IN MOLD GRAINING (IMG) –Automobile Interior Manufacturing Technologies

Table of Contents

TDL is a plastic injection mold supplier. We support worldwide automotive manufacturers with plastic mold manufacturing for exterior parts like automotive bumpers, grilles, fuel fittings, engine covers, water tanks, air tanks, lamp housing & shelter, etc. We also support automotive interior parts manufacturers with IMG tools for soft-touch feeling interior parts manufacturing, such as soft instrument panels, door trims, door armrests, IP trims, etc.

According to the different surface material structures of automotive interior parts, interior parts can be divided into three categories: hard, semi-hard, and soft. With the continuous improvement of consumers’ requirements for comfort, soft interior decoration has gradually become a trend. It mostly appears in high-end cars that pursue comfort, beauty, and luxury craft. Currently, soft skin automotive interior parts manufacturing technology on the market are in-mold graining (IMG), vacuum wrapping, laminating, PU spray, slush molding, low-pressure injection molding, etc.

MaterialInjection molded plastic partInjection molded plastic part + SkinInjection molded plastic part + foam + skin
Manufacturing TechnologyPlastic part: Injection moldingPlastic part: Injection moldingLow pressure injection moldingSkin wrapping: Hot pressing / laminating, vacuum wrappingPlastic part: Injection moldingFoam+Skin wrapping: In mold graining, PU spray, slush molding, manual wrapping, hot pressing / laminating,  edge pressing, etc
Skin/Leather, PVC, TPO skin with PP-Foam, PUR, knitted fabric with non-woven fabric, fiber felt, textile material, PULeather, PVC, TPO skin with PP-Foam, PUR, knitted fabric with non-woven fabric, fiber felt, textile material, PU
Mold/toolInjection moldInjection moldTool for vacuum adsorption or hot pressingInjection moldNickel shell moldTool for hot pressingTool for slush molding
EquipmentInjection molding machineInjection molding machineVacuum adsorption machineryOr hot pressInjection molding machineIMG vacuum adsorption machineryOr hot press

In-mold graining (IMG), low-pressure injection molding, and skin wrapping technologies are essential in producing soft-touch automotive interior parts.

What are they?

  1. Low-pressure injection molding. This process combines the interior skin and plastic parts directly by injection molding.
  2. IMG solution includes IMGS and IMGL. IMGS only makes the textured skins of the interior parts. Designers will add stitch patterns on the skin to achieve more applications. Also, to make the automotive interior parts softer, designers will add thick PP foam between the skin and molded plastic parts. IMGL customized the skin and combined the skin with the plastic parts.
  3. Wrapping technologies. Whether IMGS makes the skin, PU Spray, or slush molding, combining the skin with the plastic parts is necessary through various skin wrapping technologies. The wrapping process includes manual wrapping, vacuum adsorption wrapping, hot pressing (lamination), edge pressing wrapping, etc.

Low Pressure Injection Molding

Low-pressure injection molding is a process that injects melted plastic into molds with very low injection pressure and rapidly solidifies plastic and fabric together. In the automotive industry, this injection molding process is used to manufacture some automotive interior parts.

Low-pressure injection molding process for making automotive interior parts:

Using a minimal injection pressure (300-600Bar), add PVC skin or knitted fabric to the injection molding process to complete the composite injection molding between the surface material and the plastic skeleton material; The decorative fabrics for low-pressure injection molding are various, and the commonly used types of materials are: leather, PVC skin, TPO skin with PP-Foam, PUR skin, knitted fabric with non-woven fabric, fiber felt, etc.

Many mid-to-high-end car interior parts adopt low-pressure injection molding technology, such as Mercedes-Benz and BMW door panel inlays, and so on. Low-pressure injection molding is a unique in-mold decoration technology, similar to IMD injection molding, an injection molding process that directly injects fabric or skin into the product. If the injection pressure is too high, it will damage the fabric.

According to the different characteristics of the molding process, it can be divided into open mold low-pressure injection molding and closed mold low-pressure injection molding. The difference between these two types of injection molding is whether there is a certain gap between the movable mold and the fixed mold when the mold is closed. The precise amount of melted plastic is injected smoothly during the entire injection-filling process. Because the flow ratio of the plastic melt on the fabric is much slower than the smooth mold cavity, the low-pressure injection mold needs more gates than the ordinary mold.

