Category: Machine specifications

clamp, injection mold, Machine specifications, Mold

Injection Molding Press

Plastic injection molding presses are classified or rated based on tonnage, or more specifically, the clamping pressure or force. Presses can run in size from less than 5 tons of clamping pressure to over 6000 tons. The higher the press ton rating, the larger the machine.

A machine rated for 60 tons can deliver 60 tons of clamping pressure. This pressure keeps the mold closed during the injection process. Too much or too little pressure can cause quality issues. Too much or too little pressure can also cause flashing, where excess material appears on the part edge.

How to know the required press tonnage?

There are many factors that are taken into consideration when determining the size of the press. The size of the part, the polymer being used and something called the safety factor. The safety factor is an additional numerical percentage buffer that is added to the calculation to help avoid defects in the final part. Some recommend adding 10% to allow for the safety factor. As mentioned earlier, the MFI (Melt Flow Index) of the plastic compound will also impact the pressure needed to produce the part. Many calculations include the platen size as well as the mold and part size, however, to get an estimate of the press size your project will need, we have simplified it even further.

Reference :https://www.slideshare.net/BhushanGadilohar/know-about-injection-moulding-copy?from_action=save

injection mold, Machine Operation Terminology, Terminology of Injection Molding

Understanding the Molding Parameters

There are over 200 different parameters that must be established and controlled to achieve proper action molding of a plastic part. These parameters fall within four major areas :

4 parameter areas [1]
Reference : A guide for injection molders , by D.M. Bryce

To the troubleshooter, all the 4 areas are important, but the pressure and temperature areas are the ones most commonly considered during the troubleshooting process. Based on the requirements of any particular plastic material, the must be sufficient to inject the plastic material and to hold the mold closed. In addition, the temperature of the injected plastic and mold must be correctly maintained.

1 . PRESSURE

Pressure is found primarily in the injection area, but there is also pressure found in the clamp unit of the molding machine.

1.1 Back Pressure

The first pressure to consider is back pressure. This is pressure that is created during t he return action of the screw after injecting material. The screw turns (augers) to bring fresh material into the heating cylinder. This material is placed in front of the screw and nudges the screw backwards. A buildup of pressure is created at the front end of screw. This pressure is used for better mixing of the plastic (especially if colors are added to the press), removing small amounts of trapped air, and controlling the weight of the shot by maintaining an accurate density of a given volume of melt. The back pressure setting should start at 50 psi and be increased in 10 psi increments as needed, with a maximum setting of 300 psi. The maximum setting is needed because anything over that will cause too much shearing of tthe plastic and result in thermally degraded plastic.

The first pressure to consider is back pressure. This is pressure that is created during the returns action of the screw after injecting material. The screw turns (augers) to bring fresh material into the heating cylinder. This material is placed in front of the screw and nudges the screw backwards. A buildup of pressure is created at the front end of screw. This pressure is used for better mixing of the plastic (especially if colors are added to the press), removing small amounts of trapped air, and controlling the weight of the shot by maintaining an accurate density of a given volume of melt. The back pressure setting should start at 50 psi and be increased in 10 psi increments as needed, with a maximum setting of 300 psi. The maximum setting is needed because anything over that will cause too much shearing of the plastic and result in thermally degraded plastic.

1.2 Injection Pressure

The next type of pressure to consider is injection pressure. This is the primary pressure for injecting 95% of the molten plastic into the closed mold. Normally, the highest pressure and the fastest fill rate are the best conditions. However, high pressure will increase molded-in stress. And the stress will be released at some time. There is no question as to its being released, only as to when it will be released. And remember, the hotter the plastic, the more fluid it becomes and the lower the pressure can be to fill the mold.

The next type of pressure to consider is injection pressure. This is the primary pressure for injecting 95% of the molten plastic into the closed mold. Normally, the highest pressure and the fastest fill rate are the best conditions. However, high pressure will increase molded-in stress. And the stress will be released at some time. There is no question as to its being released, only as to when it will be released. And remember, the hotter the plastic, the more fluid it becomes and the lower the pressure can be to fill the mold.

 

 

 

 

 

 

clamp, Machine specifications

Clamping Concepts

The injection molding machine clamp is used to close the mold, hold it closed during the injection and curing of the plastic material, and open the mold for the removal of the formed part. There are 3 different types of clamp design:

  1. Straight hydraulic clamp
  2. Linkage or toggle clamp
  3. Hydromechanical clamp

1- Straight hydraulic clamp

This design uses hydraulic fluid and pressure to open and close the clamp and to develop the force required to hold the mold closed during the injection of plastic. The basic concept is to direct hydraulic fluid to the booster tube to move the clamp ram forward. Oil fills the main area by flowing from the tank through the prefill to the main area. as the ram moves forward, a slight vacuum is developed in the main area, pulling fluid from the tank into this chamber. Once the clamp is closed, the prefill valve is closed, trapping the oil in the main cylinder area. High-pressure fluid is put into this area, compressing in this area. The maximum pressure is controlled by a pressure control valves, which closely controls the clamp tonnage ( the max. hydraulic pressure times the area it pushes against.)

