Injection molding is used for mass production for plastic parts. Molten plastic is forced in to cavities and solidified to produce parts. Injection molded plastic parts are considerably cheaper than parts produced with conventional manufacturing processes. The molten plastic takes shape of the cavities before solidifying. The molten plastic enters the cavity through a gate. A gate is a critical portion in the plastic injection molding mold/tool. Any error during the selection of type, design and manufacturing of the plastic injection mold/tool are magnified in the form of injection molding flaws. Gate modifications require precision work and is usually expensive and time consuming. The selection of type, size and location of injection molding gate limits injection molding process setting or process window.
A good gate design reduces time required to validate the mold/tool, improves part quality and widens the process window.
A gate is the location where molten plastic enters the cavity. The gate is usually the smallest cross section in the entire flow path. The gate or gates are critical to the molten polymer flow path. The maximum injection pressure loss in the molten polymer flow path is across the gate. The gate location is critical to the part geometry. Injection molding flaws associated with gate are
- Flow lines
- Sink marks near the gate
- Short shots
- Parts stuck during ejection
- Silver streaking
- Weld lines
- Sink marks
- Burn marks
The critical characteristics of the gate are type, location and size.
Types of gates
- Sprue gate
- Fan gate
- Tunnel gate
- Disk gate
- Tab gate
- Pin gate
- Edge gate
- Banana gate
- Spoke gate
- Chisel gate
Factors in selecting the type of gate are:
- Cosmetic requirements of a part
- Type of plastic
- Type of mold/tool
- Flow length of the molten plastic
- Ejection system used in the mold/tool
- Part design
The selection of type of gate is dependent on one or more factors while selecting the gate type. Each of the gate types come with advantages and disadvantages. Changing gate type once the tool is built is difficult. The gate selection is critical to the tooling and molding process. Most commonly used gate types are edge, fan and tunnel gates.
The tunnel and banana gates cut off from the part when the parts and runners are ejected from the mold/tool. All the rest of the gate types need trimming parts from the runner after the ejection. The spoke gate and disk gate types are usually used for cylindrical parts.
One of the primary function of the gate is to solidify the molten plastic after the cavity is filled. The gate also functions to increase the speed of molten plastic going through and into the cavity. The flow across the gate experiences significant pressure drop. The gate also functions to spread the molten plastic in the cavity.
A gate with cold runner is blocked when the molten plastic solidifies. This is called gate freeze. The molten plastic stops moving across the gate after the gate is frozen. As the gate is smallest cross section across the flow path, so the gate is the first to solidify. The flow path with the cold runner system gate freeze occurs due to cooling system in the mold/tool. The gate freeze time cannot be altered more than 20~30%, as no operational valves are in the flow path.
The hot runner system can be added with a valve gate. The valve gate in a hot runner system can be operated with a pneumatic controls. The valve gate open and close time can be controlled externally from injection molding machine controller.
With a small cross section of the gate will achieve gate freeze in short time. For a short gate freeze time cavity needs to be filled faster and at high pressure. At the same time cycle time is shorter with shorter gate freeze time, that saves costs. When the injection molding machine is limited on speed or pressure the solution is to increase the gate size. The large gate cross section reduces required injection pressure and speed of molten plastic flowing into the cavities. At the same time increases the gate freeze time, cycle time and costs.
The gate in the injection molding process is critical to process settings and cycle time. Also the injection molding flaws are also associated with the gate. The injection molding flaws like flow line, jetting, burn marks, short shots, silver streaking, poor weld lines and warpage can be resolved by adjusting or changing the gate.
About the author:
Mr. Tushar Naik (RJG trained Master Molder)
Tushar Naik is a mechanical engineer with international experience in plastic injection molding. He has more than 17 years of experience in research and product development. A mechanical engineer by education and trained by RJG, he has worked with organizations and individuals from USA, CHINA, MEXICO, TAIWAN, KOREA, JAPAN, and most of EUROPE, in diverse industries like medical, surgical, automotive, consumer audio, professional audio, consumer appliances and consumer products. He has successfully worked on 100+ products and several patented technologies.
He started working as an intern in a custom injection molding company while doing research of injection molding process. He has worked with scientific molding technology since 2004. He shares his knowledge of scientific molding to save time and money for the plastic injection molding professionals.