The difference between low-pressure injection mold and ordinary injection molding:

  • The low-pressure injection mold has a spring plate, its function is to fix the fabric in the mold, and at the same time, the spring block presses the fabric to prevent the material from wrinkling. Common injection molds do not have.
  • The low-pressure injection mold has air claws, which are used to fix the fabric before closing the mold. Common injection molds do not have.
  • The low-pressure injection mold has a synchronous hook, which ensures that the mold moves synchronously and avoids the risk of mold collision. Common injection molds do not have.

Factors to be considered in the low-pressure injection molding process

The fabric

Fabric stretch and tension

The fabrics used for low-pressure injection molding are the bottom layer of non-woven fabric layer + foam layer/sponge layer + surface knitted fabric layer or soft plastic layer (PVC). The fabric’s performance includes longitudinal static elongation, longitudinal residual elongation, transverse static elongation, transverse residual elongation, and so on. Due to the compression of the mold and the molten plastic, the longitudinal and lateral extensibility of the fabric is different, which leads to the phenomenon reflected on the product also being different.

In general, the greater the fabric elongation, the less likely to deform, and it is prone to wrinkle problems during injection molding production. The smaller the fabric elongation, the easier to deform, and it is less prone to wrinkle defects during injection molding.

The most prominent problems are seepage, breakdown, and damage to the fabric. That is to say, when the plastic mold state is constant, and the process conditions are consistent, the fabric’s characteristics greatly influence the product’s quality. The same elongation, longitudinal and transverse, have different adaptability to different molds, some molds may require high longitudinal elongation due to design restrictions, and some may have high requirements for transverse elongation. Therefore, this issue must be considered comprehensively when trial-manufacturing new products and molds.

Mold structure

The melt index MI value is the indicator for evaluating the fluidity of plastics. Everyone knows plastic flow on the fabric must be much slower than on the smooth mold cavity. Therefore, low-pressure injection molds need more gates than ordinary molds. Low-pressure injection molds usually choose the form of point-gate sequence valves, and the number of gates is large. For example, the total length of the product is L=1000mm, the wall thickness T=2.5mm, and the recommended gate number is =1000/2.5/F

(PP material F value is less than 100, ABS or PC material is less than 80)

The slow flow rate will inevitably affect other appearance problems of the product, such as weld line marks, short shots, etc. Therefore, it is necessary to pay attention to the design of the number of gates.

The air venting design for low-pressure injection mold cavities is also very important. The PVC skin is not breathable compared with knitted fabrics. Therefore, in the structure of the low-pressure injection mold, you should pay more attention to the air venting of the plastic mold cavity and core when you use PVC skin during low-pressure injection molding.

Other technical requirements:

  • The R angle at the corner of the product is not less than 3.0;
  • The design of the thickness of plastic wall for the product should not be too thick to prevent the development from shrinking;
  • The thickness of the ribs should not exceed 3/5 of the thickness of the plastic wall;
  • Deep-hole products need to be split;
  • The edge of the product appearance needs to leave the place where the turned-up fabric works for wrapping;
  • There should be no ribs on the back of the product, etc.

Common Defects and Improvement Plans of Low-Pressure Injection Molding Process

(1) breakdown

The breakdown easily occurs at the sharp corner of the product. If there is a gap in the parting surface at the sharp corner, the molten plastic will penetrate from the gap when the material amount or speed increases. Also, it is easy to cause a breakdown when the wall thickness at the sharp corner is too thick.

(2) seeping material

Seeping is easy to occur at the corner of the product.

Improvement plan:

  1. The wall thickness at the sharp corner is too thick to cause breakdown easily, so the wall thickness at the sharp corner of the mold is relatively thinner to prevent fabric seepage or damage;
  2. Product preventive design, try to make the transition of the product shape as smooth as possible, reduce the step difference, and avoid the rapid turning on the product;
  3. Adjust the injection molding process parameters, reduce the mold temperature and melt temperature, reduce the amount of plastic injection, reduce the injection pressure, etc.

(3) Sink mark at the gate

To avoid the gate mark problem, the gate diameter should be below Φ3, and increase the length of the cold runner should be at least 15 mm to longer the cooling time.

(4) Short shots

The sharp corners of the product are prone to short shots.

The causes:

  • The mold temperature is too low;
  • The gate temperature in the short shot is too low;
  • The amount of plastic for the gate near the short shot is controlled too little;
  • The wall thickness of the sharp corner is too thin;
  • The injection speed and compression speed are low.

(5) The fabric is crushed

Usually, the mold temperature used in low-pressure injection molding is low (10-15 °C). If the anti-rust doesn`t do well, there will be rust spots on the fabric-pressing frame and the parting surface of the slide. If the pressing effect on the fabric during production is excessive, the fabric will be damaged because of limited extensibility.

