Introduction: Cast Faux Pumpkins
Cast Your Own Faux Foam Pumpkins
Mini pumpkins are so cute, but they only stick around for one season. Maybe you want a gold colored one as a table accent, or perhaps you want to make a Fall or Halloween wreath for your door or mantle. It's a shame that you will have to throw the art out as the mini pumpkins age and go bad.
Why throw away your beautiful projects.... Make them with faux mini pumpkins you make yourself.
It is relatively easy to cast mini pumpkins. All we need is to create a mold from an existing mini pumpkin and then we can cast replicas!
Once the mold is made it can be used over and over to create a venerable pumpkin patch of mini pumpkins. You even have the option of multiple materials; lightweight polyurethane foam*, epoxy, plaster, and if you use food compatible silicone molding compound (this project is not using food safe) you can make food safe molds for chocolate, jello, etc. pumpkins or even bake in the mold.
* I'm trying a new source for an old material (polyurethane foam) to make my mini pumpkins out of. Home Depot has a fence post setting material (used in place of concrete) that is polyurethane foam. Instead of ordering rigid expanding foam on the internet, I walked into Home Depot and picked up a bag of two-part fence post polyurethane. We'll see how it works!
1 - Real mini pumpkin
1- Smooth-On OOMOO 25 - FAST Curing Mold Making Silicone kit (Amazon)
1 -Sika Pro Select 33 fl. oz. Fence Post Mix* polyurethane resin (Home Depot)
2 - Glass Jars with lids (for polyurethane resin storage)
1 - Empty plastic container/bottle sized for your pumpkin (as a mold form; drink bottle, milk jug, etc.)
1 - Cooking spray (as mold release)
4 - 3/8" dia. x ~1/2 inch Wood dowels
1 - 3" Nail (10d penny; size)
1 - Mini cup
2+ - Clear drinking cups (~ 9 oz. for measuring polyurethane)
4 - Large plastic cups (~ 18 oz. for measuring mold compound)
2 - Yet again larger mixing cups for combining mold compound)
2+ - Stir Sticks ( or popsicle sticks, tongue depressors, etc. but longer are better)
1 - Black marking pen
1 - Petroleum Jelly (Vaseline; not shown)
4+ Cotton Swabs (Q-tips; not shown)
1 - Masking Tape
Multiple Large Rubber Bands (not shown)
Safety Glasses (not shown)
Hot Glue Gun and Glue Sticks
Drop cloth to protect working surfaces (not shown)
Orange, Yellow and Brown Acrylic Paint (or other finishing supplies; optional; not shown)
Air Brush set up (definitely optional; not shown)
Step 1: Evaluate Your Pumpkin for Casting
We are going to be making a mold of an existing mini pumpkin. Mold making is a science and an art. I would strongly suggest reviewing the Instructables Mold Making & Casting Class it will review the process we are going through and help gain a greater understanding of it. There is also a very nice YouTube video on Casting a Pumpkin - DIY that is helpful.
We are going to make a two-part block mold. This allows the molded pumpkin (and also the original) to be easily removed by the mold splitting into two parts.
We want to look at our pumpkin and see where it is symmetrical and also look for areas that might be undercuts that might trap the pumpkin in the mold. I noted that in the YouTube video the cast pumpkins stem broke off. I wanted to orient the pumpkin in the mold so that the mold would open (part) along the stem so that there wasn't any stress on the stem pulling the cast pumpkin out of the mold. My mold will also have the seam (part line) follow the groves on the sides of the pumpkin. So my pumpkin orientation in the mold is different than those shown in the video.
After evaluating the pumpkin I took the marking pen and put some lines where I wanted the parting line to be. This will be the limit of the first pour of the silicone mold making compound.
Step 2: Prepare the Pumpkin for Molding
We want the original pumpkin to easily come out of the mold after we have poured the mold material. Smooth the stem of any major bumps or prickly spines.
