Rotary

 

Co-injection

 

Molding

 

 

Romano Discacciati

 

 

Summary

 

This paper describes the advantages of the rotary, multi-station,

co-injection molding press for high volume molding of thick parts.

 

The co-injection process is reviewed, and the issues of the use of recycled or regrind material, thickness and appearance are addressed with examples of actual applications.


Co-injection FAQS

What is co-injection?

It is the simultaneous injection of two compatible thermoplastic materials where one material engulfs or encapsulates the other. This is accomplished with the use of two separate injection units and a single injection nozzle. The two materials flow into the mold together as skin and core and maintain this configuration throughout the injection. This process should not be confused with two-shot or two-color molding where two colors and/or two materials are injected separately.

 

How old is co-injection?

This technology is more than fifteen years old. Imperial Chemical Industries holds the original patents for this molding process. The pioneer developers of co-injection machines were Germany's Schloemann-Siemag and Italy's Presma.

 

Why use co-injection?

  • In thin wall parts, it is mostly used for technical reasons such as shielding or combining a soft skin with a rigid core or for using prime material with off-grade or scrap.
  • In thick wall parts, all of the above reasons are applicable as well as additional benefits from the physical and mechanical properties and the cost savings possible with a foamed core.

 

What are the main advantages of co-injection?

  • The skin can be hard and shiny with a Class "A" finish, or even provide a different surface such as a "soft touch" material.
  • Problems of shrinkage, sinking, warping and internal stress are greatly reduced.
  • The inferior materials of the core do not flow in direct contact with the walls of the mold which facilitates their use in the molding. For the same reason, difficult mixes such as material loaded with wood flour and fiberglass can be easily injected as the core with minimal shrinkage problems.
  • In some applications, incompatible materials may even be used in spite of the serious processing problems they pose. This is possible because the pressure exerted by the foamed core on the skin is such that an acceptable mechanical bond can be achieved in substitution of a chemical one.


Co-injection FAQS

What are the principal cost savings with co-injection?

  • As only the skin requires prime material, the core may be off-grade, recycled or otherwise inferior material, as much as 50% by weight.
  • Colorant is required only in the skin and not in the core.
  • The fact that the core is normally foamed represents a savings in the total weight of the part of approximately 15%.

What are the royalties, fees or licenses required to use this process?

  • There are none.

     

What about tools?

  • Most conventional existing tools can be used as is, or with some minor modifications. Multi-cavity molds must be balanced. Since this is a low pressure system, new tooling is typically more economical as aluminum and less expensive grades of steel are the materials of choice.

Which machine: single station or multi-station?

  • Thin wall parts with low volume requirements are best served by single station injection molding machines.
  • Multi-station machines are a must for producing thick parts and meeting high volume requirements combined with the possibility of highly varied production. Multi-station means multiple molds which can produce different products at the same time.


The Equipment :
  • Low Tonnage
  • Low Pressure

Single Station

Co-injection machines are derived from traditional high pressure, high tonnage injection molding machines. They are slightly reduced in tonnage, and an additional injection unit is added. This make-up explains why some of our colleagues have said that co-injection machines cost 56% to 73% more than conventional presses.

 

Multi Station

Co-injection, on the other hand, derives from structural foam with a completely different concept involving large platens, large injection units but a small clamp with low tonnage. Here, the make-up involves only a small increase in tonnage and the additional injection unit. The increment in the cost of the press is only in the 18-35% range.

 

Typical configurations:

A
Single Station
Multi Station
Injection capacity
4 + 4 pounds
4 + 4 pounds
In between tie bars
32 inches
32 inches
Clamping Force
500 Tons
200 Tons

 

B
Single Station
Multi Station
Injection capacity
25 + 25 pounds
25 + 25 pounds
In between tie bars
55 inches
55 inches
Clamping Force
1500 Tons
650 Tons


The Equipment :
  • Low Tonnage
  • Low Pressure

In conventional injection molding, high pressure is utilized to fill the mold cavity. In co-injection, the material is injected with low pressure, and the cavity is filled by foaming the core material which pushes the skin. Just as high pressure is not required to fill the mold cavity, the tonnage of the clamp need not be elevated. In conventional molding, molds are typically cooled to the point where sometimes material freezing can be an issue. Again, high pressure is required to make sure the material flows.

On rotary, multi-station machines, on the other hand, because cooling continues on the carousel after injection, cooling time is less critical. For this reason, molds are not normally chilled to the maximum, and the material flows smoothly into the cavity. Thus pressure and tonnage can be maintained low.

Multiple stations - 4, 5, 8, 10, etc. allow the processor to manufacture different parts or components of an item on a single machine with the same basic operating cost of a conventional molding machine. The production of these components on the same machine facilitates subsequent assembly of the final product.

This also applies to the simultaneous production of completely different items which facilitates just-in-time inventory management.


The Application :
  • Toilet Seats
There are a number of different types of toilet seats available on the market. These range from the very flimsy and inexpensive to top-of-the-line heavy, solid and shiny seats. Here, we are going to consider co-injected seats and covers - an alternative that encompasses all the features of a premium product and is meant to replace polyester coating. These seats are manufactured with a mixture of ABS and SAN resins in order to produce a hard, shiny skin which engulfs the foamed core. The characteristics of the skin make the product solid, glossy and resistant, and no paints or varnishes are required for finishing. The foamed core gives the seat thickness and weight with less material. The closed cell core does not absorb humidity, nor is it adversely affected by salts, acid and grease. Finally, this product is 100% recyclable.

