Fig 1: Pneumatic sprue cutters on an EOAT (end-of-arm-tool).

 
Fig 2: Floor-mounted degater with pneumatic sprue cutters..

 
 
Fig 3: Robot beam-mounted degater.

  
Fig 4: Floor-mounted degater with cutting blades mounted to a pneumatic cylinder.

  
Fig 5: Blade cutting assembly shown
in Fig 4.

  
Fig 6: Punch press degater.

  
Fig 7: Robot beam-mounted CNC degater.

 
Fig 8: Floor-mounted CNC degater.

  
Fig 9: Robot with rigid cutting blade.

By John Westbeld, Design Manager, and Josh Long, Design Engineer, >SAS< Automation, LLC

Although hot runner molding continues to increase in popularity, for many injection molded parts, cold runner molding is still the process of choice. With cold runner molding, of course, the challenge is to remove the runner from the molded part economically.  In many instances, the operator removes the runner by cutting edge gates with a hand-held blade or pneumatic cutter. As with any operation performed by a person, manual degating can lead to an inconsistent cut, increased operator costs and possible cycle time issues.

There are several ways to automate the gate cutting process:

• Pneumatic sprue cutters can be integrated onto a robotic EOAT (end-of-arm-tool) to cut the gate if the injection molding machine is equipped with a robot.
• Floor-mounted or robot beam-mounted degating fixtures that have pneumatic sprue cutters similar to those on EOAT, or cutting blades actuated by pneumatic cylinders.
• Guillotine-style punch presses can be used to shear gates from the parts.
• CNC gate cutters can be programmed to cut gates for several applications.
• 5- or 6-axis robots can be equipped with pneumatic cutters to cut gates.
• Lasers can also be used in gate cutting applications.

With all of these methods available, it is important to note that the equipment needed is just one part of the evaluation equation. It is prudent to spend time evaluating all gate-cutting methods based upon the level of automation needed. The most important factors in making your decision will be cost, gate vestige requirements, level of automation needed and available floor space.

Sprue cutters on robotic EOAT
Many times the most cost effective way to automate degating for any single degating application is to incorporate the sprue cutters with the EOAT (see Fig 1). This can be done by clamping the sprue cutters to the existing framework of the EOAT. This may require one or two additional air circuits and a programmed sequence from the robot.

The runner can be up to 0.75 in. dia, depending upon the material to be cut. Sprue cutters work better on cylindrical or rectangular runners rather than fan gates. If you want to secure the runner until you reach a specified drop area, a sprue plier can be mounted to the EOAT. It can come with or without a sensor to detect whether the sprue is present.

The estimated gate vestige left on the part after cutting with sprue cutters is approximately 0.015 to 0.025 in., depending on the precision of the robot, the type of plastic used and the location and cross-sectional profile of the runner.

Floor-mounted or robot beam-mounted degaters.
Floor-or robot beam-mounted degaters have pneumatic sprue cutters similar to those that would mount to the EOAT (see Figs 2 and 3) or cutting blades mounted to a pneumatic cylinder (see Figs 4 and 5). As the name indicates, the framework of the degater can be mounted to the robot beam or floor. In either case the robot secures the parts from the mold with the EOAT and then presents the parts to the degater for cutting.

Advantages of this type of degater are that the robot does not have to carry the weight of the cutters on the EOAT, which prolongs the life of your robot. In addition, the sprue cutter assemblies can have change-over tooling to accommodate the degating of several different molds with one frame.

Yet another advantage is that the degater can be equipped with valves so extra air circuits for the robot are not required. Specifications on the cutting vestige are similar to those for the sprue cutters on the EOAT.

Punch press degating
Punch presses can be used when the cutting vestige requirements are critical and range between 0.003 and 0.010 in. (see Fig 6). The punch press can be operated manually or automatically and can be integrated to the robot or press.

A punch press usually consists of pneumatic slide and ram cylinders, slide table with rods and bearings, part nests with part down switches, two hand non-tie down push buttons, hardened cutting steel, polycarbonate guarding and a control box with a PLC.

Options include light curtains, scrap chutes and part inspection sensors. Advantages of the punch press are the "flush" cut that it provides and the fact that it can be used to cut large fan gates.

CNC degating
CNC gate cutters are used when several gate-cutting applications are needed at the same press. The CNC gate cutter can be robot beam-mounted or floor-mounted. They can hold up to 20 different programs with various steps. A teach pendant is used for positional programming and x,y,z and theta axes, which are driven by stepper motors.

The robot beam-mounted version has a pneumatic sprue cutter mounted to the end of its arm (see Fig 7). The robot presents the parts to the degater on the EOAT and then sends a signal to the degater to begin its cycle. After completing the cycle, a signal is sent back to the robot to continue its cycle.

The floor-mounted version generally works in conjunction with a sprue picker (see Fig 8). The sprue picker hands the parts off to the degater, which has a pneumatic gripper to secure the sprue. The degater then presents the parts (on the runner) to a fixed mounted pneumatic sprue cutter for removal. The gate vestige left for either unit is estimated at 0.007 to 0.015 in.

Robotic degating
Five- or 6-axis robots can be equipped with a pneumatic sprue cutter or a rigid cutting blade at the end of its arm (see Fig 9). The parts are presented to the robot on the EOAT and the robot positions the cutter at the gate for cutting. the gate vestige remaining is estimated at 0.005 to 0.015 in. Advantages include unlimited programming for different gate cutting applications and high level of repeatability.

Laser Degating
CO2 laser cutting is another option for gate cutting, but is not usually cost effective for most applications. The cutting beam of the laser can be fixed or driven by pneumatic cylinder or electric motor. The gate vestige left by a laser cut is limited to the device providing motion to the laser, with estimates of 0.002 to 0.010 in. The laser does offer a "clean" cut to remove gates from class A surfaces as well as fan gates.

Reprinted with permission from Plastics Auxiliaries Magazine, April 2001