To open the clamp, hydraulic fluid is directed to the pullback side of the cylinder while the prefill valve is open, with fluid from the main cylinder being returned to the tank. One of the major advantages of the straight hydraulic clamp is its very precise control of the clamp tonnage.

2- Linkage or Toggle Clamp

This concept uses the mechanical advantage of a linkage to develop the force required to hold the mold closed during the plastic injection portion of the cycle. Normally the linkage design is done in such a way that slowdowns are built in. The advantage of a toggle clamp is that less hydraulic fluid is required to open and close the clamp. A disadvantage is that the clamp tonnage is not precisely known.

A small hydraulic cylinder travels at a constant speed with the slowdown for mold close built into the linkage. The mechanical advantage of the linkage is extremely high so the relatively small closing cylinder can develop high tonnage.

3- Hydromechanical Clamp

This design uses a mechanical means for high speed close and open. A short stroke cylinder is used to develop tonnage identical to the straight hydraulic design. This concept is said to offer the advantage of toggle clamps for high-speed close and open, and the advantage of a straight hydraulic for precise control of the clamp tonnage. The hydromechanical design normally has a high-speed clamp close and open device which is usually a hydraulic cylinder or actuator. The closing and opening mode occurs with relatively low force. Once the clamp is closed, a blocking action takes place allowing a large -diameter hydraulic cylinder to build tonnage similar to the straight hydraulic design.

When the clamp is to be opened, the blocking member is removed, and the clamp opens rapidly. The blocking action is normally a mechanical device,  and the tonnage action is done by hydraulic; hence the name hydromechanical.

Comparison of Clamp Designs

Over the years many arguments have been presented showing each clamp design concept to be superior to the others. In reality, each concept has merit.

The straight hydraulic design has proved over the years to have long-term reliability, excellent control of low-pressure mold protection, and exact control of tonnage, and it will not allow the clamp t be overstressed due to high injection forces.

The toggle clamp has extremely fast closing and opening actions and is typically lower in cost than the straight hydraulic, but this energy is small compared to the total energy usage of the machine. With good lubrication, the toggle bushings and pins last well, but they still must be reworked after several years of service.

The toggle design will also develop higher than lockup tonnage if the clamp is overpowered by the injection end, or due to temperature buildup in the mold. The hydromechanical tends to have the advantages of the straight hydraulic, whereas the toggle is more complex because of the block action required. The debate over the three clamp concepts will continue for many years.

 

 

Machine specifications

Standard For Using Interchangeable Molds

The society of plastics industries has published standards in regard to mounting molds in the injection molding machine. The obvious advantage of these standards is that they easily allow molds to be designed to run on more than one brand of machine. These standards are listed here for descriptive purposes.

1- Platen Bolting Pattern

These standards specify the location and size of tapped holes in the stationary and moving platen for the attachment of the mold. They cover 3 sizes of machines:

  • Up to 750-ton clamping capacity
  • Overy 750-to 1600-ton clamping capacity
  • Over 1600- to 4100-ton clamping capacity

2- Knockout Pin Locations

Knockout pins are used to push the molded part of the mold during the portion of the machine cycle called ejection. Since the location of these pins is critical to the mold design, the society of plastics industries has written a standard for locating the knockout pins. The location and size of the knockout pin holes are classified according to machine sizes identical to those used for mold mounting holes standards.

3- Machine Nozzle and Die Locating Ring

The die locating ring is used to align the mold to the machine platen. This alignment is needed for the knockout pins and for the injection unit to line up with the mold. The machine nozzle provides the mating necessary between the injection unit and the mold.

 

 

 

injection mold, Machine specifications, Machine start-up

Injection System Specifications

Injection Capacity (Theoretical): the maximum calculated the swept volume (or trapped volume in a plunger unit) is cubic inches that can be displaced by a single stroke of the injection plunger or screw, assuming no leakage and excluding the use of a rotating screw to displace additional volume.

Thermoset Injection Capacity: Injection capacity can be measured in cubic inches of swept volume, but as there is non-return valve on the thermoset screws, this figure cannot be used to convert to true shot weight because some material flows back over the screw during injection. The amount of backflow is dependent on variables in both the machine and molding material.

Plasticizing Capacity: the maximum quantity of a specified plastic material that can be raised to a uniform and moldable temperature in a unit of time (Pound/hr).

Recovery Rate: the volume or weight of a specified moldable material discharged from the screw per unit of time, when operating at 50% of injection capacity as determined by SPI test procedure ( cubic inches/sec).

Injection Pressure (PSI): the maximum theoretical pressure of the injection plunger or screw against the material expressed in psi.

Maximum Injection Rate: the maximum calculated rate of displacement of the injection plunger or screw, expressed in cubic inches per second.

Minimum Injection Rate: the minimum calculated rate of displacement of the injection plunger or screw, expressed in cubic inches per second.