Let us consider then the production of high-end toilet seats and covers with the following characteristics:

Skin: SAN 50% by weight
Core: foamed ABS/SAN 50% by weight
Maximum thickness: 18 mm (7/10 of an inch)
Total weight: 3.3 kgs. (Aprox. 7.2 lbs.)
Cooling time: Aprox. 6 minutes
Expected production:
  • 5 station:
42 pieces/hour;
  • Single station
10 pieces/hour


The Application :
  • Toilet Seats
Single Station
1 Machine + 1 Mold + 1 Robot
Capital Investment = 100
Production: 10 pcs/hr = 100
1 model only
5 Station Rotary
1 Machine + 5 Molds + 1 Robot
Capital Investment = 125
Production: 42 pcs/hr = 420
5 different models

As you can observe, with only a 25% additional capital investment, the processor is able to produce a greater variety of products and obtain a 320% increase in production. Naturally, by reducing the weight and thickness of the product, the cycle will change, and single station output will increase.

However, assuming these modifications, the throughput also increases on the rotary machine, and a similar advantage will be maintained. Floor Space example:


The Application :
  • Brushes
This application is definitely in the high volume category, and these processors typically use 10 station, 33 or 55 ton machines. The tools are usually 4/8 cavities, and on average run on a 30 second injection cycle with a cooling time of 5-6 minutes. Based on the above assumptions, one can conservatively estimate a daily production of a minimum of 10,000 brushes which translates into an annual production of 2,500,000 based on 20 hours x 250 days. Gas assisted molding is not a suitable technology for this application because the bristles must be inserted into solid material and not holes. These manufacturers also tend to take full advantage of the individual programming of each station in order to produce a wide variety of different articles simultaneously.

A comparison with single station molding is not possible because the production economics of these thick parts have dictated the use of rotary presses for the past 20 years. Broom blocks or industrial foam moved to rotary injection in the early 1970's, and rotary co-injection is a natural outgrowth of this preference.

Co-injection has also allowed these manufacturers to move from the industrial cleaning markets into the home products market with the appearance characteristics possible in terms of "soft touch" or hard and shiny first class finish without need of lacquering. With paintbrushes, co-injection again provides distinct advantages in terms of the surface appearance which can be achieved, either hard and glossy or "soft touch".


The Application :
  • Lawn Furniture

There are indicators of a steady decline in the resin furniture market, but this observation needs to be clarified because there are really two distinct markets involved in this sector. One is the monoblock "cheap", thin-walled resin furniture, and the other is the high-end, multi-component co-injected furniture. Unfortunately, the image of cheap plastic has also affected the quality segment of the market. However, although several manufacturers of inexpensive furniture are going out of business, those few who have invested in co-injection, in spite of the difficulties, continue to grow. Why is that?

Take, for example, a high-end folding chair. This application is an excellent example of the possibility of the simultaneous manufacture of different components of a single item on a single machine to facilitate the subsequent assembly of the final product. This in addition to the benefits of the foamed PP core and Class "A" skin. A single station co-injection machine used to make large, thick parts which take a lot of time to cool could in no way compete with the cheap plastic. On the rotary press, on the other hand, with 4-6 minute cycles, we still get parts every 30-45 seconds.

A small, 8 station 80 ton machine could produce a set similar to the one in the picture in just two complete molding cycles. Each chair requires three molds, and half a table takes two. In two cycles, 4 chairs and 1 table can be produced. All of this adds up to high quality, high volume and high income for the manufacturer.


The Application :
  • Tabletops

Here again, the results which can be achieved with normal injection in a more or less economical fashion are flimsy, and the tops require extensive ribbing for rigidity.

Just a few months ago, going against the current of the decline in resin furniture, a leading German company invested heavily in rotary co-injection buying this "giant" press capable of a 55lb. shot. In this case, in the manufacture of tabletops, there is no ribbing, and the surface is clean and rigid.

Also, with the five stations, five different sizes of top can be molded in a single cycle, obtaining one tabletop every 120 seconds.

The tops range in size up to 70" x 39". Conventional tops often break because of shrinkage as they are vulnerable at the ribbing due to the behavior of PP over significant lengths. With rotary co-injection, on the other hand, even if there is shrinkage, the part is unlikely to break because it can withstand the internal stress due to its thickness.


The Application :
  • Tabletops

Single Station Co-injection - Continuous change of molds plus limited production due to cycle times of 7-8 minutes, or 7-8 pieces per hour. There is also the additional complication of the relative difficulty of the mold changes because of the auto insert and automatic pickup of the large tabletops.

Rotary Co-injection - 660 ton, 5 station machine with an approximately equivalent cost of a single station machine produces up to 30 tops per hour with few mold changes. There is less possibility of damage to the top surface as ejection of the finished article can be done from the injection side as the mold station rotates to the discharge position.

This dramatically decreases the cost of the molds. Only one robot handles the insert of the decorative top as well as the pick up of thedifferent molded parts. Just-in-time inventory management is possible with the variety of sizes which can be produced in one complete cycle.

As is amply evident from these examples of articles currently in production, rotary co-injection is an ideal solution for many applications.

Co-injection is attractive because of the material savings possible and the physical properties of the finished products.

 

Additionally, the rotary press provides a versatile, space-saving and economical solution for high output production of thick parts with long cooling times.


 

Romano Discacciati