(6) The R corner is bright

Check whether the R angle is at a sharp corner. Does the product have a large warping angle but a slight R angle? Does the ductility of the skin meet the requirements?

(7) Skin scratched

If this problem occurs, we should consider the following:

Does the demoulding angle sufficient?

Does the surface polishing of the mold meet the requirements?

Low pressure injection molding defects and solutions chat

Sink mark / color difference around gateGate is large will lead to sink mark;Gate diameter can`t exceed 2.5mm;
 Cold gate is short, hard to cool lead to sink mark;The length of cold gate at 15mm is OK;
 Cooling time is not enough lead to sink mark. Increase the cooling time.
Fabric/leather damaged or bright spotProduct structure exceeds fabric stretch ratio.( Refer to Q/SQR.04.998-2008 4.5, static elongation of B21 fabric under 50N: warp > 25%, weft > 50%)Product design needs to refer to the stretch ratio of the fabric, and the design of sharp corners requires stretch ratio analysis.
 The mold parting surface design have many sharp points and dead corners, which exceed the stretch ratio of the fabric;The parting surface of the mold is designed to be as smooth as possible, and carry out the stretch ratio analysis.
 The temperature of the melt plastic is too high, the fluidity of the plastic is too good, and it penetrates the non-woven fabric;Lower the temperature of melt plastic material.
 The MFR value of the plastic is too large;Appropriately reduce the MFR value of the plastic.
 The thickness of non-woven fabric is too small;The non-woven fabric weight of B21 automotive pillar is suitable for 300±15g.
 Fabric stretched too much, parting surface of mold fit too tight; 
 The draft angle is too small, and the friction with the mold causes the side to shine;Appropriately increase the draft angle.
Product parting surface has sharp cornersparting surface design errorAfter the mold closed, the fabric will formed an R angle at the parting surface position, and an extended sharp angle will be formed at this R angle position during injection molding.
WrinkleWrong product structure leads to fabric stacking and wrinklingOptimize product structure.
 The design of the parting surface of the mold leads to the overlapping of fabricsDesign parting surfaces as smooth as possible.
 The fabric hanging structure is unreasonableRational arrangement of hanging nails.
 Two streams of melt plastic push the fabric to pile upSequence valve hot runner to control the melt plastic flow with a complex profile.
Short shotProblems similar to ordinary injection molds, such as pressure, flow length ratio, local wall thickness is too thin, etc. 
 The fabric stretch ratio is too small to block the flow. 
Parting surface position has flash Control the gap of the skin and the machining accuracy of the parting surface.
Defects at the parting line of the slideThe slide on the movable side of the mold causes defects in the parting line on the A side of the productReasonable design of mold structure, while manufacturing, to achieve mold processing accuracy

Advantages of low-pressure injection molding process compared with traditional cladding process.

(1) Low-pressure injection molding integrates the skin material with the plastic injection, and there is no possibility of falling off.

(2) Since no gluing process is necessary for the coating process, the low-pressure injection molding process is more environmentally friendly.

(3) The internal structure of the low-pressure double-layer injection molding part can be designed arbitrarily, and the degree of freedom of the surface shape is more significant than that for the wrapping process. The shape features are clear and structured well.

(4) Low-pressure injection molding does not require additional secondary coating, which improves production efficiency.

(5) Low-pressure injection molding products have a good feel and appearance.

(6) Low-pressure injection molding products have low injection pressure, which can effectively protect the product’s internal parts, and the scrap rate is low.


  1. The process is relatively complicated, susceptible to the influence of mold and the flexibility of the fabric/PVC skin, and the scrap rate is relatively high.
  2. Pattern retention, at high tensile places, the three-dimensional effect of the pattern is poor.
  3. The cost of mold tooling is higher, and the craft fabric is also more expensive.

Low-pressure injection molding has the following characteristics considering the above advantages and disadvantages:

  • Mold: parting surface with clamping gap, and gates are controlled by sequence valve (controlled by valve type hot runner).
  • Manipulator: It has both the function of clamping fabrics and the function of picking up products
  • Composite layer: PVC/PU, fabric, leather, etc.
  • Plastic material: modified PP and PC/ABS with good fluidity, etc.

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 process:

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:

Automotive interior manufacturers

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%.

  1. 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.

  1. 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.

  1. 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).

  1. 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℃.

  1. 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 ℃.

  1. Thickness

TPO: 0.5~1.2mm;

TPO+back foam: 2.0~3.5mm

  1. 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.

  1. 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.


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 site display:

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.

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