Q. Why is my mold not setting up?
Q. Why is the urethane sticking to the master?
Q. How do I measure the mix ratio of curative to prepolymer?
Q. Do I need to pull a vacuum on the polyurethane after I mix it?
Q. What is the proper temperature to mix polyurethane?
Q. Why are there soft spots in the mold?
Q. Can the Por-A-Kast be painted?
Q. What is the shelf life for polyurethane?
Q. How long is the mold going to last?
Q. What mold release agent should I use?
Q. Can the Por-A-Kast and Por-A-Mold polyurethane be pigmented?
Q. Why are there small pinholes on the mold surface?
Q. How much material can I mix at one time?
Q. Why do I get a different gel time then that stated on the technical bulletin?
Answers
Q. Why is my mold not setting up?
A. There are two main reasons why urethane doesn’t react fully. One is the urethane was not
mixed well enough and the other is it was mixed off ratio. The urethane may require the curative side
to be mixed previous to being measured. Some of our formulations will settle or phase thus causing
the urethane to be off ratio. If the material was mixed before it was measured out to get the
correct ratio, when the two components were mixed together it was not mixed long enough or fast
enough to get a uniform mix. Think of mixing pancake batter and you will get a good mix.
Q. Why is the urethane sticking to the master?
A. The urethane may be sticking because not enough mold release was applied to the master
and box mold. If it is the first time the master was used a heavier coat of mold release may be
required to get a good release. If the object you are casting has a lot of detail, the mold
release may not have got into the crevices. Also, if the urethane was not mixed properly, it
will have a tendency to stick to the master. Refer to mixing procedures to correct this situation.
Q. How do I measure the mix ratio of curative to prepolymer?
A. We supply the mix ratio by volume to the majority of our Por-A-Kast
and Por-A-Mold products on the technical data sheet, but the mix ratio by weight is also
available. Mixing the polyurethane by weight is always more accurate, but sometimes
a scale is not always available. Mixing by volume is ok. We allow a variance of mix
ratio of 3 to 5 percent without many issues.
Q. Do I need to pull a vacuum on the polyurethane after I mix it?
A. If the gel time on the urethane is long enough where it won’t interfere with
pouring the mold, it isn’t a bad idea to take most of the air out of the urethane. This will
eliminate most of the air bubbles in the mold and on the surface. However if the gel time less
then 5 to 7 minutes, you most likely will not have enough time to evacuate the molding material.
Most people believe in mixing the molding material slow to avoid putting air into the polyurethane,
however, it is better to have a little air entrapment then to have a poorly mixed urethane.
Q. What is the proper temperature to mix polyurethane?
A. Most of our polyurethane systems are designed to be processed at room
temperature – 75 degrees ± 5 degrees. However, it is safe to cast urethane between 5 degrees and 100 degrees F.
Keep in mind though the gel times are based on 75 degrees F. If you are casting in cooler temps the gel time will be
longer and if you are casting in warmer temps the gel time will be faster. Casting in temperatures cooler
then 65 degrees will result in the polyurethane not curing properly. It will have poor physical
properties and/or remain sticky.
Q. Why are there soft spots in the mold?
A. Soft spots are a result or poorly mixed urethane. They happen because of a pocket of
unreacted material. If you are hand mixing, try increasing the amount of time you mix or if you are mixing
large volumes of urethane (over 1/2 gallon) a mechanical mixer may be necessary.
Q. Can the Por-A-Kast be painted?
A. Yes. The Por-A-Kast can be painted if you use a wax based mold release and clean the
part after it is demolded. A wax removing cleaner is recommended. The part can then be primed with an
automotive primer, which is what we found out works best. After the part is primed, the piece may be
painted with an acrylic paint. Sometimes people prefer to spray a mold release into the mold and coat
the mold after with baby powder or calcium carbonate. This causes a matte finish to the part and roughs
up the surface to accept paint a little easier.
Q. What is the shelf life for polyurethane?
A. As a company, we guarantee a shelf life of 6 months from the manufacturing date.
However if our containers are kept sealed and not continually opened and closed, thus being exposed to
humidity in the air, a shelf life of one year or more is quite common.
Q. How long is the mold going to last?
A. This is the time old question, and a good one at that. This will depend on a number of
factors, the type of urethane used and for what application. A rule of thumb is the harder a urethane
mold is the longer it will last, but this also makes it much more difficult to demold a product with a lot of detail.
If you think of it this way, the more detail your mold has the shorter the life span. A soft mold (20 Shore A)
with a lot of detail will most likely yield about 20 to 30 castings. A harder mold ( 70 to 80 Shore A) with
minimal detail will most likely yield over 100 castings.
Q. What mold release agent should I use?
A. For the most part, mold releases are categorized into silicon and non-silicon based mold
releases. Silicon based mold releases are good for most applications. They supply superior release
properties from most substrates, although many casters debate this, and have a range of use greater
then that of wax based mold release. If you plan to paint the cast part however, it is very
difficult to remove silicon from the part. In this instance it is better to use a wax based
release because it can be cleaned off the cast piece much easier.
Q. Can the Por-A-Kast and Por-A-Mold polyurethane be pigmented?
A. Yes. The polyurethane can be pigmented to any color if blended with compatible pigment
dispersions. The suggested pigment loading is between 1 and 2 percent by weight on the curative side.
If more pigment is added a chance of flooding or phasing of the pigment may occur. Also, the more
pigment that is added the more the mix ratio gets affected. For this reason, we recommend sticking to
less then 2% pigment loading.
Q. Why are there small pinholes on the mold surface?
A. Pinholes that appear on the mold surface are usually caused from moisture on the
surface of the mold. As the urethane reacts it heats up (exothermic reaction). If moisture is present
on the surface, it evaporates and gets hung up in the urethane, thus causing the air pocket. Moisture
also can become incorporated into the chemical reaction between the polyol and isocyanate. The
isocyanate will react with water to form carbon dioxide. This CO2 gas is trapped in the urethane
thus causing an air pocket. Make sure the mold is dry before casting urethane. If casting in a humid
environment, it is a good idea to heat the mold before casting urethane into it. A heat gun or
hair drier works nicely.
Q. How much material can I mix at one time?
A. It depends on the gel time of the product you are working with. A faster
reacting urethane may require more mixing time then you have. If you are hand mixing
with a spatula, it is a good idea not to mix more then ½ gallon of material. You will
not get a good mix if you mix more than this. You are able to mix up to 3.5 gallons of
material however if you use a mechanical mixer. A jiffy mixer and a hand drill work
well in a 5 gallon pail. It is best to mix for a minute with the jiffy mixer, scrap
the sides of the pail down with a spatula, and then mix for one more minute with the
jiffy mixer. This will give you a good mix for quantities up to 3.5 gallons.
Q. Why do I get a different gel time then that stated on the technical bulletin?
A. The stated gel time on the Technical Bulletin is recorded at 75 degrees F and an average
of the recorded gel times. The gel time will shorten if the polyurethane is mixed at higher temperature.
For instance, if your urethane is 90 degrees F it may shorten to half the stated gel time. If the
conditions are cooler, it works the other way as well. At 65 degrees F, the urethane will take longer to
gel then the stated value. Keep in mind, that molding at temperatures less then 65 degrees F will result
in the polyurethane not curing properly.