The Importance of Gates Choices in Injection Molding Operations

The Importance of Gates Choices in Injection Molding Operations

The aim of every professional injection molding exercise is to outcome finished parts with excellent mechanical properties, designed to dimension and free from defects. Where injection molding designs are concerned, one of the most important considerations lie in the positioning of the gates.

Some published researches have already emphasized the influence if gate sizes along with other injection molding parameters in obtain quality molded parts. From research, we know that the smaller the gate sizes, the lower the chances of weld lines in the part. In a typical design, the injection molding gate refers to the mold opening where the molten plastic material flows into the final part. The gate is essentially the border between the scrap and the part. From gate location to size and shape, the gate design parameters and style are crucial to the finish, appearance and structural integrity of the final part.

 Types of Gates

There are many options for gate style, position and size. Each gate is suitable for certain types of application. Generally, gates will be classified as either manually trimmed or automatically trimmed gates.

The most common examples of gates in mold designs include:

1. Direct/Sprue gate

2. Side gate

3. Tab gate

4. Fan gate

5. Submarine gate

6. Banana gate

7. Pinpoint gate and

8. Edge gate

9. Hot tip gate

10. Hot runner – thermal/valve gates

For better understanding of each type of gate, we shall detail each example in our technical article on Injection molding gate design.

Considerations for choosing gates

Mold design

The nature of the mold design will determine where a gate can be positioned. This along with part orientation, action locations and show surface locations can limit or broaden the options for what types of gates and where they can be located.

Material and Size

The nature of the resin to be used, the part size itself, the gate size, volume and functionality also affect the placement of the gates in the injection molding design and process. This is because the thermal properties and reactions to heat differ from plastic to plastic. In some instances, shear heating, a phenomenon where plastic becomes overheated during molding operations, may occur. Furthermore, larger sized plastic parts should have compensating gate sizes as filling large plastic sizes with smaller gates could slow down the dynamics of the process, resulting in material freezing at the gates before other parts of the mold has been filled. It could also degrade the plastic and result in excessive shear heating. While smaller-sized gates may be more aesthetically pleasing, consider the time and pressure that it might take for the resin to fill up the mold before cooling commences.


It is widely advised that regardless of the gate type being employed, all gates should be:

i.    Placed in non-functional and non-cosmetic areas of the part

ii.   Placed at the thickest cross section area to avoid or reduce the formation of voids and sinks

iii.  Placed away from structures like pins and cores that may affect proper material flow

Gate quantity

When dealing with large parts, it is more effective to have multiple gates to help curtail the issues sited above. This will help ensure proper material flow and avoid bottlenecks that may inhibit flow rate in the mold design. Having multiple gates can also help you eliminate the need for large gate sizes and reduce the chances of flow marks and weld lines.

Bottom line

Gate design is very important in determining the outcome of any injection molding process. From what has been covered above, we can infer that small gates leave lesser trim marks, weld lines and lower the chances of defects in a part. However, small gates are likelier to cause a gate freeze and require more pressure to ensure that the resin material properly circulates the mold. Bigger gates on the other hand are likely to leave larger trim marks but ensure faster cycle times and flow rates. Other important considerations are the number of gates, automatic or manual trimming mechanism, the size of the part and nature of the plastic material to be used. Subsequently, we shall cover the examples on gates in injection molding and when to use what. As gate design remains a critical part of design, it is imperative that you partner with an expert injection molding company in China for best results.

First Part Injection Molding Services in China

FirstPart offers high quality, thermoplastic injection molding service for all your plastic part projects in China. We also offer low-volume manufacturing that serve small quantities from 50 to 10000 parts and produce for mass production. Our engineers are always available to advice on the best processes, material and design optimization to ensure cost-savings, quick turnaround and the production of parts that are 100 percent defect and issue free. Contact us to request a free quote now and submit your design for a free evaluation.