Thin-wall injection molding

Thin wall injection molding is a specialized form of conventional injection molding. The thin wall injection molding process focuses on mass producing plastic parts that are thin and light so that material cost savings can be made and cycle times can be as short as possible. Shorter cycle times means higher productivity and lower costs per part.

The definition of thin wall is really about the size of the part compared to its wall thickness. For any particular plastic part, as the wall thickness reduces the harder it is to manufacture using the injection molding process. The size of a part puts a limit on how thin the wall thickness can be. For packaging containers thin wall means wall thicknesses that are less than 0.025 inch (0.62mm) with a flow length to wall thickness greater than 200.

Processing

Standard tool and melt temperature can also be applied when dealing with thin-walled parts. In order to reduce the filling pressure, however, it is usually recommended to increase melt residence time in the barrel, this can lead to critical material reduction. In order to avoid freezing effects during the filling process, the injection time is rather short. In case of cellular phone covers, standard injection times are less than 0.5 secs.

Markets

The trend towards thin wall molding continues to increase in many plastic industries as plastic material and energy costs continue to rise and delivery lead times are squeezed. 

The following industries make use of thin wall molding:

• food packaging ( eg. food containers and lids)
• automotive (eg. both structural and non-structural car parts)
• mobile telecommunications (eg. mobile phone housings)
• medical (eg. syringes) 
• computing equipment (eg. computer housings)

Benefits

• Cheap, safe and clean plastic parts.
• Thin wall molding reduces resource consumption and cuts weight, reducing fuel usage and carbon emissions in shipping – further supporting sustainability efforts.
• Allows faster cycle times compared with thicker walled plastic parts. This is good for injection molders because it reduces their delivery lead time and cost per part.
• Lighter parts reduce fuel emissions in automotive applications.
• Made from recyclable plastics such as polypropylene (PP) in food packaging.
• Some thin wall parts can be made from sustainable plastics.

Disadvantages

• Environmental litter.
• High capital investment cost for injection molders. Thin wall molding requires specialized molding machines, injection molds and robots that can withstand the high stresses, fast cycle times and relentless 24/7 production schedules.
• To make thin wall parts requires highly skilled molding technicians and these are difficult to find and keep.

Examples

Plastic resins suitable for thin-wall molding should have high-flow properties, particularly low melt viscosity. In addition, they need to be robust enough to avoid degradation from the heat generated by high shear rates (high injection speeds)

Some plastic manufacturers make plastics specifically for thin wall applications which have excellent flow properties inside the mould cavity. For example, plastic manufacturer Sabic, has a polypropylene food contact grade plastic which is specifically designed for thin wall margarine containers and lids.

Another plastic manufacturer Bayer, make a blend of Polycarbonate (PC) and Acrylonitrile butadiene styrene (ABS) specifically designed to make thin wall mobile housings.

Equipment

• Plastic Injection Molding Machine.

Compared to conventional injection molding, thin wall molding requires molding machines that are designed and built to withstand higher stresses and injection pressures. The molding machines computer control should also be precise in order to make quality parts. For this reason these molding machines are more expensive than general purpose machines.

Thin-wall-capable machines usually also have accumulator-assisted clamps to accommodate fast cycle times.

Regular maintenance schedules must be completed so that the machine and part quality does not suffer. These machines usually work 24/7 so they need to be well maintained. 

• Injection Mold Design

As with the injection molding machines, injection molds need to be robust enough to withstand high stresses and pressures. Heavy mold construction with through hardened tool steels will ensure a long lasting mold.

The mold must also have a well designed cooling system so that heat can be quickly extracted from the hot plastic part allowing fast cycle times. To achieve this, cooling channels need to be designed close to the molding surface. Cleaning the mould on a daily basis is also a critical requirement to maintain the part quality.

                                    

STANDARD VS. THIN-WALL PROCESSING
Key Factors	Conventional	Thin-Wall
Typical Wall, in.	0.080-0.120	0.050-0.080	<0.050
Machinery	Standard	High-end	Custom
Inject. Pressure, psi	9000-14,000	16,000-20,000	20,000-35,000
Hydraulic System	Standard	Standard	Accumulators on injection & clamp units. Servo valves.
Control System	Standard	Closed-loop on injection speed, hold pressure, decompression speed, screw rpm, backpressure, and all temperatures.	Same as at left, with resolution of 0.40 in. on speed, 14.5 psi on pressure, 0.004 in. on position, 0.01 sec on time, 1 rpm on rotation, 0.10 ton on clamp force, 2° F on temperature.
Processing
Fill Time, sec	>2	1-2	0.1-1
Cycle Time, sec	40-60	20-40	6-20
Tooling	Standard	Better venting, heavier construction, more ejector pins, better polish	Extreme venting, very heavy construction, mold interlocks, precise surface preparation, extensive ejection features, mold costs 30-40% higher than standard.