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Plastic Injection Mould Runner & Gate Design

Plastic Injection Mould Runner & Gate Design

Design principles for the runner system:

1. For the single plastic part and multiple cavities, the balanced runner (except special) should be used, that is, the position of each cavity gate and the size of the gate point are the same and the length of the runner is the same. To ensure uniform molding conditions in each cavity.

2. For multi products and multi cavities use non-equilibrium runner and gate point, the size of the nozzle or runner should be adjusted according to the distance of the flow to ensure that the cavities are filled as much as possible.

3. In order to reduce heat and pressure loss, the runner should be as short and straight as possible, and the radius should be added at the corner to ensure smooth flow. The surface of the runner must be polished with more than 600# sandpaper. If the part is mirrored, the runner must be polished with 1000# sandpaper.

4. The gate system should be designed cold slug well to prevent cold material entry into the cavity and affect the quality and appearance of the parts.

5. The gate system must be vented to ensure that the plastic melt fills all corners of the cavity, so that the cavity gas can be discharged smoothly.

6. The gate system should be able to prevent the plastic parts from shrinking mark, deformation, dimensional deviation and other defects. The gates are convenient to cutoff or cut automatically and do not affect the appearance of the product.

7. In order to improve the mass production efficiency, it is necessary to minimize the subsequent processing of the plastic parts when designing the gate. Automated mass production, when the gate is taken out, the robot is used as much as possible to shorten the cycle time.

8. For the sprue angle design, the diameter of the root should be no smaller than the runner. For materials with high brittleness such as PS, a double-sided 3° draft angle should be used. Preventing the breakage of the sprue and runner, while the runner breakage will cause trouble for mass production.

9. Making runner into a circular or U-shaped that is easy to process.

10. The design of the gate system should take into account the amount of runner material. To save costs and eliminate waste, the weight of the runner is controlled below 25% and cannot exceed 50% (special consideration for special parts).

11. When there are multiple injection points on a product, the influence of the size and position of the welding line on the strength and appearance of the product should be considered.

12. For a multi-cavity plastic mould, reasonable, symmetry and compact positioning is able to prevent the eccentric load, reduce the mold base dimension and the tonnage of the molding machine.

13. For a multiple products mould which is also called family mold, the size of the gate must be adjusted according to the plastic parts dimension to ensure that each product is filled simultaneously.

14. For the complicate product, it's necessary to make mold-flow analysis to help design a good runner system and gate position.

Design standard for Sprue:

1.Injection molding machine nozzle and sprue matching

The spherical surface SR of the mold sprue should be 1-2MM larger than the nozzle SR of the injection molding machine. The SR of the nozzle can be found in the specifications of the injection molding machine.

Injection machine tonnage<130<350>350
Nozzle radius SRCheck injection machine specification
Mould sprue121423

Attention: When designing the mold, check the length of the nozzle of the molding machine that can be inserted into the mold. Should make proposal if have to use the extended nozzle.

2.Sprue diameter ⌀D setting

Injection machine tonnage






Above 550













The sprue D should be 0.5mm larger than the the nozzle of the injection molding machine.

3.To shorten the sprue, we have to:

A.For two plate mould, sprue should be sunk in to the A plate (except for special customer requirements).

B.For three plate mould, big spue bush is must if the sprue length is 25mm or more.

C.Plastic parts with appearance requirements and other special requirements use hot sprue.

Design standard for Runner:

1.Type of runner.

A.Round runner.

B.U shape runner.

2.D series: 2 2.5 3 3.5 4 4.5 5 6 7 8 (10 12 reference use)

3.The runner diameter setting

For PE, PA, the diameter can be small. For PC, PMMA, the diameter value can be a large value.

Estimation diameter according to experience

Injection machine tonnage≤60≤130≤220≤350≤4505506508501300
ABS HIPS POM PE PP PA2.0-3.03.0-4.03.5-4.54.0-5.04.5-6.05.0-7.06.0-8.08-10
PC PPS PPE2.5-3.53.5-4.54.0-5.04.5-6.05.0-7.06.0-8.07.0-1010-12

The sprue D should be 0.5mm larger than the the nozzle of the injection molding machine.


A.In the U-shaped runner, the size of the runner can be changed by

adjusting the H value.

B.Use round runner whenever possible

C.The runner must be thicker than the section of the subordinate

runner (see table below), if the secondary runner increases,

D1 diameter can be reduced according to the actual situation.

Runner ⌀D222.533.544.556
Runner ⌀D1222.533.53.544.5


Design principles for common gates:

The form, position, size and number of nozzles should fully meet the requirements of mold processing and parts production.

1.Direct gate

A. Low pressure loss, easy to fill

B. The pressure at the gate is large, which will cause deformation.

C. It is difficult to remove the nozzle

2. Common gate

A. Easy mold processing

B. Correction is easy

C. Thin and long barrel products should not be used.

D. It is necessary to take care that whether the subsequent processing of the nozzle is convenient and whether the residue after processing will affect the appearance and function of the product.

3. Pin point gate

A. Flexible to select the gate location.

B. Small residual stress near the gate

C. After the mold is opened, the gate can be pulled off by itself. The traces left on the plastic part are not obvious, and the appearance of the surface of the plastic part is not affected.

D. The flow rate of the molten plastic flow through the gate is increased, and the frictional force acts to increase the temperature of the plastic stream. In this way, a plastic part with a clear shape and a glossy surface can be obtained.

E. Injection pressure loss is large, which is not good for plastic molding.

F. The mold structure is more complicated, and the double parting line mold (three plate mold) is generally used to remove the sprue.

G. Because the high flow velocity near the gate, the molecules are highly oriented, and local stress is increased, which may cause cracking. Therefore, without affecting the function of the plastic part, increase the wall thickness of the plastic part opposite the gate.

H.Warpage wil occur when a large plastic part adopts a point gate, so multiple point gates should be used at

the same time.

4. Sub-gate

A. Gate can be located inside or outside of the plastic product, the gate is usually located at the bone position of the injection molded product, the gate is concealed and does not affect the appearance of the injection molded part.

B. Small gate mark.

C. Gate is cut automatically.

D. There is a problem of large pressure loss.

E. Large flow resistance.

5. Fan gate

A. Good liquidity.

B. Can be evenly filled to prevent deformation of plastic parts.

C. The plastic parts look good.

D. Gate is not easy separate.

E. Large gate residue

F. Suitable for large and thin-walled plastic parts.

Slug well design

The location of the slug well is generally designed at the end of the sprue and runner.

That is, the first place where the melt plastic arrives, and venting is must added.

The design of the slug well varies depending on the different gate type.

Ejection of sprue

Normally, designer should have a thorough understanding of material properties, choose the most suitable solution.

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