how to mold long fiber reinforced thermoplastics: here\'s the machinery, tooling, and process know how you need to get the most out of these high-performance materials.

Long Fiber Reinforced thermoplastic (LFRTs)
Used in injection molding applications that require a high level of mechanical performance.
Although the LFRT technology has excellent strength, stiffness and impact properties, the processing methods of these materials play an important role in determining the performance of the parts.
To successfully shape the LFRTs, it is important to understand what makes them unique.
Understanding the differences between LFRTs and traditional enhanced thermoplastic plastics drives equipment, design, and processing decisions to maximize the benefits and potential of LFRTs.
The difference between LFRTs and traditional short cut fiberglass-reinforced compounds is the length of the fiber.
In LFRTs, the length of the fiber is the same as the length of the particles.
This is because most LFRTs are produced through a pull-up process rather than a shear process.
Composite reinforcement.
In the manufacture of LFRT, continuous glass wire-
The fiber coarse yarn is pulled out through the die head, coated with thermoplastic resin and soaked on the die head.
Then chop or marinate this continuous reinforced plastic stick, usually at a length of 10-12 mm.
In contrast, traditional short glass compounds contain chopped fibers 3-
4mm long, further reduced to usually less than 2mm in one cut
Dense extruder.
The fiber length wrapped in LFRT particles helps to improve the mechanical properties of LFRT.
The impact resistance or toughness is increased while the stiffness is maintained.
As long as the fibers remain for a long time throughout the molding process, they produce an \"internal skeleton\" that provides the mechanical properties they notice.
However, a poor molding process can convert long fiber products into short fiber materials.
If the length of the fiber is affected during the molding process, it may not be possible to reach the possible and desired level of performance.
In order to maintain fiber length throughout the molding process of LFRTs, there are three important areas to consider: injection molding machines, parts and tool design, and processing conditions.
A common question about LFRT processing is whether it is possible to use an existing injection molding device to mold the material.
In the vast majority of cases, equipment for mold staple fiber compounds can also be used for mold LFRTs.
While typical short fiber forming equipment is sufficient for most LFRT parts and products, some modifications can be made to the equipment to help keep the fiber length.
With typical \"feed-compression-
Although the fiber-metering area can work well
By reducing the compression ratio of the metering part, destructive shear may be reduced.
For LFRT products, the compression ratio of about 2:1 metering parts is very good.
Screws, barrels and other equipment made of special metal alloys are not necessary because LFRTs are more chopped than traditional ones-
Glass reinforced plastic.
Another device that may benefit from the design review is the nozzle tip.
Some thermoplastic materials are easier by reverse processing
The tapered nozzle tip can produce a high shear when the material is injected into the mold cavity.
This nozzle tip significantly reduces the fiber length of long fiber compounds. A 100%\"free-
Flow \"slot nozzle-
Tip/valve assembly is recommended so that long fibers can easily enter the part through the nozzle.
In addition, the general size of the nozzle and gate should be 5. 5 mm (0. 250 in. )diam.
Or more, no sharks.
It is important to understand how the material flows through the injection molding equipment and to determine where the shear will attempt to destroy the fiber.
Good part and tool design practices for parts and tools also help to maintain the fiber length of LFRTs.
Eliminate the sharp corners around the edge of the part (
Including ribs, boss and other functions)
Avoid unnecessary stress in molded parts and reduce fiber wear.
The part should be designed as a nominal wall--
Consistent and uniform wall thickness at all times.
Large changes in wall thickness will lead to inconsistent filling and poor orientation of some fibers.
Where a thicker or thinner wall is required, avoid sudden changes in wall thickness, which may result in a high
It may damage the shear area of the fiber and is the source of stress concentration.
Always try to enter the thicker walls and flow to the thin part, keeping the thin part at the end of the filling.
Good general plastic design principles recommend keeping wall thickness below 4mm (0. 160 in. )
Will promote good, uniform flow and reduce the possibility of sink and void.
For LFRT compounds, the optimal wall thickness is usually about 3mm (0. 120 in. )
The thickness of Aminimum is 2mm (0. 080 in. ).
