Compared to metals and ceramics, resin molding can produce large quantities of high-precision products at low cost. In addition, since it is cheaper and has excellent mass production compared to processing methods such as cutting and pressing, there are many cases where parts that used aluminum or other materials at the prototype stage are changed to resin at the mass production stage.
However, there are also characteristics such as injection and blowing defects due to abnormalities such as viscosity, temperature, and pressure during molding, and shape defects due to mold wear. Furthermore, due to the characteristics of the resin, deformation may occur due to residual stress or thermal stress even after molding. These defects are fatal, especially for assembly and mating parts that require precision, and are major problems that directly lead to product malfunctions and assembly and mating failures.
This arctile introduces the main resin molding methods and types of troubles, prototype measurement in development and design, and measurement methods to prevent defective outflow in mass production. In addition, we will introduce the challenges of conventional measurement methods and solutions to solve them.
What Is Resin Molding?
Resin molding is a processing method in which melted resin is injected into a mold by heating, formed into a predetermined shape, cooled, hardened, and taken out. Taking advantage of the characteristics of various resin molding materials, we mold them in a method suitable for the application and shape.
As molding materials, pellets made of rice granules are used for thermoplastic resins, which become plastic when heated, and powdered resins are used for thermosetting resins that solidify upon heating. They may also use pellets or powders that mix the resin with additives or colorants that improve their functionality.
In order to perform resin molding stably, it is important to manage molding conditions such as mold temperature, amount of resin material, and injection speed. In addition, in each process, attention should be paid to contamination with dirt and dust, and foreign matter adhering due to static electricity.
Types Of Resin Molding
The resin molding methods can be mainly classified into injection molding, blow molding, extrusion molding, and calender molding, and among them, injection molding and blow molding are widely used as representative molding methods. Here, we will explain these two molding methods.
In injection molding, a melted resin is injected and filled into a mold to mold. Since resin is injected and filled at high pressure, even products with thin walls and complex shapes can be molded at high speed, making them suitable for mass production. It is mainly used for molding thermoplastic resins, but in rare cases it is also used for molding thermosetting resins.
The basic principles of injection molding are as follows:
(1) The material resin is added from the hopper and heated and melted to plasticize.
(2) Mold into a certain shape using a mold or the like.
(3) Cool and solidify into the shape of a mold.
After that, the molded product is removed from the equipment and made into a resin product through various processes and inspections. The main products made by injection molding are:
- Smartphone cover
- Electrical appliance housings and plastic models
- Toilet seat in the bath or toilet
- Automotive bumpers and interior panels
In this way, it is used for molding and mass production of a wide variety of resin products, from small and medium-sized products to large parts, so it can be said to be a representative resin molding method. Injection molding is further divided into the following methods, depending on the product to be manufactured:
This is a molding method in which metal screws and terminals (insert products) are inserted into a mold in advance, resin is injected around it, and molded in one piece (composite molding).
Multicolor Molding And Dissimilar Material Molding:
This is a molding method in which resins of different colors and materials are combined and molded into an integrated manner.
Film Insert Molding And Film-In-Mold Molding:
Film insert molding and film-in-mold molding are used to decorate the surface of logos and letters of resin products, membrane switches for electrical products, and automobile interiors (plastic parts such as shift panels), and are called “decorative molding.”
Film insert molding is a molding method in which a film (decorative film) printed with a pattern, gloss, matte, etc. is set in advance in a mold, and the film and resin in the mold are bonded together by heat and pressure during injection molding. Film-in-mold molding is a molding method in which a decorative film is set in a mold and the decoration of the film is transferred to the resin during injection molding.
Blow Molding (Hollow Molding, Blowing Molding)
In blow molding, heat-melted resin is extruded into a pipe shape, sandwiched between molds, and air is blown in from the inside to inflate and mold. It is suitable for the production of hollow resin molded products, and is also called “blow-in molding” or “hollow molding” because of the production method. The main products made by blow molding are:
- PET bottle
- Containers for liquid cosmetics, liquid detergents, etc.
- Piping Fittings
When filling liquids, resins may be used differently depending on the characteristics of the liquid. For example, if it is necessary to prevent oxidation due to oxygen permeation, resins with gas barrier properties are used. When chemical resistance is required, multiple molding materials containing resins that are not altered by chemicals are molded in layers.
There are various blow molding methods depending on the product to be manufactured. This section describes typical blow molding methods.
