Selecting Injection Molding Screws

Using the right injection molding screws is crucial to making quality parts consistently and with maximum production output.

To select the right screw, details of the particular part to be moulded must be known (that is, part material, weight, size and wall thickness).  The basic mould design is also important so that flow length from gate, shot weight and runner system are all taken into consideration.

Choosing a screw without knowledge of the parts is like buying a car without any preference for performance and handling requirements.

Screw Design


The basic design of any screw has 3 zones along its length:

1.       Feed zone

2.       Transition zone

3.       Metering zone

The feed zone conveys the solid plastic pellets which are fed from the hopper to the transition zone where they are compressed by a change in screw geometry. This compression forces the pellets to melt through the action of pushing up against each other. This is called shearing.  The metering zone then conveys the melt to the front of the screw ready for injection into the mould cavity. 

In the transition zone the material is compressed by the change in the depth of the screw channels from the feed zone to the metering zone.  The ratio of the change in depth is called the compression ratio and is usually between 2 and 3 for plastics such as PP and PE. The length of the transition zone is typically 4 to 7 x the screw diameter in a general purpose screw.

Another aspect of screw design is the length to diameter ratio (L/D) meaning how long it is compared to its diameter.    As an example, the L/D ratio for PP and PE is in the range 20-30:1.

When it comes to general purpose screws, longer screws are usually preferred because they will produce a better quality melt and therefore produce better quality parts

General Purpose Screws (GPS)


The advantage of a GPS is that they can be used with most plastic materials such as PP, PE, Nylon, PET and PC so they are very flexible and good for moulding companies that mould a variety of different materials.

The disadvantage is that, for some materials, part quality and productivity rates will be lower compared to more advanced injection molding screw designs such as the barrier screw. 

Barrier Screws


This type of screw provides a better quality melt at a faster rate compared with a GPS.  There are many different designs of barrier screws, the difference being in the varying of the flight depths and channel widths. 

The exact design chosen must be in line with the application.

Double Flight Screws


Although double flight screws have a different design, they are an alternative to barrier screws. They are also designed to deliver a high quality melt at fast rates.

The design ensures the plastic is fully melted before it reaches the compression zone, which is not the case in a GPS. 

Double flight injection molding screws can be used in technical parts for PP and thin wall technical parts in PA which does not plasticize well with barrier screws.

Screw Diameter


The screw diameter is important for 2 reasons. The first reason is that it determines the maximum available injection pressure, the smaller the diameter the higher the available pressure. This is critical for parts that have thin walls and a long flow length and for plastic materials that are difficult to inject.

The second reason is the diameter determines the maximum shot size available. The smaller the diameter, the smaller the shot size.

 It can be seen that there is a conflict between shot size and injection pressure when selecting a screw diameter.  Initially it might seem advantageous to choose the largest diameter so that there is more flexibility in the types and size of parts that can be made in one machine but this is the wrong way to think about it.

The screw diameter should be chosen in line with the application otherwise quality and/or productivity rates will suffer. The injection unit must be capable of generating enough injection pressure (with some in reserve) to maintain consistent fill times and as a consequence, maintain the quality. 

Wear Resistant Injection Molding Screws


Serious thought should be given to using a heat-treated screw and barrel as these will provide longer life than non heat-treated parts. This is especially important when the material contains some level of reinforcement as this is much more abrasive and will wear out the screw and barrel sooner than material without reinforcement.

Once the screw and barrel start to wear, part quality will start to suffer and it will only be a matter of time before a replacement will be needed. This is a large cost, not just because of the cost of the replacement screw but for the loss in production.

Screw Tip


The tip is a non-return valve at the front of the screw which allows the melt to pass through during the plasticizing stage but stops the melt from back flowing into the screw during the injection stage.

There are 2 basic designs the ball check valve and the sliding ring check valve.  The ring check valve is generally preferred because it allows an easier path for the melt to pass through compared to a ball check valve. Therefore, a ring check valve is suited to shear sensitive materials such PC.

However, the disadvantage of the ring valve is their tendency to wear, so the ring check valve condition should be checked on a regular basis.  A typical sign of wear is inconsistent cushioning during processing. 

Additional Comments


The fact is, in today’s competitive environment, injection molding manufacturers need to be making parts as efficiently as possible in order to keep manufacturing costs down and delivery times short.

Using the right injection molding screws for your parts will play a significant role in this.







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References


BASF Screw Designs In Injection Molding  Technical Information Guide,  June 2007

Basell Injection Molding Polypropylene Technical Guide

Double fight screw targets throughputs, Injection World Magazine Nov/Dec 2011 Engel