Basic knowledge of mold

What is basic knowledge of mold?

The basic knowledge of mold is to describe the basic structure of mold, the influence of shape on the product, and the introduction of mold model.

1. The basic composition of the mold

(1) Front mold (master mold) (fixed mold)

(2) Rear mold (male mold) (movable mold)

(3) Inserts (inserts),

(4) Row position (slider),

(5) Inclined roof

(6) Thimble

(7) Gate (water inlet)

 2. The influence of the shape of the mold product on the product

The wall thickness and geometry will affect the shrinkage of molding and the size of the draft

3. The influence of the water inlet on the shrinkage rate of the product

The larger the size of the inlet, the smaller the shrinkage, the smaller the size, the larger the shrinkage, the parallel to the material flow direction the larger the shrinkage, the vertical direction the smaller the shrinkage

4. The influence of mold wall thickness is too large and the wall thickness is too small

  The wall thickness is too large:

(1) Increase cost

(2) Extend the forming time and reduce the production efficiency

(3) Quality is difficult to control, bubbles, shrinkage holes, dents, etc. are easy to appear

The wall thickness is too small:

(1) The resistance of plastic flowing in the mold is large. If the shape is more complicated, it will be difficult to form
(2) The strength is poor

If the wall thickness of the plastic part is not uniform, it will shrink unevenly after the forming process, which will not only cause bubbles, dents, and deformation, but also have large internal stress. Avoid sharp corners at the junction of wall thickness and thin wall, and excessive convergence. The thickness should be gradually reduced along the direction of plastic flow.

5. Fillet (R position)

Set rounded corners (R position) to increase the strength, so that the plastic parts will not be deformed or cracked

6. ​​Reinforcing ribs

(1) In order to ensure the strength and rigidity of the product without making the wall thickness of the plastic part thick, reinforcing ribs can be set at the appropriate part of the plastic part to prevent deformation. In some cases, it can also improve the flow of plastic during forming.

(2) The thickness of the stiffener should not exceed 50% of the plastic part, usually about 20%

(3) The rib should be lower than the plane of the plastic part

 7. Hole

(1) The periphery of the hole is prone to weld marks, which reduces the strength of the plastic part. Note: The distance between the hole and the hole, and the distance between the hole and the plastic part should generally be more than twice the hole

(2) The edge of the hole can be strengthened by a boss

(3) The depth of the blind hole cannot exceed 4 times the diameter of the hole

(4) Special attention should be paid to the strength of the screw hole and the size of the hole diameter. If the hole diameter is too large, it will slip into the screw. If the hole diameter is too small, the screw cannot be inserted or the screw column will burst.

(5) The hole column is too long (high), pay attention to poor mold exhaust

(6) The depth of the aperture should preferably not exceed 8 times the aperture

(7) For holes with steps, the cores are fixed on both sides of the fixed and movable molds, it is difficult to ensure concentricity, and it is easy to produce burrs at the junction of the two cores. Therefore, either side of the core (aperture ) Increased by 0.5MM or more, formed by the guide at the other end

 8. Mold insert, row position, inclined top

The inserts of the mold, the row position and the inclined top are usually set on the movable mold of the mold, if they are fitted
If it is not tight, there will be burrs.


Public and private models

The public mold is public, that is, after the manufacturer opens the mold (of course, the mold belongs to the mold manufacturer), anyone can buy the products produced by this set of molds.

Private mold: There is no doubt that it refers to the mold specially customized by the customer at the manufacturer, and the ownership belongs to the customer.

Basic classification

Male models are generally divided into:

1. Two-platen mold (namely, large nozzle mold);
2. Three-plate mold (ie small nozzle mold);
3. Hot runner mold.

Generally speaking, a set of molds is divided into: mold structure, forming parts, casting system, cooling system, ejection system, and exhaust system.

1. Mold structure: it is the mold base, which is used to fix the forming parts.

2. Forming parts: mold cores, inserts, sliders, etc. used to form products.

3. Casting system: the runners and nozzles for the plastic to enter the mold.

4. Cooling system: It is used to control the mold temperature. That is, the waterway or other facilities used for cooling on the mold core. Generally speaking, there should be waterways on the mold core or the template.

5. Ejection system: refers to the mechanism and parts used to eject products such as thimble.

6. Exhaust system: It is used to exhaust the air in the mold cavity during forming to avoid bubbles and unsatisfactory filling of the product during forming.

The two-platen mold is named as two templates. A set of molds is divided into the male mold side and the male mold side. The male mold side is fixed on the forming machine during forming, so it is also called the fixed side. When the molding is completed, the male mold side will move through the mold opening system of the molding machine to open the mold, so the male mold side is also called the movable side.

working principle

1. When the mold is not working, the inner and outer stripper plates will bounce off under the action of the stripper spring, and the male die, male die and punch are protected.

