Case Study – Popcorn Twist 

Services Used

Master Model:  SLA stereolithography

Mold Tool:  Liquid Silicone Mold

Prototype:  Vacuum Casting

Material Used

            Master Model:  SOMOS 14120

            Prototype:  High Temperature Polyurethane & ABS UP 4280

About The Product

The Popcorn Twist is a fun, innovative way to make perfect popcorn at home every time. Easy to use and requiring no electricity, the secret is in the stirring paddle, which continuously moves the kernels around to avoid burnt or un-popped kernels. Using vacuum casting, the team at Popcorn Twist were able to find the best way to make and test their new product and turn this great idea into a reality.

About The Client

The Popcorn Twist team is located in Germany. After getting sick of burnt, bland, half-popped microwave popcorn, they decided to invent their new idea. After two years of design work, they’re now ready to share with the world their idea for a better way to make popcorn.

popcorn twist

Not satisfied with other prototyping services available in Europe, I reached out to Star Rapid. In one word, I would describe Star Rapid as ‘professional.’  I noticed that they had already worked on a similar prototyping project like the Loo Blade, so I felt I could trust them.  We needed a prototype quickly for a Kickstarter campaign and their response was very fast.  It was also great to work with their representative, Patrick Neikes, who’s based in Germany, as well as the rest of the team. I’m 100% satisfied and I wouldn’t change a thing!

Gottlieb Plesniarski

Product Developer, Popcorn Twist

About The Project

The team at Popcorn Twist asked us to help them make a 3D printed master model, from which we could then create a silicone mold for vacuum casting prototype copies. We were excited about this project (not just because we love popcorn) and up for the challenge of helping them manufacture their product while staying true to their design requirements.

Customer’s Manufacturing Requirements

  • Must have a production-quality fit and finish
  • The handle and the star-shaped stirring paddle would be made from separate molds
  • All plastic resins needed to be food-safe, high-temperature and strong
  • Two different types of material would be made to test different versions in different colors
  • Inexpensive tooling, easy to modify if necessary

How It Was Made

Making The 3D Printed Master Model

Vacuum casting requires the use of a master model, from which copies are then made.  We created the master by taking the client’s CAD data and 3D printing it using stereolithography (SLA).  The digital file was segmented into many separate 2D profiles, which were slowly built up one layer at a time inside a photo-curing liquid bath.  To make the master model we used SOMOS 14120 resin, which created a solid part with good mechanical properties.  Since vacuum casting was going to be used to make the prototype copies of this master we needed to ensure we took great care to sand and finish the master by hand in our model shop.

handle makred

Once the master model is made, a silicone mold can be made to prepare vacuum casted copies.

Making The Vacuum Casting Mold

The vacuum casting mold is built around the master model.  We will use it to make replicas of the original.  Molds are made one-half at a time, each half cured in an oven at 70° C for 15 hours.  It’s important at this time that our technicians carefully arrange the location of gates (for filling the mold cavity) and overflows (which let excess plastic and trapped air to escape).

Because this is a relatively long and thin piece, the mold was separated both horizontally and vertically for easier part removal.

Completed prototype handle and vacuum casting mold.

Making The Vacuum Casted Copies

Step 1: Filling The Mold

Pouring spouts are used to fill the mold cavities

Similar to plastic injection molding or pressure die casting, vacuum casting uses liquid resin poured into an empty cavity (the vacuum casting mold) to make prototype copies.  The first step is to fill this mold, using simple filling spouts that our technician removes after the part is cured. Vents inside the mold help air to escape so that the cavity is filled completely.  This air will later be evacuated out of the mold when it goes into the vacuum chamber.

Step 2: Unsealing The Prototype

Removing the mold pins

We used pins and staples to hold the mold together while the resin was cured in the oven. We also used high-temperature tape to seal all sections of the mold tightly together.  We were very careful when unsealing the mold, so as not to damage the soft silicone which can be used many times before it must be replaced.

Step 3: Opening The Mold

Opening the stirring paddle vacuum mold

Many kinds of liquid resins can be used for vacuum castings, from very soft elastomers to hard engineering-grade plastics.  For these copies, we tried polyurethane PX223 resin in black.  This plastic is safe for food handling and will resist the high temperatures involved in making popcorn.

Notice the fingers of plastic that have fanned away from the paddle, through the overflow vents.  Vacuum pressure will remove tiny air bubbles even in very narrow and thin features inside the mold.

Reusing The Mold for Up to 50 Prototypes

Notice the wavy cut pattern on the different mold sections.  This ensured they were in the correct alignment for every casting, creating identical finished parts each time. Cutting the mold in three sections also helped to release the handle without damaging the mold.  With care, a silicone mold can make 50 or more copies before it must be replaced.  Of course the master model can be used to make an indefinite number of vacuum casting molds.

open mold handle with mark

The handle is carefully opened in three sections

Testing Clarity of Part Features

Vacuum casting provides excellent fidelity even to small part features.  Notice the clarity of the “twist” logo on the handle, and how it was correctly reproduced in the casting.

Detail of the vacuum cast handle

Detail of the vacuum cast handle

Testing Different Colors

Another advantage of vacuum casting is that it’s easy to test out different resins in various colors.  The orange stirrer paddle was cast in a high-temperature polyurethane plastic.  This is a great way to go through several design iterations to find the look and feel that’s just right, which is exactly what you want a prototype for.

Two stirrer paddles in different colors and plastic compounds.

The Finished Popcorn Twist Prototype

We were able to meet the client’s ambitious schedule for their product launch, exceeding their expectations. Fast response times, with great quality at an unbeatable price.  That was true for this successful product development, and it will be true for you too when you contact us for a free quotation and project review.  Let’s get popping today!


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