"Magic" of injection molds

There is a type of injection mold, called a multi-cavity mold, which occupies half of the entire plastic product field. Demand for these plastic products is very large, some in tens of millions, some in hundreds of millions. For example: plastic hangers, plastic cheese sticks, plastic cutlery, mask buckles and so on. Even nucleic acid detection kits and nucleic acid detection sampling sticks that we often come into contact with now are in tens of billions.
Under same production conditions, under premise of maximizing quality of plastic products, how to reduce production cost of these huge quantities of plastic products is dream of plastic engineers, mold designers and entrepreneurs in injection molding industry, but reality is difficult.
Because in production process of these plastic products, there are many constraints. I call it "magic".
Let's take a look at what these "magic" are all about.

As we all know, injection molding process is to quickly and efficiently transfer molten plastic into mold cavity, continuously pressurize (hold pressure) and cool molten plastic, so that molten plastic solidifies into a shape similar to mold cavity, so as to obtain plastic products we need. In order to reduce production cost of injection molded products and reduce loss of plastic raw materials due to gates and runners, today's injection molding factories all hope to use hot runner technology, which is very good for large plastic products and beyond reproach, but for large and small plastic products, this is a bit difficult, such as: toothpicks, mask buckles, cheese great wait.

 

Due to increasingly high labor costs and raw material costs, planners of plastic products factories are walking a tightrope, making difficult choices between cold runners and hot runners. Let me give an example of a business that makes plastic cheese sticks, products and production molds are as follows.

 

 

This mold is a mold with 80 holes and uses a needle valve hot runner with 80 points. Boss of factory has been complaining that he can't make money because processing fee is too low and needle valve hot runner mold is too expensive. They originally used a large nozzle mold, but there was too much waste in gate and runner, plus labor costs, it was a loss-making business.
Later, it was changed to a needle valve type hot runner, which can be fully automated 24 hours a day, and there is no gate waste, and result is still a loss. Reason is that cheap hot runner parts are used, and there are too many downtimes for maintenance. Later, they contacted my team, and we took a plan to turn losses into profits, which will be described in detail later.

In minds of most mold designers, gate balance of a mold with multiple cavities is an iron law. If it is not balanced, in order to continue to fill underfilled mold cavity, it is necessary to increase injection pressure. This is where problem arises, as those cavities that are already filled with molten plastic continue to be filled with molten plastic under high pressure, resulting in flash.
Regarding this principle, many plastic scientists have conducted in-depth research. Through patent search, we can see that there are many patent documents in this area. The most impressive is the "inner and outer circle theory" proposed by Mr. Xu Changyu, a famous plastic scientist in Taiwan, which has formed a deep-rooted concept in minds of new generation of mold designers. Let's take a look at ranking of such a cavity.

 

Seeing ranking of this mold, everyone is very distressed, because effective area of mold parting surface is divided into a large part by gate, which is direct cost. In order to balance gate, it is necessary to achieve both number of cavities and balance of gate, and a larger injection molding machine must be selected. This is underlying thinking logic of mold designers.
After years of research, my team has broken this "magic". How can we have both? There will be a detailed introduction below.

This problem is easy to understand, because labor cost of dealing with gates is too expensive now!
In the 1970s, injection molding technology was just introduced to China. At that time, labor was not worth much, and products with multiple cavities in one mold were filled with glue from side of large nozzle, then gate was manually cut. Because plastic raw materials at that time were very expensive, although point gate can eliminate need to manually cut gate, but waste plastic produced by point gate is a pity. There is also a gate scar on product.
Wheel of history rolls to 2022. Plastic raw materials are more expensive. Unexpectedly, labor costs are also very expensive. What should we do?
In order to break this "Magic", my team made unremitting efforts to create "in-mold extrusion", and there is no trace of gate on product at all. There will be a detailed introduction below.

This sentence is a famous saying in mold design, how can it become a "Magic" again?
In order to maximize output of injection molded products under same production conditions, this is fundamental way to reduce production costs, so emergence of stack molds has become inevitable. Then problem comes. In injection molding machine of same size, mold space is a very sensitive issue. Let's take a look at a typical set of stack molds.

 

This is a mold for a set of antigen detection kits. There are two layers in total, with 32 holes in each layer. As can be seen from picture above, thickness of front mold of mold is only 80 mm, length and width are 600X500. It looks very wronged. I changed it to my own design and wanted to increase thickness of mold plate, but conditions are not allowed! A limitation of injection molding machine: maximum mold thickness is a fixed value. Conventional injection molds must have structure of mold plate (B plate), die foot (support block), ejector plate, push plate, mold bottom plate, and ejection space. No one is missing, and no board is thin.
In following content, we will introduce a new concept, a mold without mold feet and thimble plates, to break this Magic.

 

In fact, in real work, there are still many constraints that affect control of production costs. Let's take a look at above-mentioned major "Magics" and see what black-tech tools are in our toolbox that can break these "Magics".

In fact, main consideration for hot runner problem is to reduce waste of plastic raw materials and cost of processing nozzle materials due to runner, as well as secondary dust pollution generated thereby.
Our countermeasure is to adopt a very effective technology, name of patent is: In-mold dust-free crushing technology. Let's see what's going on here.