We also want to add an opening (funnel like) to pour the casting material into the mold when we make faux pumpkins. We are going to hot glue a mini cup onto the bottom of the pumpkin to act as a opening and funnel for the casting material. The cup was trimmed to size and hot glued in place on the bottom of the pumpkin.
In order to facilitate removal of the pumpkin from the mold we need to add a mold release. Spray the pumpkin with cooking spray and then wipe the excess off.
Step 3: Prepare Your Mold
Pick your mold form so that there is at least 1/2 inch of space around the pumpkin. I had to cut the top off of a plastic bottle to make my mold form.
I decided that I wanted to fix the pumpkin with a nail to firmly attach it. I had seen that some items being cast want to float in the mold material. I didn't want this to happen to my pumpkin.
Step 4: Mount the Pumpkin in the Mold
I placed the pumpkin inside the bottle and pushed the nail in from the outside. After the nail was in place I taped it to the sides of the bottle to keep it from moving. (Ooops - if I was concerned about the pumpkin floating I should have had my tape pulling the nail upward to keep the pumpkin down. - Still worked OK).
Step 5: Prepare to Pour
The Oomoo 25 silicone mold material should be stored and used at 73 degree fahrenheit. When equal volumes of the mold material are mixed it has 15 minutes of pot time (remains runny) and takes 75 minutes to cure. I let my mold halves each cure 2 hours. Always pre-stir the each mold compound separately to make sure they are thoroughly mixed before measuring out the amount you want to use and combine.
I poured water in my mold up to the level of the 1st pour; then poured it out into a measuring cup. I divided the volume into Part A and Part B volumes. I marked these volumes on the inside of two plastic cups respectively. It is a good idea to figure in a bit of extra material in case calculations/measurements are slightly off.
I poured out Part A and Part B materials into their respective marked cups and then combined them into a single larger cut and mixed them thoroughly - No trace of streaks or other signs of incomplete mixing should remain.
Pour the mixed mold compound into the mold. It should ideally be poured into a corner and be allowed to fill and surround the pumpkin. The mold compound should be poured in a very thin stream from as high a height as you feel comfortable doing it. This technique helps to remove bubbles that were introduced during the mixing of the two component parts.
Step 6: Add Dowels - Index Marks
Because we are making a two-part block mold we want the two halves to fit together snugly and line up. To insure that the mold halves line up we add indexing features.
This can easily be done by taking 1/2 inch lengths of 3/8" dowel, spray cooking spray on them to make sure they can be removed and then imbed them half way into the mold compound. After the mold has set these dowels will be removed and create a void for mold material of the 2nd half to fill and create an interlocking detail.
Step 7: Let 1st Layer Cure
After pouring the first half of the mold let it sit for at least 75 minutes at a 73 degree fahrenheit ambient. I let mine sit for 2 hours.
I mentioned to calculate a little extra material for your pour. I poured the material I had mixed into my mold and realized that I was not going to come up to half cover the stem. I quickly pushed two stir sticks into the mold material and taped them to the side of the mold (they wanted to float). The added volume of the stir sticks displaced enough material so the mold compound came up to cover half the stem, how I wanted it. So lesson learned. Now plan on a little extra.
Step 8: Remove Dowels
After the material has cured the dowels can be removed. This leaves the voids, as mentioned earlier, that will be filled by the 2nd pour to create interlocking features.
You can see the stir sticks that were cured in place. I took the complete mold to the bandsaw and cut into the corners at a 45 degree angle and cut off the stir sticks. I taped over the slots in the side of the mold container to keep the mold material from leaking out. I had to wash out the sawdust before continuing. What a pain - sawdust sticks really well to fresh silicone mold material.
Step 9: Pour 2nd Layer and Let It Cure
It is important to make sure the 1st and 2nd pours don't stick together. I applied cooking spray over the top surface of the 1st layer and on the pumpkin. However, when I got to Step 10 and tried to separate both haves I could not easily separate them. I had to cut and tear the halves apart. So I am going to suggest that a thin layer of petroleum jelly (Vaseline) first be applied to the top surface of the first pour. Then, if desired additional cooking spray can be applied to the pumpkin.