Extrusion Blow Molding (Direct Blow Molding):
The basic structure and principle of the extrusion blow molding machine are as follows:
(1) Extrude the molten resin.
(2) Cylindrical parison (hot parison) is molded with a die.
(3) Do not cool or solidify, put it in the mold.
(4) Mold by blowing air.
Injection Blow Molding (Injection Stretching/2-Axis Stretch Blow Molding):
The basic structure and principle of the injection blow molding machine are as follows: A typical product is a PET bottle made of polyethylene terephthalate (PET).
(1) The thermoplastic resin is injection molded in advance as a test tubular preform (cold parison).
(2) The preform is heated and stretched into the mold with a stretching rod.
(3) Molding by blowing high-pressure air.
Multilayer Blow Molding:
For Parison, we use materials made by co-extruding ethylene-vinyl alcohol copolymer (EVOH), which has a high gas barrier, to prevent oxidation and deterioration of the contents and to improve strength. Therefore, this molding method is suitable for molding resin containers such as cooking oil, seasonings, and gasoline tanks.
The basic principles of multi-layer blow molding are as follows:
(1) Two or more material resins are co-extruded to form a multilayer preform (cold parison).
(2) Heat the preform (cold parison).
(3) Blow molding by blowing air.
3D Blow Molding:
Unlike extrusion (direct) blow molding in which parison is inserted in the vertical direction, it is possible to mold three-dimensional shapes that avoid material drawdown and burrs.
This molding method can produce high-quality products with complex shapes with curved parts and bellows, such as hoses and piping in air conditioners.
(1) Fill the accumulator with resin with an extruder and mold the parison.
(2) Perform air suction from the opposite outlet. The parison follows the shape in the mold to the bottom of the mold.
(3) Blow molding by blowing air.
Causes Of Poor Assembly And Mating Of Resin Molded Products
Resin molding is easier to process than metal, and by using various materials, the hardness and weight can be changed, and the color and shape can be freely expressed. On the other hand, slight differences in material temperature, filling amount, and slight strain in the mold can cause warping, waviness, strain, short shots, etc., which can cause assembly defects and mating failures.
Poor mating is a defect in which resin products are not matched in size and cannot be bonded or gaps are formed. In addition, forcibly assembling parts that have a mating defect may damage the parts, and parts that require airtightness may cause leakage of contents.
- Warping (Sledding/Sledding) And Swells
The warping and undulation of resin molded products is a state in which the workpiece is curved like a bow, and is sometimes called bending or twisting. In addition, warpage may be forward or reverse warp depending on the direction.
On the other hand, waviness is an unevenness on the surface formed by a combination of warps in various directions. Warpage and waviness of resin molded products are mainly caused by the following:
- Difference in resin shrinkage
- Uneven mold temperature
- Difference in shrinkage of molten resin depending on flow direction
Difference In Resin Shrinkage:
Warpage occurs when there is a shrinkage difference in the resin due to variations in temperature and pressure in the cavity. The magnitude of the shrinkage of the resin in the cavity is proportional to temperature and inversely proportional to pressure. For this reason, for example, if one workpiece has a thin part and a thick part, the thin part shrinks less due to the low temperature, and the thick part shrinks a lot because heat is stored. This difference in temperature causes differences in shrinkage in the cavity, causing deformation called warping.
2. Mold Temperature Unevenness:
If the temperature of the mold is uneven, the resin in the cavity will shrink and warp will occur. For example, if the cooling time of a mold varies from part to part, the amount of shrinkage of the resin will vary, causing warpage.
Difference In Shrinkage Of Melt Depending On The Flow Direction:
Since the shrinkage rate differs depending on the flow direction of the resin, warping and waviness occur. When glass fiber is blended with the resin, the fibers are oriented in the direction in which the resin flows abundantly. Therefore, the shrinkage in the direction in which the resin flows is smaller, and the shrinkage in the direction perpendicular to the flow of the resin is larger. This is called “anisotropy of contraction due to fiber orientation”, and warpage occurs due to this.
Strain of resin molded products is a phenomenon in which the entire workpiece twists or warps, and is mainly caused by residual stress. Residual stress in resin molding is the internal strain that remains in the resin molded product, and there are “tensile residual stress” in which the outward force remains and “compressive residual stress” that goes inward.
“Tensile residual stress” is the force that pulls the inside toward the outside of the molded product generated when the molten resin injected into the mold under high pressure cools from the outside in the mold and solidifies (solidifies). “Compressive residual stress” is the force that the pressure applied to the resin during molding or processing tries to compress it inward.
