IN MOLD GRAINING (IMG) – Automobile Interior Manufacturing Technologies

Brief introduction of the IMG process

The IMG (In Mold Graining) process uses a nickel shell mold to form a customized textured skin by vacuum adsorption and then adsorption the skin to the injection molded plastic parts together. IMG is divided into two types. The first one is vacuum adsorption skin molding (IMG-S). This process mainly shapes the shape and texture of the skin of the part. Similar to the slush molding process. The second is the vacuum adsorption of customized skin to the composite of the molded plastic parts together(IMG-L). This process forms the textured skin and lets the skin and plastic parts adsorb together at once. Currently,  IMG-L is more popular because of its high efficiency and less investment in equipment. IMGS needs an extra wrapping process to compare with IMGL.

Application of IMG

IMG technology is mainly used in automotive interiors, such as instrument panels, upper door trim, door middle trim, door armrests, and other products.

IMG process flow

IMG tool making flow

The requirements of the IMG process for the plastic parts:

1. Common materials for injection molding skeletons include PP and ABS+PC. The injection molded parts of PP materials must be flame treated before use. ABS+PC does not require flame treatment.
2. There must be vacuum holes on the molded skeleton, the diameter is generally 0.8 ~ 1.2mm, the spacing on the large surface can be appropriately larger, and the corners can be denser. Vacuum holes can be drilled manually after injection molding or made during the injection molding. The diameter for holes are 1.2mm.
3. The surface of the injection molded parts must be textured; the pattern’s depth is 5~100m, which is good for bonding and air exhaust.

IMG’s requirements for product shape and structure:

1. Shape

The product should not be too deep, the surface should be relatively flat, and there are no areas that need to be stretched too much. Otherwise, the skin will be easily thinned or broken. When the skin does not have a back foam, the stretching amount is recommended to be around 150%, and when there is a back foam, the stretching amount is preferably around 120~140%.

2. Undercut

When applying the nickel shell to vacuum adsorption manufacturing(IMGL), the product cannot have undercuts, and the minimum draft angle must be 7 degrees. Otherwise, it is easy to have defects such as wrinkles and dark lines. While for IMGS, a certain degree of undercut is allowed, and the aspect ratio of the undercut area is at least 3:1.

3. Radius

The radius of the product is greater than 1mm.

IMG process requirements for molds:

1. Classification:

IMG-S: the cavity textured is the upper mold, and the lower mold is the core without any mechanism.

IMG-L: The cavity textured is the upper mold, and the lower mold has mechanisms such as clips or suction cups to fix the plastic parts.

IMGS make the skin and can sew natural line on it. The stitching pattern by IMGL is a fake line.

2. Materials:

At present, IMG use nickel-shell molds.

Nickel is low in cost, good in strength, and has excellent corrosion resistance and thermal conductivity (its thermal conductivity is equivalent to 4 times that of stainless steel).

3. Mold temperature:

The shrinkage of the skin becomes more significant as the mold temperature increases.

When the skin is forming(IMGS), the upper mold temperature is 60-70℃.

When the skin is forming and compounding(IMGL): the mold temperature on the side with skin texture is 60-70℃, and on the other side fixes the plastic part, the mold temperature is generally 25-35℃.

4. Vacuum hole:

The diameter of the vacuum holes is generally 0.8-1.2mm, and the layout as a rhombus is staggered. The distance between holes is 30x30mm. More details as below.

The layout of holes for air exhaustion:

1, Distance 30mm*30mm；

2, Layout: Rhombus staggered arrangement；

3, Layout at R corner or unconventional shapes is 15mm；

4, Holes as designed to be tapered, diameter on the upper surface is 1.2mm；

Requirements of the IMG process for the skin:

1. Materials

Generally, two types of skins are used in the IMG process: TPO and PVC. TPO material has a low density (about 0.9g/cm^3), is lighter than PVC, and does not contain plasticizers and halogens. It is recyclable and has better heat resistance than PVC. The use temperature can reach 120 ℃.

2. Thickness

TPO: 0.5~1.2mm;

TPO+back foam: 2.0～3.5mm

3. Stretch

The IMG process requires the skin have good ductility after heating, to sag neatly when heated and to stretch evenly and flatly. Otherwise, it will be difficult to clamp the skin, or there will be wrinkles or cracks on the product’s surface after molding.

4. Heating temperature

The upper layer of skin heating temperature (TPO): the heating temperature is controlled at 170-190 ℃.

The lower layer of skin heating temperature (PP FOAM): the heating temperature is controlled at 130-150 ℃.

The relationship between IMG molding and skins:

1: The relationship between TPO skin and molding

1. The thicker the TPO, the better the molding effect. But the price is high and the heating time is long.
2. The skin thickness of the door panel is 0.5-1.2mm, and for the instrument panel is 0.8-1.2mm. The thickness of the skin with TPO+ PP FOAM is 2.6-3.5mm.

2: The relationship between PP FOAM (back foam) and molding

1. Back foam: 15 times > 20 times > 25 times, a standard foaming ratio.
2. 15 times foaming feels hard, takes a long time to heat up, and costs a lot.
3. The door panel uses 20 times foamed skin, and the instrument panel uses 15 times foamed skin.
4. After forming, the skin is stretched and thinned after being pressed. The higher the heating temperature, the shallower the formed skin pattern and the brighter the skin.

Selection of IMG craft glue:

1. Solvent glue, this kind of glue does not require very high temperature. It will be cured in about 4 hours after forming, and the bonding will be firm. The longer the time, the better the bonding effect. The disadvantage is that the smell is heavy, and some OEMs propose not to use solvent glue.
2. Water-based glue has a better odor grade than solvent glue but has high environmental requirements. After molding, it must be matured in a constant temperature and humidity environment for 72 hours to reach the final strength.
3. Hot-melt glue is non-toxic, tasteless, and does not pollute the environment, but the price is high, and the use process is complicated.

Summarize:

The automotive interior parts made by IMG process feel good and are environmentally friendly and efficient. With the weight reduction of automobiles, the use of IMG technology in automobile interiors will become more and more extensive.

Production line of IMG manufacturing:

Vacuum adsorption equipment for IMG manufacturing.

TDL IMG solution for automotive interior parts OEMs.

Advantages of the IMG process:

1. The skin texture is clear, and there is no stretch deformation of the skin texture, especially the fake sutures can be very realistic.
2. Compared with the slush molding process, the investment is less, and the quality is stable.
3. Different patterns can be formed at one time.
4. It can form smaller rounded corners than lamination.
5. The skin thickness is thick compare with low-pressure injection molding, parts have a better soft touch feeling.

Disadvantages of the IMG process:

1. There are limitations to the design of the product shape, and it is required that the stretching should not be too large, and there are also restrictions on the undercut.
2. The minimum fillet radius is required to be greater than 1mm.
3. The cost of the mold is high (compared with the lamination), and the manufacturing cycle is long.
4. After the mold is damaged, it is almost impossible to repair.
5. The life of the mold is about 400,000 shots. It is relatively short.

TDL began to apply IMG technology in 2018 and served hundreds of automotive interior OEM manufacturers during the four years. Our IMG technology is very mature. We know how to design and manufacture nickel shell molds and know the manufacturing difficulties and production advantages of IMG. If you are a manufacturer of automotive interior parts and are interested in IMG production, TDL can help you. We provide our customers with plastic injection molds and IMG nickel shell mold manufacturing.