1.1.1 Good Product Structure
Product design should minimize unnecessary undercuts and core pulling, thereby simplifying mold structure.

 

Plastic products must have appropriate wall thickness and uniform wall thickness variations.
Too thin a wall thickness will result in excessive flow resistance and glue shortages. Too thick a wall thickness will waste raw materials and cause severe shrinkage.
Sudden changes in wall thickness will cause stress concentrations, leading to deformation. Therefore, when designing a product, try to ensure consistent wall thickness or smooth, uniform changes.
Some areas of plastic parts should be designed with rounded corners to avoid corner clearance issues during mold processing and increase mold strength.
At the same time, other areas of plastic parts, such as parting surfaces or where core and cavity meet, should not be designed with rounded corners. Doing so will make mold separation difficult and complicate mold.
1.1.2 Reasonable Dimensions
Product drawings must include complete dimensions, neither too many nor too few. Missing dimensions can leave mold designers at a loss, while excessive dimensions can lead to loss of control over critical dimensions.

 

1.1.3 Reasonable Tolerances
Excessively tight tolerances can increase mold design difficulty and mold costs. Therefore, careful analysis should be conducted before determining product dimensional tolerances.
1.1.4 Reasonable Surface Requirements
Excessively high surface requirements will increase mold processing costs. A certain etch depth corresponds to a certain draft angle. Excessive etch depth can result in scratches on product when it is released from mold.
Product drawing should indicate injection molding material used. Mold designers should determine shrinkage rate and processability of material provided by customer to design mold accordingly.
Using different injection molding materials in same mold does not guarantee dimensional stability and appearance quality of molded parts.

It's crucial to understand average shrinkage and flow properties of injection molding material.
Plastics shrink differently in filling direction and perpendicular to mold, with a difference of approximately 20% to 30%.
Different plastics also have different flow properties. For example, a burr-free ABS mold can produce very different results when used to process nylon.
Plastic types include: ABS, PC, PP, PE, PA, POM, PPS, PMMA, PS, PBT, PET, PVC, LCP, and more.
Many plastics incorporate glass fiber or carbon fiber to increase strength and toughness. Addition of these different components significantly alters shrinkage and flow properties of plastic. Typical glass fiber additions range from 15% to 40%.
Some plastics also incorporate another plastic to achieve a compromise between properties of the two plastics. For example, ABS + PC is commonly used.
PBT + 30% GF is a design material for many electronic products.
1.2.1 What properties of injection molding materials should I understand?
1.2.2 Are plastics adulterated?
Many plastics incorporate glass or carbon fibers to increase strength and toughness. Addition of these different components can significantly alter plastic's shrinkage and flow properties. Typical glass fiber additions range from 15% to 40%.
Some plastics also incorporate another plastic to achieve a compromise between properties of two plastics. For example, commonly used ABS+PC combination.

1.3.1 What is maximum shot volume?
Maximum shot volume of an injection molding machine refers to weight of plastic injected in a single shot after screw has retracted to its full stroke. It is usually measured in PS.
Mold designers should understand maximum shot volume of machine they will use for injection molding, and ideally, required shot volume should not exceed 80% of maximum shot volume.
1.3.2 What are maximum injection speed and pressure?
For some products, sufficient injection pressure and speed are essential to ensure complete filling or to prevent noticeable weld marks. For example, thin-walled products require high injection speeds to achieve full filling.
Note: Many injection molding machines specify relative pressure and speed. To determine absolute values, consult machine's manual.
1.3.3 Moving and Fixed Platen Size, Maximum and Minimum Die Clearance, and Mold Opening Stroke
Moving and fixed platen size limits mold's length and width, maximum and minimum die clearance limits mold's height, and mold opening stroke limits product's maximum ejection height.

 

1.4.1 What are principles for determining number of mold cavities?
Number of mold cavities depends on a combination of economic and technical factors.
Multiple cavities offer high efficiency and benefits, but they also present challenges for mold designers and manufacturers. Product dimensions in different cavities cannot be guaranteed to be consistent, leading to variations in product appearance and quality.
1.4.2 What are multi-cavity arrangements?
Double cavity

 

Three cavities

 

Four cavities

 

Six cavities

 

Eight cavities

 

1.5.1 Large Gate Mold

 

1.5.2 Small sprue mold base

 

Determine basic mold release method, such as push rod ejection, tube ejection, plate ejection, pneumatic ejection, or manual removal.
After determining mold base type and mold release method, you can determine specific mold base dimensions based on number of cavities.
At this point, you can order mold base from supplier!

Following chart provides a reference for plastic shrinkage rates, but it is important to consult your material supplier for accurate data.

