Common Examples:
Fig. 1
Above Fig. 1 example shows air voids in surface of the molded part, in this case the part is not gel-coat so defects show clearly on surface, other examples could be shown with void(s) under gel-coat in which case the void may not appear until part is exposed to heat.
Fig. 2
Above Fig. 2 example shows Fiber drawn to inner radii leaving outer radii resin rich, prone to air entrapment and or cracking in radii.
Initial Inspection
Common defect found in the RTM and LRTM molded parts are voids in the surface ranging in size from less than a millimeter in diameter to as large as 2 centimeters, some found in individual location, others in concession adjacent locations. Excessive heat shrink is also commonly noted in radii further indicating the resin rich areas.
Dissection of the voids will indicated a classic case of the fiber not filling the cross section of the laminate (see Fig 2 above), resulting in entrapped air during the molding process.
Resolution
The resolution of the molding defects discussed below will cover both discussion of the root cause and then in practice of how to prevent such defects in production.
It should be noted that the voids will reappear if the corrective action below is not continued with careful monitoring and process control enforcement.
Root Cause Discussion Overview –
KEY Principle to Keep in Mind
The resin flow front MUST have greater resistance then the entire wet or dynamic hydraulic area of the resin within the mold. This is to mean, that the resin must find it easier, and have every incentive, to look in every direction behind the flow front when it cannot flow ahead fast enough.
When the resin is flowing through the mold, if there are ANY areas such as radii or similar shapes which do not have complete fiber fill, then you have an area in which the resin flow front will see little or no resistance and hence there will be NO back pressure within the hydraulics of the resin as it flows in the mold, resulting in no compression of the atmospher in that area of the mold cavity.
The MOST Common Misconception of Today's Molders –
Is to assume the fiber can be pushed into place by the upper mold half.
It is IMPOSSIBLE for the fiber to be pushed into the geometry of the part cavity by the upper mold half, especially in details such as radii, curves, or “hat sections”. The result of of the upper mold pressing the fiber into place will leave the fiber formed tighter to the inner radii (see Fig 2 above)
This is the number one misconception in the closed molding industry. IT IS CRITICAL that the fiber be stretched BEYOND the actual part geometry during the forming and loading process, knowing that once stretched the fiber will “spring back”, hence proving it is impossible for the upper mold to do the work.
Driving Force Behind Air Evacuation –
First, the fact that vacuum is drawn on the vent does not mean the vacuum “sucks” the air out of the mold, that is the second most common misconception.
The driving force behind the air leaving the mold is when the air still within the mold is COMPRESSED by the resin filling the mold.
The air within the mold is at a particular atmospheric pressure before the injection of resin begins.
To illustrate, let’s say you draw -.5 bar (15” of Mercury) on the mold cavity. Thus then the air (atmosphere) within the mold is at a atmospheric pressure of -.5 bar, at that point the air within the mold is quite happy to stay within the mold as long as the incoming resin does not increase the internal atmospheric pressure over the -.5 bar.
Yet, when the resin injection begins to fill the mold we are adding additional mass to the mold cavity with the resin flow. Thus the air within the mold must either accept being compressed to a higher atmospheric pressure or it will have to SEEK a LOWER PRESSURE environment in an effort to sustain its current atmospheric pressure.
To keep from being compressed the air NATURALLY flows to the VENT which is a lower pressure environment.
Understanding What Was Just Said –
Whenever there is a void within a molded composite, that void was caused by the atmospheric pressure of the void not being exceeded by the surrounding resin hydraulic pressure. So when we consider the fact that fiber in combination with resin viscosity is what causes both the resin flow front resistance and the global hydraulic pressure of the dynamic resin flow, we can easily picture in our mind’s eye what happens when the fiber is not fully filling the cross section of the mold cavity in the thickness of the part.
Is to assume the fiber can be pushed into place by the upper mold half.