 

Above picture is a simplified schematic diagram, which is a part of upper flow channel of mold. We set a special-shaped push tube and a matching push tube needle in runner.
When molten plastic fills cavity of mold, runner is also filled with molten plastic.

 

At the moment when injection molding machine completes injection action, plastic is completely in a molten state, special-shaped push tube rises. Distance of rise is thickness of gate, and molten plastic is isolated into many small particles.

 

Solidified small particles are ejected by push pin

 

Then, these small particles are directly sucked into feeding system of injection molding machine by specially set collection system, and are mixed with new material in proportion for injection molding.
We often encounter situation where weight of gates and runners far exceeds weight of product. This technology avoids waste of plastic raw materials caused by small cavities and many runners in some products, and also avoids investment of expensive needle valve hot runners. For large-scale production of one-mold multi-cavity products, economic benefits can be imagined.

Let's take a look at picture below, which is a simplified product. Product size is: 50X12X3 (mm) single-layer 64-piece arrangement.

 

Isolator is set on 400X300 mold plate. When not injection molding, top surface of isolator is flat with shape of product

 

Just after injection molding is like this, plastic is completely molten.

 

At this point isolator rises to isolate large sheet of molten plastic into 64 individual pieces

 

After cooling and solidifying, product is ejected from mold. In this process, there are no gates and cross runners, let alone balance of gates, and no hot runners are used.

 

Glue inlet of a multi-cavity plastic product can be divided into two categories from outside, hot runner and ordinary runner.
Glue inlet of hot runner can be divided into three types: large gate, point gate, and needle valve gate.
Gates of ordinary runners can be divided into direct side entry gates, point gates, and latent gates. Our technology is evolved from a classification of latent gate: latent pin gate.
Let's look at following two pictures

 

 

Principle of submerged needle gate is to add a cylindrical waste to product. During injection molding, molten plastic enters cavity of product from this cylindrical space through latent gate. After molten plastic cools and solidifies, it becomes a product and is ejected from the mold, then find a way to remove cylinder.
Principle of in-mold extrusion gate is that after molten plastic fills cavity through space of cylinder, needle of a cylinder in lower part of cylinder rises rapidly while plastic is still molten, so that cylinder ( Top surface of needle) is level with shape of product, and molten plastic in original cylindrical space is extruded into mold cavity of product, which appropriately increases density of product, has certain benefits for improving shrinkage and deformation of product.
Of course, top surface of needle of cylinder can also be made into any shape required by product, which is in line with appearance of product.

 

Core of this technology is to add an ejection system in mold. When molten plastic just fills cavity, new ejector system forces needle, whose top surface is in line with bottom surface of product, to advance rapidly until it is level with bottom surface of product.
Of course, added ejector system can use function of injection molding machine (some injection molding machines can be customized), or mold itself can come with this system, which will be introduced in detail in following chapters.
What we want to point out in particular is that size of feeding port of common latent gate is very important, which directly affects molding quality of product, is also affected by balance of the gate. Here, plastic inlet is made relatively large. Since plastic is isolated from product when it is in a molten state, there is no need to worry about problem of cutting after solidification.
Black Technology 4: Injection mold without mold foot and ejector plate
Since traditional injection mold must have a product ejection space, this space is generated by mold foot to empty movable mold B plate. When movable mold B plate of mold is relatively large (mold B plate with one mold and multiple cavities is often large in area), rigidity of movable mold B plate is very sensitive, and a little carelessness will produce flash.
Calculation formula of "simply supported beam" in material mechanics is related to 4th power of span of die foot and thickness of formwork.
Our current method is to cancel die feet, let movable die B plate directly on die bottom plate, and make some corresponding pneumatic or hydraulic mechanisms on die bottom plate to directly support it where it is needed.
Please see picture below

 

 

 

 

This function can be customized for many brands of injection molding machines. After injection molding machine completes injection molding action, injection molding machine can start pre-melting glue while mold is cooling and other actions are in progress. This simple improvement can effectively shorten injection cycle, which is a very good measure for one-mold multi-cavity products aimed at improving production efficiency.

Realization of several "black technologies" I mentioned earlier has a premise, that is, after injection molding machine is completed, when plastic is still in a molten state, mold needs to use an external force to achieve an action, pneumatic or hydraulic, which depends on function of injection molding machine. I used to change neutron circuit of injection molding machine, or use an external pump station to achieve it. Now there are injection molding machine companies that are willing to provide this function.
Actual case one
Nucleic acid detection sampling rod mold with 320 cavities in one mold

 

Product: Nucleic acid detection sampling stick
Plastic: ABS
Injection molding machine: domestic 168-ton machine
Number of cavities: 320
Injection cycle: 10 seconds

Hour output: 2.7648 million

 

 

 

Actual case two
Disposable plastic spoon with 64 cavities in one mold
Product: Disposable Plastic Spoon
User: A company in Ningbo, Zhejiang
Plastic: food grade PP
Injection molding machine: domestic 250-ton machine
Number of cavities: 64
Injection cycle: 8 seconds

Hour output: 691,200 pieces

 

 

 

Actual case three
Magic one mold 36 cavity plastic hanger mold
Product: ordinary plastic hanger
Plastic raw material: PP
Number of cavities: 36
Injection molding machine: domestic 250 tons
Injection cycle: 15 seconds
Output per 24 hours: 524,000