Using a Pet Food Extruder to Form Treats and Soft Chews

A pet food extruder for treats, soft chews and pellets is a single-screw machine that compacts the material, pushes it through a die plate and cuts it to length at the die. Recent enquiries to our test room asked about soft chews, pet food pellets and fish feed, and each one is at bottom a forming question. The dough has to feed steadily, build enough pressure to fill the die, and hold its shape once it leaves. Whether your recipe does all three on a single-screw machine is something a trial on your own material can settle before any capital is committed.

Pet food extruders: forming and cooking machines do different jobs

Two kinds of single-screw machine turn up in pet food, and the first decision is which one your product needs.

A forming extruder compacts a dough or paste and shapes it at the die, adding little heat beyond what the screw’s own friction produces. It suits shaped treats and soft chews, where the recipe is already mixed and the job is to give it a consistent cross-section and length. Our food forming extruders page covers the range.

A cooking extruder heats the material under pressure inside the barrel, so starch cooks and the product sets or expands as it leaves the die. Extruded dry dog food is made this way, which is what the term "extruded" means on a kibble bag. Cooked or puffed pellets belong here too, and our cooking extruders page describes those machines.

Some products need both a cooking step and careful forming, and the boundary is not always obvious from the recipe. Extrusion temperature follows from that choice. A forming run stays cool enough that fats and sugars do not soften the dough past the point where it holds its shape, while a cooking run is hot by design. The right setting for your recipe is found in a trial, not read from a table.

What decides whether a soft chew will feed and hold its shape

Soft chews are difficult because they are sticky, and stickiness shows up in three places.

In the hopper, a sticky dough bridges over the screw opening or clings to the walls, so the screw turns without being filled. The feed then surges, pressure at the die swings with it, and the strands come out uneven. A live bottom hopper, which uses a moving floor to carry material to the screw, is one option for this kind of material, and a trial shows whether it is needed. Moisture matters here as well. A dough that is too dry will not compact into a solid plug, and one that is too wet gives little resistance for the screw to build pressure against.

At the die, pressure depends on how well the screw compacts the dough and how much resistance the die plate offers. Temperature moves it directly, because a warmer dough flows more easily and builds less pressure. Warming the dough can cure a feeding problem and create a shape problem further down the line.

After the die, a soft dough that has been warmed to flow well can slump or sag before it is cut or cooled. Air trapped in the dough is a further complication, since it appears as voids in the finished chew. A vacuum extruders page explains how de-airing works for materials where that matters.

The die plate and cutter set the treat

Once the dough feeds consistently, the die plate and cutter decide what the product looks like. Hole size and shape set the cross-section. The distance between holes matters more than most formulators expect, because soft strands that leave the plate close together can touch and fuse before they are cut. The length of the channel through the plate adds back pressure, and that affects both how dense the product is and how smooth its surface is.

The cutter sets length from the relationship between its speed and the rate at which material leaves the die. A sticky chew can drag on the blade or stick to it, so the cutter setup is worth testing with the real dough rather than assuming it from a similar product. Our dies and cutters page lists the options we build.

Pellets for pet food and fish feed, where drying is part of the process

A pellet is not finished when it leaves the die, and the process has to be designed with that in mind. We saw this in a published trial on marshmallow pellets, run on a cooking extruder. Drying and chilling the pellets after the die made them float, which tells us that the product leaving the machine and the product in the bag are different things. Moisture and structure are still changing downstream, and so is the weight of a pellet per unit of volume.

The same applies to pet food pellets and fish feed. If a pellet has to float or sink, or has to stay intact in water or in the bowl, the die, the cooking conditions and the drying step all contribute to that result. Treating drying as a separate problem to be solved later usually means redesigning the front of the line. A trial that includes the post-die steps gives you the whole picture.

What a trial on your recipe tells you

In a feasibility trial we run your actual material on a laboratory machine and, where it is useful, on a production-size machine. We watch how the material feeds, what pressure it builds, how it holds its shape at the die and how the cutter handles it. You then receive a written report on what worked, what did not and what is still unknown, before anyone commits to equipment. If a result is partial, the report says so, and it says what the next trial would need to test. Our feasibility testing page describes the process.

It helps to bring a few facts with you:

• The shape, size and length you need, and the output you want per hour.
• Whether the product is cooked or only formed.
• The moisture and fat range of the mix as it will reach the hopper.
• Any known trouble, such as sticking, slumping or inconsistent feed.

Send us a sample of your material, or write to ask whether a trial would settle your question. You can reach the test room at The Bonnot Company, 1301 Home Ave., Akron, OH 44310, on (330) 896-6544 or at info@thebonnotco.com.