It is IMPOSSIBLE for the fiber to be pushed into the geometry of the part cavity by the upper mold half, especially in details such as radii, curves, or “hat sections”. The result of of the upper mold pressing the fiber into place will leave the fiber formed tighter to the inner radii (see Fig 2 above)
This is the number one misconception in the closed molding industry. IT IS CRITICAL that the fiber be stretched BEYOND the actual part geometry during the forming and loading process, knowing that once stretched the fiber will “spring back”, hence proving it is impossible for the upper mold to do the work.
Driving Force Behind Air Evacuation –
First, the fact that vacuum is drawn on the vent does not mean the vacuum “sucks” the air out of the mold, that is the second most common misconception.
The driving force behind the air leaving the mold is when the air still within the mold is COMPRESSED by the resin filling the mold.
The air within the mold is at a particular atmospheric pressure before the injection of resin begins.
To illustrate, let’s say you draw -.5 bar (15” of Mercury) on the mold cavity. Thus then the air (atmosphere) within the mold is at a atmospheric pressure of -.5 bar, at that point the air within the mold is quite happy to stay within the mold as long as the incoming resin does not increase the internal atmospheric pressure over the -.5 bar.
Yet, when the resin injection begins to fill the mold we are adding additional mass to the mold cavity with the resin flow. Thus the air within the mold must either accept being compressed to a higher atmospheric pressure or it will have to SEEK a LOWER PRESSURE environment in an effort to sustain its current atmospheric pressure.
To keep from being compressed the air NATURALLY flows to the VENT which is a lower pressure environment.
Understanding What Was Just Said –
Whenever there is a void within a molded composite, that void was caused by the atmospheric pressure of the void not being exceeded by the surrounding resin hydraulic pressure. So when we consider the fact that fiber in combination with resin viscosity is what causes both the resin flow front resistance and the global hydraulic pressure of the dynamic resin flow, we can easily picture in our mind’s eye what happens when the fiber is not fully filling the cross section of the mold cavity in the thickness of the part.
Not only is the common problem of the fiber not filling the cross section, often molds are not accurate in cavity calibration when they are built and then the inaccuracy is further exasperated during the injection process when there is not proper control of the resin flow rate / pressure to prevent the lifting of the upper mold half causing a loss of flow front flow resistance and thus loss of hydraulic resin pressure over the atmospheric pressure of the air with in the mold cavity.
Putting the Solution Into Practice
Putting the Solution Into Practice
Fig 3
Above in Fig 3 we can see that the fiber is not forming precisely to the contures of the actual part shape, normally the molder has assumed at this point that the Upper Mold half will form the fiber as the mold closes. As mentioned above this is the number one misconception of the industry, again the mold closure is NOT the means or method of forming the fiber precisely to the actual part shape.
Fig 4
Fig 4 above shows the initial step in preventing air within the molding, that photo is showing fiber strips having been placed and formed directly in the radii of the mold detail.
Then the nominal fiber, will be formed by stretching BEYOND the actual shapes of the mold details. This is done by placing forming aids such as plywood or other material which can be inserted during the forming process, between the fiber and the mold surface, having this forming aid under the fiber exagerates the vertical wall length of the various mold details. When the forming aids are removed the fiber is then formed to the actual size of the detial, placing the fiber tight to the mold cavity and allowing it to fill the cross section thickness of the molded part.
Additional Consideration - Mold Half Registration
Additional Consideration - Mold Half Registration
It should also be notted that ALL MOLDS MUST HAVE LOCATING DOWELS FOR MOLD HALVES REGISTRATION, without repeatable mold halves registration IT IS IMPOSSIBLE to control mold accuracy and thus internal pressures.
At YOUR Service -
JHM Technologies, Inc. located in Fenton Michigan USA
Call the JHM Technical team for all of your RTM & LRTM molding needs. JHM carries a full line of Mold Building Accessories, Injection Systems and Technical Training.
Phone: 001-810-629-6515
Email: info@jhmtechnologies.com
No comments:
Post a Comment