 

For free dimensions, nominal dimension is target dimension.
For controlled dimensions, lower tolerance plus 75% or 25% of the total tolerance (depending on direction of mold wear) is target dimension.
For a hole position of 10 ± 0.1 on a part, target dimension during mold design can be set at 10.05, as core becomes smaller as it wears.
For a glue position of 10 ± 0.1 on a part, target dimension during mold design can be set at 9.95, as cavity becomes larger as it wears.

Principles for gate location selection include: ensuring complete mold filling; facilitating air venting; minimizing number and impact of weld marks; and minimizing influence of molecular orientation.

A flat gate is one with gate located on main parting surface. Depending on gate shape, it can be categorized as a rectangular gate, a fan gate, or a flat seam gate.
Gate height and width can be determined empirically or according to following table:

To facilitate separation of sprue material from product, flat gate can also be designed with an angled design. As shown in figure below, gate has angles in three directions.

 

Some transparent products often use fan-shaped gates. These are usually trimmed using a metal mold after injection molding to create a more aesthetically pleasing product.

 

Submerged gates are a widely used type of gate. During mold opening, part and sprue are forcibly separated, facilitating automated production. However, brittle plastics are not suitable for submerged gates.
When using a submerged gate, ensure front end of gate is machined into a frustum to facilitate thorough separation of part and sprue.

 

This is another type of submerged gate, used for softer plastics.

 

Point gates introduce water vertically from top of part, while sprue and part are removed from different parting surfaces. Mold uses a small sprue base, also known as a three-plate mold. Some brittle plastics are not suitable for small sprues.

 

① Circular: Minimum surface area, minimizes heat dissipation, and minimizes resistance. However, it must be opened simultaneously on front and rear molds, making it difficult to manufacture. Diameter: ⌀5-⌀10 mm.
② U-shaped: Easy to manufacture, produces good results, is commonly used.
③ Trapezoidal: Easy to manufacture, produces good results, is also commonly used.
④ Semicircular: Larger surface area, less commonly used.
⑤ Rectangular: Larger surface area, rarely used.

 

To remove plastic parts and gating system solids from a closed mold and to place inserts, mold is appropriately divided into two or more main parts. These separable contact surfaces are collectively referred to as parting surfaces.

When front mold insert and rear mold insert are matched, matching surface is perpendicular to movement direction.

 

When front mold insert and rear mold insert are matched, matching surface is perpendicular to movement direction, and small insert is inserted into large insert, so that small insert is accurately positioned and prevented from shifting.

 

When front and rear mold inserts mate, mating surfaces are not perpendicular to direction of motion.

 

When designing mold, avoid contact between parting surface and direction of motion.

3.3.1 Mold Structure Considerations
The most important principle in parting surface selection is that it must be located where part's cross-sectional profile is largest. Otherwise, part will be forced out.
Use CAD to check for undercuts in demolding direction.
To facilitate demolding, part should remain in core mold as much as possible during mold opening. This is accomplished by ensuring that bonding force between part and core mold is greater than that between part and core mold.
There are various methods for achieving a bonding strength between part and core mold that is greater than that between part and core mold.
By selecting parting surface, try to avoid side core pulling mechanism.
If plastic part has side holes, core pulling should be set on male mold part as much as possible to avoid core pulling of female mold, because core pulling of female mold is much more difficult than core pulling of male mold.
Because locking force of inclined slider is relatively low during mold closing, for large plastic parts with large projected areas, parting surface with larger projected area can be placed on main plane of male and female molds, while parting surface with smaller projected area serves as lateral parting surface. Otherwise, locking mechanism of inclined slider would have to be extremely large, or overflow could occur due to insufficient locking.

 

Large circle serves as main parting surface, small circle as core drawing surface.
3.3.2 Product Quality Considerations
Parts requiring high coaxiality should be molded in either male or female mold. If molded separately in male and female molds, coaxiality will be difficult to ensure due to inaccurate mold closing.

 

To ensure surface requirements of plastic part are met, parting surface should be located where it will not affect appearance of part.
3.3.3 Mold Manufacturing Difficulty Considerations
Try to simplify parting surface design, using more flat surfaces and fewer parting surfaces. This simplifies mold design and reduces mold manufacturing difficulty.

High and Low Parting Surfaces

 

Oblique Parting Surface

 

Curved Parting Surface

 

Breaking and Pillowing on Side Perforated Parting Surface

 

Side Perforated Parting Surface

 

Planar Perforated Parting Surface

 

Beveled Square Hole Parting Surface

 

Beveled Column Parting Surface

 

Side Azimuth Parting Surface