Injection molding forces molten plastic into a closed cavity in order to fill the cavity completely. The entrance into the cavity is called a ‘gate’. The gate seems like it should be a pretty easy decision on what shape and size to be, but choosing the wrong gate for your part can hinder production, affect quality, and impact profits. Do you have the right molded parts gating option selected for your part? Read on and find out.
The gate is only part of the leftover parts of injection molding. Beyond the gate there are additional considerations:
- The sprue is typically a round tapered hole that that leads the molten plastic from the nozzle of the heating barrel of the injection molding machine to the distributing runner in the mold.
- The runner is the section between the sprue and the gate into the molding cavity. The runner transfers the molten plastic.
- A vestige is a witness mark left in the plastic surface from the gate after it has been trimmed off the final molded part. Gates should be in non-cosmetic locations on the molded part, if possible. If the gate is required in a visible spot due to a technical need, steps can be taken to reduce the effect on the final part quality.
There are three basic types of gate design, and each has an advantage depending on the material being molded, the shape of the final part, a cold or hot runner, and the cosmetic quality requirement of the final molded part.
Tab gates are the common option used for plastic injection molding. They are usually large in size and appear to be a tapered rectangle connected to the exterior of the final part. They are usually placed where the part has the thickest cross-section and at the parting line of the two halves of the mold. This option is easy to trim from the final molded part and doesn’t affect surface quality or functionality. Tab gates are usually the best option for glass-filled or mineral-filled resins as those two material types can be difficult to mold.
Hot Tip Gates
There are designs that won’t allow a Tab Gate for cosmetic or functionality reasons. Perhaps the mold cavity takes up too much of the molding footprint within a designated molding machine, or a tab gate leaves too much of a witness mark on the final part. In this case, a Hot Tip gate may be the better option. Hot Tip gates use the heated nozzle to keep plastic at the optimal temperature as it flows into the molded cavity. Tab gates can allow the molten plastic to partly cool in the runner, which can cause issues with molding efficiency and other fill considerations. The Hot Tip typically leaves a vestige shaped as a round bump, typically 0.060” to 0.080” across and 0.010” to 0.020” high. It can easily be trimmed flush to the surface too. The Hot Tip is a great option for conical or dome-shaped parts that need an optimum radial flow through the gate. It is typically put near the center of the A-surface of the finished part and can offer additional benefits to molding in equipment wear, molding wear and damage, and flash that would need to be trimmed from the finished part.
While Hot Tip gates have a number of design options that give them an advantage, certain instances make them a less than perfect choice. Some designs requiring difficult geometries or a difficult material to mold may be better with a Pin gate. Pin gates – also known as Post or Tunnel gates – can be used when the finished part cannot afford to have gate vestiges on the parting line (as a Tab gate allows) or on the cosmetic surface (as a Hot Tip gate allows). Pin gates inject molten resin into the molding cavity through a cone-shaped hole that intersects an ejector pin in the mold. Once the plastic has hardened in the mold, the ejector pin pushes the finished part from the mold and shears the material left in the gate. That leftover material can then be removed from the finished part for a clean surface. Some of the disadvantages of the Pin gate include the use of glass-filled or resins that require a slow injection speed. They can sits inside the injection machine barrel and degrade under intense heat, and can allow the gate to become plugged easier than a faster injected material. Pin gates can also increase the stress level that the mold sees during injection and lead to more maintenance requirements over the life if the mold itself.
Knowing the type of gates available is a good start, but there are other factors that may influence the final gate design used:
- Runner type – Hot or cold – Both runners have options to choose from. Cold runners typically use tab and pin gates, while hot runners do well with Hot Tip gates.
- Chosen material – The resin chosen for the part can influence the gate design, its location, and size. Glass-filled and mineral-filled plastic materials may require a larger gate to avoid blockage or heat degradation. Materials like PET (Polyethylene Terephthalate, the most common thermoplastic polymer resin) will require a larger gate because they are sensitive to shear forces during ejection.
- Part shape – The shape of the molded part can influence the gate design as certain shapes do best with a certain gate type. Tab gates are a common inclusion for flat parts, while Hot Tip gates might be the best option for a round part like a lid or container.
- Cosmetic requirements – Certain gate types leave minimal residual witness marks, while others may leave a significant vestige that needs to be dealt with. If the part has stringent cosmetic requirements, a Pin or Hot Tip gate may be a better choice if other factors like material and runner type work with those gates. Tab gates are low cost, but can leave a large gate mark that should be placed on a non-cosmetic surface.
The type of gate used is very dependent on the molding application. Every part will need to be studied on a case-by-case basis to determine the material being used, the runner type, part packing needed, injection speed, how much gate vestige can remain, and what surface quality is required. In some cases, one gate may not be enough, and a second or third may allow your part to be molded to meet your requirements. If you’re unsure of the best option for your new product, give our experts a call at (425) 339-0288 or email us at email@example.com. We can offer you advice on the best option to use, the best materials to meet your product demands, and how to navigate through each development stage with ease.
Matthias Poischbeg was born and raised in Hamburg, Germany. Matt moved to Everett, Wash., after finishing his bachelor’s degree in business in 1995 to work for Sea-Dog Corporation, a manufacturer, and distributor of marine and rigging hardware established in 1923.
In 1999, Matt took over the reins at Sea-Lect Plastics Corporation, a sister company of Sea-Dog and a manufacturer of plastic injection molded products with an in-house tool & die shop. Matthias Poischbeg is also a contributor to Grit Daily.