2. Under the action of the punch press, the upper die begins to descend. When the inner and outer stripper plates are in contact with the male and male die, the punch slider continues to descend, the inner and outer stripper plates are forced to compress the spring, and the male die and the male die, the punch and the male die are punched and sheared. Function, material separation is completed.

3. After the punch slider drops to the bottom dead center, it starts to rise. Under the action of the spring, the inner and outer stripper plates bounce off the male die and punch that have been cut into the material, and the die stripping is completed.

4. The punching machine rises to the top dead center, and the entire punching process is completed.

The design of the male model

The corner shape and chamfer shape of the drawn male die are very important design items. The shape and characteristics of the corner and chamfer are as follows:

When the R angle value of the drawn male mold is large, it is easier to draw and process, but it also produces wrinkles on the surface of the drawn product, and the thickness of the side wall of the drawn product is greater than the thickness of the plate. When drawing thick plates and difficult ejection, the R value of the male mold should be small, about 1-2 times the thickness of the plate. Generally, most of the drawing parts of the cylinder and square barrel drawing male molds are made straight, in order to prevent For purposes such as burning, destroying the lubricating oil film and reducing the ejection force, it is advisable to have an escape design under the straight section. Especially in the case of shrinking processing, it is necessary to minimize the number of punches in this straight section. Also called punches, upper molds, male molds, punching needles, etc., and punches are divided into A-type punches, T-type punches, and special-shaped punches. shower.

The punch is a metal part installed on the stamping die, which is used in direct contact with the material to deform and cut the material. Die punches generally use high-speed steel and tungsten steel as materials, such as high-speed steel punches and tungsten steel punches, and high-speed steel is the most commonly used material.

Overview of front mold and back mold

The mold cavity is the mold term of the manufacturing industry, that is, the parts that make up the product space are called molded parts (ie, the overall mold), and the (mold) parts on the outer surface of the molded product are called cavities.

The concave part is the cavity, also known as the front mold or the master mold.
The corresponding convex part is called the core (Core). Also known as back mold or male mold.
The cavity of the mold is closed with the core, and the gap in the middle is the product.

The terminology of the plastics industry has various names. Taiwan and Japan are generally called male mold and female mold, and Hong Kong is generally called front mold and back mold. In fact, male mold = back mold, female mold = front mold, domestically. There are also called dynamic and fixed molds.

A mold is composed of an upper mold and a lower mold. The back mold is called the concave mold or the upper mold. The design of the male mold is very important, and the structural characteristics of the female mold itself vary with the structure of the product and the processing method of the mold. Have considerable flexibility.

But for the cavity with complex shape, it is more difficult to process if the integral structure is adopted. Therefore, a combined die structure is adopted. At the same time, the performance of the material at the edge of the rear mold can be lower than that of the front mold, which avoids the uneconomical use of the same material for the integral cavity mold. The inlay structure of the cavity mold can facilitate exhaust through the gap and reduce the thermal deformation of the front mold. For the easily worn parts of the front mold, the inlay type is adopted, which can facilitate the maintenance of the mold and avoid the scrap of the entire front mold.

The following introduces the design type of the front mold and the type of the front mold at the time of design.

When designing the shape of the die after punching, the factors that should be considered when designing are: die life and the shape of the escape angle, the shear angle of the back mold, and the division of the back mold. Mold life and shape of escape angle: This design is a very important matter. If the design is not correct, it will cause punch damage, blockage or floating of punching chips, and burrs.

Shear angle of the back mold: to reduce the punching force when the shape is punched, the back mold can be designed with a shear angle. When the shear angle is large, the punching force will be reduced greatly, but it is easy to cause the product to recurve and deform.

Dividing of the back mold: The back mold must be subjected to finishing processing such as forming and grinding. Because it is a concave shape, the grinding tool is not easy to enter, so it must be divided.

The design of the front mold for bending The front mold for bending is designed to prevent springback and excessive bending. The shape of the front mold for U-shaped bending is a combination of double R and straight parts, and it is best to approximate the R shape. The shape of R part should be polished after forming grinding or NC electric discharge machining.

Precautions for post-mold processing

1. The gap value between the pressure plate and the punch and the length of the punch guide;
2. The installation standard of auxiliary guide post and pressing plate and the design of escape part of pressing plate;
3. The movable pressing plate is a countermeasure to prevent the occurrence of tilt during the stamping process;
4. The size relationship between the fixed guide plate and the guide pin hole of the pressure plate;
5. The relationship between the material guide part of the fixed pressing plate and the width of the processed material. [1]

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