Determining Gate Locations for your Mold
Determining the proper location for gating a part is key in mold design and development, and provides optimum molding. A “gate” is the point at which the melted plastic enters the cavities of the mold, and allows the plastic to make a transition from low restricted runners to actually filling the cavity. Except for the sprue gate, all examples covered here depict gate styles which are of a constrictive nature. According to Tech Mold Inc.’s book “What Is a Mold?” there are basically two scenarios which may be reason to incorporate restrictive gates into a mold design:
1. The smaller or thinner entry point is easier to detach from the plastic part as well as providing for a less noticeable blemish.
2. The restriction, which occurs when the plastic moves through the entry point of gate, builds friction in the plastic causing the plastic to re-heat itself. At times when plastic travels long distances to the cavities, this re-heating may be crucial to filling the mold cavities.
There are a number of options when gating a part, including: (See graph below for gate styles)
1. Pinpoint gate – an extremely small, round gate typically used in micro-molded parts or with parts that are cosmetically sensitive and requiring an extremely tiny gate vestige for aesthetic reasons.
2. Edge gate – generally used when it may be an advantage to have multiple plastic parts attached to the runner for orientation or others of part control. It may also be utilized if blemishes left on the plastic part side walls would be more objectionable than blemishes on the parting line. This is more widely used gate style.
3. Sub-gate – probably the second most popular cold runner gate overall, although likely the most widely used in high cavitation molds. It is a labor-saving feature which provides for plastic part and runner separation within the molding cycle. Sub-gates are not practical on extremely shallow parts.
4. Sprue gate – commonly used on larger molded parts in single cavity molds. It generally provides a lower stressed higher strength part. Its disadvantage is that the sprue vestige will be noticeable whether the sprue is cut or machined after molding.
5. Fan gate – used in an effort to deliver plastic from the runner to a wide area of a plastic part so as to minimize backfilling, thereby reducing surface imperfections and stresses in the plastic parts. These gates generally must be trimmed from runners following molding.
6. Flash gate – takes the intent of the fan gate to the maximum. It differs in that a longer gate may be used because the runner is cut parallel to the edge of the part and the gate is cut at 90 degrees to the runner. It generally also must be trimmed from the runner following molding.
7. Ring gate – usually used in a situation where a round part will need to be molded without plastic part weakening weld lines. A criteria for a ring gate to best be effective is that the part must require a hold be formed in the center. Molding gases are collected simultaneously in the center of the part and must be evacuated through the hole formed in the center. This is done by grinding a 360 degree vent in the core pin forming the center hole.
8. Banana gate – a gate that is directed under the part and into the bottom of the part. Some-times called a cashew gate. Some mold component suppliers make inserts specifically for these types of gates to lift up and release the banana gate at the end of the cycle. It’s a modification of an edge gate because it is edge gated on the bottom side of the part.
9. Tab gate – also a modification of an edge gate in which you gate into a tab first, and then cut off or machine off the tab. The reason for doing this is that gives you better dispersal of material by upsetting the flow – making the flow turbulent – giving you a better fill across the part. This type of gate is often used in optical lens or clear parts to enhance clarity.
Gate Location to Increase Product Strength
Gate location is important for the ability to fill the mold. Some large cavities may require two or more gates to properly fill, which of course affects the cost of the hot runner system.
The gate location is also important because it determines the direction in which the plastic molecules will be oriented within the molded product to a large extent. In general, the strength of plastic in the direction of the flow is greater than in the direction across the flow. Near the gate when the plastic flows in all directions, the strength is relatively low and can be compared with the low strength at a weld line. Therefore if maximum longitudinal strength is required in an elongated product, gating near the end of the product is very important. Thin-walled products are especially vulnerable to this difference in strength, because the thinner plastic cools faster and does not have a chance to re-align the molecules randomly for better strength in all directions, as is the case with heavier-wall products where the plastic stays hot longer between the cooled outer layers and gives the inside core of the plastic time to re-align.
It is important to recognize the importance of the gate location. It may be easier and cheaper to locate the gates so as to employ the shortest runner length (cold or hot runner), but this could result in lower strength of the product, particularly with elongated products, which are subjected to bending forces in use. There are also other issues that affect the gate location.
· The gate needs to be hidden or recessed, so that any protruding gate mark cannot hurt (scratch) anyone using the product (this is very important in medical products.)
· A protruding gate mark could also be in the way of a paper label, which will be applied there.
· Some protruding gate marks are protected with gate rings for strength, typically at the bottom of large containers.
Choosing the right option depends upon the part criteria and which gate will provide optimum molding with the least amount of secondary operations required to remove the gate. Contact your AMBA member mold manufacturer to receive advice on gating options for your particular application.
[Parts of this article were excerpted from “Selecting Injection Molds”, by Herbert Rees and Bruce Catoen.]
Also, thanks to Tech Mold for providing assistance with this article. To order the book, “What Is A Mold?” go to www.techmold.com .