corners on the parting surface increase the difficulty of mold making and leave weld line on the product surface, affecting appearance.

Parting line should not have rounded corners
The parting line should not have rounded corners

4. Sharp edges are not allowed on surfaces that can be touched on the outside and inside of the product. If necessary, chamfer the edges to a minimum radius of 0.30mm to prevent cutting fingers, especially important in the design of handheld electronic products.

Round corners to prevent scratching
Round corners to prevent scratching

Holes

Holes are common in product structure design and are typically categorized into two types: circular and non-circular holes. When designing the position of holes, the goal should be to minimize the difficulty of mold processing without compromising the strength of the plastic part.

Common Design Requirements for Holes

Dimensional specifications (excluding the inner holes of screw posts):

Dimensional specifications of holes
Dimensional specifications of holes

Dimension A is the distance between holes. If the diameter of the hole is less than 3.00mm, it is recommended that the value of A is not less than D; if the diameter exceeds 3.00mm, then A can be 0.70 times the diameter of the hole.

Dimension B is the distance from the hole to the edge, and it is recommended that the value of B is not less than D.

Relationship Between Hole Diameter and Depth

Dimensional specifications (excluding the inner holes of screw posts):

Relationship Between Hole Diameter and Depth
Relationship Between Hole Diameter and Depth

Dimension A is the depth of a blind hole, recommended to not exceed 5D. Generally, A is less than 2D with a length-to-diameter ratio not exceeding 4mm.

If D ≤ 1.5mm, then A ≤ D. The thickness of the bottom wall of the blind hole should be ≥ 1/6D.

Dimension B is the depth of a through-hole, recommended to not exceed 10D.

Step Holes

Step holes are composed of multiple coaxially connected holes of different diameters, with the depth of the hole being longer than that of a single-diameter hole, as shown in diagrams.

Step hole
Step hole

Angled Holes

Aligning the axis of the hole with the direction of the mold opening can avoid the need for core pulling. For forming methods of angled holes and complex-shaped holes, a split core can be used to avoid lateral core-pulling structures.

Side Holes and Indentations

When side holes and indentations appear on plastic products, sliders or side core-pulling structures must be set for easy demolding, which complicates the mold structure and increases costs. The product structure can be improved accordingly. As shown in picture below, changing from a design with side holes (a) to one with side indentations (b).

Improved side hole structure
Improved side hole structure

Design of Screw Head Holes

As shown in the picture below, the preferred form for screw head holes is illustrated in (a). If the structure requires the form shown in (b), the tapered surface should be below the end face by no less than 0.50mm to prevent cracking of the hole surface.

Design of Screw Head Hole
Design of Screw Head Hole

Edge Structure of Holes

Designing a complete chamfer or radius at the edge of a hole is impractical; the edge of the hole should have at least a 0.4mm straight feature.

Edge Structure of Holes
Edge Structure of Holes

Bosses

Bosses are typically used for the assembly of two plastic products via shaft-hole fitting or for the assembly of self-tapping screws. When a boss is not very tall and is ejected using an ejector sleeve in the mold, it may not need a draft angle. However, when the boss is tall, it’s common to add cross ribs (reinforcements) on its exterior. These cross ribs usually have a draft angle of 1-2 degrees, and the boss itself may also require a draft angle depending on the situation.

When a boss is paired with a post (or another boss), the fitting gap is usually set to a unilateral 0.05-0.10 to accommodate positional errors that may occur during the processing of each boss. When a boss is used for the assembly of self-tapping screws, its inner hole should be 0.1-0.2 mm smaller than the diameter of the screw on one side to ensure the screw can be securely fastened. For instance, when assembling with an M3.0 self-tapping screw, the inner hole of the boss is typically made to be Ф2.60-2.80 mm.

Inserts

In the plastic molding process, metal or other material parts such as bolts and terminals embedded during or after molding are collectively referred to as inserts within the plastic parts. Inserts can enhance the functionality of the product or serve decorative purposes.

Inserts in plastic parts are often used as fasteners or support elements. Additionally, inserts are a common assembly method when the product design requires ease of repair, ease of replacement, or reusability. However, regardless of whether they are used for functional or decorative purposes, the use of inserts should be minimized. The reason is that incorporating inserts requires additional processing steps, increasing production costs. Inserts are typically made of metal, with copper being a common material choice.

Shape and Structural Requirements for Inserts

1. Metal inserts are made through cutting or stamping processes, so their shapes must be conducive to manufacturing.

2. They must possess sufficient mechanical strength (material, dimensions).

3. There must be adequate bonding strength between the insert and the plastic matrix to prevent the insert from pulling out or rotating during use. The surface of the insert should have annular grooves or crosshatching; sharp angles should be avoided to prevent damage caused by stress concentration. Where possible, round or symmetrical shapes should be used to ensure uniform shrinkage.

4. For easy placement and positioning within the mold, the portion of the insert extending outside (the part placed in the mold) should be cylindrical, as circular holes are the easiest for mold machining.

5. To prevent flash, inserts should have structures such as sealing bosses.

6. The design should facilitate secondary processing of the insert after molding, such as threading, end face cutting, flanging, etc.

Inserts in structual design
Inserts in structural design

When designing plastic products with inserts, it is crucial to ensure that the inserts can be precisely and reliably positioned within the mold. It’s also important to consider that the insert must form a strong connection with the molded part, which can be challenging when the encapsulating material is too thin. Additionally, the design must prevent any leakage of plastic.

Product Surface Texture

The surface of plastic products can be smooth (polished mold surface), spark-etched (copper EDM processed mold cavity), various patterned etched surfaces (patterned surfaces), and engraved surfaces. When the depth of the texture is significant or there are many textures, the demolding resistance increases, necessitating a corresponding increase in the demolding angle.

Text and Patterns

Text and patterns on plastic products come in two forms: raised and recessed surfaces. There are generally two processing methods: small text and patterns are obtained by mold etching, while slightly larger text and patterns are directly machined into the mold. The size of the text must be conducive to molding and avoid sharp angles.

1. It’s best to use raised surfaces for text and patterns on plastic products, making them recessed on the mold, which simplifies mold processing. If the structure requires that the surface must not have any raised features, you can create a recessed area where the text or pattern is located to a certain depth, and then raise the text or pattern within the recess. This meets the structural requirements while facilitating mold-making.

The text and pattern on plastic products are best used to protrude the surface
The text and pattern on plastic products are best used to protrude the surface

2. For plastic products, the height of raised text and patterns is generally between 0.15 and 0.30mm, while the depth of recessed text and patterns is between 0.15 and 0.25mm.

3. Text Size Specifications:

Text size description in structural design
Text size description in structural design
  • Dimension A is the width of the text stroke, recommended to be no less than 0.25mm.
  • Dimension B is the spacing between two characters, recommended to be no less than 0.40mm.
  • Dimensions C and D are the distances from the characters to the edge, recommended to be no less than 0.60mm.
Indice dei contenuti
Tag
Commenti

Lascia un commento

Il tuo indirizzo email non sarà pubblicato. I campi obbligatori sono contrassegnati *