Plastic material’s usage is growing across the automotive industry. There are various methods used to manufacture plastic components based on the requirements. However, it is seen that Injection moulding is the most common process employed.
Let us try and understand the kind of defects which are observed in components manufactured by injection moulding. Components manufactured by using injection moulding process may have the following defects.
- Tearing of the component during ejection.
- Cracking of component during ejection or solidification.
- Local shrinkage (cavity formation on surfaces).
- Underfilling of the component.
- Loss of stiffness at some zones.
- Tearing of component at corners
- Non-uniform solidification resulting into variations in strength at different zones.
To minimize these defects, production engineer uses number of different methods like ensuring correct temperature & viscosity of molten plastics, maintaining correct duration of injection and so on. However, the focus of our discussion is to control these defects through design parameters.
Introduction of “Draft”
Plastic components are delicate due to their very small thickness. Component surfaces which are parallel to the direction of mould separation and component ejection are prone to get torn off. This happens due to rubbing of the component surfaces at the cavity mould separation (1) and at the core mould during ejection (2) of the component from the core.
These locations are required to have a Draft angle around 1 to 3 inches. This draft helps the part to get released from the mould easily without rubbing.
Introduction of Appropriate “Corner radius”
A very sharp corner is more prone to tearing. Further, due to proportionately larger surrounding area for cooling, the corner cooling is proportionately faster and creates more tendency for cracking. Instead, a relatively smooth and blunt radius at the corner offers ease at separation from moulds, relatively steady solidification and ease at propagation of molten plastic while filling of the mould cavity.
Accordingly, a minimum corner radius of round 1 to 2 times the component thickness is required. Still larger corner radii like 10 mm, 15mm are further preferred; however, they generate a major change at the component shape. These corner radii are called as fillets and at times they are a “3-D” features also.
Avoiding Non-uniform Thickness at Junctions
Junctions of wall when consist of different wall thicknesses, undergo non-uniform solidification and cooling. Smaller wall thickness leads to faster solidification whereas larger wall thickness leads to slower solidification. Accordingly, locations like (1) are more prone for cracking and cations like (2) are more prone for local shrinkages formation of blind cavities. However, if the wall thicknesses at the junctions are kept same then the possibility of local cracks and local shrinkage gets avoided
Avoiding Abrupt Changes in the Wall Thickness
Changes at wall thickness like (A) are prone troubles like local shrinkage or cracks. They should basically be avoided as far as possible. However, if they are unavoidable, they must be converted to generate a gradual change like B.
The change “B” is not trouble-free, but the troubles shall be lesser as compared to A.
Avoiding too small thickness for a longer zone
Molten plastic must not be compelled to travel a large distance through a small passage component thickness. In such cases the leading plastic may face a problem of getting solidified before the molten plastic reaches its destination. Hence, smaller thicknesses around 1 mm should never have a longer zone for plastic propagation. If the thickness is more like 2-4 mm then the longer zones can be accommodated.
Provision of Ribs
Plastic wall thickness undergoing a 90° change in the orientation at times are not sufficiently strong to bear the loads. The ribs with appropriate base and height can be provided. The thickness at any rib is expected to be same as the parent wall thickness. The ribs normally are of a triangular shape. However, they can even be concave if one is likely to create some fouling at the assembly.
Provision of Embossing
Large plain surfaces with no change in orientation cannot allow any ribs. In such places, use of embossing can improve the strength and stiffness of the object. The embossing can have a decorative appearance, or it can be in the form of the manufacturer’s logo or emblem.
Recruiter’s expectations are moving up
You know that as a mechanical engineering graduate, you are going to apply for a job in companies working in different domains like automobile, plastic, heavy machines, aviation etc. It is obvious that companies working in plastic domain would look for engineering grads with some basic understanding on plastic component manufacturing processes and design considerations.
Few years back, learning about domain of plastic manufacturing was an on-job learning activity. After getting on to the job, engineers were trained for it. But, now-a-days, expectations of recruiting companies are moving up. Even at the level of fresher, they are looking for some basic knowledge about plastic component design and manufacturing processes. Pursuing a course like CADCAMGURU Product and tool Design Engineer can prepare you for such risen expectations.
With engineering drawing, tolerances and GD&T, if you know basics of plastic manufacturing, it will be an additional weapon in your arsenal. And this can surely help you to differentiate yourself from the crowd in the job selection process.