Plastic buckets are used for industrial, commercial and household applications. They are highly functional items widely used to store and transport products in the form of liquid, pellets, powders, granules and other forms. Paints, oils, cleaning products, yoghurts, tablets, grains, and seeds are some examples of products stored in buckets. The sizes of buckets can vary from a litre to tens of litres. They can be produced from different materials, most commonly plastics.
Injection molding has long been the industry standard process for bucket manufacturing. This article reviews some common issues that may occur in injection molding of plastic buckets and how they are troubleshooted
Materials for Manufacturing Buckets
While plastics have become the most common materials for manufacturing buckets, other materials have been used over the years. These include metal, wood and rubber.
Due to their lightweight, waterproof nature and durability, plastics have become more popular materials for production of buckets. Polypropylene and high density polyethylene are the two most common plastics used in the production of buckets.
PP buckets are better suited for applications where higher temperature tolerance may be required for example in pharmaceutical or food products where the bucket may need to be used to carry hot products following cooking or sterilization.
HDPE buckets are chosen for their impact resistance, leakage proof and resistance to a wide range of chemicals. HDPE is also preferred for low temperature applications such as ice cream buckets
Buckets are also manufactured using ABS and LDPE. These are generally buckets intended for light duty, single use applications requiring less durability.
Key features of a Bucket
A bucket can be distinguished from other containers such as barrels, drums, and jerrican based on:
-Size
-Shape
-Parts
Innovators have patented different designs of plastic buckets over the years to meet different functions and aesthetics.
Buckets are generally between 10 litres on the small end to 100 litres on the larger end for industrial uses. Most common sizes for household and general applications are between 20 and 50 litres.
They are typically round in shape with a handle which typically runs from a point on the circumference to the opposite point of the circumference. The handle is typically made from the same type of plastic or from a metal. They can be either open top where they either have no covers or the cover simply fits on or gets fitted with a screw.
For buckets with multiple parts, an assembly stage is required after the molding stage. Some injection molding processes may use multi cavity systems to mold all parts in one cycle or have dedicated injection molding machines for each part (bucket, cover, handle) separately.
Process for Manufacturing Plastic Buckets
Plastic buckets can be manufactured using a number of different plastics processing technologies. Most common and well established ones are compression molding, thermoforming, blow molding, and injection molding.
Each process has its advantages and disadvantages. Factors that determine the best suited process for a particular design include size, thickness and design complexity. Injection Molding is particularly preferred for large scale manufacturing of durable buckets with good strength. Injection Molding can be used for both simple and complex bucket designs.
The Challenges in Bucket Injection Molding
Screw cap buckets
Injection molding of screwed edges always poses mold design challenges. Simple push covers which simply fit on are less complicated to mold. The main challenge with screw cover is designing the ejection system.
Buckets with Thin Walls
Heavy duty buckets may have thicker walls to resist impact and bear heavy loads. Buckets intended for lighter duties or single use are generally made with thinner walls. This could be for a combination of reasons like minimizing materials cost, portability, and application requirements. For example ice cream buckets or buckets used for food packaging which are intended for single use or more temporary use. These walls pose specific injection molding challenges and may lead to some of the defects discussed in the next sections.
Undercuts
The rolled rim of the bucket is the most prominent example of undercut in injection molding of plastic buckets. The puncture hole for fitting the handle, ribs that may feature on the edges to improve strength and other special features are also examples of undercuts in plastic buckets. These require more sophisticated ejection systems in the mold design.
Thickness non uniformity
Buckets may require the base to be thicker than the sides of the buckets. This nonuniformity poses design and processing challenges. Furthermore, a minimum draft angle is generally required to aid product ejection. Therefore other mold design parameters like position and size of gate, filling pressure and temperature must take this into consideration.
Assembly
Parts must retain dimensional stability to fit perfectly even when molded separately. For example the pin hole for the handle must have a diameter that corresponds with the connection point on the handle. If too much shrinkage, warpage or loss of dimensional stability occurs or shrinkage is not properly considered on the mold design then you might find a whole batch of buckets with covers or handles that don’t fit. This leads to losses.
Possible Problems and Fixes in Bucket Injection Molding
Even after setting the optimum parameters for your bucket injection molding, oftentimes one may still run into some issues that will require adjusting some parameters and doing a few checks, most likely in the first cycle. This is because simulations and calculations don’t always fully represent reality. Below we look at some problems peculiar to bucket manufacturing and how to troubleshoot such.
Short shots
This is when the melt injected into the mold is not sufficient to fill the mold cavity. Troubleshoot this by adjusting melt temperature and injection pressure. If these don’t fix it adjusts the check valve to increase melt volume for the cycle. Ensure proper venting to make sure the melt flow isn’t getting obstructed by air pockets.
Flash
Here, a thin sheet of plastic protrudes from the finished part through the parting line of the mold or gap in ejector pins.
Address this by increasing clamping pressure to prevent leakage, check that mold is not damaged or reduce filling pressure.
Internal voids
This defect occurs as cavities within the part structure. This causes structural weakness. It may occur as a result of insufficient filling or clamping pressure. Troubleshoot this by adjusting the pressure and ensure proper venting
Warping
This occurs when the part does not conform to the design shape. It is caused by residual pressure and uneven stress and temperature distribution. To fix this, ensure even temperature distribution by adjusting melt and mold temperature. Adjust filling and clamping pressure.
Burn mark
As the name implies, this is as a result of overheated parts getting burnt and applying dark spots on the part. To fix this Ensure you use well compounded pellets, even temperature distribution by adjusting cooling channels and the mold temperature.
Sink marks
Depressions of the surface of the buckets caused by low pressure, high melt temperature or inefficient cooling system. Fix this by adjusting the melt temperature and increasing mold pressure and ensure optimized uniform cooling.
Color streaking
This can be observed as uneven coloration on the part. This is often due to poor blending of pigment and additives with the plastic. To address this, ensure you use compatible additives and use well blended compounds.
Weld lines
This occurs when two flowing streams of fluid fail to merge and blend at a meeting point. This is often due to low melt temperature or insufficient injection pressure. Troubleshoot these by adjusting temperature and pressure accordingly
Conclusion
Plastic buckets pose some peculiar manufacturing challenges due to their design requirements and key features. With good understanding of the manufacturing process and implementation of adequate measures, defects can be avoided.