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| Rotomolding
Division - PVC Answer Center |
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Can we use iron dies for Roto moulding, if yes then what oven temperature
should be used, or is there any other technical issues while using iron
dies instead of aluminum and how to solve them.
?
: I'm not sure if you mean cast iron, or carbon steel molds. If you
Are using cast iron, DO NOT USE THEM FOR PVC. At elevated temperatures the
PVC will let off hydrogen chloride gas that will eat right through them.
The same goes for carbon steel. Stainless steel is acceptable for use with
PVC. If you are using either of the above mentioned mold materials for
Polyethylene, they will both work. I have never heard of cast iron molds
Being used, because they will not transmit heat readily and tend to be very
Brittle. Carbon steel molds are used occasionally, but tend to rust even
Internally between product runs. Stainless is widely used for molds.
Although it does not transmit heat as readily as aluminum, the mold walls
Can be much thinner, so oven time and temps can be pretty close to each
Other. Stainless is used extensively in open-flame molding of water tanks,
And holds up quite well, in spite of the temperature extremes.
What temperature do you mold pvc ?
As a starting point, assuming a 1/4" wall aluminum mold, the OVEN
temp and residence time for a plastisol would be as follows:
3mm pvc wall thickness: 550f oven temp. 8 minutes
6mm pvc wall thickness: 550f oven temp. 12 minutes
Have you a formulation for transparent or clear plastisol?
Yes we do. The plastisol used for anesthetic masks is clear,
although it does not have optical clarity. The thicker the molded part
is, the more translucent it becomes.
Is rotation as critical for PVC as LDPE?
YES ! Probably more so. The viscosity of plastisol changes continuously as it is heated, unlike LDPE. Consequently, the rate of deposition also changes. The greatest danger is running too high a rotation speed. This can cause the plastisol to cascade, trapping air in the material as it folds-over on itself. Try running a rotation ratio similar to what you would use for the given part geometry in PE. Run the lowest rotation speed that you can to get a uniform wall thickness.
Can PVC be over cured and what is the result? Acetone checks under cured. What will check for over-cure?
See answer below for splitting/cracking.
What would be the shrinkage for soft 60A or rigid PVC parts?
Although shrinkage values will vary with both processing parameters and the plastisol components used to create the durometers listed, we use the following shrinkage values in-house; soft 60 A .006 in/in , rigid 70D .016 in/in.
What would cause a split or crack in the molded piece?
This would depend on a number of factors. Assuming that the plastisol has been compounded correctly, it is either under-cooked, or extremely over-cooked. If it is over-cooked to this degree, you will see discoloration of the part and it will give off an acrid “aroma”. The smell will be that of hydrochloric acid fumes (HCL split-off from the breakdown of the vinyl chloride) It is an unmistakable smell.
To check for an under-cook situation (in a flexible part) cut open a room temperature part and bend it back tight on itself, exposing the interior wall. Pinch the creased portion down tight, putting strain on the crease. You can then either submerge the part, or wipe/brush onto the surface either Ethyl Acetate or MEK, keeping it wet for about 15 seconds. If the part is fully cured, nothing will happen. If it is underfused it will crack. The speed and severity of the cracking will give you an indication of the degree of underfusion. If all that you see is a little surface crazing, you are not far off. If the part splits in two immediately, you have a long way to go. Be sure to observe proper safety procedures with these solvents. Obviously this won’t work on a rigid part.
Your best bet for a rigid part is to push the heat until it just begins to over-cook and then back off a little time and/or temperature until the color change or smell disappear.
What colors are available & is clear or semi-transparent PVC available?
Color possibilities are absolutely unlimited. (check out PVC parts used in auto interiors) Although optical clarity isn’t possible, “clear” materials are readily available. They will typically have a slight tan or blue haze to them.
What would be the shrinkage for soft 60A or rigid rotomolded PVC parts?
Depending on color, fillers and processing parameters, you can figure on .005 to .007 inch per inch of "free" shrinkage for your 60A material. It is also very easy to stretch soft parts out of shape or tolerance when de-molding them hot.
Rigid plastisols will shrink a little more, typically in the .007 to .010 inch per inch range.
Your best bet is always to get a sample of the intended material and run a test mold with a calibration grid scribed into it. That way you can also determine the effects that processing temperature changes will have on final part size. If the geometry of your part will restrict the shrinkage, try to simulate that too.
I'm an Industrial Designer. Is rotomolding transparent and soft plastisol possible? I need to design a (part) with a single opening for a design competition. (Must this opening) be made before or after (the) molding process?
Both soft and transparent qualities are possible. The degree of transparency is dependant upon the wall thickness. Although optical clarity is not possible with today's grades of plastisol, a polished mold will result in a transparent part, although it may have a haze. The degree of softness can be as low as a fake fishing worm. The softer it is though, the lower its tensile and tear strength are.
If the "opening" that you mentioned is in a single-wall construction, it will most likely need to be made post-molding. A "tear-seal" can be built into the tool to facilitate removal of the molded section that spans the intended opening. Otherwise, it would need to be cut out.
Is PVC used in rotomolding for products other than toys?
When rotomolding was first commercialized, the only material
available was vinyl plastisol, and at the time the largest market
was toys. Since then, new varieties have allowed the process
to mature into markets ranging from medical applications (anesthetic
masks) to automotive (arm and head rests) to industrial components
(traffic cones and tank bladders).
How hard and how soft can rotomolded
pvc parts be?
The range is from about 30 Shore A durometer to about 70 Shore
D. Although the harder varieties are referred to as " rigid",
they are not as stiff as pvc pipe. Some amount of plasticizer
is present, making them somewhat more flexible.
What type of mold release is required
when molding plastisol?
The plasticizer contained in a plastisol acts as an
internal mold release. Consequently, with "softer" grades, no
additional release is required. "Rigid", and filled plastisols
may require a sacrificial release, or Teflon ® coated molds.
Acrylic modified materials require an aggressive release, even
on Teflon ®.
Check back here frequently for answers to some of the most perplexing
questions in PVC rotomolding.
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