Measure, mix and pour the second layer of mold compound. I used the remainder of the material so I did not have to measure. Luckily there was just the right amount of material for the second pour.
Remember to pour from a height to help remove air bubbles.
Step 10: Take the Mold Apart - Remove Pumpkin
After the mold material has cured, take the mold apart. I cut off additional material from the top of the bottle (mold) as this necked down area acted as a undercut and needed to be removed in order to get the block of material out of the remainder of the bottle (mold).
I cut a hole in the bottom to allow air in so the block would not be held in place by vacuum. Cutting down the side of the bottle freed the block of material from the bottle.
Separate the halves and carefully remove the pumpkin.
Now you have an empty two-part mold. Let's put something in it........!
Step 11: Prepare Mold for Final Pumpkin Pour
Apply Petroleum Jelly (Vaseline) to the inside of the mold cavity. Use the cotton swabs to get Vaseline into the stem area.
Put the mold back together and hold it together will lots of rubber bands.
I covered the area around the opening of the mold with masking tape to keep excess polyurethane from sticking.
Step 12: Try a New Source for Polyurethane
I have used two-part polyurethane before for a different purpose and was keen on trying it in a mold.
I was also intrigued to see if the new post hole filling material I found could be used for crafting. The material comes as two liquids packaged separately in a two compartment pouch. When ready to use for post holes the bottom pouch of material is squeezed until the barrier between the pouches bursts and the materials flow together and are quickly mixed (kneeded for 15 seconds max) and poured into the post hole.
DO NOT SQUEEZE THE POUCHES. DO NOT MIX THE LIQUIDS as instructed for Post Holes!
We will be 'harvesting' the resin components by cutting off the corners of each pouch separately and pouring out the resin components into glass jars. Cut a corner off of one pouch and pour the contents into a glass jar. I used a cardboard tube to act as a squeegee to try to get all the material out. Cap the jar. Repeat for the other pouch, pouring its contents into an empty jar.
From the amount of material removed it looks as if the ratio is about 1 to 1 by volume. There was slightly more material in the bottom pouch, but I think this may be to insure equal mixing by allowing residual material to be left behind in the bottom pouch.
The last photo is of a test mix of about 35ml of each material ~ 70ml total. It completely overflowed the cup and completely engulfed it. I decided to use much less resin for the pour into the pumpkin mold! :-)
Step 13: Mix and Pour Polyurethane
I used approximately 12 ml (cc) of both resin and hardener for a total of approximately 24ml. I marked my measuring cup with two equal volume lines. I poured one material up to the first line and then added the second material to take the volume up to the second line. I then mixed quickly for 15 seconds and then poured the material into the mold.
As can be seen by the last photo the foaming polyurethane exceeded the volume of the mold, which was desired.
Step 14: Let Polyurethane Cure and Remove the Faux Pumpkin
I let the polyurethane cure for 30 minutes to get hard, even though the post hold instructions state it will be hard in as soon as 3 minutes. My experience was that the resin was still tacky and soft at 3 minutes. After 30 minutes the resin was cured hard. Cut the excess foam material off and open the mold. Remove the faux pumpkin.
The polyurethane was pretty frothy and could be crushed pretty easily. Even so they could still be useful for making lightweight pumpkins for use on a wreath.
I will have to try using the polyurethane at slightly cooler than room temperature. If my memory serves me, the finished foam should have smaller bubbles and have more structural strength.
I will have to try using plaster of paris to make some decorative pumpkins - maybe even gold and silver ones.
Additionally, I will have to try mixing up some epoxy and swirling it around in the mold until it sets up and creates a hollow epoxy pumpkin.
Candles? Cast mortar? Etc., etc., etc.
Lots of opportunities to experiment with a mini faux pumpkin mold!
Runner Up in the