Resin products with residual stress not only impair dimensional accuracy due to shrinkage and stretching, but also cause deformation and cracking due to the reaction of residual stress of resin products during machining such as cutting and grinding, heating such as welding, and processing and processing with paints and solvents. In addition, not only during manufacturing and processing, but also during aging, strain may occur and the shape may change, so special attention is required.
For this reason, a treatment called “annealing (annealing)” may be performed to remove residual stress.
- Short Shot
A short shot is a phenomenon in which the resin does not reach every corner of the cavity of the mold, causing some parts of the molded part to become incomplete.
The main causes are “fluid tip solidification,” in which the resin cools and solidifies before reaching the end and a part of the molded product becomes incomplete, and “air traps,” in which the resin cannot flow smoothly into the mold.
Challenges Of Measuring Resin-Molded Mating Parts
It is very important to confirm that the resin molded product has the desired dimensions (within tolerances) and shape. In particular, the mating part of a part with a complex shape affects the combination work and the degree of sealing, so accurate and quantitative measurement of 3D shape is required.
In the past, measurements were made using coordinate measuring machines and calipers, but it was difficult to measure accurately with 3D measuring machines, and there were various problems with calipers, such as variations depending on the operator.
Challenges Of Measuring Mating Parts With Coordinate Measuring Machines
In general, in order to measure warpage and waviness with a coordinate measuring machine, it is necessary to place the contactor of the probe tip at four or more corners of the surface to be measured on the object.
For example, in the case of plate members, it is common to measure 6~8 points. If the measurement range is wide, more measurement points can be used to obtain measurements for more locations and improve measurement accuracy. However, in the measurement of warpage and waviness, there were the following problems.
- Since measurements must be made in contact with points, it is difficult to grasp the shape of the entire object.
- Multi-point measurements to get more measurements take a lot of time and don’t give you a detailed picture of the whole picture.
Challenges Of Measuring Mating Parts With Calipers
Hand tools such as calipers are very easy to measure. However, there are several factors that can cause measurement errors and variability of measured values.
For example, the amount of calipers measured differs depending on the person, such as the force of pressing the caliper against the measurement point by hand (measurement force) and the variation in the measurement point. As a result, the measured values vary and quantitative measurements become difficult. In addition, warping and swelling over a large area required measuring many points, which took a long time.
How To Solve The Problem Of Measuring Resin Molded Mating Parts
A review of the challenges of conventional measuring machines reveals a commonality. That is, it is measured while contacting a three-dimensional object or measurement point with a point or line.
3D scanner-type coordinate measuring machines solve these measurement problems. The 3D shape of the object can be captured without contact and accurately in the plane. You can 3D scan an object on the stage in as little as 1 second to measure the 3D shape with high accuracy. Therefore, it is possible to perform quantitative measurement instantaneously without variation in measurement results. Specific benefits include:
Advantage 1: Scan 3D shapes on surfaces. Warping and undulation are obvious at a glance
Measuring resin products with complex shapes with a coordinate measuring machine or calipers required a long time because many points had to be measured. In addition, it was difficult to measure the 3D shape because it was measured in points. With the 3D scanner, all you have to do is place the object on the stage and scan it. Non-contact and no positioning required, it captures the 3D shape of the entire object in a plane. Since it is possible to display the entire object and measure the profile of any part, it is possible to visualize and grasp the defective shape part and its detailed numerical value.
As a result, it is possible to smoothly check the mold and molding conditions of the trial product, and to identify the cause of assembly defects in mass-produced products and take countermeasures. Of course, since quantitatively measured shape data can be obtained, warpage and waviness can be managed with tolerance values (tolerances) and used for trend analysis.
Advantage 2: Visualization of differences from 3D-CAD data in color
The acquired data can be compared with the designed 3D-CAD data, and the actual finish of the workpiece can be visualized against the design. By comparing the acquired data with the acquired data, it is possible to clarify defects that were previously unknown, even for workpieces that were difficult to measure in the past, so it is effective for investigating the part of the mating defect and pursuing the cause. In addition, it can handle various dimensional measurements, and by precisely measuring the dimensions of the part of concern, it is possible to analyze the problem in detail.
Advantage 3: Cross-sectional analysis is possible without cutting the sample
Cross-sections, which were difficult to measure in the past, can be non-destructively created and measured and analyzed in detail. The reference surface can be freely set from the 3D shape data, and cross-sectional measurement from all directions can be performed.