Category: injection mold

clamp, injection mold, Moldflow, Terminology of Injection Molding

CAE analysis of injection molding

Nowadays there are mechanical engineering softwares that help the mold makers and molding professionals to understand to evaluate the part design for plastic injection molding and also check the tool design before cutting the steel. Although the simulation software’s results are not 100% reliable, it saves time and cost by predicting the week area of the design.

Although the results are not reflecting 100% accurate results, it gives an idea to the designers to observe the tool functionality and evaluate their design before cutting the tool. Also, it will reduce development time and save money for companies. Moldflow and Moldex are two examples of software that are very useful to simulate injection molding of plastic.

Below you can see two graphs produced by Mold Flow analysis.

 

 

 

 

 

 

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.

 

 

 

 

 

 

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.

injection mold, Machine specifications

Screw Terminology

Screw: 

a helically flighted shaft that when rotated within the barrel mechanically works and advances the material being processed.

screw

Screw Flight:

the helical metal thread of the screw.

Screw Root:

the continuous central shaft, usually cylindrical or conical in shape.

Flight Land:

the surface at the radial extremity of the flight constituting the periphery or outside diameter of the screw.

Screw Shank:

the rear protruding portion of the screw to which the driving force is applied.

Feed Section of Screw:

the portion of a screw that picks up the material at the feed opening (throat) plus an additional portion downstream. Many screws have an initial constant lead and depth section, all of which is considered the feed section.

Transition Section of Screw:

the portion of a screw between the feed section and metering section in which the flight depth decreases in the direction of discharge.

Metering Section of Screw:

a relatively shallow portion of the screw between the feed section and metering section in which the flight depth decreases in the direction of discharge.

Screw Diameter:

the diameter developed by the rotating flight land about the screw axis.

Helix Angle:

the angle of the flight at its periphery relative to a plane perpendicular to the screw axis.

Axial Flight Land Width:

the distance in an axial direction across one flight land in a direction perpendicular to the flight.

Flight Lead:

the distance in an axial direction from the center of a flight at its outside diameter to the center of the same flight on turn away. The location of measurement should be specified.

Full Flighted Length os screw:

overall axial length of the flighted portion of the screw, excluding non-return valves, smear head, etc.

Screw Channel: 

with the screw in the barrel, the space bounded by the surface of flights, the root of the screw, and the bore of the barrel. This is the space through which the stock is conveyed and pumped.

Enclosed Volume of Screw Channel:

the volume of screw channel starting from the forward edge of the feed opening to the discharge end of the screw channel.

Screw Efficiency: 

the volume of material discharged from the machine during one revolution of the screw expressed as a percentage of the developed volume of the last turn of the screw channel.

Screw Speed:

the number of revolutions of the screw per minute.