When the material enters the tool, a wall thickness of less than 2mm increases the possibility of fiber breakage.
Part design is only one aspect of tooling;
It is also important to consider how materials enter the mold.
Since the runnersand gate directs the material to the mold cavity, a large amount of fiber degradation may occur if these areas are improperly designed.
When designing a tool for LFRT compounds
The circle is preferred, with a minimum diameter of 5. 5 mm (0. 250 in. ).
Except for a complete
Round runners will have sharp corners which may increase the stress and damage of the glass
Fiber reinforcement during molding.
Open channel of heat
The runner system is acceptable.
The minimum thickness of the door should be 2mm (0. 080 in. ).
If possible, position the gate along the edge where it flows unimpeded into the cavity.
The part that Gates faces will need a 90 [degrees]
The optical fiber breaks and the mechanical performance is reduced.
Finally, know the location of the knitting lines and how they are associated with the parts area that will bear the load (or stress)when inuse.
The position of the door should be careful to move these knitting lines to areas with a lower expected stress level. A computermold-
Filling analysis can help identify places where knitting lines will be late.
Finite element analysis of structure (FEA)
Can be used to compare the position of the high stress and the position of the knitting line in the mold
Filling analysis.
It should be noted that these parts and tool design suggestions are just those--Suggestions.
There are many examples of the thinner parts of the wall
Changes in thickness, and fine or detail features that utilize LFRT compounds for good performance.
However, further deviations from these proposals will require more time and effort to ensure that the full benefits of long fiber technology are realized.
Processing conditions are key to the success of LFRTs.
As long as the correct processing conditions are used, it is possible to create good parts from LFRT materials using a general injection molding machine and properly designed tools.
However, even with the proper equipment and tool design, the fiber length is affected if poor processing conditions are used.
Similarly, pay attention to the situation that the fiber will encounter during the molding process, and determine the area that may lead to excessive shear.
First, monitor the back pressure.
High reverse pressure creates a huge shear force on the material and reduces the length of the fiber.
Consider starting with zero back pressure and only increasing to the point where the screw comes back evenly and consistently during the feeding process.
The back pressure of 1. 5 to 2. 5 bar (20-50 psi)
Usually enough to get consistent feeding.
High screw speed will also have adverse effects.
The faster the screws turn, the more likely the solid and unmelted material will enter the screw compression zone and cause damage to the fibers.
Similar to the suggestion of back pressure, try to keep the screw speed at the lowest level required for consistent filling of the screw.
When forming the LFRT compound, the screw speed is usually 30 to 70 RPM.
During the injection molding process, melt by combining the two factors of shear and heating.
Since the purpose is to maintain the fiber length in the LFRTs by reducing the shear, then more heat will be consumed.
According to the different resin system, The lfrt compound is processed 10-30[degrees]
F is higher than conventional molding compounds.
However, try to reverse the barrel temperature curve before fully increasing the barrel temperature.
Usually, the barrel temperature rises when the material moves from the hopper to the nozzle;
But for LFRTs, it is recommended that the Hopper temperature be higher.
The torsional temperature distribution will soften and melt before LFRT pellet enters the high temperature
Shear Zone of screw compression zone.
This is a great help to improve the fiber.
Keep the length.
The last item on processing involves the use of re-grinding.
Grinding molded parts or channels often lead to much lower fiber lengths;
Therefore, no re-grinding can be added without affecting the overall fiber length.
In the case of no significant reduction in mechanical performance, the maximum grinding amount of 5% can be used.
Higher levels of re-grinding can have a negative impact on impact strength and other mechanical properties.
Karl groupe is a senior product development engineer at Winona RTP, Minnesota.
He has led the structural materials development team since 2001 and has been patented for rtp vlf materials. Contact: (507)474-5367;
Khoppe @ rtpcompany. com; rtpcompany.
Bruce Vettel is a Midwest Technology Company in RTP
Service rendering.
He has 37 years.
Experience working with tooling and tooling engineer. Contact: (507)474-5306;
Bvietor @ rtpcompanycom.